Coupling between landward and seaward fringes of sandy beaches: algal deposits on the upper beach influence biogeochemistry and faunal assemblages in the swash zone.

Coupling between landward and seaward fringes of sandy beaches: algal deposits on the upper beach influence biogeochemistry and faunal assemblages in the swash zone. | 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 Research Article Coupling between landward and seaward fringes of sandy beaches: algal deposits on the upper beach influence biogeochemistry and faunal assemblages in the swash zone. Mariano Lastra Valdor, L De Pablo, J López, F Soliño, TA Schlacher This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4744352/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 03 Feb, 2025 Read the published version in Marine Biology → Version 1 posted 5 You are reading this latest preprint version Abstract Energy subsidies from the sea typically underpin ocean-exposed sandy beach ecosystems. Strandings of detached macro-algae – ‘wrack’ – can be a spectacular form of such cross-ecosystem transfers of organic matter that sustain consumers in the recipient shore system; this has given rise to a model of wrack promoting the diversity and abundance of invertebrates, with scaling effect on upper trophic levels. However, most wrack is often wave-cast to the upper beach, whereas a distinct part of the shore fauna is limited to the ocean fringe of beaches – the ‘swash zone’. This creates a spatial asymmetry between the location of subsidies (landwards fringe) and the location of the putative recipients (ocean fringe). Here, we tested whether the fauna of the swash zone can benefit from wrack subsidies, sampling fauna and algal deposits on a range of beaches in NW Spain. We also measured the potential functional link between algal wrack and nutrients released from wrack during decay. Wrack decay increased nutrient concentrations, and it is the combination of wrack cover, nutrient levels, and sediment coarseness that jointly drove variation in the assemblage structure of the swash fauna among beaches. Similarly, the density of the swash fauna and species richness increased markedly at higher nutrient levels and wrack cover. These findings expand the ‘wrack enhancement’ model to include the promotion of consumers at the ocean edge of sandy shores; it also contains a cross-shore linkage via decomposition processes that favourable change the nutrient regime across all the beach face and thereby couple the swash zone with the upper strandline. wrack decay nutrients biodiversity sediment swash zone Figures Figure 1 1. Introduction Ocean-exposed sandy beaches are conventionally viewed as spatially subsidised ecosystems, their food webs being chiefly underpinned by imports of organic matter from the sea (Schlacher et al. 2008 ; Hyndes et al. 2022 ). Organic matter in the form of detached macro-algae and seagrass – ‘wrack’ - is often the pivotal part of these subsidies (Dugan et al. 2003 ). Algal wrack primarily originates from nearby rocky shores and reefs (Griffiths et al. 1983 ; Colombini and Chelazzi 2003 ; Dugan et al. 2003 ; Orr et al. 2005 ). Once stranded, this material is the substrate of intense biogeochemical processing that determines rates of nutrient cycling and substrate metabolism (Coupland et al. 2007 ; Dugan et al. 2011 ; Barreiro et al. 2013 ; Lastra et al. 2015 ). Wrack imports can be a key driver of ecological traits in beaches, such as enhancing biodiversity and the biomass of consumers (e.g., Ince et al. 2007 ; Olabarría et al. 2007 ; Crawley et al. 2009 ; Spiller et al. 2010 ; Wilson and Wolkovich 2011 ). The trophic effects of organic matter imports in the coupled surf-beach-dune systems extend to the subtidal parts (Crawley et al. 2006 ), the intertidal shores (Soares et al. 1997 ; Dugan et al. 2003 ) and the supratidal zone and seawards parts of dunes (Dugan et al. 2003 ; Lastra et al. 2008 ; Duarte et al. 2014 ; Ruiz-Delgado et al. 2017; Schlacher et al. 2013a ; 2013b ). Consumption by herbivores is the initial step within the biological processing of algal wrack supplies. For example, in temperate latitudes, large populations of supratidal crustaceans and insects use the wrack deposits as food and shelter, and this can occur within the first hours once the material reaches the beach. These populations frequently comprise up to 90% of the beach macrofauna (Stenton-Dozey and Griffiths 1983 ; Dugan et al. 2003 ; Duarte et al. 2014 ; Ruiz-Delgado et al. 2015 ). Algal wrack consumption by macroinvertebrates involves fragmentation and excretion, which facilitates decomposition and mineralisation of the organic materials by lower biotic compartments, such as meiofauna and bacteria (Koop et al. 1982 ; Mews et al. 2006 ; Coupland and McDonald 2008 ; Anschutz et al. 2009 ; Salathe and Riera 2012). Once nutrients are incorporated in the interstitial environment, the enriched beach runoff can enhance the productivity of coastal waters through discharge with nitrate, nitrite, ammonia and phosphorous (Maier and Pregnall 1990; Brooks et al. 2008 ; Dugan et al. 2011 ; Barreiro et al. 2011 ). The seaward fringe of the tidal beach is the swash zone, which is the part of the beach alternately covered and exposed by uprush and backwash, thus playing a pivotal role in connecting beach processes with surrounding coastal ecosystems. Swash zones frequently contain more species of benthic invertebrates, also often at greater abundance, compared to other parts of the intertidal beach (Degraer et al 2003 ; Dugan et al. 2003 ). Wrack becomes typically stranded at the turn of the ebbing tide, resulting in most of the wave-cast algal deposits being located on the upper beach near the strandline (Orr et al. 2005 ). By contrast, the swash zone is always situated at the seaward edge of the beach. Thus, there exists on many beaches a spatial asymmetry between a putative wrack subsidy being concentrated typically at the landward edge of the beach and the fauna in the swash zone at the ocean end. Whether this spatial separation between wrack and swash animals weakens the documented relationship between wrack subsidies and the fauna of the upper shore or the full intertidal is unknown. It is plausible that ‘wrack effects can propagate down-shore via nutrient regeneration and organic matter enrichment during wrack decay. Again, whether changes to the chemical milieu of beach habitats originating from wrack can couple the upper beach with swash consumers is unknown. Thus, our principal aims here are two-fold: a) to examine how wrack strandings change nutrient availability in beach habitats, and b) to test for associations/linkages between the structure, diversity and density of swash fauna with variations in wrack supply wrack-derived nutrients, and geomorphological beach features. 2. Methods 2.1 Study site We tested the association between algal wrack and swash fauna by sampling eight beach ecosystems between October and December 2020, located on the NW coast of Spain, spanning roughly 200 km of the coastline of Galicia. Beaches range from reflective to dissipative morphodynamic states (Lastra et al. 2006 ) and vary in size from small pocket beaches to more extensive shores up to 7 km long, frequently fringed by rocky headlands or cliffs. Three of the chosen beaches (Samil, America and Panxon) were included in a suburban environment, being affected by irregular and occasional mechanical grooming during the tourist season (June to August). Land-use seawards of three sites was urban to para-urban, replacing the foredunes with a seawall (Samil, America, Panxon); all other sites were backed by vegetated, quasi-natural foredunes. The climate is transitional between the Atlantic and Mediterranean zones, with seasonal variation in seawater temperature ranging between 10 o C during winter to 20 ºC during summer (Mounier 1979 ). 2.2 Field sampling Faunal sampling was designed to evaluate the role of wrack supply in structuring faunal assemblages of the swash zone. To this end, a shore parallel stretch, 100 m long, was selected at the centre of each beach to avoid the boundary effect of rocky headlands. Along those 100 m stretch of beach, 27 sediment samples were collected randomly ( www.random.org ; Randomness and Integrity Services Ltd., Dublin), with at least 1 m of separation from one another. A 12 cm diameter PVC corer, pushed 15 cm deep into the sediment, was used to collect the samples during spring low tides at the edge of the lower swash zone, which was located by positioning the maximum seaward retreat among all the bores occurring within a 10 minutes lapse. Corer samples were pooled every consecutive 3 (as done by Dugan et al. 2003 ; Schooler et al. 2024 ), which translates to 0.034 m 2 of surface unit per core and 0.3 m 2 of total surface sampled at each beach. Sediment cores were sieved in mesh bags with a 1 mm aperture size to extract the macrobenthos. Individuals were identified to the lowest taxonomic level when possible. Wrack coverage was measured along six shore-perpendicular transects randomly spaced alongshore within the same 100 m central section of beach where the swash fauna was collected. Transects extended from the base of the foredune to the upper swash level. We used the line-intercept method (Dugan et al. 2003 ) to quantify the across-shore distance of the intertidal beach face covered by wrack. With this, algal wrack deposits were mapped across the beach face, and the position of the accumulated 90% of the wrack coverage was calculated for each transect; results were expressed in % of beach width where 90% of algal coverage was reached, starting at the dune base (0%) and ending in the upper swash zone (100%). The sediment samples for the nutrient concentration, granulometry and moisture were collected at the upper edge of the swash zone at the same six alongshore locations where the wrack survey transects were positioned. At each position, one corer 5 cm diameter was pushed 8 cm deep into the sediment following Canion et al. ( 2014 ). Sediment cores were also collected to quantify total protein content as a proxy of the food quality of the organic matter within the sedimentary environment. Likewise, DNA in the top-8 cm of sediment was obtained to estimate the bacterial and fungal communities contributing to the decomposition and mineralisation of organic matter (Agnelli et al. 2004 ). To analyze the nutrient concentration in intertidal pore water, samples were taken next to the positions where sediment samples were collected; to do this, holes were dug to an adequate depth to allow pore water to fill the bottom ( ca . 