An Annual Blue Carbon Budget for Kelp Forests and Seagrass Beds

An Annual Blue Carbon Budget for Kelp Forests and Seagrass Beds | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article An Annual Blue Carbon Budget for Kelp Forests and Seagrass Beds Kira Krumhansl, Melisa Wong, Manon Picard, Meredith Fraser, Carrie-Ellen Gabriel, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6473319/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 02 Jan, 2026 Read the published version in Communications Earth & Environment → Version 1 posted You are reading this latest preprint version Abstract Coastal vegetated ecosystems (e.g. kelp forests, seagrass meadows) potentially play a significant role in oceanic carbon sequestration, yet existing estimates have high uncertainty because they are compilations of few data from disparate species and regions, or focus on individual stocks or fluxes. We use empirical data and modeling to generate detailed carbon budgets for kelp forests and seagrass meadows in a single region (Nova Scotia, Canada). We estimate kelp forests to sequester more carbon (0.313 ± 0.144 Tg C y − 1 , 27 ± 19% of annual net primary production [NPP]) than seagrass meadows (0.011 ± 0.004 Tg C y − 1 , 9.5 ± 5.2% of NPP) by an order of magnitude. A substantially higher proportion of carbon sequestered by kelp forests is from the production and export of dissolved organic carbon (DOC) (98.5 ± 60.1% of carbon sequestered) than for seagrass meadows, which primarily sequester carbon through shelf burial and the export of particulate organic carbon (POC) to the deep sea (63.6 ± 43%). Our results show how detailed carbon budgeting can highlight undervalued or overlooked carbon pathways (e.g. POC to DOC conversion), and support more robust valuation and management of blue carbon ecosystems for carbon sequestration. Earth and environmental sciences/Biogeochemistry/Carbon cycle Earth and environmental sciences/Climate sciences/Biogeochemistry/Carbon cycle Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Coastal vegetated ecosystems (e.g. seagrass, salt marsh, mangroves, kelp forests) cycle and store carbon in our oceans, and are increasingly being recognized for their contribution to long-term carbon sequestration [ 1 – 3 ]. These ecosystems (i.e. blue carbon ecosystems, BCEs) are threatened by a variety of stressors, including climate change, fisheries, and coastal development, which can reduce their capacity to sequester carbon and release carbon stored in sediments, further exacerbating climate change [ 4 , 5 ]. The protection, restoration, and natural enhancement of these ecosystems have therefore been proposed to help limit further emissions and draw down additional carbon as a climate change mitigation tool [ 6 ]. Further, there is increasing interest in including coastal BCEs in national carbon inventories and voluntary carbon markets. However, knowledge of the contribution of BCEs to long-term carbon sequestration is not well resolved largely because we lack basic data on carbon fluxes and their links to pools of stored carbon [ 7 , 8 ]. Existing estimates of the global contribution of kelp forests and seagrass beds to carbon sequestration are coarse compilations of studies from disparate geographic regions, species, and time periods, within which most flux estimates were derived from a small number of studies [ 2 , 3 ], focus on only a subset of fluxes [ 9 ] [ 10 ], or rely mainly on quantifying sediment carbon stocks [ 11 ]. Applying first order global estimates on local and regional scales may be a good first approximation, but can lead to over- or underestimating the contribution of BCEs to climate change mitigation, which can misguide efforts to limit further warming. Detailed accounting of the fluxes and stocks of carbon within a given BCE kelp forest or seagrass system (i.e. a carbon budget) is now needed to reduce uncertainty in estimates of carbon sequestration. Considering a system as a whole provides a more accurate picture of the relative importance of different carbon pathways and may yield contrasting patterns to general paradigms. It can also provide insights into sources of variability that need to be characterized to improve overall estimates, and can be used to direct research towards the most critical parameters to resolve. Ultimately, this information provides insights into carbon pathways that may be most at risk from human activities, and inform how BCEs should be managed to maintain their capacity to cycle and store carbon in the long-term. Critically, this information supports a more robust appraisal of the potential for activities like BCE protection, enhancement, and restoration to help mitigate climate change [ 7 , 10 , 12 ]. Results Carbon stocks and Net Primary Production We developed detailed carbon budgets for kelp forests and seagrass beds along the Atlantic coast of Nova Scotia, Canada using new and existing field measurements, lab and field experiments, modeling, and a literature synthesis to reduce uncertainty in regional estimate of carbon sequestration and demonstrate the broad utility of carbon budgeting. We first estimate carbon stocks contained in sediments and living biomass and net primary production (NPP) rates for the two dominant kelp species ( Saccharina latissima and Laminaria digitata ) and eelgrass ( Zostera marina ). All estimates are from field measurements scaled to the region using species distribution models (Krumhansl et al. in revision ) [ 13 ]. Kelps are estimated to occupy nearly 6 times the area (1400 km 2 ) of seagrass (240 km 2 ) in our region (Fig. 1 A-C), and contain more living biomass by an order of magnitude, though this is not a long-term carbon sink (Figs. 2.1, 3.1) [ 14 , 15 ]. Seagrass beds also contain stored carbon in the underlying sediments, comprised of locally fixed carbon (i.e. autochthonous, e.g. seagrass leaves, roots, and rhizomes) and carbon imported from adjacent marine and terrestrial ecosystems (i.e. allochthonous). We estimate the regional carbon stock in eelgrass sediments at 1.77 ± 1.34 Tg C, which exceeds carbon contained in living biomass by either species group (Figs. 1 D, E, 3.2). From literature carbon burial rates [ 16 ] and source contributions to buried carbon [ 15 ] we estimate that 1.6 ± 0.95% of seagrass NPP (25% of total carbon buried) (Fig. 3.3) and 0.23 ± 0.25% of kelp NPP (35.5% of total carbon buried) (Fig. 2.2) is buried within seagrass beds on an annual basis [ 15 ]. Kelps have a higher NPP than seagrass per m 2 and for the bioregion (kelp = 985 ± 516 g C m − 2 y − 1 ) (seagrass = 465.17 ± 188.11 g C m − 2 y − 1 ) (Fig. 1 F, 1 G, 2.3, 3.4). NPP for kelps and seagrass exceed that of phytoplankton on the Scotian Shelf per m 2 (123 g C m − 2 y − 1 ) [ 17 ] and represent ~ 10% and 1% of annual NPP (11.58 Tg C y − 1 estimated using a study domain of 94,193 km 2 ) [ 17 ]. Grazing accounts for a relatively small percent of NPP for both species groups (kelp: 0.7 ± 1.1%, seagrass: 7.5 ± 4.2%) (Figs. 2.4, 3.5) [ 18 ] [ 19 ]. Particulate Organic Carbon The majority of primary production by kelps is eroded off the distal ends of blades as particulate organic carbon (POC) [ 9 , 20 ], accounting for 53 ± 51% of NPP annually (Fig. 2.5). Kelp mortality through dislodgement also accounts for a significant influx of POC to the system [ 21 ] (40 ± 32% of NPP, Fig. 2.6). POC production rates peak in summer when warm temperatures and storms cause senescence and loss [ 18 ] (Figs. 4 A, B). The majority of POC is labile or semi-labile [ 22 ] (hereafter referred to as labile) (Figs. 2.7), with only ~ 14% of dislodged kelp stipe biomass (2.7% of total POC) estimated to be refractory after a year of degradation [ 23 ] (1.1 ± 2.5% of kelp NPP, Figs. 2.8, 4 C). If POC is exported to deep water masses (> 1000 m) or buried in soft sediments before it is remineralized, it is considered isolated from the atmosphere for climate relevant time scales (100 years +) [ 9 ]. POC is negatively buoyant (i.e. sinking) for the kelp species in our region, resulting in limited offshore dispersal in particle distribution models from insufficient cross-shelf transport mechanisms (0% export to the shelf break) (Figs. 2.9, 2.10, 4D). (Krumhansl et al. in revision ). We estimate that 7.9 ± 2.5% of kelp POC is transported to nearshore seagrass beds, mainly in fall (Fig. 4 E), amounting to a deposition rates of 0.051 ± 0.13% of NPP as refractory POC (RPOC) annually (Fig. 2.11, 4 E). This refractory portion accounts for about half as much kelp carbon we estimated to be buried per year in seagrass beds (Fig. 2.2), indicating that some labile POC (Fig. 2.12) may be buried before remineralization. 27.9 ± 7.7% of kelp POC is transported to bare muddy sediment in the nearshore and on the shelf (Krumhansl et al. in revision ), totaling deposition rates of 0.16 ± 0.41% of NPP annually as RPOC (Figs. 2.13, 4 F) and 12 ± 9.4% of NPP as LPOC (Figs. 2.14). We consider our estimated deposition rates of RPOC on mud sediments on the shelf a conservative first order estimate of the burial rate of this carbon (Fig. 2.15) (compared to 0.9% of NPP globally) [ 3 ]. This quantity accounts for ~ 0.8% of carbon buried annually on the Scotian Shelf (0.741 Tg C year − 1 for the 128,487 km 2 domain) [ 24 ]. Kelp has been estimated to contribute 10–32% of the carbon stock in shelf sediments dating > 100 years old elsewhere, indicating that nearshore burial of kelp carbon can contribute to long-term carbon sequestration [ 25 , 26 ]. The majority of seagrass NPP is also released as POC in the form of dislodged or senescing leaves, with seasonal peaks in summer and fall (Fig. 3.6, 4 A, B). Seagrass POC production rates are two orders of magnitude lower than those estimated for kelps in our region (Fig. 4 A, B), though a greater percentage of seagrass POC is estimated to be refractory (5.5%) [ 27 , 28 ] accounting for 2.8 ± 1.4% of NPP (Figs. 3.7, 4 C). The remaining POC is estimated to be degraded within one year (49 ± 24% of NPP, Fig. 3.8) [ 23 ]. We estimated 12.4 ± 12.2% of dislodged leaves to be transported to nearby seagrass beds (Krumhansl et al. in revision ), mainly in summer (Fig. 4 E), amounting to 0.35 ± 0.29% of NPP deposited as RPOC (Fig. 3.9). This is significantly lower than our estimated contribution of seagrass to the carbon burial rate within beds (Fig. 3.3), indicating that burial of local or imported seagrass LPOC (i.e. leaves) (Fig. 3.10), and/or roots and rhizomes that remain in place also occurs. 21% ± 7.4% of seagrass POC is estimated to be deposited in muddy sediments in the nearshore and on the shelf, (Figs. 3.11, 3.12, 4F). The refractory portion of this accounts for 0.08% of the total carbon buried on the shelf annually (Fig. 3.13) [ 24 ]. In contrast to kelp POC, seagrass leaves are buoyant for a period of about 3 weeks, depending on temperature, allowing them to disperse by winds and surface currents before they lose buoyancy during degradation [ 29 ]. Due to this, export rates of seagrass POC are predicted to be significantly higher than for kelps, with ~ 2.1 ± 3.3% of floating seagrass leaves estimated to reach the Scotian shelf break within the first few weeks following dislodgement (Fig. 4 D) (Krumhansl et al. in revision ). At this export rate, 0.34 ± 0.41% of NPP is exported as LPOC (Fig. 3.14) and 0.020 ± 0.024% of NPP is exported as RPOC (Fig. 3.15, 4 D). Once seagrass leaves lose buoyancy they rapidly sink to the seabed, after which point dispersal is limited (Krumhansl et al. in revision ). Seagrass leaves that lose buoyancy beyond the shelf break may sink to deep water, though some of this carbon is likely remineralized during downward transport. Our estimates therefore represent the maximum potential POC sequestration through POC export to deep water (Figs. 2.09, 2.10 for kelp; Figs. 3.14, 3.15 for seagrass). Summing this with POC burial in seagrass beds and muddy sediments for each species group (Figs. 2.2, 2.15, 3.3, 3.13) yields an estimated carbon sequestration by POC for kelp of 0.00462 ± 0.00530 and 0.00293 ± 0.000917 Tg C year − 1 for seagrass, representing 0.40 ± 0.50 and 2.6 ± 1.3% of NPP, respectively (Fig. 5 A). Dissolved Organic Carbon Dissolved organic carbon (DOC) is released directly by live kelps and seagrass during primary production, fragmentation and senescence, and during POC degradation [ 30 , 31 ]. We measured DOC production from live S. latissima, L. digitata , and Z. marina in incubation chambers in situ, and found comparable DOC production rates between kelps (64.0 ± 34.5 g C m − 2 y − 1 ) and seagrass (61.9 ± 27.0 g C m − 2 y − 1 ) per m 2 (Fig. 6 A), but higher rates for kelps on a regional scale (Figs. 2.16, 2.17, 3.16, 3.17, 6B). This DOC production accounts for 6.6 ± 5.1% and 13.3 ± 7.9% of NPP for kelp and seagrass, respectively, although DOC production did not follow seasonal trends in NPP (Figs. 2.16, 2.17, 3.16, 3.17, 6A, B). Previous studies have found that 35.5% of kelp DOC [ 32 ] and 10% of seagrass DOC [ 33 ] is refractory, resisting degradation for at least one year. Using these estimates, the production of refractory DOC (RDOC) accounts for 2.5 ± 1.9% (Fig. 2.16, 6 C) and 1.3 ± 0.8% (Fig. 3.16, 6 C) of NPP for kelp and seagrass. Only a few studies have measured DOC release during POC degradation, estimating that on average 59% and 25% of carbon in POC is released as DOC for kelp and seagrass within the first two weeks [ 34 ] [ 33 ]. Using these percentages, we estimate that the release of DOC from POC degradation is a more significant carbon pathway than the release of DOC from attached, growing plants, accounting for 54 ± 42% (Fig. 2.18, 2.19, 6D) and 12.8 ± 6.3% (Figs. 3.18, 3.19, 6D) of annual NPP in kelps and seagrass, respectively, 35.5% and 10% of which is assumed to be refractory (Figs. 2.18, 3.18). Export of neutrally buoyant DOC is rapid and extensive, with 33.6 ± 19.9% of kelp DOC and 12.0 ± 9.5% of seagrass DOC being exported to the shelf break within 90 days of release (Krumhansl et al. in revision ). We modeled degradation of the labile portion of DOC (LDOC) during export using decay functions in the literature [ 32 , 35 ], and merged these with export rates to estimate that 0.055 ± 0.042% of kelp NPP (Figs. 2.20, 