Desert Coral Reefs: A possible sink of atmospheric CO2 in a rapidly warming World

Desert Coral Reefs: A possible sink of atmospheric CO2 in a rapidly warming World | 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 Desert Coral Reefs: A possible sink of atmospheric CO 2 in a rapidly warming World Hamish McGowan, Shai Abir, Nadav Lensky, Yonathan Shaked This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7005193/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Eddy covariance (EC) measurements of air-sea CO 2 exchange (Net Ecosystem Exchange) over desert fringing coral reefs in the Gulf of Eilat (GoE), Israel show these ecosystems may be net sinks of atmospheric CO 2 . This result contrasts with marine productivity models and bulk formula calculations based on for example, water chemistry used to then determine the magnitude and direction of the CO 2 flux. These studies have for more than three decades often concluded that coral reefs are net sources of CO 2 to the atmosphere with only rare cases finding otherwise. Our EC measurements find coral reefs in the GoE sequester 3 to 10 times more CO 2 than other marine and terrestrial ecosystems including tropical rainforests. This highlights the need for further direct measurements of air-sea CO 2 exchanges over coral reefs in different environmental settings and climate regimes so that the role of coral reefs in the global carbon cycle can be accurately quantified. Earth and environmental sciences/Climate sciences Earth and environmental sciences/Ocean sciences Coral Reefs CO2 Ecosystems Air-sea exchange Direct measurement Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Understanding net ecosystem – atmosphere CO 2 exchange (Net Ecosystem Exchange (NEE)) is crucial to inform policy responses to anthropogenic global warming. Quantification of NEE has therefore become a major focus of initiatives to directly measure ecosystem CO 2 exchanges. The eddy covariance (EC) method provides the only direct representative measure that captures the multi-scaled and complex processes controlling ecosystem – atmosphere trace gas fluxes including CO 2 . These span a spectrum of time and space scales of 15 orders of magnitude [ 1 , 2 , 3 ] . EC is therefore the cornerstone of AmeriFlux [ 4 ] , EUROFLUX [ 5 ] , OzFlux [ 6 ] and Asiaflux [ 7 ] which seek to quantify NEE by direct measurement and contribute to the global FLUXNET program [ 8 ] .Substantial advances have therefore been made in understanding terrestrial CO 2 fluxes. Yet, direct measurement of CO 2 flux over lacustrine and marine ecosystems remains dominated by time and space limited observations at disparate locations, none more so than over coral reefs – the rainforests of the oceans [ 9 , 10 ] . Coral reefs represent 0.1 to 0.25% of the global marine environment with at least 1 billion people benefiting from their ecosystem services [ 11 , 12 , 13 ] . They are at risk from warming sea surface temperatures (SSTs), ocean acidification, overfishing, pollution, mining, and tourism. While the ecological importance of coral reefs is unquestionable, their role in carbon budgets is uncertain. Previous research suggested coral reefs may absorb ≈ 2% of annual anthropogenic CO 2 (-720 Mt), while others have stated coral reefs are sources of atmospheric CO 2 (+ 144 Mt to + 504 Mt) [ 14 , 15 , 16 ] . These estimates of CO 2 exchange were based on marine productivity models or bulk formula that use calculations of the CO 2 partial pressure ( p CO 2 ) difference across the air-sea interfacial boundary multiplied by an estimated gas transfer velocity [ 17 , 18 ] . Other studies have used point measurements of water pH, temperature, and chemistry to estimate reef water p CO 2 which is then compared against atmospheric CO 2 concentration to determine the CO 2 flux gradient [ 19 , 20 ] . Direct measurements of air-sea CO 2 exchange using EC with measurement footprints of > 20,000 m 2 have found coral reefs to be net sinks of CO 2 , at times an order of magnitude larger than reported by bulk formula and productivity models [ 21 , 22 , 23 ] . The large measurement footprint of EC ensures truly representative measurements of NEE. However, these studies were conducted for only a few days to several weeks and may not have captured the full range of hydrodynamic and meteorological conditions expected on coral reefs. Nonetheless, they highlighted the spatial heterogeneity of air-sea CO 2 exchange such as at Heron Reef on the humid subtropical southern Great Barrier Reef, Australia. Here the reef flat was identified as a net source of CO 2 to the atmosphere, while concurrently adjacent lagoons were found to be net CO 2 sinks [ 21 ] . Using data on water chemistry, tropical fringing coral reefs at Mo'orea, Tahiti were found to be CO 2 sinks (up to 10 mmol m − 2 day − 1 ), whereas a neighbouring barrier reef flat was a CO 2 source. This was attributed to the possible effect of the relative coverage of algae with coral dominated reefs hypothesized to be sources of CO 2 , while those with substantial algae coverage were CO 2 sinks [ 24 ] . More recently, analysis of water chemistry over fringing coral reefs on the South China coast during summer found coral reefs in this region to be CO 2 sinks with mean values of 12.3 to 19.2 mmol CO 2 m − 2 d − 1 . These air-sea CO 2 exchanges were believed to be dependent on hydrodynamics and algae cover [ 25 ] . This highlights the importance of community structure of coral reef ecosystems in air – sea CO 2 exchange. Factors that may influence this observation either directly or indirectly include currents, coastal development, river discharges, deposition of aerosols, nutrients from aquiculture, changes in atmospheric CO 2 concentrations, and synoptic meteorology. Corals affect air-sea CO 2 transfers through changing the p CO 2 in the water overlying the reef via photosynthesis/respiration and calcification/dissolution across the day–night cycle [ 26 , 27 ] . During the day, photosynthetic activity consumes CO 2 , while at night, respiration consumes O 2 and produces CO 2 . Calcification and dissolution raise the p CO 2 in the water with the precipitation of 1 mol of CaCO 3 producing 1 mol of CO 2 [ 25 ] with around 60% escaping to the atmosphere [ 28 ] . Variability in the p CO 2 in the water overlying a coral reef will therefore reflect biogeochemical, hydrodynamic, and meteorology processes. For example, dust deposited on the fringing coral reefs at Eilat, Israel was found to supply nanomolar amounts of essential bio-elements to the coral symbionts including iron, manganese, zinc and copper from natural, industrial, and agricultural processes [ 29 ] . This fertilization by dust enhanced chlorophyll concentrations and photosynthesis by coral symbionts, lowering the p CO 2 in the water overlying the reef and leading to a net influx of atmospheric CO 2 [ 22 ] . Here we present results from the longest direct measurement record of air-sea CO 2 exchange over a coral reef made using the EC method. Research was conducted in the Gulf of Eilat (GoE), Israel. In this hyper-arid environment evaporative cooling of water provides a thermal refugia for corals, which have evolutionary traits believed to further protect them from extreme water temperatures [ 30 , 31 , 32 ] . Results are compared to previously published EC CO 2 fluxes for coral reefs in humid environments, and CO 2 exchanges over other marine and terrestrial ecosystems. 2. Results 2.1 Air-sea meteorology and CO 2 exchange. Direct EC measurements of air-sea CO 2 exchange over the fringing coral reef at UOE was undertaken from the 3 September 2020 to 2 March 2021 (181 days) with concurrent measurements of associated meteorology (Fig. 1 ). The seasonal transition from hot summer conditions with afternoon air temperatures of around 40°C (Fig. 1 d) and water temperatures greater than 30°C (Fig. 1 c) to cooler winter conditions was marked by an abrupt transition in early November 2020. This is most evident in ΔT (Fig. 1 h) as a step change from conditions where the air was warmer than the water to where the water was predominantly warmer than the air. There was no concurrent effect on CO 2 flux (Fig. 1 f) with this rapid “flip” in ΔT. Daytime wind speed maxima were at their minimum during winter reflecting a weaker local sea breeze (Fig. 1 a). The northerly wind that regularly blows from the Arava Valley onto the northern GoE as a result of the valley channelling synoptic winds and the inland penetration of the Mediterranean Sea breeze was also weaker. CO 2 flux was dominated by negative values (blue shading) indicating influx (sequestration of CO 2 from the atmosphere) (Fig. 1 f). This influx doesn’t show obvious correlation with other variables including aerosol (PM10) (Fig. 1 e) which is primarily minerogenic aerosol entrained from the surrounding deserts. It has previously been linked to fertilization of the coral reefs in the GoE and possible CO 2 sequestration [ 22 , 29 ] . PM10 concentrations averaged 49.8 µg m − 3 over the observation period with a maximum 30-minute concentration of 1200 µg m − 3 . The absence of clear evidence of correlation between CO 2 flux and PM10 (r = 0.06) may reflect that such relationships are event specific and linked to dust chemistry, time of day, and prevailing coastal hydrodynamics. For example, at 0930 hrs 17 November 2020 CO 2 flux reached − 18.90 µmols m − 2 s − 1 (influx) and then rapidly increased to + 4.9 µmols m − 2 s − 1 (efflux) at 2100hrs 18 November 2020 following a dust event in the morning of the 18 November 2020, when PM10 concentrations peaked at 314 µg m − 3 at 1100 hrs. Net CO 2 fluxes then remained positive (efflux) over the following days. In comparison, one month later PM10 concentrations peaked at 295 µg m − 3 at 0600 hrs on 15 December 2020 after which net CO 2 fluxes became negative at 1900 hrs on 15 December 2020 and then increasing negative (influx) until early January 2021 (Fig. 2 ). Incoming solar irradiance was not correlated with CO 2 flux (r -0.07). The introduction of a 3-hour lag between CO 2 flux and positive solar irradiance to allow for coral symbionts to respond to solar irradiance resulted in a change of sign with r = 0.08. CO 2 flux was negatively correlated with windspeed (r = -0.49) and relative humidity (r = -0.32). Accordingly, as windspeed increased, CO 2 influx increased. CO 2 flux displayed a positive correlation with air temperature (r = 0.40) indicating as temperature decreased CO 2 influx increased. Over the entire 181-day monitoring period the mean 30-minute CO 2 flux measured by EC over the fringing coral reefs at Eilat was − 2.07 µmols m − 2 s − 1 and displayed a weak trend of increasing influx from late summer into early winter (Fig. 2 ). This trend was not significantly correlated with the increase in daily Chlorophyll-a concentrations from late October to mid-December 2020 (Fig. 2 b) as confirmed by an R 2 value of 0.1. This suggests that algae in the water over the coral reef did not influence air-sea CO 2 exchange. 