10 cm); then, interstitial water samples of 150 mL were taken and stored in polypropylene bottles. Water samples at the landward edge of the surf zone were also collected at the 6 positions. Once in the lab, all the water samples were filtered to remove any particulate material (quantitative filter of 1–3 µm) and immediately stored at -20 o C until analysis. 2.3 Laboratory analyses The sediment samples (10 g) for nutrient analyses were shaken for 2 hours in 25 mL 0.01 mol/L KCl solution for inorganic N extraction; the solution was then filtered (quantitative filter paper of 1–3 µm) and stored at -20 o C until processing (Barreiro et al. 2013 ). Nutrients were quantified by continuous flow analysis in a Bran Luebbe Nutrient Analyzer AA3. Ammonium was measured fluorometrically at 460 nm following excitation at 370 nm according to the method of Kerouel and Aminot ( 1997 ); the samples were reacted with ophthalaldehyde (OPA) at 75 o C in the presence of borate buffer and sodium sulphite, to form a fluorescent species in a quantity that is proportional to the ammonium concentration. NO 2 − and NO 3 − were analysed via the diazo-reaction based on the methods of Armstrong et al. ( 1967 ) and Grasshoff et al. ( 1983 ). This automated procedure involves reduction of NO 2 − to NO 3 − by a copper-cadmium reductor column; the NO 2 − then reacts with sulphanilamide and N-1-napthylethyleneidiamine dihydrochloride under acidic conditions; the concentrations were determined colorimetrically at 550 nm. Nitrification products hereafter will be referred as the sum of NO 2 − +NO 3 − (as in Brooks et al. 2008 ). Total inorganic N (hereafter TIN) was calculated as the sum of NH 4 + , NO 2 − and NO 3 − . Phosphate (PO 4 3− ) analyses was based on the colorimetric method of Murphy and Riley ( 1962 ), in which a blue colour is formed by the reaction of orthophosphate, molybdate ions, and antimony ions followed by reduction with ascorbic acid at a pH of 1; the reduced blue phospho-molybdenum complex is determined colorimetrically at 880 nm. Data units for the inorganic nutrients in the sediment were in µM per g of dry sediment. Contents in total proteins were measured as a proxy of the edible organic matter received by the sedimentary environment; more conventional methods as dry-combustion in a CHN-LECO were not used to avoid interference from carbonates. Total proteins were determined using methods from Lowry and Rosebrough ( 1951 ), modified by Markwell et al. ( 1978 ). Concentrations were calculated as bovine serum albumin equivalents. Three g of sediment were used for the analyses of each sample. For each test, blanks were made using the same sediments previously treated in a muffle furnace (500 o C 6h). The “EZNA DNA Soil extraction kit” (Omega Biotek Ref. D5625-01), followed by Qubic Fluorometric quantitation, were used to analyse total DNA in 200 mg sediment samples, representative of the top-8 cm of sediment; results were a proxy of the bacterial and fungal communities contributing to organic matter decomposition (Agnelli et al. 2004 ). DNA is expressed as ng·g − 1 of sediment. 2.4 Numerical analyses The chief question of interest was to test whether variation in the swash fauna can be explained by the corresponding variations in environmental variables, particularly wrack cover and changes in nutrient and organic matter conditions in the shore habitats. To this end, the principal method was distance-based linear models (DISTLM) as implemented in Primer (Anderson et al 2008 ). Fundamentally, the modelling technique analyses the relationship between the fauna data and the set of environmental variables measured in terms of variation in the fauna data explained by individual variables (‘marginal tests’) and by models containing multiple predictors. The model selection criterion for the latter was the corrected Akaike Information Criteria (AICs). The main fauna resemblance matrix modelled was ‘community structure’ (species composition and the density of all component species) quantified by Bray-Curtis resemblance coefficients calculated on square-root transformed density data; an Euclidean matrix of total density and species richness complemented the models of community structure. 3. Results 3.1 Geo-Bio-Chemical Attributes of Beaches Wrack deposits were concentrated along the different drift lines associated with the hightide water marks, with more than 90% of the wrack coverage concentrated in the upper 25% of the beach face in all the beaches, except Panxon (at the upper 33%). The cover of algal wrack varied 17-fold across the region, ranging between a minimum of 0.18 m (± 0.1) at Barra to a maximum of 3.12 m (± 0.83) at America (Fig. 1 A, Table SU1). Mean wrack cover does not differ between beaches that are occasionally and irregularly groomed (mean wrack cover = 1.66 m, ± 0.71) and those from which wrack is not removed (mean wrack cover = 1.12 m, ± 0.51). PERMANOVA, Pseudo-F = 2.421, P = 0.146). The spatial variation in wrack cover was mirrored by a 10-fold variation in total inorganic nitrogen concentrations in the sediments, with the lowest and largest mean values recorded at the same sites (min Barra = 0.77 ± 0.15 µg g − 1 ); max America = 10.59 ± 1.56 µg g − 1 ). Across all sites, more wrack covering a beach was associated with significantly higher concentrations of N and P in the sediment and in the interstitial water (Fig. 1 B, Table SU1). By contrast, correlations between wrack and N & P in the surf-zone water were weaker, as were the correlations between wrack and sedimentary proteins and DNA (Fig. SU2). PO 4 concentrations in the porewater showed a massive (270 times) variation between 3.43 µg L − 1 (± 1.37) at Barra and 927.82 µg L − 1 (± 435.2) at Panxon (Table SU1). By contrast, the range of observed PO 4 values in the sediment was considerably smaller, ranging two-fold between Barra (1.09 ± 0.09 µg g − 1 ) and Corrubedo (2.40 ± 0.40 µg g − 1 ). Similar ranges in spatial variation among beaches were observed for total inorganic N in the sediments (min Barra = 215.24 ± 38.6 µg g − 1 ; max Carnota = 804.77 ± 51.4 µg g − Table SU1). Concentrations of DIN in the surf-zone water were lower than that in the intertidal porewater, ranging between 161 µg L − 1 (± 117.6) at Lanzada and 486 µg L − 1 (± 218.0) at Carnota. Proteins and DNA in the sediments showed a similar range (ca. 3 times) of spatial variation, with the lowest mean protein content recorded at Panxon (30.70, ± 3.75) and highest at Barra (84.4 ± 24.2). Total DNA in the sediment varied between 17 ng g − 1 (± 2.9) at Lanzada and 59 ng g − 1 (± 12.25) at Corrubedo (Table SU1 ) . The width of the studied beaches ranged between 65 m (Barra) and 120 m (Carnota and Corrubedo), with slopes between 2.21° (Corrubedo) and 5.08° (Barra). The finest sediments were recorded at Carnota (mean grains size 336 ± 41.6 µm), and the coarsest sands at Nerga (1276 ± 159.25 µm; Table SU1). Across the full suite of geomorphological and chemical variables measured, every beach differs significantly from all other beaches (PERMANOVA, Pseudo-F = 16.99, p < 0.001; Table SU1; Fig. SU3); i.e. each beach has a unique set of environmental conditions. America Beach was characterised by a high cover of algal wrack, and elevated concentrations of DIN in the sediments, PO 4 in the surf zone and sediment proteins (Table SU1; Fig. SU3). Wrack cover was also high on Carnota Beach, complemented by a wider beach face, a gentle gradient and high DIN in the porewater (Table SU1; Fig. SU3). Corrubedo Beach sloped very gradually over a wide beach face and had high values of DNA and PO 4 in the sands. Elevated PO 4 values in both the porewater and surf zone were typical for Panxon Beach, complemented by higher concentrations of surf-zone DIN. The distinguishing feature of Nerga Beach was coarse sand, which resulted in low values for Dean’s parameter. Sediments were also coarse at Samil Beach, but they contained comparatively high values of DNA and PO 4 . Barra beach was narrow and steep, its sediments rich in protein but poor in interstitial DIN. A high Dean’s Parameter value characterised Lanzada Beach but lower concentrations of nutrients in both the porewater and the surf zone (Table SU1; Fig. SU3). 