6 E) and 0.06 ± 0.04% of seagrass NPP (Fig. 3.20, 6 E) is exported to the shelf break as LDOC annually (remineralization in Figs. 2.21, 3.21). The export of LDOC from POC degradation is estimated to account 3.3 ± 2.0% of NPP for kelp (Fig. 2.22) and 0.10 ± 0.075% of NPP in seagrass (Fig. 3.22) (remineralization in Figs. 2.23, 3.23, 6F). During lateral transport and once this LDOC reaches the shelf break it has the potential to reach deeper water through downward advection, adsorption and sinking [ 30 ]. Our estimates of LDOC export represent a maximum upper bound of this form of carbon potentially sequestered through deep ocean export from our region as a portion is likely deposited on the shelf or remineralized before reaching deep waters. Using coastal residence times predicted for our region, we expect all RDOC to be advected from coastal waters in our domain to the open ocean in 181–365 days [ 36 ], indicating that all this carbon is likely exported beyond the 200 m isobath within the time frame over which its persistence has been measured (1 year) [ 32 , 33 ]. We therefore consider the release of RDOC by live and degrading kelps and seagrass to contribute to long-term carbon sequestration, and our estimates of total flux through this pathway represent an upper bound of carbon sequestration via this mechanism (Figs. 2.16, 2.18, 3.16, 3.18). Total potential sequestration through the formation of RDOC (Figs. 2.20, 2.22, 3.20, 3.22) and export of LDOC before remineralization (2.17, 2.19, 3.16, 3.18) is 0.308 ± 0.143 Tg C y − 1 and 0.00373 ± 0.000915 Tg C y − 1 for kelp and seagrass respectively, representing 26 ± 19% and 3.3 ± 1.6% of NPP (Figs. 5 B, C). For kelp, DOC sequestration from the release of DOC from POC degradation (Fig. 5 C) is an order of magnitude higher than from the production of DOC from live plants (Fig. 5 B). Discussion The value of kelp forests and seagrass beds to carbon sequestration Summing RDOC from live kelp and degrading POC (Figs. 2.16, 2.18), the export of LDOC, LPOC, and RPOC to the shelf break (Fig. 2.24), and RPOC deposition/burial in soft sediments on the shelf (Figs. 2.2, 2.15) yields a total potential sequestration from kelp forests in our domain of 0.313 ± 0.144 Tg C y − 1 , accounting for 27 ± 19% of annual NPP (Fig. 2.25). For seagrass beds, our calculation also incorporates non-kelp allochthonous carbon burial (Figs. 3.3, 3.13, 3.16, 3.18, 3.24, quantity in Fig. 3.25 minus quantity in 2.2), yielding a total sequestration rate of 0.011 ± 0.004 Tg C y − 1 , accounting for 9.5 ± 5.2% of NPP (Fig. 3.26). On a per unit area basis, kelps are also more efficient at sequestering carbon (161.0 ± 68.9 Tg C m − 2 year − 1 for kelp vs. 45.1 ± 15.8 Tg C m − 2 year − 1 for seagrass), and a higher percent of their NPP is estimated to be sequestered on an annual basis (Figs. 2.25, 3.26). The higher contribution of kelp to carbon sequestration in our region relative to seagrass is therefore due to a higher sequestration efficiency as well as the greater habitat area occupied in the study domain. Implications Macroalgal forests have been undervalued relative to other BCEs for their potential contribution to ocean carbon sequestration due to their inability to trap and bury carbon locally. Our study estimates a higher contribution of kelp to ocean carbon sequestration by an order of magnitude and strong differences in the pathways by which this carbon is stored compared with seagrass beds. DOC accounts for the majority of carbon sequestered by kelp forests in Nova Scotia, representing a higher percentage than has been estimated for kelp forests globally (98.5 ± 60.1% of carbon sequestered vs. 64% globally)[ 3 ], with POC export and burial accounting for very little stored carbon (1.5 ± 1.8% vs 31% globally) [ 9 , 37 ]. Higher contribution by DOC in our region is partially due to the fact that we include the conversion from kelp POC to DOC in our budget [ 34 ], which has not been considered for macroalgal systems but may account for a significant flux of carbon to global oceanic DOC pools. Our results suggest that conservation measures that maintain, promote, or enhance the living biomass and productivity of kelps also promote higher DOC production and export, which can have positive effects on carbon sequestration rates even over short time scales. Most carbon is sequestered by seagrass beds in our region through the burial of allochthonous and autochthonous POC within meadows and on soft sediments on the shelf (63.6 ± 43%) [ 15 ], contrasting dominant pathways of carbon sequestration for kelp forests. Focusing solely on POC burial by seagrasses will significantly underestimate carbon sequestration, however, with DOC production emerging as a more significant flux of carbon than has been documented for seagrasses elsewhere (36.4 ± 15.5% vs 10.6% of sequestration globally) [ 2 ]. Discrepancies between our regional carbon budgets and existing global budgets are due to natural spatial and intraspecific variability as well as inadequate parameterization on a global scale. Applying generalized estimates to individual regions can therefore lead to inaccuracies in carbon accounting, and focusing on single carbon pathways can distort the relative value of BCEs for carbon sequestrations. Our results also lay bare significant deficiencies in our knowledge of ocean carbon cycling, in particular the production, transformation, and dispersal of DOC from live and decaying coastal macrophytes as a critical area for future research and key carbon parameters to measure to quantify sequestration in BCEs. Further, carbon emissions from BCEs have gone largely uncharacterized. Broadly, our result show how carbon accounting can highlight undervalued carbon pathways, reduce uncertainty in existing global estimates of carbon sequestration, and support more robust carbon accounting and strategies for the management of BCEs. Methods Study design The study domain encompassed the mainland Atlantic Coast of Nova Scotia, ranging from Cape Sable Island (43.38959, -65.62084) in the southwest and Cape Canso in the Northeast (45.344620, -60.909754) to the edge of the Scotian Shelf break (Fig. 1 ). Carbon budgets were generated for the three dominant macrophyte species in the region, including Zostera marina, Laminaria digitata , and Saccharina latissima [ 38 ]. Estimates are a compilation and reanalysis of new and existing experimental and observational data from our domain. When using existing studies, priority was given to studies from our region and species, where possible. Budget estimates are given as seasonal or annual totals, on a per m 2 or bioregion (defined as the study domain) basis. Details of how each carbon stock and flux were calculated are below. Note, we have not included emissions of greenhouse gasses (e.g. methane, halocarbons) in our budgets due to a lack of data. Carbon Stocks Estimates of carbon stocks contained in living biomass and sediments were generated for kelps (living biomass only) and seagrass using modeled suitable habitat area [ 13 ] combined with field measurements of carbon stocks per unit area. For S. latissima and L. digitata , biomass and density data were measured using quadrat sampling by SCUBA divers spanning all seasons at 13 sites within the study domain from 2021–2023. At each site, divers sampled 6–8, 0.75 m 2 quadrats along 1–2 depth strata (3–5 m, 7–9 m). Divers counted the density of kelp stipes (one stipe represents one individual) and then all kelps were collected and placed in a mesh bag. Kelp species were separated and weighed (wet weight, ww) at the surface using a fish scale (± 0.01 kg). For carbon stock calculations, kelp biomass (kg ww) and density (ind m − 2 ) data were averaged across all sites and depths, and converted to measures of carbon content per unit area (kg C m − 2 ) using a wet weight to dry weight (dw) conversion factor for each species [ 18 ] and a dw to carbon conversion factor of 0.3 [ 39 ]. For Z. marina , above and below ground biomass data were collected at four sites within the study domain [ 40 ], including a deep (3.2 m) and shallow (1.8 m) site at two of the four sampled sites. Above and below ground biomass was estimated using 6 hand cores (10.8 cm diameter) at each site. Both biomass components were summed for each core, converted to an estimate of biomass per m 2 , converted to carbon using a conversion factor of 0.36 [ 41 ], and then averaged across sites. Sediment carbon content within beds of Z. marina was calculated using data from [ 42 ], and summed with above and below ground biomass to generate a per m 2 estimate of carbon stock contained within live biomass and sediments within seagrass beds. For seagrass and kelp, these per m 2 measurements were then multiplied by the suitable habitat area for each species (Krumhansl et al. in revision ) [ 13 ] in the region to estimate total regional carbon stock within each habitat. Suitable habitat area was estimated using probabilities from species distribution models, thresholded to a binary presence/absence as described in (Krumhansl et al. in revision ) [ 13 ]. Net Primary Production (NPP) Estimates of net primary production were generated from field growth rate measurements of kelp for S. latissima and L. digitata using the hole punch method for each species at 7 sites during all four seasons [ 18 ]. The data set includes field growth measurements taken in 2008–2009 [ 18 ], supplemented with new field data collected using the same methods at two additional sites (Sober Island and Broad Cove) in 2022–2023. In short, blade growth was measured by punching a hole at the base of the kelp blade (n = 10–20 individuals per species) near the meristem, and measuring the distance the hole moved over a period of 2–4 weeks. This distance is converted to a measure of new tissue production (g dw) by excising the blade section after the growth period, drying the tissue at 60°C for 24–48 hours, and then weighing the tissue. The amount of new tissue produced per day was then calculated by dividing the g dw of the tissue section by the time elapsed, and then converting this to an estimate of carbon production per individual (g C d − 1 ) using a conversion factor of 0.3 [ 39 ]. Individual growth measurements were then averaged across replicates and scaled to g C m − 2 d − 1 estimate by multiplying this number by the density of each kelp species as collected in diver quadrats (see above). This rate was converted to a seasonal or annual total by multiplying it by the number of days in each season, and then summed it across the time period. The annual NPP value for each species was then multiplied by the area of suitable habitat in the domain (Krumhansl et al. in revision ) to generate an annual estimate for the entire region. Hatcher [ 43 ] found that growth represented by movement of the punched hole at the base of kelp blades of S. latissima represented 45% of net carbon assimilation. Therefore, we corrected our field growth measures to NPP by multiplying field growth rates by a factor of 2.22 [ 43 ]. For seagrass, NPP was measured in the field at two representative sites in May 2023–2024 using the plastochrone growth method [ 44 ]. For each sampling period, 20–30 Z. marina shoots at each site were marked with a hole through the middle of the sheath and collected 3–9 weeks later depending on the season. After collection, the length (mm) of the most mature leaf (leaf three) was measured and converted to biomass (g dry mass) using established site-specific length-weight relationships [ 45 ]. The number of new leaves since marking (i.e., leaves with no mark) were then determined and used to calculate the plastochrone interval (number of new leaves divided by number of days since marked). Shoot growth (g C shoot − 1 d − 1 ) was calculated by dividing the weight of the third leaf (dry g shoot − 1 ) by the plastochrone interval, then converting dry mass to carbon using a factor of 0.34 (g C per g dry mass; Wong, unpublished data). To scale individual shoot growth to growth per unit area (g C m − 2 d − 1 ), mean shoot growth (g C shoot − 1 d − 1 ) was multiplied by the shoot density (number of shoots m − 2 ) corresponding to that sampling period (where shoot density was determined using 3–5 quadrats at each site). Finally, growth (g C m − 2 d − 1 ) was then averaged across the 2 sites, multiplied by the number of days in each season, and then summed for each season and the whole year. The seasonal and annual values for each species was then multiplied by the area of suitable habitat in the domain [ 13 ] to generate estimates for the entire region. We compared field growth measures to measurements of NPP in the lab from oxygen evolution experiments [ 46 ] and found that they represent on average 61% of net carbon assimilation across seasons at a representative site in Nova Scotia (M. Wong, unpublished data). We therefore corrected our field growth measures to NPP by multiplying seasonal and annual values per m 2 and for the region by a factor of 1.64. POC production and export POC production as blade erosion from kelps (i.e. senescence and breakage of blade material from the distal tip) was measured on the same individuals as for the field growth measurements in 2022–2023, and again utilizing previously collected data and methodologies from 2008–2009 [ 18 ]. Total blade length was measured at the start and end of the measurement period, and the difference in blade length, accounting for growth, is considered the amount of blade material lost to the distal erosion of tissues. The length of this section was converted to carbon by generating a g/cm conversion factor from a section of blade tissue of known length that is excised, dried, and weighed. This is then multiplied by the length of tissue eroded, and then converted to carbon using the conversion factor of 0.3 [ 39 ]. Individual erosion rate (g C d − 1 ) was then calculated by dividing the amount of eroded tissue by the time elapsed between measurements. Individual measurements were then averaged across replicates and converted to a per m 2 and per region estimate using the same method as for growth. POC production from the mortality of whole kelp individuals was measured by marking 10–20 individual