2.2 Diurnal CO 2 fluxes over coral reefs Diurnal CO 2 fluxes measured over the coral reefs at Eilat displayed a clear cycle with a maximum influx in the early to mid-morning and maximum efflux early evening after sunset. Respiration and calcification from early afternoon onward including possible dark (nocturnal) calcification following sunset with associated cooling of the water surface increases the reef water p CO 2 relative to the atmosphere. This results in a decrease in CO 2 influx which in late summer (September) switched to an efflux by early afternoon which continued into the evening (Fig. 3 a). Following sunset (i.e. approx. 18:00 hrs) the p CO 2 in the water overlying the coral reef gradually began to decrease resulting in an increasing influx of CO 2 from atmosphere to the coral reef (Fig. 3 ). This influx of CO 2 increases throughout the early morning from midnight peaking around 11AM for our entire 181-day data set (Fig. 3 b) is more pronounced in winter (January) as seen in Fig. 3 a. The effect of photosynthesis on CO 2 influx is particularly evident by the rapid increase in CO 2 influx in January (Fig. 3 a) around 6:30AM. During this cooler time of year, the coral reef at Eilat behaves as a significant sink for atmospheric CO 2 throughout the diel cycle. 2.3 CO 2 fluxes over coral reefs and other ecosystems CO 2 flux measured by EC over the fringing coral reefs at Eilat are extrapolated to a mean annualised value for comparison with CO 2 flux over other ecosystems including commercial pine plantations and tropical rainforest measured also by EC. These ecosystems are regularly highlighted as important global carbon sinks [ 33 , 34 , 35 ] (Table 1 ). Our results show that CO 2 influx over the coral reefs of Eilat range from 3 times to an order of magnitude greater than CO 2 influx measured over sites in mangrove forest, pine plantations, cool temperate forests, and Brazilian rainforest (Table 1 ). They are significantly greater than measured over the ocean in both tropical and high latitude locations (Table 1 ) although caution is warranted given the short observation periods of these measurements and their extrapolation to annual values. Table 1 Examples of CO 2 exchange (NEE) over different ecosystems compared to measurements made over coral reefs in the GoE including this study. All CO 2 flux measurements were made using EC systems and therefore represent direct ecosystem – atmosphere CO 2 exchange. Published CO 2 fluxes have been converted to annual values for ease of comparison. Location (duration of observation) Ecosystem t C ha yr − 1 (negative influx) Eilat, Israel (3 Sept 2020–2 March 2021: 181 days) [This study] Healthy fringing coral reefs only. -28.7 Eilat, Israel (1 April to 27 August of 2009: 142 days) [ 23 ] Healthy fringing coral reefs and adjacent ocean. -18.04 Durham, North Carolina, USA (1 year) [ 36 ] Forest ( Pinus taeda L. ) -6.05 Maine, USA. (7 yrs) [ 37 ] ∼90-year-old spruce dominated forest -1.74 Changbai Mountain Natural Reserve, northeastern China (1 year) [ 38 ] Montane forest (Dominant species Pinus koraiensis Sieb. et Zucc. (34%), Tilia amurensis Rupr. (42%)). -1.88 Brazilian Amazon Basin - mean of 3 sites. (3–5 yrs) [ 39 ] Brazilian rainforest -10 Central Japan (1 year) [ 40 ] Cool-temperate deciduous forest -2.14 Bribie Island, Subtropical eastern Australia (2.5 yrs) [ 41 ] Coastal wetland (Dominant species - Melaleuca quinquenervia ) -5.61 Bribie Island, Subtropical eastern Australia (2.5 yrs) [ 41 ] Pine plantation ( Pinus elliottii var. elliottii x Pinus caribaea var. hondurensis ) -6.31 Arctic Ocean (30 Jun–1 Aug 2019) [ 42 ] Arctic Ocean -1.62 Arctic Ocean (5 Aug–29 Sep 2019) [ 42 ] Arctic Ocean -2.62 Tropical Atlantic (9–16 Oct 2018) [ 42 ] Tropical Atlantic Ocean -0.40 Pichavaram, southeast India (1 year) [ 43 ] Mangrove forest -1.83 3. Discussion Ecosystem productivity models, microscale chamber gas flux exchange measurements, and water – atmosphere chemistry studies have typically concluded that coral reefs are net sources of CO 2 to the atmosphere [ 15 , 16 , 18 , 44 ] . Direct EC measurement of coral reef – atmosphere CO 2 flux at ecosystem scale presented here show that the fringing coral reefs of the GoE are net sinks of CO 2 at -2.07 µmol m − 2 s − 1 (-28.7 t C ha yr − 1 ). Net Ecosystem Exchange over coral reefs at Eilat show they sequester 3 to 10 times more atmospheric CO 2 than Brazilian rainforest [ 39 ] , pine plantations [ 36 , 41 ] and mangrove forest [ 43 ] . Our results are aligned with previous independent EC measurements of CO 2 flux in the GoE. Over 142 days from April to August 2009 these measured a mean CO 2 influx of -1.3 µmol m − 2 s − 1 (-18.04 t C ha yr − 1 ) (Table 1 ) [ 23 ] at a site approximately 280 m south from our UOE site with a measurement footprint that extended beyond the coral reef and over the adjacent ocean. These ocean waters with depths from 40 to 550 m sequester around 50% less CO 2 on average than the fringing coral reefs [ 22 ] . Notably, these measurements were made through late spring and summer, namely outside the months of our measurements, but interestingly they showed the coral reef of Eilat to also be a net CO 2 sink in these seasons [ 23 ] . CO 2 fluxes were found to be positively correlated with air temperature but negatively correlated with wind speed. Namely, as air temperature (and water skin temperature) decrease CO 2 solubility increases, while increase in windspeed may further increase influx through evaporative cooling of the water surface and mixing. Disparity between published coral reef CO 2 flux studies showing coral reefs as sources of CO 2 and results presented here showing coral reefs in the GoE are very significant net sinks of atmospheric CO 2 likely arise from: i) the very small measurement footprints (cm 3 to a few m 2 ) of non-EC research, ii) short duration measurements (minutes to a few days), iii) incorrect assumptions about gas transfer velocities and, iv) air – sea p CO 2 gradients calculated from point sampling of water chemistry and assumed atmospheric CO 2 concentrations from measurements made 10s to 100s of kilometers from the water sampling sites [ 19 , 45 , 46 , 47 , 48 ] . These studies that have often presented microscale site and time specific observations do not capture the heterogenous properties of complex coral reef ecosystems and their environment. Site factors such as ecosystem composition, hydrodynamics, wave environment, terrestrial runoff and aquifer seepage all influence air – sea CO 2 flux and are captured by EC measurements because of their large measurement footprint. At Heron Reef on the southern Great Barrier Reef, Australia concurrent EC measurements found the shallow lagoon to be a net CO 2 sink (-2.27 µg m − 2 s − 1 ), while the adjacent reef flat was a net source of CO 2 (+ 3.40 µg m − 2 s − 1 ) to the atmosphere [ 21 ] . Here, dissolution of calcium carbonate on the reef flat along with seepage of CO 2 enriched water at low tide from the coral cay (Heron Island) was believed to contribute to supersaturation of reef flat waters with CO 2 resulting in net evasion of CO 2 to the atmosphere. Concurrently over the shallow lagoon, photosynthesis by the benthic microalgae and corals was thought responsible for a net CO 2 influx [ 21 ] . Such variability highlights the need for ecosystem scale and multi-seasonal to annual continuous direct measurements of CO 2 flux over coral reefs. While the majority of non-EC coral reef CO 2 flux studies have reported coral reefs as net sources of CO 2 , scleractinian coral communities at two sites on Dapeng Peninsula in the South China Sea have also been found to be very weak net CO 2 sinks (0.99 ± 0.08 to 1.66 ± 0.40 mmol m − 2 day − 1 ) [ 49 ] . Coral reefs at Lombok, Indonesia have also been found to be net CO 2 sinks (0.14 to 1.1 mmol m − 2 day − 1 ) during October, but they switched to net sources in April [ 50 ] . Recently interest has emerged in potentially establishing new or restoring coral reefs to sequester CO 2 . It has been estimated that floating coral nurseys with 10 6 coral colonies per 1 km 2 could potentially sequester the equivalent of 110 t of CO 2 per year [ 51 ] . Thus, such coral reef restoration initiatives would not only have direct ecological benefits but would sequester atmospheric CO 2 . A clearer understanding of the role of coral reefs in the carbon cycle as sources or sinks in different geographic and climate regimes, and their net impact on global atmospheric carbon is therefore required. It is clear that there remain very substantial differences in the determination of whether coral reefs are net sources or sinks of atmospheric CO 2 , and the factors that may cause coral reefs to switch from source to sink and vice versa. For example, during our measurement period we found no clear and