3.2 Faunal assemblages & Environmental Drivers A total of 26 operational taxonomic units (OTUs) were recorded across all sites, comprising six phyla, six classes, 11 orders, 19 families, 20 genera and 21 species. Species richness per beach ranged between 5 at Barra and 14 at America, with a grand mean of 9.38 species (± 3.15) per beach across the region (Table SU2). Densities (total number of individuals) varied 37-fold among beaches, with the lowest mean density of 285 ind.m − 2 (± 285) being recorded at Barra and the highest at 10561 ind.m − 2 (± 1836) at America (Table SU2). Filter feeders and deposit feeders represent between 70 and 99% of the total abundance at each beach (Table SU2). Donax trunculus was the main contributor to the filter feeders’ group, while peracarids (amphipods, isopods and cumaceans) were the main representatives for the depositivorous guild (Table SU2). Variation in the assemblage structure of the swash fauna was jointly explained by a combination of wrack cover, wrack-derived nitrogen in the sediments and porewater, and sediment grain size (Fig. 1 C; Table 1 ). Each of these variables explained a similar proportion of variance, and all of them were included in the best model (Fig. 1 C; Table 1 ). By contrast, amongst the other chemical factors examined, neither nutrients in the surf zone nor proteins and DNA in the sediments were correlated with spatial patterns in assemblage structure (Table 1 ). All the geomorphic attributes (e.g. beach width, slope), wave properties (height, period), and the composite index of beach morphodynamic state (Dean’s parameter) were very weak predictors of faunal community structure (Table 1 ). Table 1 Summary of distance-based linear modelling (DISTLM) relating variation in community structure (i.e. species composition and density of species) of the swash fauna to a suite of environmental predictors. (Pseudo-F, P and Prop. refer to marginal tests; ‘top models’ are solutions with a delta AICc of 2). Variable Pseudo-F P Prop. # in ‘Top Models’ Sediment, Grain Size (mean, microns) 3.09 0.00 0.34 5 Sediment, Total Inorganic Nitrogen (µg g − 1 ) 2.39 0.05 0.29 2 Porewater, Dissolved Inorganic N (µg L − 1 ) 2.26 0.06 0.27 2 Wrack Cover (m) 2.69 0.03 0.31 1 Surf-zone, Dissolved Inorganic N (µg L − 1 ) 1.79 0.14 0.23 2 Wave Period (s) 1.59 0.19 0.21 2 Pore-water, PO 4 (µg L − 1 ) 2.03 0.09 0.25 1 Beach Width (m) 1.89 0.12 0.24 1 Beach-Slope (deg) 1.43 0.24 0.19 1 Sediment, PO 4 (µg g − 1 ) 1.33 0.29 0.18 1 Dean’s Parameter (Ω) 1.20 0.34 0.17 1 Surf-zone, PO 4 (µg L − 1 ) 1.04 0.39 0.15 0 Wave Height (m) 0.80 0.57 0.12 0 Sediment, DNA (ng g − 1 ) 0.38 0.89 0.06 0 Sediment, Proteins (µg g − 1 ) 0.35 0.89 0.06 0 Variation in total fauna density among beaches was best explained by corresponding variation in wrack cover and nitrogen in the sediment (Fig. 1 D; Table 1 ). All other tested environmental predictors were either not significant, not included in top models, or explained a very small fraction of variation (Table 1 ). Similar to density, species richness was best predicted by wrack cover and wrack-derived P & N (Table 1 ). By contrast, the traditionally favoured geomorphological, hydrodynamic, and morphodynamic variables (i.e. grain size, wave properties, Dean’s Parameter) were very poor predictors of species numbers in the swash infauna (Table 1 ). 4. Discussion The results obtained showed the relevance of algal wrack supplies on the ecology and function of exposed sandy beaches. This link has been demonstrated in previous studies where faunal assemblages were associated with macroalgal patches stranded along the intertidal beach (see Hyndes et al. 2022 and references within). Most of the species responding to wrack are talitrid amphipods, isopods, coleopterans and dipterans, among others. Our results expand the hypotheses with regard to the effect of algal wrack subsidies on beach ecology: besides consequences on tidal levels where algal strandings primarily occurs, the lower intertidal and the swash zone, defined as the lower tidal limit and shallow submerged zones, also respond to the presence or absence of algal deposits that decompose in the upper beach around the drift line. Wrack affects the sediment metabolism and inorganic nutrient release (Barreiro et al. 2013 ; Dugan et al. 2003 ), which enhances N and P drainage to coastal water, fertilising primary producers, including phytoplankton and phytobenthos (Page and Lastra 2003) and macroalgae. The precise mechanism behind the increase of invertebrate consumers belonging to a range of trophic guilds and taxonomic groups in the swash zone with increasing wrack subsidies is under discussion. Isotopic analyses to analyse food sources and to untangle trophic web at community scale are scarce (e.g. Colombini et al. 2011 ). Although the study of Colombini et al. ( 2011 ) does not include the swash zone, these authors remarked that macroalgal 13C and 15N signatures present in the infauna occur at several trophic levels and vary with tidal position across the beach-dune ecosystem gradient. At the population scale, Olabarria et al. ( 2009 ) highlighted the intra-species variability in the food sources used by Talitrus saltator in the Atlantic coast of Spain, using 13C and 15N isotopic signature as tracers. Using C and N counterbalance, Soares et al. ( 1997 ) demonstrate that the standing stock of the filter feeder clam Donax serra is supported mainly by particulate organic matter formed by erosion and decay of stranded macroalgae, with alive sources located in the kelp beds growing massively along the West coast of South Africa. In our study, although filter feeders are a significant compartment of the trophic web, deposit feeders and several polychaetes and peracarid species function as secondary consumers. We hypothesise that these non-filter feeder groups also feed on wrack-derived particulate organic matter that accumulates on the sediment surface or percolates to the interstitial space. Previous studies in the area (Lastra et al. 2006 ) have demonstrated that ocean productivity measured as Chl-a is coupled with the number of infaunal species and abundance in the entire active sands zone (i.e., the zone where waves and tides transport sand grains), including the upper swash zone. In this regard, the high primary productivity in the NW coast of the Iberian Peninsula is related to the increase in nutrients supplied by a seasonal upwelling event (Tenore et al. 1982 ; Blanton et al. 1987 ; Bode et al. 1996 ; Alvarez-Salgado et al. 2001 ), which means that biodiversity, abundance and biomass of the exposed sandy beaches of this area are above those obtained in neighbouring regions (Lastra et al. 2006 ), irrespective of the morphodynamical state. The physical and macrofaunal data collected make it possible to test the general hypothesis that beach morphodynamics, including slope, beach width, wave height, wave period and sediment features, affect faunal communities. Low correlations between the number of species and abundances with physical and morphodynamic variables (Bally, 1987 ; Dexter 1992 ; Jaramillo and McLachlan 1993 ; McLachlan et al. 1996 ; 1998 ; Hacking 1998 ; Nel 2001 ) are usually explained by latitudinal effects, biotic interactions, biogeography or human impact (Defeo 1996 ; Defeo et al. 1997 ; Brazeiro and Defeo 1999 ; Dugan 1999 ; Nel 2001 ; Schoeman and Richardson 2002 ). The beach locations, sampling dates, and characteristics of the studied beaches allow us to exclude latitudinal and seasonal effects from the interpretation of the results. Hence, the weak correlation between beach morphodynamics, swash climate and other factors with macrofaunal assemblages in the swash zone could only be explained by a bottom-up effect of nutrients provided, directly or indirectly, by the macrophyte subsidies stranded in the upper beach. Declarations Acknowledgement This research was supported by the Consellería de Cultura, Educación e Ordenación Universitaria, Xunta de Galicia, Project ED431C 2021/42. Funding This study received financial support from XUNTA de Galicia, España (Projects Ref.: ED431C 2017/46-GRC and ED431C 2018/54 -GRC). Competing interest The authors whose names are listed in the Authors list certify that they have NO affiliations with or involvement in any organization or entity with any financial interest, or non-financial interest, in the subject matter or materials discussed in this manuscript. Authors statement I confirm that the manuscript has been read and approved by all named authors. I further confirm that the order of authors listed in the manuscript has been approved by all of us. Author Contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Mariano Lastra, Jesús López , Luis De Pablo and Thomas Schlacher. The first draft of the manuscript was written by Mariano Lastra. Luis De Pablo and Thomas Schlacher commented and improve on previous versions of the manuscript. All authors read and approved the final manuscript. Data availability The datasets generated during and/or analyzed for the current study are available from the corresponding author upon request. References Anderson MJ, Gorley RN, Clarke KR (2008) PERMANOVA+ for PRIMER: Guide to software and statistical methods, PRIMER-E, Plymouth, 214 pp Agnelli A, Ascher J, Corti G, Ceccherini MT, Nannipieri P, Pietramellara G (2004) Distribution of microbial communities in a forest soil profile investigated by microbial biomass, soil respiration and DGGE of total and extracellular DNA. 