kelps of each species with a numbered cable tie at Broad Cove and Sober Island at monthly intervals, and then tracking the survival of each individual between measurement periods. For the data from 2008–2009 [ 18 ], we used the number of tagged individuals missing between the start and end of the growth measurement periods to estimate loss. The proportion of missing individuals was calculated for each month as a monthly mortality rate (including an estimated 25% relocation error rate) and averaged across months for each season. This mortality rate was corrected to account for only the losses relating to a single year of production by multiplying loss rates by the proportion of individuals in a single cohort estimated to be lost within a year (77% for S latissima 50% for L. digitata ) [ 47 ]. This mortality rate was then multiplied by a per individual average biomass (calculated by dividing quadrat biomass by density) to generate a per m 2 estimate of POC production through mortality. This was then converted to g C [ 39 ] to generate a carbon loss rate in g C season − 1 per m 2 , and multiplied by suitable habitat area for each species to generate an estimate for the whole region. We note that density estimates used to calculate POC production from erosion were corrected with mortality rate to avoid overestimating POC production. POC production from erosion and dislodgement were then summed to generate an estimate of total seasonal and annual POC production for each kelp species. Since mortality contributes carbon to the system from multiple years of NPP, only POC from erosion was related as a percent of annual NPP in our budget. We calculated the amount of this POC that is refractory (RPOC) as 14% of the stipe biomass [ 23 ], assuming that all blade material is degraded within 12 months given aerobic conditions on the Scotian Shelf [ 48 ]. We calculated stipe biomass using a stipe to blade ratios recorded previously for Saccharina latissima and Laminaria digitata [ 49 , 50 ] multiplied by biomass lost through plant mortality and corrected to 14%. The remaining POC is assumed to be labile (LPOC). POC production from senescing and dislodged seagrass leaves was estimated for each growth sampling interval as the difference between plant production (described above) (g dw m − 2 ) and the change in aboveground standing biomass (g dw m − 2 ) [ 51 ]. Aboveground standing biomass (g dw m − 2 ) is estimated by multiplying shoot density (number of shoots m − 2 ) by the average aboveground biomass per shoot (specific to site) [ 45 ]. POC production estimates were converted to g C using a conversion factor of 0.34, summed across season to generate a carbon loss rate (g C season − 1 m − 2 ), and multiplied by suitable habitat area to generate an estimate for the whole region. 5.5% of this POC is assumed to be refractory [ 27 ]. The amount of POC exported to the shelf break was then calculated for each species by multiplying % POC particle export rates for each season (Krumhansl et al in revision ) by the seasonal LPOC and RPOC production estimates for the bioregion, and then summing these quantities across seasons to generate an annual total POC export estimate in g C region year − 1 . We note that degradation times at the bottom temperatures experienced in our region would result in negligible losses of biomass over the 90-day export period modeled, so we do not account for degradation in our carbon export estimates (Krumhansl et al in revision ). We also generated estimates of the amount of carbon exported to seagrass beds and muddy sediments in the nearshore and on the shelf. This was done by multiplying seasonal % export estimates to these areas (Krumhansl et al. in revision ) by RPOC and LPOC production, and then summing these quantities for the year. We estimated the amount of carbon originating from Z. marina, S. latissima and L. digitata that is buried in seagrass beds using carbon burial rates (g C m − 2 y − 1 ) measured for Z. marina beds [ 16 ] multiplied by the proportion of sediment carbon that is Z. marina or macroalgae in origin [ 15 ]. Allochthonous carbon burial in seagrass beds is accounted for in our budget as the total estimated carbon burial rate minus the burial of carbon originating from Z. marina. Sufficient data to estimate the amount of POC from Z. marina , S. latissima , and L. digitata buried in shelf sediments do not exist for our region, but we consider our estimates of RPOC deposition on muddy sediments as a first order estimate of the burial rate of this carbon. All quantities were then multiplied by the area of the bioregion to yield a regional total. The amount of LPOC and RPOC originating from kelp and Z. marina not deposited on muddy sediments or seagrass beds was assumed to be remineralized. DOC production and export DOC production was measured for L. digitata, S. latissima , and Z. marina in situ using incubation experiments conducted in the spring, summer, and fall (winter experiments were not conducted due to logistical limitations). Whole seagrass (including leaves, rhizomes and roots ) and kelp (including blades and holdfasts) individuals were collected at two sites each for both species groups. Plants chosen for the incubations were clear of visible epiphytes and epifauna, and were in good condition (i.e. little visible damage and/or degradation of tissues). Plants were collected the week prior to each experimental period to allow for wound healing from collections and stored in ambient seawater until incubations began. DOC production was then measured by containing whole plants in clear plastic bags (16 L) anchored to a lead line on the substratum (25 m in length, plants separated by 50 cm) at 4 m depth. The experimental design consisted of 5 replicates of each species from 2 sites each (n = 10 for each species), and 10 controls (n = 40 total replicates) during each experimental run (except summer where 4 replicates per site, 8 controls, and only 1 seagrass site were included). Seagrass individuals were bundled into groups of 10 plants (5 plants in summer) within experimental bags, as DOC production from one individual was expected to be too low to detect relative to the volume of the bag. For each experimental run, seagrass, kelp or a bare tag (experimental control) was randomly added inside each bag and the bags were filled with surrounding water (9.0 ± 2.2 L; mean ± SD, Fig. 2 ) and sealed. In addition to the controls affixed to the lead line, we also collected 5 control bags filled from the surrounding waters at the start and end each deployment. These controls were brought to shore immediately for chemical analysis. DOC production was assessed during 1 daytime period and 1 nighttime period during each experimental run. The deployment duration varied between seasons depending on daylength and time for sunrise and sunset. The night deployment was done during the entire dark phase, with experimental bags deployed within an hour after sunset and retrieved within an hour before sunrise (duration range: 11-14.5 hours). The day deployment was shorter and aimed to be within the peak daylight 10 − 2 pm (duration range: 4–7 hours). Upon retrieval of the nighttime experiment, bags were returned to shore and kept in the dark before processing (1.5 hours max). Bags were weighed using a digital scale (©Berkley, precision 10 g), and then dissolved oxygen (DO; YSI ODO), temperature and salinity (YSI Pro 30) were measured inside each bag (except for the summer when the YSI ODO was not used). Samples were immediately filtered using a peristaltic pump maintained below 10 psi with 0.45 micron nitrocellulose mixed ester membrane filters. Samples were poisoned with concentrated HgCl2 upon return to the lab and kept at 4°C until analysis. For one replicate of each treatment and control type, we also measured DO using the Winkler method [ 52 ]. The same kelp and seagrass individuals were used for the night and daytime experimental runs, and were kept in a cooler on ice between deployments. Within 24 hours of the end of the experiment, we measured the wet weight of each plant and tags before drying the them in a drying oven (60°C) for 48 hours and measuring their dry weight (DW). The quantity of DOC produced was measured with a OI Analytical Aurora 1030W TOC analyzer with a model 1080 autosampler and a combustion unit. Briefly, dried gas from sparged acidified samples was detected using infrared gas analysis with an analytical precision of 0.4 ppm or better (Jan Veizer Stable Isotope Laboratory, University of Ottawa). DOC concentrations (mg C L -1 ) were then converted to production/consumption rates (mg C L -1 h -1 ) by first subtracting the start concentration for each deployment, and dividing this amount by the experimental duration. This amount was then multiplied by the volume per bag to create a measure of DOC production per hour (mg C h -1 ). To account for the biological activity of the surrounding seawater in the treatment bags, the mean DOC production/consumption rate in the control bags was then subtracted from the DOC production/consumption rate of each experimental bag. The DOC production/consumption hourly rate per individual (mg C ind -1 h -1 ) was then calculated by dividing the DOC production/consumption hourly rate (mg C h -1 ) by the number of individuals in each bag (seagrass only). To estimate the daily DOC production/consumption rates per m 2 , we extracted information on the total duration in hours above light saturation allowing for photosynthesis (HSat) for kelp and seagrass for each day in each season assuming the following conditions (depth seagrass = 1m, depth kelp = 5m, attenuation coefficient = 0.33, saturation irradiance (Ik) IK seagrass = 100 or 250 umol photons.m -2 .s -1 [ 53 ], Ik kelp = 38 umol photons.m -2 .s -1 [ 53 ] at a central geographical location for the south shore of Nova Scotia (latitude: 44.4° N). The hourly DOC production/consumption rate per individual (mg C ind -1 h -1 ) was then converted to a daily rate for each day of the season by multiplying the daytime DOC production/consumption rate by HSat, and then adding this quantity to the product of nighttime DOC production/consumption rate and 24 – Hsat (i.e. hours under saturating irradiance). Summer data were missing at nighttime due to logistical limitations, therefore we used a 10% decrease of daytime DOC production/consumption rate per biomass to estimate the nighttime value (nighttime DOC production was 10% lower than daytime production during other seasons). We did not run an experiment in winter, so we applied DOC production/consumption rates for spring with winter daily HSat values to calculate winter DOC production. We assume winter is most similar to spring in terms of DOC production given that growth exceeds senescence in both seasons, whereas the opposite is true in summer and fall [ 18 ]. Daily DOC production values per gram dw were multiplied by the per individual biomass in each bag and then multiplied by the density of plants at each site [ 40 ], and then summed to generate a total DOC production rate for each season. These quantities were then summed across seasons to generate an annual rate (g C m -2 y -1 ), and multiplied by the area of suitable habitat for each species to generate an estimate of DOC production for the bioregion. The amount of DOC in refractory form was then estimated by multiplying each seasonal and annual quantity by the estimated amount of this carbon that is refractory, as measured from prior studies: 37.5% for kelp [ 32 ], and 10% for seagrass [ 31 ]. The difference between the total DOC production and the RDOC quantity was considered the labile fraction (LDOC). The amount of DOC released during kelp and seagrass POC degradation was estimated by multiplying the quantity of LPOC produced during each season by the percent of this carbon released as DOC within the first two weeks of degradation, as measured by experimental studies (59% for kelp [ 34 ] and 25% for seagrass [ 33 ]). 37.5% and 10% were used, as above, to convert this DOC quantity to RDOC for kelp and seagrass, respectively. To quantify export of the LDOC fractions released from live and decaying individuals, we modeled decay of LPOC quantities using an exponential relationship and decay constants found in the literature: k = -0.06% day − 1 for kelp [ 32 ] and − 0.01% day − 1 for seagrass [ 35 ]). Daily estimates of the % of particles (i.e. carbon) reaching the shelf break (Krumhansl et al. in revision ) were then multiplied by the quantity of LDOC remaining on each day to estimate the amount of this DOC exported at each time step. This quantity was then subtracted from the amount available for decay and export during the following time step. The total amount of LDOC exported to the shelf break during each season was then calculated by summing this quantity across the 90 days of the season, and then across seasons for an annual total. Using coastal residence times predicted for our region, we expect all RDOC to be advected from coastal waters in our domain to the open ocean in 181–365 days [ 36 ], indicating that all this carbon is likely exported beyond the 200 m isobath within the time frame over which its persistence has been measured (1 year) [ 32 , 33 ]. Declarations Author contribution statement KK, MW, and KA-S conceived of the study, KK, MW, MP, MF, C-EG, YW, and KA-S designed field measurements and experiments, KK, MP, MF, C-EG collected field data and conducted experiments, KK, MW, MP, MF analyzed the data, KK, MP, MF reviewed the literature, KK, MW, MP, MF wrote the manuscript, KK, KA-S, MW, MP, MF, C-EG, YW edited and provided input on the manuscript, KA-S oversaw the work. Acknowledgements We acknowledge Katie Thistle, Shawn Roach, Thomas Baker, Cody Brooks, Danielle Davenport, Claudio DiBacco, Katherine Lee, and Nick Jeffery for their support in the field. We also thank Wendy Gentleman for feedback on the work. KK discloses support for this work from a Fisheries and Oceans Canada Competitive Science Research Fund grant to KK and KA-S. The authors declare no competing interests. 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Marine Biology, 1972. 