persistent association between dust fertilization of corals in the GoE and CO 2 flux as previously postulated and supported by in aquaria studies with GoE corals and dust [ 22 , 29 ] . This may reflect variability in dust source chemistry, prevailing meteorology and hydrodynamics over the GoE coral reefs and the associated impact of coral photosynthesis. EC provides the only direct measurement method that can accurately quantify air-sea CO 2 exchange at ecosystem scale that accounts for changes in water chemistry including that caused by the deposition of aerosols, coral reef hydrodynamics, meteorology, and coral reef ecology. The debate as to whether coral reefs act as net sources or sink of atmospheric CO 2 has prevailed for more three decades. However, there is growing evidence through research as we present here that when continuous direct measurement is undertaken at ecosystem scale, coral reefs such as those in the GoE bordered by a hyper-arid desert are net sinks of CO 2 , potentially far exceeding CO 2 sequestration by rainforests and commercial forestry plantations. Mean air – sea CO 2 flux (NEE) measured by EC from late summer to the end of winter (181 days) over the fringing coral reefs in the GoE was − 2.07 µmol m − 2 s − 1 which equates to -28.7 t C ha yr − 1 assuming CO 2 flux remains of similar magnitude during spring and summer – an assumption supported by an independent study [ 23 ] . These results highlight the value of ecosystem services provided by coral reefs and should strengthen the justification for their protection from a range of threats including global warming. Our research also highlights the need to extend direct measurement of air – sea CO 2 exchanges over coral reefs to other locations and climate regimes such as the Indian, Pacific and Atlantic Oceans. Such measurements should be made over one full year to capture all meteorological, hydrodynamic and biophysical processes that can reasonably be expected to affect CO 2 flux over coral reefs. This should include the possible influence of aerosol deposition on the productivity of coral reefs such as Ningaloo Reef – the world’s largest fringing coral reef located under the northwest dust transport pathway from Australia. At locations where fertilization of coral reefs occurs by the deposition of aerosols in low-nutrient, low-chlorophyll marine environments, then the ability of coral reefs to sequester atmospheric CO 2 may be most evident. 4. Materials and Methods 4.1 Study Site This study was conducted along the northwest shore of the GoE (Fig. 4 a), an almost rectangular region of the Gulf roughly 6 × 10 km with steep lateral boundaries with a maximum depth of ∼800 m [ 52 ] . Along this section of coast more than 40 coral genera are regularly identified, with the most common genera comprising of Stylophora, Acropora, Montipora, Echinopora, Cyphastrea, Goniastrea, Porites, Dipsastrea and Stylophora pistillata. Coral coverage averages approximately 25%, with rock making up around 20%, dead coral 5%, and the remaining being loose substrate of sand and shell fragments [ 53 ] . The alongshore current is predominantly from north to south in response to prevailing northerly winds blowing from the Arava Valley, with a semidiurnal and diurnal barotropic tide range of ~ 1 m [ 54 ] . Fish farms that operated for almost 20 years approximately 7 km northeast of our study site were removed in June 2008, and within six months sediment organic matter decreased [ 55 ] . Subsequent storm induced sediment overturning events enabled the removal of all residual nutrients from the northern GoE. 4.2 Direct Measurement of Air-Sea Croal Reef Ecosystem CO 2 exchange CO 2 flux (F) is defined as: where ρ is the mole density of dry air (mol m − 3 ), c is the CO 2 concentration (dry mixing ratio, µmol mol − 1 ) measured by a fast-response infra-red gas analyser (IRGA) (open or closed path), and the vertical wind velocity w (m s − 1 ) is measured by a sonic anemometer. The prime denotes the fluctuations from the mean, while the overbar indicates time average [ 56 ] . Measurement of air-sea CO 2 exchanges over coral reefs in the northern GoE was made using an EC system attached to the pier of Eilat's Coral World Underwater Observatory (UOE) (29°30′15.43″N and 34°55′6.68″E) at 2.5 m above mean sea level (Fig. 4 b). The EC system consisted of a 3D sonic anemometer (RM Young 81000), an open-path IRGA (Li-Cor 7500), a net radiometer (Kipp and Zonen CNR1), and ancillary sensors [ 22 , 30 ] . The Li-Cor 7500 open-path gas analyzer has been used in many geographic locations to provide direct measurement of CO 2 in coastal and marine environments [ 21 , 22 , 57 , 58 , 59 ] . To minimize the potential influence of salt, dust, and biogenic films on CO 2 measurements, the Li-Cor 7500 gas analyzer was regularly washed to ensure the optical sensors were clean to minimize hygroscopic contamination leading to erroneously high CO 2 measurements. The hyper-arid climate of the GoE field site also minimizes potential cross-sensitivity of CO 2 measurements to water vapor. Half-hourly CO 2 fluxes were calculated using EddyPro® Software v7.0 and included correction for anemometer tilt, air density variability [ 60 ] , frequency response, and sensor separation [ 56 , 61 ] . Wind speed measurements were standardized to a height of 10 m above the surface. A wind direction filter was applied to exclude measurements made when wind was directed onto the EC measurement footprint from the deep sea (> 40 m) and land [ 62 ] . The 90% isopleth of the EC measurement footprint overlaid on the fringing coral reefs in water depths from 0 to 40 m is shown in Figure (4c). Low quality data (flags ≥ 3) were removed from analysis as determined by steady state and developed turbulence tests [ 63 ] . Finally, manual spike removal was conducted so that CO 2 flux was restricted to ± 20 µmol m − 2 s − 1 [ 22 ] . Gaps in the CO 2 flux record were filled using an Artificial Neural Network (ANN) algorithm [ 64 ] . The algorithm was implemented with python sklearn.neural_network MLPRegressor package when the hyperparameters (learning rate, hidden layer, and maximum iteration number) were chosen by a grid search cross-validation algorithm. Following the gap-filling procedure, the resulting percentage recovery of 30-minute mean CO 2 data was > 99%. 4.3 Particle concentration in the atmosphere PM10 (aerosol < 10 µm in diameter) concentrations were obtained from the Israel Ministry of Environmental Protection monitoring program at Eilat, which uses Thermo Scientific FH62-C14 continuous particulate monitors. These measure the mass concentration of suspended particulate matter using beta attenuation, with average half hourly PM10 values reported here. Declarations Competing interests The authors declare no competing interests. Author contributions HM initiated the research and wrote the manuscript with contributions from NL and SA. SA analysed EC data and prepared Fig. 1. NL and SA managed instrumentation, while HM and NL obtained funding to support the research. Acknowledgements The authors thank Yoni Shaked, Asaph Rivlin, Modi Pilersdorf, The Interuniversity Institute for Marine Sciences in Eilat for access to infrastructure and services. Dr. Assaf Zvuloni and Chen Toufikian of Israel's Nature and Parks Authority for their assistance. The research was supported by the Israel Science Foundation (Grant ISF-2018/1471), PI–NGL, and by funds for PIs HM, and NGL from the Hebrew University of Jerusalem–Zelman Cowen Academic Initiatives (ZCAI) Joint Projects 2021 (2022–2024). Data Availability Statement The data used in the research is available from The University of Queensland at https://espace.library.uq.edu.au/view/UQ:b11504f References Baldocchi, D. Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: past, present and future. Global Change Biol. , 9 , 479–492 (2003). Baldocchi, D. 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Gross primary production and net ecosystem exchange of a cool-temperate deciduous forest estimated by the eddy covariance method. Agric. For. Meteorol. , 112 (3-4), 203-215 (2002). Lowry, A. L., McGowan, H.A. & Gray, M. A. Multi-year carbon and water exchanges over contrasting ecosystems on a sub-tropical sand island. Agric. For. Meteorol. , https://doi.org/10.1016/j.agrformet.2021.108404 (2021). Dong, Y., Yang, M., Bakker, D.C., Kitidis, V. & Bell, T.G. Uncertainties in eddy covariance air–sea CO 2 flux measurements and implications for gas transfer velocity parameterisations. Atmos. Chem. Phys. , 21 (10), 8089-8110 (2021). Gnanamoorthy, P., et al. Seasonal variations of net ecosystem (CO 2 ) exchange in the Indian tropical mangrove forest of Pichavaram. Estuarine Coastal Shelf Sci. , 243 , p.106828. (2020). Gattuso, J.P., Frankignoulle, M. & Smith, S.V. Measurement of community metabolism and significance in the coral reef CO 2 source-sink debate. Proc. Natl. Acad. Sci. U.S.A. , 96 (23), 13017-13022 (1999). Yan, H., et al. Coral reef ecosystems in the South China Sea as a source of atmospheric CO 2 in summer. Chin. Sci. Bull. , 56, 676-684 (2011). Yan, H., et al. Seasonal variations of seawater pCO 2 and sea-air CO 2 fluxes in a fringing coral reef, northern South China Sea, J. Geophys. Res.