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Role of substratum preferences and competition on abundance and distribution patterns of cirolanid isopods in Uruguayan beaches. Estuar Coast Shelf Sci 45: 453-462. Degraer S, Volckaert A, Vincx M (2003) Macrobenthic zonation patterns along a morphodynamical continuum of macro-tidal, low tide bar/rip and ultra-dissipative beaches. Estuar Coast Shelf Sci 56: 459-468 Dexter DM (1992) Sandy beach community structure: the role of exposure and latitude. J Biogeogr 19: 59-66. doi.org/10.2307/2845620 Duarte C, Acuña K, Navarro JM, Gómez I, Jaramillo E, Quijón P (2014) Variable feeding behavior in Orchestoidea tuberculata (Nicolet 1849) exploring the relative importance of macroalgal traits. J Sea Res 87: 1–7. doi.org/10.1016/j.seares.2013.12.003 Dugan JE (1999) Utilisation of sandy beaches by shorebirds: relationships to population characteristics of macrofauna prey species and beach morphodynamics. Draft Final Study Report to Minerals Management Service and the University of California Coastal Marine Institute, Santa Barbara, California. Dugan JE, Hubbard DM, McCrary MD, Pierson MO (2003) The response of macrofauna communities and shorebirds to macrophyte wrack subsides on exposed sandy beaches of southern California. Estuar Coast Shelf Sci 58: 25-40. doi.org/10.1016/S0272-7714(03)00045-3 Dugan JE, Hubbard DM, Page HM, Schimel JP (2011) Marine macrophyte wrack inputs and dissolved nutrients in beach sands. Estuaries Coast 34: 839–850. doi.org/10.1007/s12237-011-9375-9 Grasshoff K, Ehrhardt M, Kremling K (Eds) (1983) Methods of Seawater Analysis, second revised and extended edition, John Wiley & Sons, Weinheim. 600 pp. doi:10.1002/9783527613984 Griffiths CL, Stenton-Dozey JME, Koop K (1983) Kelp wrack and energy flow through a sandy beach In: Sandy beaches as ecosystems (eds. McLachlan A, Erasmus T), pp. 547-556. W Junk, The Hague. Hacking N (1998) Macrofaunal community structure of beaches in northern New South Wales, Australia. Mar Freshwater Res 49: 47-53. doi:10.1071/MF96130 Hyndes GA, Berdan EL, Duarte C, Dugan JE, Emery KA, Hambäck PA, Henderson CJ, Hubbard DM, Lastra M, Mateo MA, Olds A, Schlacher TA (2022) The role of inputs of marine wrack and carrion in sandy-beach ecosystems: a global review. Biol Rev 97: 2127–2161. doi: 10.1111/brv.12886 Ince R, Hyndes GA, Lavery PS, Vanderklift MA (2007) Marine macrophytes directly enhance abundances of sandy beach fauna through provision of food and habitat. Estuar Coast Shelf Sci 74: 77–86. doi.org/10.1016/j.ecss.2007.03.029 Jaramillo E, McLachlan A (1993) Community and population responses of the macroinfauna to physical factors over a range of exposed sandy beaches in south-central Chile. Estuar Coast Shelf Sci 37: 615-624. doi.org/10.1006/ecss.1993.1077 Kerouel R, Aminot A (1997) Fluorometric determination of ammonia in sea and estuarine waters by direct segmented flow analysis. Mar Chem 57: 265-275. doi.org/10.1016/S0304-4203(97)00040-6 Koop K, Newell R, Lucas M (1982) Biodegradation and carbon flow based on kelp ( Ecklonia maxima ) debris in a sandy beach microcosm. Mar Ecol Prog Ser 7: 315-326. https://doi.org/10.3354/meps007315 Lastra M, De La Huz R, Sánchez-Mata A, Rodil I, Aerts K, Beloso S (2006) Ecology of exposed sandy beaches in Northern Spain: Environmental factors controlling macrofauna communities. J Sea Res 55: 128-140. doi.org/10.1016/j.seares.2005.09.001 Lastra M, Page HM, Dugan JE, Hubbard DM, Rodil IF (2008) Processing of allochthonous macrophyte subsidies by sandy beach consumers: estimates of feeding rates and impacts on food resources. Mar Biol 154: 163–174. doi.org/10.1007/s00227-008-0913-3 Lastra M, López J, Neves G (2015) Algal decay, temperature and body size influencing trophic behaviour of wrack consumers in sandy beaches. Mar Biol 162:221–33. doi:10.1007/s00227-014-2562-z Lowry OH, Rosebrough NJ (1951) Protein measurement with the folin-phenol reagent. J Biol Chem 193: 265-275. DOI:10.1016/S0021-9258(19)52451-6 LMaier C, Pregnall AM (1990) Increased macrophyte nitrate reductase activity as a consequence of groundwater input of nitrate through sandy beaches. Mar Biol 10: 263-271. doi.org/10.1007/BF01319825 Markwell MAK, Hass SM, Bieber LM, Tolbert ME (1978) A modification of the Lowry procedure to simplify protein determination in membrane and lipoprotein samples. Annals of Biochemistry 87: 206-210. DOI:10.1016/0003-2697(78)90586-9 McLachlan A, De Ruyck A, Hacking N (1996) Community structure on sandy beaches: patterns of richness and zonation in relation to tide range and latitude. Rev Chil Hist Nat 69: 451-467. McLachlan A, Fisher M, Al-Habsi HN, Al-Shukairi SS, Al-Habsi AM (1998) Ecology of sandy beaches in Oman. J Coast Conserv 4: 181-190. doi:10.1007/BF02806510 Mews M, Zimmer M, Jelinski DE (2006) Species-specific decomposition rate of beach-cast wrack in Barkeley Sound, British Columbia, Canada. Mar Ecol Prog Ser 328: 155-160. doi.org/10.3354/meps328155 Mounier J (1979) La diversité des climats oceániques de la Penínsule Iberique. La Meteorologie serie 6, 106: 205-227. Murphy J, Riley IP (1962) A modified single solution method for the determination of phosphate in natural waters. Anal Chim 27: 31–36. https://doi.org/10.1016/S0003-2670(00)88444-5 Nel P (2001) Physical and biological factors structuring sandy beach macrofauna communities. Ph.D. Thesis, University of Cape Town, South Africa. Olabarría C, Lastra M, Garrido J (2007) Succession of macrofauna on macroalgal wrack of an exposed sandy beach: Effects of patch size and site. Mar Environ Res 63: 19-40. doi:10.1016/j.marenvres.2006.06.001 Olabarria C, Incera M, Garrido J, Rodil IF, Rossi F (2009) Intraspecific diet shift in Talitrus saltator inhabiting exposed sandy beaches. Estuar Coast Shelf Sci (2009). doi:10.1016/j.ecss.2009.06.021 Orr M, Zimmer R, Jelinski DE, Mews M (2005) Wrack deposition on different beach types: spatial and temporal variation in the pattern of subsidy. Ecology 86: 1496-1507. doi:10.1890/04-1486 Ruiz-Delgado MC, Reyes-Martínez MJ, Sánchez-Moyano JE, López-Perez J, García-García FJ (2015). Distribution patterns of supralittoral arthropods: Wrack deposits as a source of food and refuge on exposed sandy beaches (SW Spain). Hydrobiologia 742: 205–219. doi.org/10.1007/s10750-014-1986-2 Salathé R, Riera P (2012) The role of Talitrus saltator in the decomposition of seaweed wrack on sandy beaches in northern Brittany: An experimental mesocosm approach. Cah Biol Mar 53: 1–8. Schoeman DS, Richardson AJ (2002) Investigating biotic and abiotic factors affecting the recruitment of an intertidal clam on an exposed sandy beach using a generalised additive model. J Exp Mar Biol Ecol 276: 67-81. Schlacher TA, Schoeman DS, Dugan JE, Lastra M, Jones A, Scapini F, McLachlan A (2008) Sandy beach ecosystems: key features, sampling issues, management challenges and climate change impacts. Mar Ecol Evol Persp 29: 70−90 Schlacher TA, Strydom S, Connolly RM (2013a) Multiple scavengers respond rapidly to pulsed carrion resources at the land ocean interface. Acta Oecol 48: 7–12. doi.org/10.1016/j.actao.2013.01.007 Schlacher TA, Strydom S, Connolly RM, Schoeman D (2013b) Donor-control of scavenging food webs at the land-ocean interface. PLoS ONE 8(6): e68221. doi:10.1371/journal.pone.0068221 Schooler NK, Emery KA, Dugan JE, Miller RJ, Schroeder DM, Madden JR, Page HM (2024) Cross-ecosystem subsidies to sandy beaches support surf zone fish. Mar Biol (in press) Soares AG, Schlacher TA, Mclachlan A (1997) Carbon and nitrogen exchange between sandy beach clams ( Donax serra ) and kelp beds in the Benguela coastal upwelling region. Mar Biol 127: 657–664 Spiller DA, Piovia-Scott J, Wright AN, Yang LH, Takimoto G, Schoener TW, Iwata T (2010) Marine subsidies have multiple effects on coastal food webs. Ecology 91: 1424-1434. doi.org/10.1890/09-0715.1 Stenton-Dozey JM, Griffiths CL (1983) The fauna associated with kelp stranded on a sandy beach. In McLachlan A, Erasmus T (Eds.) Sandy beaches as ecosystems (pp. 557–568). The Hague: W Junk. doi.org/10.1007/978-94-017-2938-3 Tenore KR, Boyer LK, Cal RM, Corral J, García-Fernandez C, Gonzalez N, González-Gurriarán E, Hanson RB, Iglesias J, Krom M, López‐Jamar E, McClain J, Pamatmat MM, Pérez A, Rhoads DC, de Santiago G, Tietjen J, Westrich J, Windom HL (1982) Coastal upwelling in the Rías Bajas, NW Spain: Contrasting the benthic re gimes of the Rias de Arosa and the Muros. J Mar Res 40: 701-772. doi.org/10.1016/0022-0981(83)90056-4 Wilson EE, Wolkovich EM (2011) Scavenging, how carnivores and carrion structure communities. Trends Ecol Evol 26: 129-135. doi.org/10.1016/j.tree.2010.12.011 Supplementary Files SupplementaryMaterial.docx Cite Share Download PDF Status: Published Journal Publication published 03 Feb, 2025 Read the published version in Marine Biology → Version 1 posted Editorial decision: Acceptable after minor revision 31 Jul, 2024 Reviewers agreed at journal 17 Jul, 2024 Reviewers invited by journal 17 Jul, 2024 Editor assigned by journal 16 Jul, 2024 First submitted to journal 15 Jul, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-4744352","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":328463019,"identity":"1725b053-015e-4a21-afff-42287e38f32f","order_by":0,"name":"Mariano Lastra Valdor","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAz0lEQVRIiWNgGAWjYDACdgglx8DA2ABm8RHUwgzEBxgYjOFa2IjVktgAEyCoRbeZ+fHnj3vs0jfcSG5+8YGhVp6gFrPDbGYSB54l5264kdhmOYPhuGEbYS0MZgwHDjDnbjhzsM2Yh+EYIxFa2D9/OHCgPt0AqsWeCC08BhIHDhxOMDje2PyYh6EmkRgtZRJnDhw3nHm8sY1xhsGBZMJajrdv/lBxoFqe7zD74w8fKups+wlpQQZsEgwGh0nRAIzVDwwMdaRpGQWjYBSMghEBAGjxROlbEpKXAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0003-2738-2056","institution":"Universidade de Vigo - Campus Lagoas Marcosende: Universidade de Vigo","correspondingAuthor":true,"prefix":"","firstName":"Mariano","middleName":"Lastra","lastName":"Valdor","suffix":""},{"id":328463020,"identity":"eb609bd0-2d29-468c-aa42-65ec98ef14ea","order_by":1,"name":"L De Pablo","email":"","orcid":"","institution":"University of