12 : p. 1-10. Wong, M.C. and M. Dowd, The role of short‐term temperature variability and light in shaping the phenology and characteristics of seagrass beds. Ecosphere, 2023. 14 (11). Postlethwaite, V.R., et al., Low blue carbon storage in eelgrass (Zostera marina) meadows on the Pacific Coast of Canada. PLoS One, 2018. 13 (6): p. e0198348. Christensen, M.S., Estimating blue carbon storage capacity of Canadas eelgrass beds , in Zoology . 2023, University of British Columbia. Hatcher B.G., Chapman A.R.O., and M. K.H., An Annual Carbon Budget for the Kelp Laminaria longicruris. Marine Biology, 1977. 44 : p. 85-96. Gaeckle, J.L. and F.T. Short, A plastochrone merhod for measuring leaf growth in Eelgrass, Zostera marina. Bulletin of Marine Science, 2002. 71 (3): p. 1237-1246. Thomson, J.A., B. Vercaemer, and M.C. Wong, Non-destructive biomass estimation for eelgrass (Zostera marina): Allometric and percent cover-biomass relationships vary with environmental conditions. Aquatic Botany, 2025. 198 . Collier, C.J., et al., Optimum Temperatures for Net Primary Productivity of Three Tropical Seagrass Species. Front Plant Sci, 2017. 8 : p. 1446. Chapman, A.R.O., Reproduction, recruitment, and mortality in two species of Laminaria in southwest Nova Scotia. J Exp Mar Biol Ecol, 1984. 78 : p. 99-109. Hebert, D., et al., Physical Oceanographic Conditions on the Scotian Shelf and in the Gulf of Maine during 2023. Canadian Technical Report of Hydrography and Ocean Sciences, 2024. 380 : p. vi+ 71 p. Grebe, G.S., et al., The effect of distal‐end trimming onSaccharina latissimamorphology, composition, and productivity. Journal of the World Aquaculture Society, 2021. 52 (5): p. 1081-1098. Ronowicz, M., P. Kukliński, and M. Włodarska-Kowalczuk, Morphological variation of kelps (Alaria esculenta, cf. Laminaria digitata, and Saccharina latissima) in an Arctic glacial fjord. Estuarine, Coastal and Shelf Science, 2022. 268 . Romero, J., et al., The Detritic Compartment in aPosidonia oceanicaMeadow: Litter Features, Decomposition Rates, and Mineral Stocks. Marine Ecology, 2008. 13 (1): p. 69-83. Jones, E., F. Zemlyak, and P. Stewart, Operating manual for the Bedford Institute of Oceanography Automated Dissolved Oxygen Titration System. Canadian Technical Report of Hydrography and Ocean Sciences, 1992. 138 . Lee, K., S. Park, and Y. Kim, Effects of irradiance, temperature, and nutrients on growth dynamics of seagrasses: A review. Journal of Experimental Marine Biology and Ecology, 2007. 350 (1-2): p. 144-175. Additional Declarations There is NO Competing Interest. Supplementary Files Carbonbudgetdataprocessing.txt Data processing code Datasetscarbonbudget.xlsx Original data used in budget calculations Cite Share Download PDF Status: Published Journal Publication published 02 Jan, 2026 Read the published version in Communications Earth & Environment → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6473319","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":454418204,"identity":"91d2ecaa-8d78-4a38-ae03-5fb35adac56d","order_by":0,"name":"Kira Krumhansl","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0001-7377-3360","institution":"Fisheries and Oceans Canada","correspondingAuthor":true,"prefix":"","firstName":"Kira","middleName":"","lastName":"Krumhansl","suffix":""},{"id":454418205,"identity":"d18174fb-95da-4eba-b980-e406c012803d","order_by":1,"name":"Melisa Wong","email":"","orcid":"","institution":"Fisheries and Oceans Canada","correspondingAuthor":false,"prefix":"","firstName":"Melisa","middleName":"","lastName":"Wong","suffix":""},{"id":454418206,"identity":"c410597f-e4ab-4768-8d04-f52f553b144c","order_by":2,"name":"Manon Picard","email":"","orcid":"","institution":"Fisheries and Oceans Canada","correspondingAuthor":false,"prefix":"","firstName":"Manon","middleName":"","lastName":"Picard","suffix":""},{"id":454418207,"identity":"6696a22f-952f-4d20-b357-e9f2a31b896c","order_by":3,"name":"Meredith Fraser","email":"","orcid":"","institution":"Fisheries and Oceans Canada","correspondingAuthor":false,"prefix":"","firstName":"Meredith","middleName":"","lastName":"Fraser","suffix":""},{"id":454418208,"identity":"14247865-8f28-45bb-a016-7a77b4276c0a","order_by":4,"name":"Carrie-Ellen Gabriel","email":"","orcid":"","institution":"Fisheries and Oceans Canada","correspondingAuthor":false,"prefix":"","firstName":"Carrie-Ellen","middleName":"","lastName":"Gabriel","suffix":""},{"id":454418209,"identity":"28a8b0bb-5da8-4f9a-b198-b6a49800aefd","order_by":5,"name":"Yongsheng Wu","email":"","orcid":"","institution":"Fisheries and Oceans Canada","correspondingAuthor":false,"prefix":"","firstName":"Yongsheng","middleName":"","lastName":"Wu","suffix":""},{"id":454418210,"identity":"ec27d2c7-06d1-4b16-8488-72c392f861d8","order_by":6,"name":"Kumiko Azetsu-Scott","email":"","orcid":"","institution":"Bedford Institute of Oceanography","correspondingAuthor":false,"prefix":"","firstName":"Kumiko","middleName":"","lastName":"Azetsu-Scott","suffix":""}],"badges":[],"createdAt":"2025-04-17 16:06:50","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6473319/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6473319/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s43247-025-03122-2","type":"published","date":"2026-01-02T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":83283275,"identity":"58d6a836-65e3-48f2-be84-708dcbcc44da","added_by":"auto","created_at":"2025-05-22 10:43:53","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":343517,"visible":true,"origin":"","legend":"\u003cp\u003eA-C. Maps of suitable habitat for the two dominant kelps (\u003cem\u003eLaminaria digitata, Saccharina latissima\u003c/em\u003e)\u003cem\u003e \u003c/em\u003eand the seagrass \u003cem\u003eZostera marina\u003c/em\u003e along the Atlantic Coast of Nova Scotia. Habitat extent shows our study domain, which spans the coast from Cape Sable Island in the southwest to Cape Canso in the northeast. D, E shows the carbon stock contained within living biomass and sediments within each suitable habitat area per m\u003csup\u003e2 \u003c/sup\u003e(D), and for the whole bioregion (E) and each blue carbon species. F, and G show bioregional net primary production estimates for each season and an annual total for each blue carbon species per m\u003csup\u003e2 \u003c/sup\u003e(F), and for the whole bioregion (G). Note, y-axis scales vary across parameter to facilitate comparisons across species and seasons and the lower limit is constrained at 0. Data are means or totals ± 1 standard deviation.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/3bd203e1e652e9e1f0afcad4.png"},{"id":83283281,"identity":"61b07115-60ba-46b2-ac54-c8302132495a","added_by":"auto","created_at":"2025-05-22 10:43:53","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":412068,"visible":true,"origin":"","legend":"\u003cp\u003eCarbon budget totals for both kelp species (\u003cem\u003eSaccharina latissima \u003c/em\u003eand\u003cem\u003e Laminaria digitata\u003c/em\u003e)\u003cem\u003e \u003c/em\u003eincluding the total amount (upper bound) of carbon estimated to be sequestered by kelp in the bioregion, estimated by summing quantities in the blue boxes only. Dashed lines indicate unknown quantities. Line weights indicate the relative flux through each pathway. DOC is dissolved organic carbon, POC is particulate organic carbon, L is labile, and R is refractory. Data are annual totals or means, or the percent of annual NPP ± 1 standard deviation.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/b347ed96b2e96b1af96a3cf8.jpeg"},{"id":83284303,"identity":"e00b3b25-0594-493f-983c-5c0d27d66252","added_by":"auto","created_at":"2025-05-22 10:59:53","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":381057,"visible":true,"origin":"","legend":"\u003cp\u003eCarbon budget totals for the seagrass \u003cem\u003eZostera marina\u003c/em\u003e, including the total amount (upper bound) of carbon estimated to be sequestered by seagrass in the bioregion, estimated by summing quantities in the blue boxes only. Dashed lines indicate unknown quantities. Line weights indicate the relative flux through each pathway. DOC is dissolved organic carbon, POC is particulate organic carbon, L is labile, and R is refractory. Data are annual totals or means ± 1 standard deviation.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/10d5514458f74e1d2cc0fa7a.jpeg"},{"id":83283605,"identity":"c2349ce3-aca6-461c-b34f-538ba8f48529","added_by":"auto","created_at":"2025-05-22 10:51:53","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":406968,"visible":true,"origin":"","legend":"\u003cp\u003eMeasures of particulate organic carbon (POC) production and export for \u003cem\u003eL. digitata, S. latissima, Z. marina\u003c/em\u003e, including A. POC production per m\u003csup\u003e2\u003c/sup\u003e, B. total POC production for the bioregion per season or year, C. total refractory particulate organic carbon (RPOC) production for the bioregion per season or year, D. Bioregion export of POC to the shelf break (total LPOC and RPOC), E. total deposition of RPOC in muddy sediments on the shelf for the bioregion per season or year, and F. total deposition of RPOC in seagrass beds for the bioregion per season or year. Note, y-axis scales vary across parameter to facilitate comparisons across species and seasons and the lower limit is constrained at 0. Data are means ± 1 standard deviation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/5e553c071b20bd97fc0aecfe.png"},{"id":83284305,"identity":"44d1a2a6-79a7-417a-96cc-5eea3157f47a","added_by":"auto","created_at":"2025-05-22 10:59:53","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":277046,"visible":true,"origin":"","legend":"\u003cp\u003eMeasures of the maximum potential amount of carbon sequestered as A. POC (sum of POC export and RPOC deposited in mud and seagrass beds), B. DOC produced from live plants (sum of BDOC export and total RDOC), and D. DOC produced from degrading POC (sum of BDOC export and total RDOC). Note, y-axis scales vary across parameter to facilitate comparisons across species and seasons and the lower limit is constrained at 0. Data are means ± 1 standard deviation.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/544c09956f43335ce2bd39b1.jpeg"},{"id":83283287,"identity":"abee7a24-6048-4140-9b20-d09451be8b11","added_by":"auto","created_at":"2025-05-22 10:43:53","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":328526,"visible":true,"origin":"","legend":"\u003cp\u003eMeasures of dissolved organic carbon (DOC) production and export for \u003cem\u003eL. digitata, S. latissima, \u003c/em\u003eand \u003cem\u003eZ. marina\u003c/em\u003e, including A. DOC production per m\u003csup\u003e2 \u003c/sup\u003efrom live plants, B. total DOC production from live plants for the bioregion per season or year, C. total RDOC production from live plants for the bioregion per season or year, D. total DOC released from POC degradation for the bioregion per season or year, E. total bioregion export to the shelf break of labile DOC (LDOC) released from live plants, and F. total bioregion export to the shelf break of BDOC released from POC degradation. Note, y-axis scales vary across parameter to facilitate comparisons across species and seasons and the lower limit is constrained at 0. Data are means ± 1 standard deviation.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/fcdf730b80953ec1c3598edd.jpeg"},{"id":101481225,"identity":"03249162-99bd-4698-a83c-6d8be8e38940","added_by":"auto","created_at":"2026-01-30 08:10:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2800753,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/65e2d54f-2494-46cd-95cc-034d620be80f.pdf"},{"id":83283278,"identity":"3c58d2f3-332e-4fe4-a6dc-65bb61aa73dd","added_by":"auto","created_at":"2025-05-22 10:43:53","extension":"txt","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":107456,"visible":true,"origin":"","legend":"Data processing code","description":"","filename":"Carbonbudgetdataprocessing.txt","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/24187a5cc63fddc19c5b3f82.txt"},{"id":83283603,"identity":"125bc762-5e3f-4972-b2bf-5263f8bfcb73","added_by":"auto","created_at":"2025-05-22 10:51:53","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":61703,"visible":true,"origin":"","legend":"Original data used in budget calculations","description":"","filename":"Datasetscarbonbudget.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6473319/v1/e2ff50891d3fb6999100a3f4.xlsx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"An Annual Blue Carbon Budget for Kelp Forests and Seagrass Beds","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCoastal vegetated ecosystems (e.g. seagrass, salt marsh, mangroves, kelp forests) cycle and store carbon in our oceans, and are increasingly being recognized for their contribution to long-term carbon sequestration [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. These ecosystems (i.e. blue carbon ecosystems, BCEs) are threatened by a variety of stressors, including climate change, fisheries, and coastal development, which can reduce their capacity to sequester carbon and release carbon stored in sediments, further exacerbating climate change [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The protection, restoration, and natural enhancement of these ecosystems have therefore been proposed to help limit further emissions and draw down additional carbon as a climate change mitigation tool [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Further, there is increasing interest in including coastal BCEs in national carbon inventories and voluntary carbon markets.\u003c/p\u003e \u003cp\u003eHowever, knowledge of the contribution of BCEs to long-term carbon sequestration is not well resolved largely because we lack basic data on carbon fluxes and their links to pools of stored carbon [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Existing estimates of the global contribution of kelp forests and seagrass beds to carbon sequestration are coarse compilations of studies from disparate geographic regions, species, and time periods, within which most flux estimates were derived from a small number of studies [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], focus on only a subset of fluxes [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], or rely mainly on quantifying sediment carbon stocks [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Applying first order global estimates on local and regional scales may be a good first approximation, but can lead to over- or underestimating the contribution of BCEs to climate change mitigation, which can misguide efforts to limit further warming.