-Oceans , 121 ,998–1008, doi:10.1002/2015JC0114 (2016). Massaro, R.F., et al. Multiple factors driving variability of CO 2 exchange between the ocean and atmosphere in a tropical coral reef environment. Aquat. Geochem. , 18 , 357-386 (2012). Lønborg, C., Calleja, M.L., Fabricius, K.E., Smith, J.N. & Achterberg, E.P. The Great Barrier Reef: A source of CO 2 to the atmosphere, Mar. Chem. , 210 , 24-33 (2019). Yang, B., et al. Multiple factors driving carbonate system in subtropical coral community environments along Dapeng Peninsula, South China Sea. Atmosphere , 14 (4), p.688 (2023). Afdal, Bengen, D.G., Wahyudi, A.J., Rastina, Prayitno, H.B. & Koropitan, A.F. Variation of CO 2 fluxes, net ecosystem production, and calcification in tropical waters of seagrass and coral reef. Reg. Stud. Mar. Sci. , 68 , p.103290 (2023). Zhang, C., et al. Eco-engineering approaches for ocean negative carbon emission. Sci. Bull. , 67 (24), 2564-2573. https://doi.org/10.1016/j.scib.2022.11.016 (2022). Carlson, D.F., et al. Observations of tidal currents in the northern Gulf of Eilat/Aqaba (Red Sea). J. Mar. Syst. , 102–104 , 14-28 (2012). Shaked, Y. & Genin, A. Gulf of Eilat National Monitoring Report 2019, Israel Ministry of Environmental Protection, 187p (2020). Shaked, Y. & Genin, A. Gulf of Eilat National Monitoring Report 2017, Israel Ministry of Environmental Protection, 209p (2018). Oron, S., Angel, D., Goodman-Tchernov, B., Merkado, G., Kiflawi, M. & Abramovich, S. Benthic foraminiferal response to the removal of aquaculture fish cages in the Gulf of Aqaba-Eilat, Red Sea. Marine Micropaleontology , 107 , 8-17 (2014). Burba, G. Eddy Covariance Method for Scientific, Industrial, Agricultural, and Regulatory Applications: A Field Book on Measuring Ecosystem Gas Exchange and Areal Emission Rates. LI-COR Biosciences, Lincoln, NE, USA, 331pp (2013). Andersson, A., Rutgersson, A. & Sahlée, E. Using eddy covariance to estimate air–sea gas transfer velocity for oxygen. J. Mar. Syst. , 159 , 67-75 (2016). Chien, H., Zhong, Y-Z., Yang, K-H. & Cheng, H-Y. Diurnal variability of CO 2 flux at coastal zone of Taiwan based on eddy covariance observation. Cont. Shelf Res. , 162 , 27-38 (2018). Rutgersson, A., et al. Using land-based stations for air–sea interaction studies. Tellus A: Dyn. Meteorol. Oceanogr. , 72 (1), pp.1-23 (2020). Webb, E. K., Pearman, G. I. & Leuning, R. Correction of flux measurements for density effects due to heat and water vapour transfer. Q. J. R. Meteorolog. Soc. , 106 , 85–100 (1980). Massman, W. Concerning the measurement of atmospheric trace gas fluxes with open-and closed-path eddy covariance system: the WPL terms and spectral attenuation. In Handbook of micrometeorology: a guide for surface flux measurement and analysis (pp. 133-160). Dordrecht: Springer Netherlands (2004). Kljun, N., Calanca, P., Rotach, M.W. & Schmid, H.P. A simple parameterisation for flux footprint predictions. Boundary Layer Meteorol. , 112 , 503-523 (2004). Mauder, M. & Foken, T. Documentation and instruction manual of the eddy covariance software package TK2. Arbeitsergebn, Univ Bayreuth, Abt Mikrometeorol, ISSN 1614-8916. 26:42pp (2004). Mahabbati, A., et al. A comparison of gap-filling algorithms for eddy covariance fluxes and their drivers. Geosci. Instrum. Methods Data Syst. , 10 (1), 123-140 (2021). Additional Declarations There is NO Competing Interest. 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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-7005193","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":482821977,"identity":"0a2001cf-2f74-43e9-ae43-9bf91ae21047","order_by":0,"name":"Hamish McGowan","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1UlEQVRIiWNgGAWjYBACAziLvQFEMpOihecAyVokEojUYs7AnfjxS8VheXPJN4Y3GCqsExvYzxjg1WLZwLtZWubMYcOds3OMLRjOpCc28OTg12JwgHcbs2TbbcYNt3PMJBjbDic2MBCpxX7DzTNALf+AWvjfENbC+LHtduKGGzxALQ1ALRKEbDkM9AvDmf/JG86kFVskHEs3bpN4VoBfy/HejR9/VKTZbjh+eOONDzXWsv38yRvwagFFBDMPlA2OGjb86iGA8QdMCzGqR8EoGAWjYOQBACd9Rwj25pXSAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-2844-2084","institution":"University of Queensland","correspondingAuthor":true,"prefix":"","firstName":"Hamish","middleName":"","lastName":"McGowan","suffix":""},{"id":482821978,"identity":"8457c573-378e-4e34-96a6-01d2c3296933","order_by":1,"name":"Shai Abir","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Shai","middleName":"","lastName":"Abir","suffix":""},{"id":482821979,"identity":"6a22246c-0bd8-4b05-8635-1e34a14acdc0","order_by":2,"name":"Nadav Lensky","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Nadav","middleName":"","lastName":"Lensky","suffix":""},{"id":482821980,"identity":"386385d5-bc0e-474f-8274-8412a2c6f6ea","order_by":3,"name":"Yonathan Shaked","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yonathan","middleName":"","lastName":"Shaked","suffix":""}],"badges":[],"createdAt":"2025-06-30 01:00:28","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7005193/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7005193/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86829261,"identity":"e7e1aa7d-126c-41f6-94a7-c694c72c1210","added_by":"auto","created_at":"2025-07-16 05:40:05","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":495629,"visible":true,"origin":"","legend":"\u003cp\u003eHeat map of measured wind speed (Ws) (a), incoming shortwave solar irradiance (SW) (b), water temperature (Tw) (c), air temperature (Ta) (d),\u0026nbsp; particulate matter 10 microns in diameter or less (PM10) (e), carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) (f), relative humidity (RH) (g), and difference between air and water temperature (ΔT) (h), for the period 3 September 2020 to 2 March 2021 measured at the UOE field site.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7005193/v1/19d363ab11472e5fff14985e.jpeg"},{"id":86828789,"identity":"4fb1cade-3fbf-4887-9cff-cd9f994fa06e","added_by":"auto","created_at":"2025-07-16 05:32:05","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":130659,"visible":true,"origin":"","legend":"\u003cp\u003eTime series (24 hr moving average) of EC CO\u003csub\u003e2\u003c/sub\u003e flux measurements made over the coral reef at the UOE field site (a), and PM10 concentrations measured at Eilat approximately 6.25 km to the northeast of the EC site, and Chl-a concentrations measured in water samples collected between 7 to 9 am daily approximately 200 m south of the UOE EC site.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7005193/v1/64cc2453e6a76c3985434d03.png"},{"id":86828793,"identity":"0acf73a8-af1c-49cf-ab86-acdbb9e88af9","added_by":"auto","created_at":"2025-07-16 05:32:05","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":44007,"visible":true,"origin":"","legend":"\u003cp\u003eDiurnal CO2 flux for late summer (September 2020) and winter (January 2021) (a), and the full 181-day measurement period (b).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7005193/v1/8aac58195d20c62c4a45b38d.png"},{"id":86829260,"identity":"3143cf5b-808b-4870-874b-ef25b11e2754","added_by":"auto","created_at":"2025-07-16 05:40:05","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1134654,"visible":true,"origin":"","legend":"\u003cp\u003eLocation map of the EC field site in the northern GoE (a) with the EC instrumentation (b) and EC measurement footprint with the 10%, 70% and 90% isopleths shown over the coral reef (c). The EC system is represented by the orange triangle (c).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7005193/v1/7c7fd137fd7c2dc3e1a4b5dd.png"},{"id":93407485,"identity":"e9e5798c-1e25-49a8-a67d-aa0144524a13","added_by":"auto","created_at":"2025-10-13 13:55:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2819054,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7005193/v1/26131c07-0cf2-4284-b194-ccfbf42d99a5.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eDesert Coral Reefs: A possible sink of atmospheric CO\u003csub\u003e2\u003c/sub\u003e in a rapidly warming World\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eUnderstanding net ecosystem \u0026ndash; atmosphere CO\u003csub\u003e2\u003c/sub\u003e exchange (Net Ecosystem Exchange (NEE)) is crucial to inform policy responses to anthropogenic global warming. Quantification of NEE has therefore become a major focus of initiatives to directly measure ecosystem CO\u003csub\u003e2\u003c/sub\u003e exchanges. The eddy covariance (EC) method provides the only \u003cem\u003edirect\u003c/em\u003e representative measure that captures the multi-scaled and complex processes controlling ecosystem \u0026ndash; atmosphere trace gas fluxes including CO\u003csub\u003e2\u003c/sub\u003e. These span a spectrum of time and space scales of 15 orders of magnitude\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. EC is therefore the cornerstone of AmeriFlux\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e, EUROFLUX\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e, OzFlux\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e and Asiaflux\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e which seek to quantify NEE by direct measurement and contribute to the global FLUXNET program\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e.Substantial advances have therefore been made in understanding terrestrial CO\u003csub\u003e2\u003c/sub\u003e fluxes. Yet, direct measurement of CO\u003csub\u003e2\u003c/sub\u003e flux over lacustrine and marine ecosystems remains dominated by time and space limited observations at disparate locations, none more so than over coral reefs \u0026ndash; the rainforests of the oceans\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eCoral reefs represent 0.1 to 0.25% of the global marine environment with at least 1\u0026nbsp;billion people benefiting from their ecosystem services\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. They are at risk from warming sea surface temperatures (SSTs), ocean acidification, overfishing, pollution, mining, and tourism. While the ecological importance of coral reefs is unquestionable, their role in carbon budgets is uncertain. Previous research suggested coral reefs may absorb\u0026thinsp;\u0026asymp;\u0026thinsp;2% of annual anthropogenic CO\u003csub\u003e2\u003c/sub\u003e (-720 Mt), while others have stated coral reefs are sources of atmospheric CO\u003csub\u003e2\u003c/sub\u003e (+\u0026thinsp;144 Mt to +\u0026thinsp;504 Mt)\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. These estimates of CO\u003csub\u003e2\u003c/sub\u003e exchange were based on marine productivity models or bulk formula that use calculations of the CO\u003csub\u003e2\u003c/sub\u003e