Colorado at Boulder: University of Colorado Boulder","correspondingAuthor":false,"prefix":"","firstName":"L","middleName":"","lastName":"De Pablo","suffix":""},{"id":328463021,"identity":"4330b270-2c29-4996-b6c1-5589a43342d4","order_by":2,"name":"J López","email":"","orcid":"","institution":"Universidade de Vigo - Campus Lagoas Marcosende: Universidade de Vigo","correspondingAuthor":false,"prefix":"","firstName":"J","middleName":"","lastName":"López","suffix":""},{"id":328463022,"identity":"b5a9bbe8-e6f3-411d-8898-9899a025c83a","order_by":3,"name":"F Soliño","email":"","orcid":"","institution":"Universidade de Vigo - Campus Lagoas Marcosende: Universidade de Vigo","correspondingAuthor":false,"prefix":"","firstName":"F","middleName":"","lastName":"Soliño","suffix":""},{"id":328463023,"identity":"e27ea44a-1f26-4a3d-a011-64a179171dcb","order_by":4,"name":"TA Schlacher","email":"","orcid":"","institution":"University of the Sunshine Coast - Sunshine Coast Campus: University of the Sunshine Coast","correspondingAuthor":false,"prefix":"","firstName":"TA","middleName":"","lastName":"Schlacher","suffix":""}],"badges":[],"createdAt":"2024-07-15 16:41:18","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4744352/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4744352/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00227-024-04524-0","type":"published","date":"2025-02-03T15:57:17+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":62266456,"identity":"c9556256-4f9d-4f9c-9a7f-b095d7102d27","added_by":"auto","created_at":"2024-08-12 09:27:27","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":314540,"visible":true,"origin":"","legend":"\u003cp\u003eWave-cast algal material (‘wrack’) deposited on the upper part of beaches significantly changes habitat conditions and the fauna of the lower shore, thereby coupling the landward and seaward edge of ocean shores. A) Location of study sites in Galicia (arguably part of Spain); the size and colour-coding (green = low, blue = moderate, red = high) of site markers indexes variation in wrack cover (cf. Table SU1 for values). B) Wrack that is wave-cast onto beaches significantly increases the levels of nitrogen and phosphorous in the beach habitat (sediment and porewater); C) The structure of fauna assemblages in the swash zone (the lower part of beaches regularly inundated by waves bores during low tides) varies markedly between beaches. It is the combination of wrack cover, nutrient levels, and sediment coarseness that jointly drives this variation of the swash fauna (the size of segments is proportional to wrack cover (green), sediment size (brown) and nitrogen levels in the sediments and porewater (red/pink)). D) Both the density and species richness of swash fauna rise strongly with increasing levels of wrack-derived nutrients.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4744352/v1/5318be09a4ee3c0527aad361.png"},{"id":75930373,"identity":"a0637eca-d7d6-45a6-ba3f-e3e92c9e60aa","added_by":"auto","created_at":"2025-02-10 16:10:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":964690,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4744352/v1/00b62071-7116-4cde-bee3-55ec7239cdcc.pdf"},{"id":62265615,"identity":"a3982f1c-71fc-4bc0-8b26-c4cd27a3ed67","added_by":"auto","created_at":"2024-08-12 09:19:27","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":839038,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-4744352/v1/11e429ad16df1262fdb507e6.docx"}],"financialInterests":"","formattedTitle":"Coupling between landward and seaward fringes of sandy beaches: algal deposits on the upper beach influence biogeochemistry and faunal assemblages in the swash zone.","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eOcean-exposed sandy beaches are conventionally viewed as spatially subsidised ecosystems, their food webs being chiefly underpinned by imports of organic matter from the sea (Schlacher et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Hyndes et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Organic matter in the form of detached macro-algae and seagrass \u0026ndash; \u0026lsquo;wrack\u0026rsquo; - is often the pivotal part of these subsidies (Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Algal wrack primarily originates from nearby rocky shores and reefs (Griffiths et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Colombini and Chelazzi \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Orr et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Once stranded, this material is the substrate of intense biogeochemical processing that determines rates of nutrient cycling and substrate metabolism (Coupland et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Dugan et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Barreiro et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lastra et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWrack imports can be a key driver of ecological traits in beaches, such as enhancing biodiversity and the biomass of consumers (e.g., Ince et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Olabarr\u0026iacute;a et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Crawley et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Spiller et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Wilson and Wolkovich \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The trophic effects of organic matter imports in the coupled surf-beach-dune systems extend to the subtidal parts (Crawley et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), the intertidal shores (Soares et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) and the supratidal zone and seawards parts of dunes (Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Lastra et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Duarte et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Ruiz-Delgado et al. 2017; Schlacher et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2013a\u003c/span\u003e; \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2013b\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eConsumption by herbivores is the initial step within the biological processing of algal wrack supplies. For example, in temperate latitudes, large populations of supratidal crustaceans and insects use the wrack deposits as food and shelter, and this can occur within the first hours once the material reaches the beach. These populations frequently comprise up to 90% of the beach macrofauna (Stenton-Dozey and Griffiths \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Duarte et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Ruiz-Delgado et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Algal wrack consumption by macroinvertebrates involves fragmentation and excretion, which facilitates decomposition and mineralisation of the organic materials by lower biotic compartments, such as meiofauna and bacteria (Koop et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Mews et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Coupland and McDonald \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Anschutz et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Salathe and Riera 2012). Once nutrients are incorporated in the interstitial environment, the enriched beach runoff can enhance the productivity of coastal waters through discharge with nitrate, nitrite, ammonia and phosphorous (Maier and Pregnall 1990; Brooks et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Dugan et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Barreiro et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe seaward fringe of the tidal beach is the swash zone, which is the part of the beach alternately covered and exposed by uprush and backwash, thus playing a pivotal role in connecting beach processes with surrounding coastal ecosystems. Swash zones frequently contain more species of benthic invertebrates, also often at greater abundance, compared to other parts of the intertidal beach (Degraer et al \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWrack becomes typically stranded at the turn of the ebbing tide, resulting in most of the wave-cast algal deposits being located on the upper beach near the strandline (Orr et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). By contrast, the swash zone is always situated at the seaward edge of the beach. Thus, there exists on many beaches a spatial asymmetry between a putative wrack subsidy being concentrated typically at the landward edge of the beach and the fauna in the swash zone at the ocean end. Whether this spatial separation between wrack and swash animals weakens the documented relationship between wrack subsidies and the fauna of the upper shore or the full intertidal is unknown. It is plausible that \u0026lsquo;wrack effects can propagate down-shore via nutrient regeneration and organic matter enrichment during wrack decay. Again, whether changes to the chemical milieu of beach habitats originating from wrack can couple the upper beach with swash consumers is unknown. Thus, our principal aims here are two-fold: a) to examine how wrack strandings change nutrient availability in beach habitats, and b) to test for associations/linkages between the structure, diversity and density of swash fauna with variations in wrack supply wrack-derived nutrients, and geomorphological beach features.