\u003c/p\u003e \u003cp\u003eDetailed accounting of the fluxes and stocks of carbon within a given BCE kelp forest or seagrass system (i.e. a carbon budget) is now needed to reduce uncertainty in estimates of carbon sequestration. Considering a system as a whole provides a more accurate picture of the relative importance of different carbon pathways and may yield contrasting patterns to general paradigms. It can also provide insights into sources of variability that need to be characterized to improve overall estimates, and can be used to direct research towards the most critical parameters to resolve. Ultimately, this information provides insights into carbon pathways that may be most at risk from human activities, and inform how BCEs should be managed to maintain their capacity to cycle and store carbon in the long-term. Critically, this information supports a more robust appraisal of the potential for activities like BCE protection, enhancement, and restoration to help mitigate climate change [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eCarbon stocks and Net Primary Production\u003c/h2\u003e \u003cp\u003eWe developed detailed carbon budgets for kelp forests and seagrass beds along the Atlantic coast of Nova Scotia, Canada using new and existing field measurements, lab and field experiments, modeling, and a literature synthesis to reduce uncertainty in regional estimate of carbon sequestration and demonstrate the broad utility of carbon budgeting. We first estimate carbon stocks contained in sediments and living biomass and net primary production (NPP) rates for the two dominant kelp species (\u003cem\u003eSaccharina latissima\u003c/em\u003e and \u003cem\u003eLaminaria digitata\u003c/em\u003e) and eelgrass (\u003cem\u003eZostera marina\u003c/em\u003e). All estimates are from field measurements scaled to the region using species distribution models (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Kelps are estimated to occupy nearly 6 times the area (1400 km\u003csup\u003e2\u003c/sup\u003e) of seagrass (240 km\u003csup\u003e2\u003c/sup\u003e ) in our region (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA-C), and contain more living biomass by an order of magnitude, though this is not a long-term carbon sink (Figs.\u0026nbsp;2.1, 3.1) [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Seagrass beds also contain stored carbon in the underlying sediments, comprised of locally fixed carbon (i.e. autochthonous, e.g. seagrass leaves, roots, and rhizomes) and carbon imported from adjacent marine and terrestrial ecosystems (i.e. allochthonous). We estimate the regional carbon stock in eelgrass sediments at 1.77\u0026thinsp;\u0026plusmn;\u0026thinsp;1.34 Tg C, which exceeds carbon contained in living biomass by either species group (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD, E, 3.2). From literature carbon burial rates [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] and source contributions to buried carbon [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] we estimate that 1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.95% of seagrass NPP (25% of total carbon buried) (Fig.\u0026nbsp;3.3) and 0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.25% of kelp NPP (35.5% of total carbon buried) (Fig.\u0026nbsp;2.2) is buried within seagrass beds on an annual basis [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eKelps have a higher NPP than seagrass per m\u003csup\u003e2\u003c/sup\u003e and for the bioregion (kelp\u0026thinsp;=\u0026thinsp;985\u0026thinsp;\u0026plusmn;\u0026thinsp;516 g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (seagrass\u0026thinsp;=\u0026thinsp;465.17\u0026thinsp;\u0026plusmn;\u0026thinsp;188.11 g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eF, \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eG, 2.3, 3.4). NPP for kelps and seagrass exceed that of phytoplankton on the Scotian Shelf per m\u003csup\u003e2\u003c/sup\u003e (123 g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] and represent\u0026thinsp;~\u0026thinsp;10% and 1% of annual NPP (11.58 Tg C y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e estimated using a study domain of 94,193 km\u003csup\u003e2\u003c/sup\u003e) [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Grazing accounts for a relatively small percent of NPP for both species groups (kelp: 0.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1%, seagrass: 7.5\u0026thinsp;\u0026plusmn;\u0026thinsp;4.2%) (Figs.\u0026nbsp;2.4, 3.5) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eParticulate Organic Carbon\u003c/h3\u003e\n\u003cp\u003eThe majority of primary production by kelps is eroded off the distal ends of blades as particulate organic carbon (POC) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], accounting for 53\u0026thinsp;\u0026plusmn;\u0026thinsp;51% of NPP annually (Fig.\u0026nbsp;2.5). Kelp mortality through dislodgement also accounts for a significant influx of POC to the system [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] (40\u0026thinsp;\u0026plusmn;\u0026thinsp;32% of NPP, Fig.\u0026nbsp;2.6). POC production rates peak in summer when warm temperatures and storms cause senescence and loss [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, B). The majority of POC is labile or semi-labile [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] (hereafter referred to as labile) (Figs.\u0026nbsp;2.7), with only\u0026thinsp;~\u0026thinsp;14% of dislodged kelp stipe biomass (2.7% of total POC) estimated to be refractory after a year of degradation [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] (1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;2.5% of kelp NPP, Figs.\u0026nbsp;2.8, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eC).\u003c/p\u003e \u003cp\u003eIf POC is exported to deep water masses (\u0026gt;\u0026thinsp;1000 m) or buried in soft sediments before it is remineralized, it is considered isolated from the atmosphere for climate relevant time scales (100 years +) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. POC is negatively buoyant (i.e. sinking) for the kelp species in our region, resulting in limited offshore dispersal in particle distribution models from insufficient cross-shelf transport mechanisms (0% export to the shelf break) (Figs.\u0026nbsp;2.9, 2.10, 4D). (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e). We estimate that 7.9\u0026thinsp;\u0026plusmn;\u0026thinsp;2.5% of kelp POC is transported to nearshore seagrass beds, mainly in fall (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eE), amounting to a deposition rates of 0.051\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13% of NPP as refractory POC (RPOC) annually (Fig.\u0026nbsp;2.11, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eE). This refractory portion accounts for about half as much kelp carbon we estimated to be buried per year in seagrass beds (Fig.\u0026nbsp;2.2), indicating that some labile POC (Fig.\u0026nbsp;2.12) may be buried before remineralization. 27.9\u0026thinsp;\u0026plusmn;\u0026thinsp;7.7% of kelp POC is transported to bare muddy sediment in the nearshore and on the shelf (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e), totaling deposition rates of 0.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.41% of NPP annually as RPOC (Figs.\u0026nbsp;2.13, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eF) and 12\u0026thinsp;\u0026plusmn;\u0026thinsp;9.4% of NPP as LPOC (Figs.\u0026nbsp;2.14). We consider our estimated deposition rates of RPOC on mud sediments on the shelf a conservative first order estimate of the burial rate of this carbon (Fig.\u0026nbsp;2.15) (compared to 0.9% of NPP globally) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. This quantity accounts for ~\u0026thinsp;0.8% of carbon buried annually on the Scotian Shelf (0.741 Tg C year \u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the 128,487 km\u003csup\u003e2\u003c/sup\u003e domain) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Kelp has been estimated to contribute 10\u0026ndash;32% of the carbon stock in shelf sediments dating\u0026thinsp;\u0026gt;\u0026thinsp;100 years old elsewhere, indicating that nearshore burial of kelp carbon can contribute to long-term carbon sequestration [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe majority of seagrass NPP is also released as POC in the form of dislodged or senescing leaves, with seasonal peaks in summer and fall (Fig.\u0026nbsp;3.6, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, B). Seagrass POC production rates are two orders of magnitude lower than those estimated for kelps in our region (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, B), though a greater percentage of seagrass POC is estimated to be refractory (5.5%) [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] accounting for 2.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4% of NPP (Figs.\u0026nbsp;3.7, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eC). The remaining POC is estimated to be degraded within one year (49\u0026thinsp;\u0026plusmn;\u0026thinsp;24% of NPP, Fig.\u0026nbsp;3.8) [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. We estimated 12.4\u0026thinsp;\u0026plusmn;\u0026thinsp;12.2% of dislodged leaves to be transported to nearby seagrass beds (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e), mainly in summer (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eE), amounting to 0.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.29% of NPP deposited as RPOC (Fig.\u0026nbsp;3.9). This is significantly lower than our estimated contribution of seagrass to the carbon burial rate within beds (Fig.\u0026nbsp;3.3), indicating that burial of local or imported seagrass LPOC (i.e. leaves) (Fig.\u0026nbsp;3.10), and/or roots and rhizomes that remain in place also occurs. 21% \u0026plusmn; 7.4% of seagrass POC is estimated to be deposited in muddy sediments in the nearshore and on the shelf, (Figs.\u0026nbsp;3.11, 3.12, 4F). The refractory portion of this accounts for 0.08% of the total carbon buried on the shelf annually (Fig.\u0026nbsp;3.13) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn contrast to kelp POC, seagrass leaves are buoyant for a period of about 3 weeks, depending on temperature, allowing them to disperse by winds and surface currents before they lose buoyancy during degradation [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Due to this, export rates of seagrass POC are predicted to be significantly higher than for kelps, with ~\u0026thinsp;2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;3.3% of floating seagrass leaves estimated to reach the Scotian shelf break within the first few weeks following dislodgement (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eD) (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e). At this export rate, 0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.41% of NPP is exported as LPOC (Fig.\u0026nbsp;3.14) and 0.020\u0026thinsp;\u0026plusmn;\u0026thinsp;0.024% of NPP is exported as RPOC (Fig.\u0026nbsp;3.15, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eD). Once seagrass leaves lose buoyancy they rapidly sink to the seabed, after which point dispersal is limited (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e). Seagrass leaves that lose buoyancy beyond the shelf break may sink to deep water, though some of this carbon is likely remineralized during downward transport. Our estimates therefore represent the maximum potential POC sequestration through POC export to deep water (Figs.\u0026nbsp;2.09, 2.10 for kelp; Figs.\u0026nbsp;3.14, 3.15 for seagrass). Summing this with POC burial in seagrass beds and muddy sediments for each species group (Figs.\u0026nbsp;2.2, 2.15, 3.3, 3.13) yields an estimated carbon sequestration by POC for kelp of 0.00462\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00530 and 0.00293\u0026thinsp;\u0026plusmn;\u0026thinsp;0.000917 Tg C year \u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for seagrass, representing 0.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.50 and 2.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3% of NPP, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA).\u003c/p\u003e\n\u003ch3\u003eDissolved Organic Carbon\u003c/h3\u003e\n\u003cp\u003eDissolved organic carbon (DOC) is released directly by live kelps and seagrass during primary production, fragmentation and senescence, and during POC degradation [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. We measured DOC production from live \u003cem\u003eS. latissima, L. digitata\u003c/em\u003e, and \u003cem\u003eZ. marina\u003c/em\u003e in incubation chambers in situ, and found comparable DOC production rates between kelps (64.0\u0026thinsp;\u0026plusmn;\u0026thinsp;34.5 g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e ) and seagrass (61.9\u0026thinsp;\u0026plusmn;\u0026thinsp;27.0 g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) per m\u003csup\u003e2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA), but higher rates for kelps on a regional scale (Figs.\u0026nbsp;2.16, 2.17, 3.16, 3.17, 6B). This DOC production accounts for 6.6\u0026thinsp;\u0026plusmn;\u0026thinsp;5.1% and 13.3\u0026thinsp;\u0026plusmn;\u0026thinsp;7.9% of NPP for kelp and seagrass, respectively, although DOC production did not follow seasonal trends in NPP (Figs.\u0026nbsp;2.16, 2.17, 3.16, 3.17, 6A, B). Previous studies have found that 35.5% of kelp DOC [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] and 10% of seagrass DOC [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e] is refractory, resisting degradation for at least one year. Using these estimates, the production of refractory DOC (RDOC) accounts for 2.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.9% (Fig.