partial pressure (\u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e) difference across the air-sea interfacial boundary multiplied by an estimated gas transfer velocity\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. Other studies have used point measurements of water pH, temperature, and chemistry to estimate reef water \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e which is then compared against atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration to determine the CO\u003csub\u003e2\u003c/sub\u003e flux gradient\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eDirect measurements of air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange using EC with measurement footprints of \u0026gt;\u0026thinsp;20,000 m\u003csup\u003e2\u003c/sup\u003e have found coral reefs to be net sinks of CO\u003csub\u003e2\u003c/sub\u003e, at times an order of magnitude larger than reported by bulk formula and productivity models\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. The large measurement footprint of EC ensures truly representative measurements of NEE. However, these studies were conducted for only a few days to several weeks and may not have captured the full range of hydrodynamic and meteorological conditions expected on coral reefs. Nonetheless, they highlighted the spatial heterogeneity of air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange such as at Heron Reef on the humid subtropical southern Great Barrier Reef, Australia. Here the reef flat was identified as a net source of CO\u003csub\u003e2\u003c/sub\u003e to the atmosphere, while concurrently adjacent lagoons were found to be net CO\u003csub\u003e2\u003c/sub\u003e sinks\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eUsing data on water chemistry, tropical fringing coral reefs at Mo'orea, Tahiti were found to be CO\u003csub\u003e2\u003c/sub\u003e sinks (up to 10 mmol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), whereas a neighbouring barrier reef flat was a CO\u003csub\u003e2\u003c/sub\u003e source. This was attributed to the possible effect of the relative coverage of algae with coral dominated reefs hypothesized to be sources of CO\u003csub\u003e2\u003c/sub\u003e, while those with substantial algae coverage were CO\u003csub\u003e2\u003c/sub\u003e sinks\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e. More recently, analysis of water chemistry over fringing coral reefs on the South China coast during summer found coral reefs in this region to be CO\u003csub\u003e2\u003c/sub\u003e sinks with mean values of 12.3 to 19.2 mmol CO\u003csub\u003e2\u003c/sub\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003ed\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. These air-sea CO\u003csub\u003e2\u003c/sub\u003e exchanges were believed to be dependent on hydrodynamics and algae cover\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. This highlights the importance of community structure of coral reef ecosystems in air \u0026ndash; sea CO\u003csub\u003e2\u003c/sub\u003e exchange. Factors that may influence this observation either directly or indirectly include currents, coastal development, river discharges, deposition of aerosols, nutrients from aquiculture, changes in atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentrations, and synoptic meteorology.\u003c/p\u003e\u003cp\u003eCorals affect air-sea CO\u003csub\u003e2\u003c/sub\u003e transfers through changing the \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e in the water overlying the reef via photosynthesis/respiration and calcification/dissolution across the day\u0026ndash;night cycle\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. During the day, photosynthetic activity consumes CO\u003csub\u003e2\u003c/sub\u003e, while at night, respiration consumes O\u003csub\u003e2\u003c/sub\u003e and produces CO\u003csub\u003e2\u003c/sub\u003e. Calcification and dissolution raise the \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e in the water with the precipitation of 1 mol of CaCO\u003csub\u003e3\u003c/sub\u003e producing 1 mol of CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e with around 60% escaping to the atmosphere\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Variability in the \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e in the water overlying a coral reef will therefore reflect biogeochemical, hydrodynamic, and meteorology processes. For example, dust deposited on the fringing coral reefs at Eilat, Israel was found to supply nanomolar amounts of essential bio-elements to the coral symbionts including iron, manganese, zinc and copper from natural, industrial, and agricultural processes\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. This fertilization by dust enhanced chlorophyll concentrations and photosynthesis by coral symbionts, lowering the \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e in the water overlying the reef and leading to a net influx of atmospheric CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eHere we present results from the longest direct measurement record of air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange over a coral reef made using the EC method. Research was conducted in the Gulf of Eilat (GoE), Israel. In this hyper-arid environment evaporative cooling of water provides a thermal refugia for corals, which have evolutionary traits believed to further protect them from extreme water temperatures\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. Results are compared to previously published EC CO\u003csub\u003e2\u003c/sub\u003e fluxes for coral reefs in humid environments, and CO\u003csub\u003e2\u003c/sub\u003e exchanges over other marine and terrestrial ecosystems.\u003c/p\u003e"},{"header":"2. Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 \u003cem\u003eAir-sea meteorology and CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e \u003cem\u003eexchange.\u003c/em\u003e\u003c/h2\u003e\u003cp\u003eDirect EC measurements of air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange over the fringing coral reef at UOE was undertaken from the 3 September 2020 to 2 March 2021 (181 days) with concurrent measurements of associated meteorology (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe seasonal transition from hot summer conditions with afternoon air temperatures of around 40\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed) and water temperatures greater than 30\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) to cooler winter conditions was marked by an abrupt transition in early November 2020. This is most evident in ΔT (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eh) as a step change from conditions where the air was warmer than the water to where the water was predominantly warmer than the air. There was no concurrent effect on CO\u003csub\u003e2\u003c/sub\u003e flux (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef) with this rapid \u0026ldquo;flip\u0026rdquo; in ΔT.\u003c/p\u003e\u003cp\u003eDaytime wind speed maxima were at their minimum during winter reflecting a weaker local sea breeze (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The northerly wind that regularly blows from the Arava Valley onto the northern GoE as a result of the valley channelling synoptic winds and the inland penetration of the Mediterranean Sea breeze was also weaker. CO\u003csub\u003e2\u003c/sub\u003e flux was dominated by negative values (blue shading) indicating influx (sequestration of CO\u003csub\u003e2\u003c/sub\u003e from the atmosphere) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef). This influx doesn\u0026rsquo;t show obvious correlation with other variables including aerosol (PM10) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee) which is primarily minerogenic aerosol entrained from the surrounding deserts. It has previously been linked to fertilization of the coral reefs in the GoE and possible CO\u003csub\u003e2\u003c/sub\u003e sequestration\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. PM10 concentrations averaged 49.8 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e over the observation period with a maximum 30-minute concentration of 1200 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e. The absence of clear evidence of correlation between CO\u003csub\u003e2\u003c/sub\u003e flux and PM10 (r\u0026thinsp;=\u0026thinsp;0.06) may reflect that such relationships are event specific and linked to dust chemistry, time of day, and prevailing coastal hydrodynamics. For example, at 0930 hrs 17 November 2020 CO\u003csub\u003e2\u003c/sub\u003e flux reached \u0026minus;\u0026thinsp;18.90 \u0026micro;mols m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (influx) and then rapidly increased to +\u0026thinsp;4.9 \u0026micro;mols m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (efflux) at 2100hrs 18 November 2020 following a dust event in the morning of the 18 November 2020, when PM10 concentrations peaked at 314 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e at 1100 hrs. Net CO\u003csub\u003e2\u003c/sub\u003e fluxes then remained positive (efflux) over the following days. In comparison, one month later PM10 concentrations peaked at 295 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e at 0600 hrs on 15 December 2020 after which net CO\u003csub\u003e2\u003c/sub\u003e fluxes became negative at 1900 hrs on 15 December 2020 and then increasing negative (influx) until early January 2021 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIncoming solar irradiance was not correlated with CO\u003csub\u003e2\u003c/sub\u003e flux (r -0.07). The introduction of a 3-hour lag between CO\u003csub\u003e2\u003c/sub\u003e flux and positive solar irradiance to allow for coral symbionts to respond to solar irradiance resulted in a change of sign with r\u0026thinsp;=\u0026thinsp;0.08. CO\u003csub\u003e2\u003c/sub\u003e