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Study site\u003c/h2\u003e \u003cp\u003eWe tested the association between algal wrack and swash fauna by sampling eight beach ecosystems between October and December 2020, located on the NW coast of Spain, spanning roughly 200 km of the coastline of Galicia. Beaches range from reflective to dissipative morphodynamic states (Lastra et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and vary in size from small pocket beaches to more extensive shores up to 7 km long, frequently fringed by rocky headlands or cliffs. Three of the chosen beaches (Samil, America and Panxon) were included in a suburban environment, being affected by irregular and occasional mechanical grooming during the tourist season (June to August). Land-use seawards of three sites was urban to para-urban, replacing the foredunes with a seawall (Samil, America, Panxon); all other sites were backed by vegetated, quasi-natural foredunes. The climate is transitional between the Atlantic and Mediterranean zones, with seasonal variation in seawater temperature ranging between 10 \u003csup\u003eo\u003c/sup\u003eC during winter to 20 \u0026ordm;C during summer (Mounier \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e1979\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Field sampling\u003c/h2\u003e \u003cp\u003eFaunal sampling was designed to evaluate the role of wrack supply in structuring faunal assemblages of the swash zone. To this end, a shore parallel stretch, 100 m long, was selected at the centre of each beach to avoid the boundary effect of rocky headlands. Along those 100 m stretch of beach, 27 sediment samples were collected randomly (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e\u003ca href=\"https://orcid.org/0000-0003-2738-2056\" target=\"_blank\"\u003ewww.random.org\u003c/a\u003e\u003c/span\u003e\u003cspan address=\"http://www.random.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e; Randomness and Integrity Services Ltd., Dublin), with at least 1 m of separation from one another. A 12 cm diameter PVC corer, pushed 15 cm deep into the sediment, was used to collect the samples during spring low tides at the edge of the lower swash zone, which was located by positioning the maximum seaward retreat among all the bores occurring within a 10 minutes lapse. Corer samples were pooled every consecutive 3 (as done by Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Schooler et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), which translates to 0.034 m\u003csup\u003e2\u003c/sup\u003e of surface unit per core and 0.3 m\u003csup\u003e2\u003c/sup\u003e of total surface sampled at each beach. Sediment cores were sieved in mesh bags with a 1 mm aperture size to extract the macrobenthos. Individuals were identified to the lowest taxonomic level when possible.\u003c/p\u003e \u003cp\u003eWrack coverage was measured along six shore-perpendicular transects randomly spaced alongshore within the same 100 m central section of beach where the swash fauna was collected. Transects extended from the base of the foredune to the upper swash level. We used the line-intercept method (Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) to quantify the across-shore distance of the intertidal beach face covered by wrack. With this, algal wrack deposits were mapped across the beach face, and the position of the accumulated 90% of the wrack coverage was calculated for each transect; results were expressed in % of beach width where 90% of algal coverage was reached, starting at the dune base (0%) and ending in the upper swash zone (100%).\u003c/p\u003e \u003cp\u003eThe sediment samples for the nutrient concentration, granulometry and moisture were collected at the upper edge of the swash zone at the same six alongshore locations where the wrack survey transects were positioned. At each position, one corer 5 cm diameter was pushed 8 cm deep into the sediment following Canion et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Sediment cores were also collected to quantify total protein content as a proxy of the food quality of the organic matter within the sedimentary environment. Likewise, DNA in the top-8 cm of sediment was obtained to estimate the bacterial and fungal communities contributing to the decomposition and mineralisation of organic matter (Agnelli et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2004\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTo analyze the nutrient concentration in intertidal pore water, samples were taken next to the positions where sediment samples were collected; to do this, holes were dug to an adequate depth to allow pore water to fill the bottom (\u003cem\u003eca\u003c/em\u003e. 10 cm); then, interstitial water samples of 150 mL were taken and stored in polypropylene bottles. Water samples at the landward edge of the surf zone were also collected at the 6 positions. Once in the lab, all the water samples were filtered to remove any particulate material (quantitative filter of 1\u0026ndash;3 \u0026micro;m) and immediately stored at -20 \u003csup\u003eo\u003c/sup\u003eC until analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Laboratory analyses\u003c/h2\u003e \u003cp\u003eThe sediment samples (10 g) for nutrient analyses were shaken for 2 hours in 25 mL 0.01 mol/L KCl solution for inorganic N extraction; the solution was then filtered (quantitative filter paper of 1\u0026ndash;3 \u0026micro;m) and stored at -20 \u003csup\u003eo\u003c/sup\u003eC until processing (Barreiro et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Nutrients were quantified by continuous flow analysis in a Bran Luebbe Nutrient Analyzer AA3. Ammonium was measured fluorometrically at 460 nm following excitation at 370 nm according to the method of Kerouel and Aminot (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1997\u003c/span\u003e); the samples were reacted with ophthalaldehyde (OPA) at 75 \u003csup\u003eo\u003c/sup\u003eC in the presence of borate buffer and sodium sulphite, to form a fluorescent species in a quantity that is proportional to the ammonium concentration. NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e were analysed via the diazo-reaction based on the methods of Armstrong et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1967\u003c/span\u003e) and Grasshoff et al. (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1983\u003c/span\u003e). This automated procedure involves reduction of NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e by a copper-cadmium reductor column; the NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e then reacts with sulphanilamide and N-1-napthylethyleneidiamine dihydrochloride under acidic conditions; the concentrations were determined colorimetrically at 550 nm. Nitrification products hereafter will be referred as the sum of NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e+NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (as in Brooks et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Total inorganic N (hereafter TIN) was calculated as the sum of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. Phosphate (PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e) analyses was based on the colorimetric method of Murphy and Riley (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1962\u003c/span\u003e), in which a blue colour is formed by the reaction of orthophosphate, molybdate ions, and antimony ions followed by reduction with ascorbic acid at a pH of 1; the reduced blue phospho-molybdenum complex is determined colorimetrically at 880 nm. Data units for the inorganic nutrients in the sediment were in \u0026micro;M per g of dry sediment.\u003c/p\u003e \u003cp\u003eContents in total proteins were measured as a proxy of the edible organic matter received by the sedimentary environment; more conventional methods as dry-combustion in a CHN-LECO were not used to avoid interference from carbonates. Total proteins were determined using methods from Lowry and Rosebrough (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1951\u003c/span\u003e), modified by Markwell et al. (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1978\u003c/span\u003e). Concentrations were calculated as bovine serum albumin equivalents. Three g of sediment were used for the analyses of each sample. For each test, blanks were made using the same sediments previously treated in a muffle furnace (500 \u003csup\u003eo\u003c/sup\u003eC 6h).\u003c/p\u003e \u003cp\u003eThe \u0026ldquo;EZNA DNA Soil extraction kit\u0026rdquo; (Omega Biotek Ref. D5625-01), followed by Qubic Fluorometric quantitation, were used to analyse total DNA in 200 mg sediment samples, representative of the top-8 cm of sediment; results were a proxy of the bacterial and fungal communities contributing to organic matter decomposition (Agnelli et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). DNA is expressed as ng\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of sediment.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Numerical analyses\u003c/h2\u003e \u003cp\u003eThe chief question of interest was to test whether variation in the swash fauna can be explained by the corresponding variations in environmental variables, particularly wrack cover and changes in nutrient and organic matter conditions in the shore habitats. To this end, the principal method was distance-based linear models (DISTLM) as implemented in Primer (Anderson et al \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Fundamentally, the modelling technique analyses the relationship between the fauna data and the set of environmental variables measured in terms of variation in the fauna data explained by individual variables (\u0026lsquo;marginal tests\u0026rsquo;) and by models containing multiple predictors. The model selection criterion for the latter was the corrected Akaike Information Criteria (AICs). The main fauna resemblance matrix modelled was \u0026lsquo;community structure\u0026rsquo; (species composition and the density of all component species) quantified by Bray-Curtis resemblance coefficients calculated on square-root transformed density data; an Euclidean matrix of total density and species richness complemented the models of community structure.