\u0026nbsp;2.16, \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eC) and 1.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8% (Fig.\u0026nbsp;3.16, \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eC) of NPP for kelp and seagrass. Only a few studies have measured DOC release during POC degradation, estimating that on average 59% and 25% of carbon in POC is released as DOC for kelp and seagrass within the first two weeks [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Using these percentages, we estimate that the release of DOC from POC degradation is a more significant carbon pathway than the release of DOC from attached, growing plants, accounting for 54\u0026thinsp;\u0026plusmn;\u0026thinsp;42% (Fig.\u0026nbsp;2.18, 2.19, 6D) and 12.8\u0026thinsp;\u0026plusmn;\u0026thinsp;6.3% (Figs.\u0026nbsp;3.18, 3.19, 6D) of annual NPP in kelps and seagrass, respectively, 35.5% and 10% of which is assumed to be refractory (Figs.\u0026nbsp;2.18, 3.18).\u003c/p\u003e \u003cp\u003eExport of neutrally buoyant DOC is rapid and extensive, with 33.6\u0026thinsp;\u0026plusmn;\u0026thinsp;19.9% of kelp DOC and 12.0\u0026thinsp;\u0026plusmn;\u0026thinsp;9.5% of seagrass DOC being exported to the shelf break within 90 days of release (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e). We modeled degradation of the labile portion of DOC (LDOC) during export using decay functions in the literature [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], and merged these with export rates to estimate that 0.055\u0026thinsp;\u0026plusmn;\u0026thinsp;0.042% of kelp NPP (Figs.\u0026nbsp;2.20, \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eE) and 0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04% of seagrass NPP (Fig.\u0026nbsp;3.20, \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eE) is exported to the shelf break as LDOC annually (remineralization in Figs.\u0026nbsp;2.21, 3.21). The export of LDOC from POC degradation is estimated to account 3.3\u0026thinsp;\u0026plusmn;\u0026thinsp;2.0% of NPP for kelp (Fig.\u0026nbsp;2.22) and 0.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.075% of NPP in seagrass (Fig.\u0026nbsp;3.22) (remineralization in Figs.\u0026nbsp;2.23, 3.23, 6F). During lateral transport and once this LDOC reaches the shelf break it has the potential to reach deeper water through downward advection, adsorption and sinking [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Our estimates of LDOC export represent a maximum upper bound of this form of carbon potentially sequestered through deep ocean export from our region as a portion is likely deposited on the shelf or remineralized before reaching deep waters.\u003c/p\u003e \u003cp\u003eUsing coastal residence times predicted for our region, we expect all RDOC to be advected from coastal waters in our domain to the open ocean in 181\u0026ndash;365 days [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], indicating that all this carbon is likely exported beyond the 200 m isobath within the time frame over which its persistence has been measured (1 year) [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. We therefore consider the release of RDOC by live and degrading kelps and seagrass to contribute to long-term carbon sequestration, and our estimates of total flux through this pathway represent an upper bound of carbon sequestration via this mechanism (Figs.\u0026nbsp;2.16, 2.18, 3.16, 3.18). Total potential sequestration through the formation of RDOC (Figs.\u0026nbsp;2.20, 2.22, 3.20, 3.22) and export of LDOC before remineralization (2.17, 2.19, 3.16, 3.18) is 0.308\u0026thinsp;\u0026plusmn;\u0026thinsp;0.143 Tg C y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 0.00373\u0026thinsp;\u0026plusmn;\u0026thinsp;0.000915 Tg C y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for kelp and seagrass respectively, representing 26\u0026thinsp;\u0026plusmn;\u0026thinsp;19% and 3.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6% of NPP (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB, C). For kelp, DOC sequestration from the release of DOC from POC degradation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eC) is an order of magnitude higher than from the production of DOC from live plants (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eThe value of kelp forests and seagrass beds to carbon sequestration\u003c/h2\u003e \u003cp\u003eSumming RDOC from live kelp and degrading POC (Figs.\u0026nbsp;2.16, 2.18), the export of LDOC, LPOC, and RPOC to the shelf break (Fig.\u0026nbsp;2.24), and RPOC deposition/burial in soft sediments on the shelf (Figs.\u0026nbsp;2.2, 2.15) yields a total potential sequestration from kelp forests in our domain of 0.313\u0026thinsp;\u0026plusmn;\u0026thinsp;0.144 Tg C y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, accounting for 27\u0026thinsp;\u0026plusmn;\u0026thinsp;19% of annual NPP (Fig.\u0026nbsp;2.25). For seagrass beds, our calculation also incorporates non-kelp allochthonous carbon burial (Figs.\u0026nbsp;3.3, 3.13, 3.16, 3.18, 3.24, quantity in Fig.\u0026nbsp;3.25 minus quantity in 2.2), yielding a total sequestration rate of 0.011\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004 Tg C y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, accounting for 9.5\u0026thinsp;\u0026plusmn;\u0026thinsp;5.2% of NPP (Fig.\u0026nbsp;3.26). On a per unit area basis, kelps are also more efficient at sequestering carbon (161.0\u0026thinsp;\u0026plusmn;\u0026thinsp;68.9 Tg C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for kelp vs. 45.1\u0026thinsp;\u0026plusmn;\u0026thinsp;15.8 Tg C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for seagrass), and a higher percent of their NPP is estimated to be sequestered on an annual basis (Figs.\u0026nbsp;2.25, 3.26). The higher contribution of kelp to carbon sequestration in our region relative to seagrass is therefore due to a higher sequestration efficiency as well as the greater habitat area occupied in the study domain.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eImplications\u003c/h2\u003e \u003cp\u003eMacroalgal forests have been undervalued relative to other BCEs for their potential contribution to ocean carbon sequestration due to their inability to trap and bury carbon locally. Our study estimates a higher contribution of kelp to ocean carbon sequestration by an order of magnitude and strong differences in the pathways by which this carbon is stored compared with seagrass beds. DOC accounts for the majority of carbon sequestered by kelp forests in Nova Scotia, representing a higher percentage than has been estimated for kelp forests globally (98.5\u0026thinsp;\u0026plusmn;\u0026thinsp;60.1% of carbon sequestered vs. 64% globally)[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], with POC export and burial accounting for very little stored carbon (1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8% vs 31% globally) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Higher contribution by DOC in our region is partially due to the fact that we include the conversion from kelp POC to DOC in our budget [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], which has not been considered for macroalgal systems but may account for a significant flux of carbon to global oceanic DOC pools. Our results suggest that conservation measures that maintain, promote, or enhance the living biomass and productivity of kelps also promote higher DOC production and export, which can have positive effects on carbon sequestration rates even over short time scales.\u003c/p\u003e \u003cp\u003eMost carbon is sequestered by seagrass beds in our region through the burial of allochthonous and autochthonous POC within meadows and on soft sediments on the shelf (63.6\u0026thinsp;\u0026plusmn;\u0026thinsp;43%) [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], contrasting dominant pathways of carbon sequestration for kelp forests. Focusing solely on POC burial by seagrasses will significantly underestimate carbon sequestration, however, with DOC production emerging as a more significant flux of carbon than has been documented for seagrasses elsewhere (36.4\u0026thinsp;\u0026plusmn;\u0026thinsp;15.5% vs 10.6% of sequestration globally) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Discrepancies between our regional carbon budgets and existing global budgets are due to natural spatial and intraspecific variability as well as inadequate parameterization on a global scale. Applying generalized estimates to individual regions can therefore lead to inaccuracies in carbon accounting, and focusing on single carbon pathways can distort the relative value of BCEs for carbon sequestrations.\u003c/p\u003e \u003cp\u003eOur results also lay bare significant deficiencies in our knowledge of ocean carbon cycling, in particular the production, transformation, and dispersal of DOC from live and decaying coastal macrophytes as a critical area for future research and key carbon parameters to measure to quantify sequestration in BCEs. Further, carbon emissions from BCEs have gone largely uncharacterized. Broadly, our result show how carbon accounting can highlight undervalued carbon pathways, reduce uncertainty in existing global estimates of carbon sequestration, and support more robust carbon accounting and strategies for the management of BCEs.\u003c/p\u003e \u003c/div\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eStudy design\u003c/h2\u003e \u003cp\u003eThe study domain encompassed the mainland Atlantic Coast of Nova Scotia, ranging from Cape Sable Island (43.38959, -65.62084) in the southwest and Cape Canso in the Northeast (45.344620, -60.909754) to the edge of the Scotian Shelf break (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Carbon budgets were generated for the three dominant macrophyte species in the region, including \u003cem\u003eZostera marina, Laminaria digitata\u003c/em\u003e, and \u003cem\u003eSaccharina latissima\u003c/em\u003e [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Estimates are a compilation and reanalysis of new and existing experimental and observational data from our domain. When using existing studies, priority was given to studies from our region and species, where possible. Budget estimates are given as seasonal or annual totals, on a per m\u003csup\u003e2\u003c/sup\u003e or bioregion (defined as the study domain) basis. Details of how each carbon stock and flux were calculated are below. Note, we have not included emissions of greenhouse gasses (e.g. methane, halocarbons) in our budgets due to a lack of data.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eCarbon Stocks\u003c/h2\u003e \u003cp\u003eEstimates of carbon stocks contained in living biomass and sediments were generated for kelps (living biomass only) and seagrass using modeled suitable habitat area [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] combined with field measurements of carbon stocks per unit area. For \u003cem\u003eS. latissima\u003c/em\u003e and \u003cem\u003eL. digitata\u003c/em\u003e, biomass and density data were measured using quadrat sampling by SCUBA divers spanning all seasons at 13 sites within the study domain from 2021\u0026ndash;2023. At each site, divers sampled 6\u0026ndash;8, 0.75 m\u003csup\u003e2\u003c/sup\u003e quadrats along 1\u0026ndash;2 depth strata (3\u0026ndash;5 m, 7\u0026ndash;9 m). Divers counted the density of kelp stipes (one stipe represents one individual) and then all kelps were collected and placed in a mesh bag. Kelp species were separated and weighed (wet weight, ww) at the surface using a fish scale (\u0026plusmn;\u0026thinsp;0.01 kg). For carbon stock calculations, kelp biomass (kg ww) and density (ind m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) data were averaged across all sites and depths, and converted to measures of carbon content per unit area (kg C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) using a wet weight to dry weight (dw) conversion factor for each species [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] and a dw to carbon conversion factor of 0.3 [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFor \u003cem\u003eZ. marina\u003c/em\u003e, above and below ground biomass data were collected at four sites within the study domain [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], including a deep (3.2 m) and shallow (1.8 m) site at two of the four sampled sites. Above and below ground biomass was estimated using 6 hand cores (10.8 cm diameter) at each site. Both biomass components were summed for each core, converted to an estimate of biomass per m\u003csup\u003e2\u003c/sup\u003e, converted to carbon using a conversion factor of 0.36 [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], and then averaged across sites. Sediment carbon content within beds of \u003cem\u003eZ. marina\u003c/em\u003e was calculated using data from [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e], and summed with above and below ground biomass to generate a per m\u003csup\u003e2\u003c/sup\u003e estimate of carbon stock contained within live biomass and sediments within seagrass beds. For seagrass and kelp, these per m\u003csup\u003e2\u003c/sup\u003e measurements were then multiplied by the suitable habitat area for each species (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] in the region to estimate total regional carbon stock within each habitat. Suitable habitat area was estimated using probabilities from species distribution models, thresholded to a binary presence/absence as described in (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eNet Primary Production (NPP)\u003c/h2\u003e \u003cp\u003eEstimates of net primary production were generated from field growth rate measurements of kelp for \u003cem\u003eS. latissima\u003c/em\u003e and \u003cem\u003eL. digitata\u003c/em\u003e using the hole punch method for each species at 7 sites during all four seasons [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The data set includes field growth measurements taken in 2008\u0026ndash;2009 [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], supplemented with new field data collected