flux was negatively correlated with windspeed (r = -0.49) and relative humidity (r = -0.32). Accordingly, as windspeed increased, CO\u003csub\u003e2\u003c/sub\u003e influx increased. CO\u003csub\u003e2\u003c/sub\u003e flux displayed a positive correlation with air temperature (r\u0026thinsp;=\u0026thinsp;0.40) indicating as temperature decreased CO\u003csub\u003e2\u003c/sub\u003e influx increased. Over the entire 181-day monitoring period the mean 30-minute CO\u003csub\u003e2\u003c/sub\u003e flux measured by EC over the fringing coral reefs at Eilat was \u0026minus;\u0026thinsp;2.07 \u0026micro;mols m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and displayed a weak trend of increasing influx from late summer into early winter (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This trend was not significantly correlated with the increase in daily Chlorophyll-a concentrations from late October to mid-December 2020 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) as confirmed by an R\u003csup\u003e2\u003c/sup\u003e value of 0.1. This suggests that algae in the water over the coral reef did not influence air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 \u003cem\u003eDiurnal CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e \u003cem\u003efluxes over coral reefs\u003c/em\u003e\u003c/h2\u003e\u003cp\u003eDiurnal CO\u003csub\u003e2\u003c/sub\u003e fluxes measured over the coral reefs at Eilat displayed a clear cycle with a maximum influx in the early to mid-morning and maximum efflux early evening after sunset. Respiration and calcification from early afternoon onward including possible dark (nocturnal) calcification following sunset with associated cooling of the water surface increases the reef water \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e relative to the atmosphere. This results in a decrease in CO\u003csub\u003e2\u003c/sub\u003e influx which in late summer (September) switched to an efflux by early afternoon which continued into the evening (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Following sunset (i.e. approx. 18:00 hrs) the \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e in the water overlying the coral reef gradually began to decrease resulting in an increasing influx of CO\u003csub\u003e2\u003c/sub\u003e from atmosphere to the coral reef (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This influx of CO\u003csub\u003e2\u003c/sub\u003e increases throughout the early morning from midnight peaking around 11AM for our entire 181-day data set (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) is more pronounced in winter (January) as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. The effect of photosynthesis on CO\u003csub\u003e2\u003c/sub\u003e influx is particularly evident by the rapid increase in CO\u003csub\u003e2\u003c/sub\u003e influx in January (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) around 6:30AM. During this cooler time of year, the coral reef at Eilat behaves as a significant sink for atmospheric CO\u003csub\u003e2\u003c/sub\u003e throughout the diel cycle.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 \u003cem\u003eCO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e \u003cem\u003efluxes over coral reefs and other ecosystems\u003c/em\u003e\u003c/h2\u003e\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e flux measured by EC over the fringing coral reefs at Eilat are extrapolated to a mean annualised value for comparison with CO\u003csub\u003e2\u003c/sub\u003e flux over other ecosystems including commercial pine plantations and tropical rainforest measured also by EC. These ecosystems are regularly highlighted as important global carbon sinks\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Our results show that CO\u003csub\u003e2\u003c/sub\u003e influx over the coral reefs of Eilat range from 3 times to an order of magnitude greater than CO\u003csub\u003e2\u003c/sub\u003e influx measured over sites in mangrove forest, pine plantations, cool temperate forests, and Brazilian rainforest (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). They are significantly greater than measured over the ocean in both tropical and high latitude locations (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) although caution is warranted given the short observation periods of these measurements and their extrapolation to annual values.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eExamples of CO\u003csub\u003e2\u003c/sub\u003e exchange (NEE) over different ecosystems compared to measurements made over coral reefs in the GoE including this study. All CO\u003csub\u003e2\u003c/sub\u003e flux measurements were made using EC systems and therefore represent direct ecosystem \u0026ndash; atmosphere CO\u003csub\u003e2\u003c/sub\u003e exchange. Published CO\u003csub\u003e2\u003c/sub\u003e fluxes have been converted to annual values for ease of comparison.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLocation (duration of observation)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eEcosystem\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003et C ha yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\u003cp\u003e(negative influx)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEilat, Israel (3 Sept 2020\u0026ndash;2 March 2021: 181 days) [This study]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHealthy fringing coral reefs only.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-28.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEilat, Israel (1\u003c/p\u003e\u003cp\u003eApril to 27 August of 2009: 142 days)\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHealthy fringing coral reefs and adjacent ocean.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-18.04\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDurham, North Carolina, USA (1\u0026nbsp;year)\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eForest (\u003cem\u003ePinus taeda L.\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-6.05\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMaine, USA. (7 yrs)\u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u0026sim;90-year-old spruce dominated forest\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-1.74\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eChangbai Mountain Natural Reserve, northeastern China (1\u0026nbsp;year)\u003csup\u003e[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMontane forest (Dominant species Pinus koraiensis Sieb. et Zucc. (34%), Tilia amurensis Rupr. (42%)).\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-1.88\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBrazilian Amazon Basin - mean of 3 sites. (3\u0026ndash;5 yrs)\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBrazilian rainforest\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-10\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCentral Japan (1\u0026nbsp;year)\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCool-temperate deciduous forest\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-2.14\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBribie Island, Subtropical eastern Australia (2.5 yrs)\u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCoastal wetland (Dominant species - \u003cem\u003eMelaleuca quinquenervia\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-5.61\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBribie Island, Subtropical eastern Australia (2.5 yrs)\u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePine plantation (\u003cem\u003ePinus elliottii var. elliottii x Pinus caribaea var. hondurensis\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-6.31\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eArctic Ocean (30 Jun\u0026ndash;1 Aug 2019)\u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eArctic Ocean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-1.62\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eArctic Ocean (5 Aug\u0026ndash;29 Sep 2019)\u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eArctic Ocean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-2.62\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTropical Atlantic (9\u0026ndash;16 Oct 2018)\u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTropical Atlantic Ocean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-0.40\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePichavaram, southeast India (1\u0026nbsp;year)\u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMangrove forest\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-1.83\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Discussion","content":"\u003cp\u003eEcosystem productivity models, microscale chamber gas flux exchange measurements, and water \u0026ndash; atmosphere chemistry studies have typically concluded that coral reefs are net sources of CO\u003csub\u003e2\u003c/sub\u003e to the atmosphere\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e. Direct EC measurement of coral reef \u0026ndash; atmosphere CO\u003csub\u003e2\u003c/sub\u003e flux at ecosystem scale presented here show that the fringing coral reefs of the GoE are net sinks of CO\u003csub\u003e2\u003c/sub\u003e at -2.07 \u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (-28.7 t C ha yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Net Ecosystem Exchange over coral reefs at Eilat show they sequester 3 to 10 times more atmospheric CO\u003csub\u003e2\u003c/sub\u003e than Brazilian rainforest\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e, pine plantations\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e and mangrove forest\u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e. Our results are aligned with previous independent EC measurements of CO\u003csub\u003e2\u003c/sub\u003e flux in the GoE. Over 142 days from April to August 2009 these measured a mean CO\u003csub\u003e2\u003c/sub\u003e influx of -1.3 \u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (-18.04 t C ha yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e at a site approximately 280 m south from our UOE site with a measurement footprint that extended beyond the coral reef and over the adjacent ocean. These ocean waters with depths from 40 to 550 m sequester around 50% less CO\u003csub\u003e2\u003c/sub\u003e on average than the fringing coral reefs\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. Notably, these measurements were made through late spring and summer, namely outside the months of our measurements, but interestingly they showed the coral reef of Eilat to also be a net CO\u003csub\u003e2\u003c/sub\u003e sink in these seasons\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. CO\u003csub\u003e2\u003c/sub\u003e fluxes were found to be positively correlated with air temperature but negatively correlated with wind speed. Namely, as air temperature (and water skin temperature) decrease CO\u003csub\u003e2\u003c/sub\u003e solubility increases, while increase in windspeed may further increase influx through evaporative cooling of the water surface and mixing.