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Geo-Bio-Chemical Attributes of Beaches\u003c/h2\u003e \u003cp\u003eWrack deposits were concentrated along the different drift lines associated with the hightide water marks, with more than 90% of the wrack coverage concentrated in the upper 25% of the beach face in all the beaches, except Panxon (at the upper 33%). The cover of algal wrack varied 17-fold across the region, ranging between a minimum of 0.18 m (\u0026plusmn;\u0026thinsp;0.1) at Barra to a maximum of 3.12 m (\u0026plusmn;\u0026thinsp;0.83) at America (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA, Table SU1). Mean wrack cover does not differ between beaches that are occasionally and irregularly groomed (mean wrack cover\u0026thinsp;=\u0026thinsp;1.66 m, \u0026plusmn; 0.71) and those from which wrack is not removed (mean wrack cover\u0026thinsp;=\u0026thinsp;1.12 m, \u0026plusmn; 0.51). PERMANOVA, Pseudo-F\u0026thinsp;=\u0026thinsp;2.421, P\u0026thinsp;=\u0026thinsp;0.146).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe spatial variation in wrack cover was mirrored by a 10-fold variation in total inorganic nitrogen concentrations in the sediments, with the lowest and largest mean values recorded at the same sites (min Barra\u0026thinsp;=\u0026thinsp;0.77\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e); max America\u0026thinsp;=\u0026thinsp;10.59\u0026thinsp;\u0026plusmn;\u0026thinsp;1.56 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Across all sites, more wrack covering a beach was associated with significantly higher concentrations of N and P in the sediment and in the interstitial water (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB, Table SU1). By contrast, correlations between wrack and N \u0026amp; P in the surf-zone water were weaker, as were the correlations between wrack and sedimentary proteins and DNA (Fig. SU2).\u003c/p\u003e \u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e concentrations in the porewater showed a massive (270 times) variation between 3.43 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u0026plusmn;\u0026thinsp;1.37) at Barra and 927.82 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u0026plusmn;\u0026thinsp;435.2) at Panxon (Table SU1). By contrast, the range of observed PO\u003csub\u003e4\u003c/sub\u003e values in the sediment was considerably smaller, ranging two-fold between Barra (1.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and Corrubedo (2.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Similar ranges in spatial variation among beaches were observed for total inorganic N in the sediments (min Barra\u0026thinsp;=\u0026thinsp;215.24\u0026thinsp;\u0026plusmn;\u0026thinsp;38.6 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; max Carnota\u0026thinsp;=\u0026thinsp;804.77\u0026thinsp;\u0026plusmn;\u0026thinsp;51.4 \u0026micro;g g\u003csup\u003e\u0026minus;\u003c/sup\u003eTable SU1). Concentrations of DIN in the surf-zone water were lower than that in the intertidal porewater, ranging between 161 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u0026plusmn;\u0026thinsp;117.6) at Lanzada and 486 \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e(\u0026plusmn;\u0026thinsp;218.0) at Carnota. Proteins and DNA in the sediments showed a similar range (ca. 3 times) of spatial variation, with the lowest mean protein content recorded at Panxon (30.70, \u0026plusmn; 3.75) and highest at Barra (84.4\u0026thinsp;\u0026plusmn;\u0026thinsp;24.2). Total DNA in the sediment varied between 17 ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u0026plusmn;\u0026thinsp;2.9) at Lanzada and 59 ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u0026plusmn;\u0026thinsp;12.25) at Corrubedo (Table SU1\u003cb\u003e)\u003c/b\u003e. The width of the studied beaches ranged between 65 m (Barra) and 120 m (Carnota and Corrubedo), with slopes between 2.21\u0026deg; (Corrubedo) and 5.08\u0026deg; (Barra). The finest sediments were recorded at Carnota (mean grains size 336\u0026thinsp;\u0026plusmn;\u0026thinsp;41.6 \u0026micro;m), and the coarsest sands at Nerga (1276\u0026thinsp;\u0026plusmn;\u0026thinsp;159.25 \u0026micro;m; Table SU1).\u003c/p\u003e \u003cp\u003eAcross the full suite of geomorphological and chemical variables measured, every beach differs significantly from all other beaches (PERMANOVA, Pseudo-F\u0026thinsp;=\u0026thinsp;16.99, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; Table SU1; Fig. SU3); i.e. each beach has a unique set of environmental conditions. America Beach was characterised by a high cover of algal wrack, and elevated concentrations of DIN in the sediments, PO\u003csub\u003e4\u003c/sub\u003e in the surf zone and sediment proteins (Table SU1; Fig. SU3). Wrack cover was also high on Carnota Beach, complemented by a wider beach face, a gentle gradient and high DIN in the porewater (Table SU1; Fig. SU3). Corrubedo Beach sloped very gradually over a wide beach face and had high values of DNA and PO\u003csub\u003e4\u003c/sub\u003e in the sands. Elevated PO\u003csub\u003e4\u003c/sub\u003e values in both the porewater and surf zone were typical for Panxon Beach, complemented by higher concentrations of surf-zone DIN. The distinguishing feature of Nerga Beach was coarse sand, which resulted in low values for Dean\u0026rsquo;s parameter. Sediments were also coarse at Samil Beach, but they contained comparatively high values of DNA and PO\u003csub\u003e4\u003c/sub\u003e. Barra beach was narrow and steep, its sediments rich in protein but poor in interstitial DIN. A high Dean\u0026rsquo;s Parameter value characterised Lanzada Beach but lower concentrations of nutrients in both the porewater and the surf zone (Table SU1; Fig. SU3).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Faunal assemblages \u0026amp; Environmental Drivers\u003c/h2\u003e \u003cp\u003eA total of 26 operational taxonomic units (OTUs) were recorded across all sites, comprising six phyla, six classes, 11 orders, 19 families, 20 genera and 21 species. Species richness per beach ranged between 5 at Barra and 14 at America, with a grand mean of 9.38 species (\u0026plusmn;\u0026thinsp;3.15) per beach across the region (Table SU2). Densities (total number of individuals) varied 37-fold among beaches, with the lowest mean density of 285 ind.m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (\u0026plusmn;\u0026thinsp;285) being recorded at Barra and the highest at 10561 ind.m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (\u0026plusmn;\u0026thinsp;1836) at America (Table SU2). Filter feeders and deposit feeders represent between 70 and 99% of the total abundance at each beach (Table SU2). \u003cem\u003eDonax trunculus\u003c/em\u003e was the main contributor to the filter feeders\u0026rsquo; group, while peracarids (amphipods, isopods and cumaceans) were the main representatives for the depositivorous guild (Table SU2).\u003c/p\u003e \u003cp\u003eVariation in the assemblage structure of the swash fauna was jointly explained by a combination of wrack cover, wrack-derived nitrogen in the sediments and porewater, and sediment grain size (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Each of these variables explained a similar proportion of variance, and all of them were included in the best model (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). By contrast, amongst the other chemical factors examined, neither nutrients in the surf zone nor proteins and DNA in the sediments were correlated with spatial patterns in assemblage structure (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). All the geomorphic attributes (e.g. beach width, slope), wave properties (height, period), and the composite index of beach morphodynamic state (Dean\u0026rsquo;s parameter) were very weak predictors of faunal community structure (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\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\u003eSummary of distance-based linear modelling (DISTLM) relating variation in community structure (i.e. species composition and density of species) of the swash fauna to a suite of environmental predictors. (Pseudo-F, P and Prop. refer to marginal tests; \u0026lsquo;top models\u0026rsquo; are solutions with a delta AICc of 2).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVariable\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePseudo-F\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eProp.