using the same methods at two additional sites (Sober Island and Broad Cove) in 2022\u0026ndash;2023. In short, blade growth was measured by punching a hole at the base of the kelp blade (n\u0026thinsp;=\u0026thinsp;10\u0026ndash;20 individuals per species) near the meristem, and measuring the distance the hole moved over a period of 2\u0026ndash;4 weeks. This distance is converted to a measure of new tissue production (g dw) by excising the blade section after the growth period, drying the tissue at 60\u0026deg;C for 24\u0026ndash;48 hours, and then weighing the tissue. The amount of new tissue produced per day was then calculated by dividing the g dw of the tissue section by the time elapsed, and then converting this to an estimate of carbon production per individual (g C d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) using a conversion factor of 0.3 [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Individual growth measurements were then averaged across replicates and scaled to g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e estimate by multiplying this number by the density of each kelp species as collected in diver quadrats (see above). This rate was converted to a seasonal or annual total by multiplying it by the number of days in each season, and then summed it across the time period. The annual NPP value for each species was then multiplied by the area of suitable habitat in the domain (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e) to generate an annual estimate for the entire region. Hatcher [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e] found that growth represented by movement of the punched hole at the base of kelp blades of \u003cem\u003eS. latissima\u003c/em\u003e represented 45% of net carbon assimilation. Therefore, we corrected our field growth measures to NPP by multiplying field growth rates by a factor of 2.22 [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFor seagrass, NPP was measured in the field at two representative sites in May 2023\u0026ndash;2024 using the plastochrone growth method [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. For each sampling period, 20\u0026ndash;30 \u003cem\u003eZ. marina\u003c/em\u003e shoots at each site were marked with a hole through the middle of the sheath and collected 3\u0026ndash;9 weeks later depending on the season. After collection, the length (mm) of the most mature leaf (leaf three) was measured and converted to biomass (g dry mass) using established site-specific length-weight relationships [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. The number of new leaves since marking (i.e., leaves with no mark) were then determined and used to calculate the plastochrone interval (number of new leaves divided by number of days since marked). Shoot growth (g C shoot\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was calculated by dividing the weight of the third leaf (dry g shoot\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) by the plastochrone interval, then converting dry mass to carbon using a factor of 0.34 (g C per g dry mass; Wong, unpublished data). To scale individual shoot growth to growth per unit area (g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), mean shoot growth (g C shoot\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was multiplied by the shoot density (number of shoots m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) corresponding to that sampling period (where shoot density was determined using 3\u0026ndash;5 quadrats at each site). Finally, growth (g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was then averaged across the 2 sites, multiplied by the number of days in each season, and then summed for each season and the whole year. The seasonal and annual values for each species was then multiplied by the area of suitable habitat in the domain [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] to generate estimates for the entire region. We compared field growth measures to measurements of NPP in the lab from oxygen evolution experiments [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e] and found that they represent on average 61% of net carbon assimilation across seasons at a representative site in Nova Scotia (M. Wong, unpublished data). We therefore corrected our field growth measures to NPP by multiplying seasonal and annual values per m\u003csup\u003e2\u003c/sup\u003e and for the region by a factor of 1.64.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003ePOC production and export\u003c/h2\u003e \u003cp\u003ePOC production as blade erosion from kelps (i.e. senescence and breakage of blade material from the distal tip) was measured on the same individuals as for the field growth measurements in 2022\u0026ndash;2023, and again utilizing previously collected data and methodologies from 2008\u0026ndash;2009 [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Total blade length was measured at the start and end of the measurement period, and the difference in blade length, accounting for growth, is considered the amount of blade material lost to the distal erosion of tissues. The length of this section was converted to carbon by generating a g/cm conversion factor from a section of blade tissue of known length that is excised, dried, and weighed. This is then multiplied by the length of tissue eroded, and then converted to carbon using the conversion factor of 0.3 [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Individual erosion rate (g C d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was then calculated by dividing the amount of eroded tissue by the time elapsed between measurements. Individual measurements were then averaged across replicates and converted to a per m\u003csup\u003e2\u003c/sup\u003e and per region estimate using the same method as for growth.\u003c/p\u003e \u003cp\u003ePOC production from the mortality of whole kelp individuals was measured by marking 10\u0026ndash;20 individual kelps of each species with a numbered cable tie at Broad Cove and Sober Island at monthly intervals, and then tracking the survival of each individual between measurement periods. For the data from 2008\u0026ndash;2009 [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], we used the number of tagged individuals missing between the start and end of the growth measurement periods to estimate loss. The proportion of missing individuals was calculated for each month as a monthly mortality rate (including an estimated 25% relocation error rate) and averaged across months for each season. This mortality rate was corrected to account for only the losses relating to a single year of production by multiplying loss rates by the proportion of individuals in a single cohort estimated to be lost within a year (77% for \u003cem\u003eS latissima\u003c/em\u003e 50% for \u003cem\u003eL. digitata\u003c/em\u003e) [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. This mortality rate was then multiplied by a per individual average biomass (calculated by dividing quadrat biomass by density) to generate a per m\u003csup\u003e2\u003c/sup\u003e estimate of POC production through mortality. This was then converted to g C [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e] to generate a carbon loss rate in g C season\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e per m\u003csup\u003e2\u003c/sup\u003e, and multiplied by suitable habitat area for each species to generate an estimate for the whole region. We note that density estimates used to calculate POC production from erosion were corrected with mortality rate to avoid overestimating POC production. POC production from erosion and dislodgement were then summed to generate an estimate of total seasonal and annual POC production for each kelp species. Since mortality contributes carbon to the system from multiple years of NPP, only POC from erosion was related as a percent of annual NPP in our budget.\u003c/p\u003e \u003cp\u003eWe calculated the amount of this POC that is refractory (RPOC) as 14% of the stipe biomass [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], assuming that all blade material is degraded within 12 months given aerobic conditions on the Scotian Shelf [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. We calculated stipe biomass using a stipe to blade ratios recorded previously for \u003cem\u003eSaccharina latissima\u003c/em\u003e and \u003cem\u003eLaminaria digitata\u003c/em\u003e [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e] multiplied by biomass lost through plant mortality and corrected to 14%. The remaining POC is assumed to be labile (LPOC).\u003c/p\u003e \u003cp\u003ePOC production from senescing and dislodged seagrass leaves was estimated for each growth sampling interval as the difference between plant production (described above) (g dw m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) and the change in aboveground standing biomass (g dw m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Aboveground standing biomass (g dw m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) is estimated by multiplying shoot density (number of shoots m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) by the average aboveground biomass per shoot (specific to site) [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. POC production estimates were converted to g C using a conversion factor of 0.34, summed across season to generate a carbon loss rate (g C season\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e ), and multiplied by suitable habitat area to generate an estimate for the whole region. 5.5% of this POC is assumed to be refractory [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe amount of POC exported to the shelf break was then calculated for each species by multiplying % POC particle export rates for each season (Krumhansl et al \u003cem\u003ein revision\u003c/em\u003e) by the seasonal LPOC and RPOC production estimates for the bioregion, and then summing these quantities across seasons to generate an annual total POC export estimate in g C region year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. We note that degradation times at the bottom temperatures experienced in our region would result in negligible losses of biomass over the 90-day export period modeled, so we do not account for degradation in our carbon export estimates (Krumhansl et al \u003cem\u003ein revision\u003c/em\u003e). We also generated estimates of the amount of carbon exported to seagrass beds and muddy sediments in the nearshore and on the shelf. This was done by multiplying seasonal % export estimates to these areas (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e) by RPOC and LPOC production, and then summing these quantities for the year. We estimated the amount of carbon originating from \u003cem\u003eZ. marina, S. latissima\u003c/em\u003e and \u003cem\u003eL. digitata\u003c/em\u003e that is buried in seagrass beds using carbon burial rates (g C m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) measured for \u003cem\u003eZ. marina\u003c/em\u003e beds [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] multiplied by the proportion of sediment carbon that is \u003cem\u003eZ. marina\u003c/em\u003e or macroalgae in origin [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Allochthonous carbon burial in seagrass beds is accounted for in our budget as the total estimated carbon burial rate minus the burial of carbon originating from \u003cem\u003eZ. marina.\u003c/em\u003e Sufficient data to estimate the amount of POC from \u003cem\u003eZ. marina\u003c/em\u003e, \u003cem\u003eS. latissima\u003c/em\u003e, and \u003cem\u003eL. digitata\u003c/em\u003e buried in shelf sediments do not exist for our region, but we consider our estimates of RPOC deposition on muddy sediments as a first order estimate of the burial rate of this carbon. All quantities were then multiplied by the area of the bioregion to yield a regional total. The amount of LPOC and RPOC originating from kelp and \u003cem\u003eZ. marina\u003c/em\u003e not deposited on muddy sediments or seagrass beds was assumed to be remineralized.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eDOC production and export\u003c/h2\u003e \u003cp\u003eDOC production was measured for \u003cem\u003eL. digitata, S. latissima\u003c/em\u003e, and \u003cem\u003eZ. marina\u003c/em\u003e in situ using incubation experiments conducted in the spring, summer, and fall (winter experiments were not conducted due to logistical limitations). Whole seagrass (including leaves, rhizomes and roots ) and kelp (including blades and holdfasts) individuals were collected at two sites each for both species groups. Plants chosen for the incubations were clear of visible epiphytes and epifauna, and were in good condition (i.e. little visible damage and/or degradation of tissues). Plants were collected the week prior to each experimental period to allow for wound healing from collections and stored in ambient seawater until incubations began. DOC production was then measured by containing whole plants in clear plastic bags (16 L) anchored to a lead line on the substratum (25 m in length, plants separated by 50 cm) at 4 m depth. The experimental design consisted of 5 replicates of each species from 2 sites each (n\u0026thinsp;=\u0026thinsp;10 for each species), and 10 controls (n\u0026thinsp;=\u0026thinsp;40 total replicates) during each experimental run (except summer where 4 replicates per site, 8 controls, and only 1 seagrass site were included). Seagrass individuals were bundled into groups of 10 plants (5 plants in summer) within experimental bags, as DOC production from one individual was expected to be too low to detect relative to the volume of the bag.