\u003c/p\u003e\u003cp\u003eDisparity between published coral reef CO\u003csub\u003e2\u003c/sub\u003e flux studies showing coral reefs as sources of CO\u003csub\u003e2\u003c/sub\u003e and results presented here showing coral reefs in the GoE are very significant net sinks of atmospheric CO\u003csub\u003e2\u003c/sub\u003e likely arise from: i) the very small measurement footprints (cm\u003csup\u003e3\u003c/sup\u003e to a few m\u003csup\u003e2\u003c/sup\u003e) of non-EC research, ii) short duration measurements (minutes to a few days), iii) incorrect assumptions about gas transfer velocities and, iv) air \u0026ndash; sea \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e gradients calculated from point sampling of water chemistry and assumed atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentrations from measurements made 10s to 100s of kilometers from the water sampling sites\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/sup\u003e. These studies that have often presented microscale site and time specific observations do not capture the heterogenous properties of complex coral reef ecosystems and their environment. Site factors such as ecosystem composition, hydrodynamics, wave environment, terrestrial runoff and aquifer seepage all influence air \u0026ndash; sea CO\u003csub\u003e2\u003c/sub\u003e flux and are captured by EC measurements because of their large measurement footprint. At Heron Reef on the southern Great Barrier Reef, Australia concurrent EC measurements found the shallow lagoon to be a net CO\u003csub\u003e2\u003c/sub\u003e sink (-2.27 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), while the adjacent reef flat was a net source of CO\u003csub\u003e2\u003c/sub\u003e (+\u0026thinsp;3.40 \u0026micro;g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) to the atmosphere\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. Here, dissolution of calcium carbonate on the reef flat along with seepage of CO\u003csub\u003e2\u003c/sub\u003e enriched water at low tide from the coral cay (Heron Island) was believed to contribute to supersaturation of reef flat waters with CO\u003csub\u003e2\u003c/sub\u003e resulting in net evasion of CO\u003csub\u003e2\u003c/sub\u003e to the atmosphere. Concurrently over the shallow lagoon, photosynthesis by the benthic microalgae and corals was thought responsible for a net CO\u003csub\u003e2\u003c/sub\u003e influx\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. Such variability highlights the need for ecosystem scale and multi-seasonal to annual continuous direct measurements of CO\u003csub\u003e2\u003c/sub\u003e flux over coral reefs.\u003c/p\u003e\u003cp\u003eWhile the majority of non-EC coral reef CO\u003csub\u003e2\u003c/sub\u003e flux studies have reported coral reefs as net sources of CO\u003csub\u003e2\u003c/sub\u003e, scleractinian coral communities at two sites on Dapeng Peninsula in the South China Sea have also been found to be very weak net CO\u003csub\u003e2\u003c/sub\u003e sinks (0.99\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08 to 1.66\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40 mmol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003csup\u003e[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]\u003c/sup\u003e. Coral reefs at Lombok, Indonesia have also been found to be net CO\u003csub\u003e2\u003c/sub\u003e sinks (0.14 to 1.1 mmol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) during October, but they switched to net sources in April\u003csup\u003e[\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]\u003c/sup\u003e. Recently interest has emerged in potentially establishing new or restoring coral reefs to sequester CO\u003csub\u003e2\u003c/sub\u003e. It has been estimated that floating coral nurseys with 10\u003csup\u003e6\u003c/sup\u003e coral colonies per 1 km\u003csup\u003e2\u003c/sup\u003e could potentially sequester the equivalent of 110 t of CO\u003csub\u003e2\u003c/sub\u003e per year\u003csup\u003e[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]\u003c/sup\u003e. Thus, such coral reef restoration initiatives would not only have direct ecological benefits but would sequester atmospheric CO\u003csub\u003e2\u003c/sub\u003e. A clearer understanding of the role of coral reefs in the carbon cycle as sources or sinks in different geographic and climate regimes, and their net impact on global atmospheric carbon is therefore required.\u003c/p\u003e\u003cp\u003eIt is clear that there remain very substantial differences in the determination of whether coral reefs are net sources or sinks of atmospheric CO\u003csub\u003e2\u003c/sub\u003e, and the factors that may cause coral reefs to switch from source to sink and vice versa. For example, during our measurement period we found no clear and persistent association between dust fertilization of corals in the GoE and CO\u003csub\u003e2\u003c/sub\u003e flux as previously postulated and supported by in aquaria studies with GoE corals and dust\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. This may reflect variability in dust source chemistry, prevailing meteorology and hydrodynamics over the GoE coral reefs and the associated impact of coral photosynthesis. EC provides the only direct measurement method that can accurately quantify air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange at ecosystem scale that accounts for changes in water chemistry including that caused by the deposition of aerosols, coral reef hydrodynamics, meteorology, and coral reef ecology.\u003c/p\u003e\u003cp\u003eThe debate as to whether coral reefs act as net sources or sink of atmospheric CO\u003csub\u003e2\u003c/sub\u003e has prevailed for more three decades. However, there is growing evidence through research as we present here that when continuous direct measurement is undertaken at ecosystem scale, coral reefs such as those in the GoE bordered by a hyper-arid desert are net sinks of CO\u003csub\u003e2\u003c/sub\u003e, potentially far exceeding CO\u003csub\u003e2\u003c/sub\u003e sequestration by rainforests and commercial forestry plantations. Mean air \u0026ndash; sea CO\u003csub\u003e2\u003c/sub\u003e flux (NEE) measured by EC from late summer to the end of winter (181 days) over the fringing coral reefs in the GoE was \u0026minus;\u0026thinsp;2.07 \u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which equates to -28.7 t C ha yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e assuming CO\u003csub\u003e2\u003c/sub\u003e flux remains of similar magnitude during spring and summer \u0026ndash; an assumption supported by an independent study\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. These results highlight the value of ecosystem services provided by coral reefs and should strengthen the justification for their protection from a range of threats including global warming.\u003c/p\u003e\u003cp\u003eOur research also highlights the need to extend direct measurement of air \u0026ndash; sea CO\u003csub\u003e2\u003c/sub\u003e exchanges over coral reefs to other locations and climate regimes such as the Indian, Pacific and Atlantic Oceans. Such measurements should be made over one full year to capture all meteorological, hydrodynamic and biophysical processes that can reasonably be expected to affect CO\u003csub\u003e2\u003c/sub\u003e flux over coral reefs. This should include the possible influence of aerosol deposition on the productivity of coral reefs such as Ningaloo Reef \u0026ndash; the world\u0026rsquo;s largest fringing coral reef located under the northwest dust transport pathway from Australia. At locations where fertilization of coral reefs occurs by the deposition of aerosols in low-nutrient, low-chlorophyll marine environments, then the ability of coral reefs to sequester atmospheric CO\u003csub\u003e2\u003c/sub\u003e may be most evident.\u003c/p\u003e"},{"header":"4. Materials and Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e4.1 \u003cem\u003eStudy Site\u003c/em\u003e\u003c/h2\u003e\n \u003cp\u003eThis study was conducted along the northwest shore of the GoE (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea), an almost rectangular region of the Gulf roughly 6 \u0026times; 10 km with steep lateral boundaries with a maximum depth of \u0026sim;800 m\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e]\u003c/sup\u003e. Along this section of coast more than 40 coral genera are regularly identified, with the most common genera comprising of Stylophora, Acropora, Montipora, Echinopora, Cyphastrea, Goniastrea, Porites, Dipsastrea and Stylophora pistillata. Coral coverage averages approximately 25%, with rock making up around 20%, dead coral 5%, and the remaining being loose substrate of sand and shell fragments\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e53\u003c/span\u003e]\u003c/sup\u003e. The alongshore current is predominantly from north to south in response to prevailing northerly winds blowing from the Arava Valley, with a semidiurnal and diurnal barotropic tide range of ~\u0026thinsp;1 m\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e]\u003c/sup\u003e. Fish farms that operated for almost 20 years approximately 7 km northeast of our study site were removed in June 2008, and within six months sediment organic matter decreased\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e55\u003c/span\u003e]\u003c/sup\u003e. Subsequent storm induced sediment overturning events enabled the removal of all residual nutrients from the northern GoE.