\u003c/p\u003e \u003cp\u003eFor each experimental run, seagrass, kelp or a bare tag (experimental control) was randomly added inside each bag and the bags were filled with surrounding water (9.0\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2 L; mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and sealed. In addition to the controls affixed to the lead line, we also collected 5 control bags filled from the surrounding waters at the start and end each deployment. These controls were brought to shore immediately for chemical analysis. DOC production was assessed during 1 daytime period and 1 nighttime period during each experimental run. The deployment duration varied between seasons depending on daylength and time for sunrise and sunset. The night deployment was done during the entire dark phase, with experimental bags deployed within an hour after sunset and retrieved within an hour before sunrise (duration range: 11-14.5 hours). The day deployment was shorter and aimed to be within the peak daylight 10\u0026thinsp;\u0026minus;\u0026thinsp;2 pm (duration range: 4\u0026ndash;7 hours). Upon retrieval of the nighttime experiment, bags were returned to shore and kept in the dark before processing (1.5 hours max). Bags were weighed using a digital scale (\u0026copy;Berkley, precision 10 g), and then dissolved oxygen (DO; YSI ODO), temperature and salinity (YSI Pro 30) were measured inside each bag (except for the summer when the YSI ODO was not used). Samples were immediately filtered using a peristaltic pump maintained below 10 psi with 0.45 micron nitrocellulose mixed ester membrane filters. Samples were poisoned with concentrated HgCl2 upon return to the lab and kept at 4\u0026deg;C until analysis. For one replicate of each treatment and control type, we also measured DO using the Winkler method [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. The same kelp and seagrass individuals were used for the night and daytime experimental runs, and were kept in a cooler on ice between deployments. Within 24 hours of the end of the experiment, we measured the wet weight of each plant and tags before drying the them in a drying oven (60\u0026deg;C) for 48 hours and measuring their dry weight (DW).\u003c/p\u003e \u003cp\u003eThe quantity of DOC produced was measured with a OI Analytical Aurora 1030W TOC analyzer with a model 1080 autosampler and a combustion unit. Briefly, dried gas from sparged acidified samples was detected using infrared gas analysis with an analytical precision of 0.4 ppm or better (Jan Veizer Stable Isotope Laboratory, University of Ottawa). DOC concentrations (mg C L\u003csup\u003e-1\u003c/sup\u003e) were then converted to production/consumption rates (mg C L\u003csup\u003e-1\u003c/sup\u003e h\u003csup\u003e-1\u003c/sup\u003e) by first subtracting the start concentration for each deployment, and dividing this amount by the experimental duration. This amount was then multiplied by the volume per bag to create a measure of DOC production per hour (mg C h\u003csup\u003e-1\u003c/sup\u003e). To account for the biological activity of the surrounding seawater in the treatment bags, the mean DOC production/consumption rate in the control bags was then subtracted from the DOC production/consumption rate of each experimental bag. The DOC production/consumption hourly rate per individual (mg C ind\u003csup\u003e-1\u003c/sup\u003e h\u003csup\u003e-1\u003c/sup\u003e) was then calculated by dividing the DOC production/consumption hourly rate (mg C h\u003csup\u003e-1\u003c/sup\u003e) by the number of individuals in each bag (seagrass only).\u003c/p\u003e \u003cp\u003eTo estimate the daily DOC production/consumption rates per m\u003csup\u003e2\u003c/sup\u003e, we extracted information on the total duration in hours above light saturation allowing for photosynthesis (HSat) for kelp and seagrass for each day in each season assuming the following conditions (depth \u003csub\u003eseagrass\u003c/sub\u003e = 1m, depth \u003csub\u003ekelp\u003c/sub\u003e = 5m, attenuation coefficient\u0026thinsp;=\u0026thinsp;0.33, saturation irradiance (Ik) IK \u003csub\u003eseagrass\u003c/sub\u003e = 100 or 250 umol photons.m\u003csup\u003e-2\u003c/sup\u003e.s\u003csup\u003e-1\u003c/sup\u003e [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e], Ik \u003csub\u003ekelp\u003c/sub\u003e = 38 umol photons.m\u003csup\u003e-2\u003c/sup\u003e.s\u003csup\u003e-1\u003c/sup\u003e [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e] at a central geographical location for the south shore of Nova Scotia (latitude: 44.4\u0026deg; N). The hourly DOC production/consumption rate per individual (mg C ind\u003csup\u003e-1\u003c/sup\u003e h\u003csup\u003e-1\u003c/sup\u003e) was then converted to a daily rate for each day of the season by multiplying the daytime DOC production/consumption rate by HSat, and then adding this quantity to the product of nighttime DOC production/consumption rate and 24 \u0026ndash; Hsat (i.e. hours under saturating irradiance). Summer data were missing at nighttime due to logistical limitations, therefore we used a 10% decrease of daytime DOC production/consumption rate per biomass to estimate the nighttime value (nighttime DOC production was 10% lower than daytime production during other seasons). We did not run an experiment in winter, so we applied DOC production/consumption rates for spring with winter daily HSat values to calculate winter DOC production. We assume winter is most similar to spring in terms of DOC production given that growth exceeds senescence in both seasons, whereas the opposite is true in summer and fall [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Daily DOC production values per gram dw were multiplied by the per individual biomass in each bag and then multiplied by the density of plants at each site [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], and then summed to generate a total DOC production rate for each season. These quantities were then summed across seasons to generate an annual rate (g C m\u003csup\u003e-2\u003c/sup\u003e y\u003csup\u003e-1\u003c/sup\u003e), and multiplied by the area of suitable habitat for each species to generate an estimate of DOC production for the bioregion.\u003c/p\u003e \u003cp\u003eThe amount of DOC in refractory form was then estimated by multiplying each seasonal and annual quantity by the estimated amount of this carbon that is refractory, as measured from prior studies: 37.5% for kelp [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], and 10% for seagrass [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The difference between the total DOC production and the RDOC quantity was considered the labile fraction (LDOC). The amount of DOC released during kelp and seagrass POC degradation was estimated by multiplying the quantity of LPOC produced during each season by the percent of this carbon released as DOC within the first two weeks of degradation, as measured by experimental studies (59% for kelp [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] and 25% for seagrass [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]). 37.5% and 10% were used, as above, to convert this DOC quantity to RDOC for kelp and seagrass, respectively. To quantify export of the LDOC fractions released from live and decaying individuals, we modeled decay of LPOC quantities using an exponential relationship and decay constants found in the literature: k = -0.06% day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for kelp [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] and \u0026minus;\u0026thinsp;0.01% day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for seagrass [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]). Daily estimates of the % of particles (i.e. carbon) reaching the shelf break (Krumhansl et al. \u003cem\u003ein revision\u003c/em\u003e) were then multiplied by the quantity of LDOC remaining on each day to estimate the amount of this DOC exported at each time step. This quantity was then subtracted from the amount available for decay and export during the following time step. The total amount of LDOC exported to the shelf break during each season was then calculated by summing this quantity across the 90 days of the season, and then across seasons for an annual total. Using coastal residence times predicted for our region, we expect all RDOC to be advected from coastal waters in our domain to the open ocean in 181\u0026ndash;365 days [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], indicating that all this carbon is likely exported beyond the 200 m isobath within the time frame over which its persistence has been measured (1 year) [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKK, MW, and KA-S\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003econceived of the study, KK, MW, MP, MF, C-EG, YW, and KA-S designed field measurements and experiments, KK, MP, MF, C-EG collected field data and conducted experiments, KK, MW, MP, MF analyzed the data, KK, MP, MF reviewed the literature, KK, MW, MP, MF wrote the manuscript, KK, KA-S, MW, MP, MF, C-EG, YW edited and provided input on the manuscript, KA-S oversaw the work.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe acknowledge Katie Thistle, Shawn Roach, Thomas Baker, Cody Brooks, Danielle Davenport, Claudio DiBacco, Katherine Lee, and Nick Jeffery for their support in the field. We also thank Wendy Gentleman for feedback on the work. KK discloses support for this work from a Fisheries and Oceans Canada Competitive Science Research Fund grant to KK and KA-S. The authors declare no competing interests. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data and processing code will be made available on a public repository upon publication, but have been made available as supplementary material for this submission.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eNellemann, C., Corcoran, E., Duarte, C. M., Vald\u0026eacute;s, L., De Young, C., Fonseca, L., Grimsditch, G. , \u003cem\u003eBlue Carbon, A Rapid Response Assessment\u003c/em\u003e. United Nations Environment Programme, ed. C. Nellemann, Corcoran, E., Duarte, C. 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Kukliński, and M. Włodarska-Kowalczuk, \u003cem\u003eMorphological variation of kelps (Alaria esculenta, cf. Laminaria digitata, and Saccharina latissima) in an Arctic glacial fjord.\u003c/em\u003e Estuarine, Coastal and Shelf Science, 2022. \u003cstrong\u003e268\u003c/strong\u003e.\u003c/li\u003e\n\u003cli\u003eRomero, J., et al., \u003cem\u003eThe Detritic Compartment in aPosidonia oceanicaMeadow: Litter Features, Decomposition Rates, and Mineral Stocks.\u003c/em\u003e Marine Ecology, 2008. \u003cstrong\u003e13\u003c/strong\u003e(1): p. 69-83.\u003c/li\u003e\n\u003cli\u003eJones, E., F. Zemlyak, and P. Stewart, \u003cem\u003eOperating manual for the Bedford Institute of Oceanography Automated Dissolved Oxygen Titration System.\u003c/em\u003e Canadian Technical Report of Hydrography and Ocean Sciences, 1992. \u003cstrong\u003e138\u003c/strong\u003e.\u003c/li\u003e\n\u003cli\u003eLee, K., S. Park, and Y. Kim, \u003cem\u003eEffects of irradiance, temperature, and nutrients on growth dynamics of seagrasses: A review.\u003c/em\u003e Journal of Experimental Marine Biology and Ecology, 2007. \u003cstrong\u003e350\u003c/strong\u003e(1-2): p. 144-175.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6473319/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6473319/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCoastal vegetated ecosystems (e.g. kelp forests, seagrass meadows) potentially play a significant role in oceanic carbon sequestration, yet existing estimates have high uncertainty because they are compilations of few data from disparate species and regions, or focus on individual stocks or fluxes. We use empirical data and modeling to generate detailed carbon budgets for kelp forests and seagrass meadows in a single region (Nova Scotia, Canada). We estimate kelp forests to sequester more carbon (0.313\u0026thinsp;\u0026plusmn;\u0026thinsp;0.144 Tg C y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 27\u0026thinsp;\u0026plusmn;\u0026thinsp;19% of annual net primary production [NPP]) than seagrass meadows (0.011\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004 Tg C y\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 9.5\u0026thinsp;\u0026plusmn;\u0026thinsp;5.2% of NPP) by an order of magnitude. A substantially higher proportion of carbon sequestered by kelp forests is from the production and export of dissolved organic carbon (DOC) (98.5\u0026thinsp;\u0026plusmn;\u0026thinsp;60.1% of carbon sequestered) than for seagrass meadows, which primarily sequester carbon through shelf burial and the export of particulate organic carbon (POC) to the deep sea (63.6\u0026thinsp;\u0026plusmn;\u0026thinsp;43%). Our results show how detailed carbon budgeting can highlight undervalued or overlooked carbon pathways (e.g. POC to DOC conversion), and support more robust valuation and management of blue carbon ecosystems for carbon sequestration.\u003c/p\u003e","manuscriptTitle":"An Annual Blue Carbon Budget for Kelp Forests and Seagrass Beds","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-22 10:43:48","doi":"10.21203/rs.3.rs-6473319/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-earth-and-environment","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsenv","sideBox":"Learn more about [Communications Earth and Environment](https://www.nature.com/commsenv/)","snPcode":"","submissionUrl":"","title":"Communications Earth \u0026 Environment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f84c7e64-4ede-450e-b0aa-0b2315ae14ef","owner":[],"postedDate":"May 22nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":48322103,"name":"Earth and environmental sciences/Biogeochemistry/Carbon cycle"},{"id":48322104,"name":"Earth and environmental sciences/Climate sciences/Biogeochemistry/Carbon cycle"}],"tags":[],"updatedAt":"2026-01-30T08:09:46+00:00","versionOfRecord":{"articleIdentity":"rs-6473319","link":"https://doi.org/10.1038/s43247-025-03122-2","journal":{"identity":"communications-earth-and-environment","isVorOnly":false,"title":"Communications Earth \u0026 Environment"},"publishedOn":"2026-01-02 05:00:00","publishedOnDateReadable":"January 2nd, 2026"},"versionCreatedAt":"2025-05-22 10:43:48","video":"","vorDoi":"10.1038/s43247-025-03122-2","vorDoiUrl":"https://doi.org/10.1038/s43247-025-03122-2","workflowStages":[]},"version":"v1","identity":"rs-6473319","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6473319","identity":"rs-6473319","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}