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e4.2 \u003cem\u003eDirect Measurement of Air-Sea Croal Reef Ecosystem CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e \u003cem\u003eexchange\u003c/em\u003e\u003c/h2\u003e\n \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e flux (F) is defined as:\u003c/p\u003e\n \u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"162\" height=\"46\"\u003e\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003ewhere \u003cem\u003e\u0026rho;\u003c/em\u003e is the mole density of dry air (mol m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), \u003cem\u003ec\u003c/em\u003e is the CO\u003csub\u003e2\u003c/sub\u003e concentration (dry mixing ratio, \u0026micro;mol mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) measured by a fast-response infra-red gas analyser (IRGA) (open or closed path), and the vertical wind velocity \u003cem\u003ew\u003c/em\u003e (m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is measured by a sonic anemometer. The prime denotes the fluctuations from the mean, while the overbar indicates time average\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\n \u003cp\u003eMeasurement of air-sea CO\u003csub\u003e2\u003c/sub\u003e exchanges over coral reefs in the northern GoE was made using an EC system attached to the pier of Eilat\u0026apos;s Coral World Underwater Observatory (UOE) (29\u0026deg;30\u0026prime;15.43\u0026Prime;N and 34\u0026deg;55\u0026prime;6.68\u0026Prime;E) at 2.5 m above mean sea level (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb). The EC system consisted of a 3D sonic anemometer (RM Young 81000), an open-path IRGA (Li-Cor 7500), a net radiometer (Kipp and Zonen CNR1), and ancillary sensors\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. The Li-Cor 7500 open-path gas analyzer has been used in many geographic locations to provide direct measurement of CO\u003csub\u003e2\u003c/sub\u003e in coastal and marine environments\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e58\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e59\u003c/span\u003e]\u003c/sup\u003e. To minimize the potential influence of salt, dust, and biogenic films on CO\u003csub\u003e2\u003c/sub\u003e measurements, the Li-Cor 7500 gas analyzer was regularly washed to ensure the optical sensors were clean to minimize hygroscopic contamination leading to erroneously high CO\u003csub\u003e2\u003c/sub\u003e measurements. The hyper-arid climate of the GoE field site also minimizes potential cross-sensitivity of CO\u003csub\u003e2\u003c/sub\u003e measurements to water vapor.\u003c/p\u003e\n \u003cp\u003eHalf-hourly CO\u003csub\u003e2\u003c/sub\u003e fluxes were calculated using EddyPro\u0026reg; Software v7.0 and included correction for anemometer tilt, air density variability\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e60\u003c/span\u003e]\u003c/sup\u003e, frequency response, and sensor separation\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e61\u003c/span\u003e]\u003c/sup\u003e. Wind speed measurements were standardized to a height of 10 m above the surface. A wind direction filter was applied to exclude measurements made when wind was directed onto the EC measurement footprint from the deep sea (\u0026gt;\u0026thinsp;40 m) and land\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e62\u003c/span\u003e]\u003c/sup\u003e. The 90% isopleth of the EC measurement footprint overlaid on the fringing coral reefs in water depths from 0 to 40 m is shown in Figure (4c). Low quality data (flags\u0026thinsp;\u0026ge;\u0026thinsp;3) were removed from analysis as determined by steady state and developed turbulence tests\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e63\u003c/span\u003e]\u003c/sup\u003e. Finally, manual spike removal was conducted so that CO\u003csub\u003e2\u003c/sub\u003e flux was restricted to \u0026plusmn;\u0026thinsp;20 \u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1 [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. Gaps in the CO\u003csub\u003e2\u003c/sub\u003e flux record were filled using an Artificial Neural Network (ANN) algorithm\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e64\u003c/span\u003e]\u003c/sup\u003e. The algorithm was implemented with python sklearn.neural_network MLPRegressor package when the hyperparameters (learning rate, hidden layer, and maximum iteration number) were chosen by a grid search cross-validation algorithm. Following the gap-filling procedure, the resulting percentage recovery of 30-minute mean CO\u003csub\u003e2\u003c/sub\u003e data was \u0026gt;\u0026thinsp;99%.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e4.3 \u003cem\u003eParticle concentration in the atmosphere\u003c/em\u003e\u003c/h2\u003e\n \u003cp\u003ePM10 (aerosol\u0026thinsp;\u0026lt;\u0026thinsp;10 \u0026micro;m in diameter) concentrations were obtained from the Israel Ministry of Environmental Protection monitoring program at Eilat, which uses Thermo Scientific FH62-C14 continuous particulate monitors. These measure the mass concentration of suspended particulate matter using beta attenuation, with average half hourly PM10 values reported here.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting interests\u003c/h2\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003ch2\u003eAuthor contributions\u003c/h2\u003e\n\u003cp\u003eHM initiated the research and wrote the manuscript with contributions from NL and SA. SA analysed EC data and prepared Fig.\u0026nbsp;1. NL and SA managed instrumentation, while HM and NL obtained funding to support the research.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThe authors thank Yoni Shaked, Asaph Rivlin, Modi Pilersdorf, The Interuniversity Institute for Marine Sciences in Eilat for access to infrastructure and services. Dr. Assaf Zvuloni and Chen Toufikian of Israel\u0026apos;s Nature and Parks Authority for their assistance. The research was supported by the Israel Science Foundation (Grant ISF-2018/1471), PI\u0026ndash;NGL, and by funds for PIs HM, and NGL from the Hebrew University of Jerusalem\u0026ndash;Zelman Cowen Academic Initiatives (ZCAI) Joint Projects 2021 (2022\u0026ndash;2024).\u003c/p\u003e\n\u003ch2\u003eData Availability Statement\u003c/h2\u003e\n\u003cp\u003eThe data used in the research is available from The University of Queensland at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://espace.library.uq.edu.au/view/UQ:b11504f\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBaldocchi, D. Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: past, present and future. \u003cem\u003eGlobal Change Biol.\u003c/em\u003e, \u003cstrong\u003e9\u003c/strong\u003e, 479\u0026ndash;492 (2003).\u003c/li\u003e\n\u003cli\u003eBaldocchi, D. 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Methods Data Syst.\u003c/em\u003e, \u003cstrong\u003e10\u003c/strong\u003e(1), 123-140 (2021).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Coral Reefs, CO2, Ecosystems, Air-sea exchange, Direct measurement","lastPublishedDoi":"10.21203/rs.3.rs-7005193/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7005193/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEddy covariance (EC) measurements of air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange (Net Ecosystem Exchange) over desert fringing coral reefs in the Gulf of Eilat (GoE), Israel show these ecosystems may be net sinks of atmospheric CO\u003csub\u003e2\u003c/sub\u003e. This result contrasts with marine productivity models and bulk formula calculations based on for example, water chemistry used to then determine the magnitude and direction of the CO\u003csub\u003e2\u003c/sub\u003e flux. These studies have for more than three decades often concluded that coral reefs are net sources of CO\u003csub\u003e2\u003c/sub\u003e to the atmosphere with only rare cases finding otherwise. Our EC measurements find coral reefs in the GoE sequester 3 to 10 times more CO\u003csub\u003e2\u003c/sub\u003e than other marine and terrestrial ecosystems including tropical rainforests. This highlights the need for further direct measurements of air-sea CO\u003csub\u003e2\u003c/sub\u003e exchanges over coral reefs in different environmental settings and climate regimes so that the role of coral reefs in the global carbon cycle can be accurately quantified.\u003c/p\u003e","manuscriptTitle":"Desert Coral Reefs: A possible sink of atmospheric CO2 in a rapidly warming World","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-16 05:32:00","doi":"10.21203/rs.3.rs-7005193/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0dabc189-afaa-4bf6-897e-5d8fd9c2db32","owner":[],"postedDate":"July 16th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":51602055,"name":"Earth and environmental sciences/Climate sciences"},{"id":51602056,"name":"Earth and environmental sciences/Ocean sciences"}],"tags":[],"updatedAt":"2025-10-13T13:47:26+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-16 05:32:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7005193","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7005193","identity":"rs-7005193","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}