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e# in \u0026lsquo;Top Models\u0026rsquo;\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eSediment, Grain Size (mean, microns)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e0.00\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eSediment, Total Inorganic Nitrogen (\u0026micro;g g\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u0026thinsp;1\u003c/b\u003e\u003c/sup\u003e\u003cb\u003e)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e0.05\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003ePorewater, Dissolved Inorganic N (\u0026micro;g L\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u0026thinsp;1\u003c/b\u003e\u003c/sup\u003e\u003cb\u003e)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e0.06\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eWrack Cover (m)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e0.03\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSurf-zone, Dissolved Inorganic N (\u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWave Period (s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePore-water, PO\u003csub\u003e4\u003c/sub\u003e (\u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBeach Width (m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBeach-Slope (deg)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSediment, PO\u003csub\u003e4\u003c/sub\u003e (\u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDean\u0026rsquo;s Parameter (Ω)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSurf-zone, PO\u003csub\u003e4\u003c/sub\u003e (\u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWave Height (m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSediment, DNA (ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSediment, Proteins (\u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eVariation in total fauna density among beaches was best explained by corresponding variation in wrack cover and nitrogen in the sediment (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). All other tested environmental predictors were either not significant, not included in top models, or explained a very small fraction of variation (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Similar to density, species richness was best predicted by wrack cover and wrack-derived P \u0026amp; N (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). By contrast, the traditionally favoured geomorphological, hydrodynamic, and morphodynamic variables (i.e. grain size, wave properties, Dean\u0026rsquo;s Parameter) were very poor predictors of species numbers in the swash infauna (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe results obtained showed the relevance of algal wrack supplies on the ecology and function of exposed sandy beaches. This link has been demonstrated in previous studies where faunal assemblages were associated with macroalgal patches stranded along the intertidal beach (see Hyndes et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e and references within). Most of the species responding to wrack are talitrid amphipods, isopods, coleopterans and dipterans, among others. Our results expand the hypotheses with regard to the effect of algal wrack subsidies on beach ecology: besides consequences on tidal levels where algal strandings primarily occurs, the lower intertidal and the swash zone, defined as the lower tidal limit and shallow submerged zones, also respond to the presence or absence of algal deposits that decompose in the upper beach around the drift line.\u003c/p\u003e \u003cp\u003eWrack affects the sediment metabolism and inorganic nutrient release (Barreiro et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Dugan et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e), which enhances N and P drainage to coastal water, fertilising primary producers, including phytoplankton and phytobenthos (Page and Lastra 2003) and macroalgae. The precise mechanism behind the increase of invertebrate consumers belonging to a range of trophic guilds and taxonomic groups in the swash zone with increasing wrack subsidies is under discussion. Isotopic analyses to analyse food sources and to untangle trophic web at community scale are scarce (e.g. Colombini et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Although the study of Colombini et al. (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) does not include the swash zone, these authors remarked that macroalgal 13C and 15N signatures present in the infauna occur at several trophic levels and vary with tidal position across the beach-dune ecosystem gradient. At the population scale, Olabarria et al. (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) highlighted the intra-species variability in the food sources used by \u003cem\u003eTalitrus saltator\u003c/em\u003e in the Atlantic coast of Spain, using 13C and 15N isotopic signature as tracers. Using C and N counterbalance, Soares et al. (\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) demonstrate that the standing stock of the filter feeder clam \u003cem\u003eDonax serra\u003c/em\u003e is supported mainly by particulate organic matter formed by erosion and decay of stranded macroalgae, with alive sources located in the kelp beds growing massively along the West coast of South Africa. In our study, although filter feeders are a significant compartment of the trophic web, deposit feeders and several polychaetes and peracarid species function as secondary consumers. We hypothesise that these non-filter feeder groups also feed on wrack-derived particulate organic matter that accumulates on the sediment surface or percolates to the interstitial space. Previous studies in the area (Lastra et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) have demonstrated that ocean productivity measured as Chl-a is coupled with the number of infaunal species and abundance in the entire active sands zone (i.e., the zone where waves and tides transport sand grains), including the upper swash zone. In this regard, the high primary productivity in the NW coast of the Iberian Peninsula is related to the increase in nutrients supplied by a seasonal upwelling event (Tenore et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Blanton et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Bode et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Alvarez-Salgado et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), which means that biodiversity, abundance and biomass of the exposed sandy beaches of this area are above those obtained in neighbouring regions (Lastra et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), irrespective of the morphodynamical state.\u003c/p\u003e \u003cp\u003eThe physical and macrofaunal data collected make it possible to test the general hypothesis that beach morphodynamics, including slope, beach width, wave height, wave period and sediment features, affect faunal communities. Low correlations between the number of species and abundances with physical and morphodynamic variables (Bally, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Dexter \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Jaramillo and McLachlan \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; McLachlan et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Hacking \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Nel \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) are usually explained by latitudinal effects, biotic interactions, biogeography or human impact (Defeo \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Defeo et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Brazeiro and Defeo \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Dugan \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Nel \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Schoeman and Richardson \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). The beach locations, sampling dates, and characteristics of the studied beaches allow us to exclude latitudinal and seasonal effects from the interpretation of the results. Hence, the weak correlation between beach morphodynamics, swash climate and other factors with macrofaunal assemblages in the swash zone could only be explained by a bottom-up effect of nutrients provided, directly or indirectly, by the macrophyte subsidies stranded in the upper beach.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAcknowledgement\u003c/p\u003e\n\u003cp\u003eThis research was supported by the Conseller\u0026iacute;a de Cultura, Educaci\u0026oacute;n e Ordenaci\u0026oacute;n Universitaria, Xunta de Galicia, Project ED431C 2021/42.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThis study received financial support from XUNTA de Galicia, Espa\u0026ntilde;a (Projects Ref.: ED431C 2017/46-GRC and ED431C 2018/54 -GRC).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eCompeting interest\u003c/p\u003e\n\u003cp\u003eThe authors whose names are listed in the Authors list certify that they have NO affiliations with or involvement in any organization or entity with any financial interest, or non-financial interest, in the subject matter or materials discussed in this manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAuthors statement\u003c/p\u003e\n\u003cp\u003eI confirm that the manuscript has been read and approved by all named authors. I further confirm that the order of authors listed in the manuscript has been approved by all of us.\u003c/p\u003e\n\u003cp\u003eAuthor Contributions\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Mariano Lastra, Jes\u0026uacute;s L\u0026oacute;pez , Luis De Pablo and Thomas Schlacher. The first draft of the manuscript was written by Mariano Lastra. Luis De Pablo and Thomas Schlacher commented and improve on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003eData availability\u003c/p\u003e\n\u003cp\u003eThe datasets generated during and/or analyzed for the current study are available from the corresponding author upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAnderson MJ, Gorley RN, Clarke KR (2008) PERMANOVA+ for PRIMER: Guide to software and statistical methods, PRIMER-E, Plymouth, 214 pp\u003c/li\u003e\n\u003cli\u003eAgnelli A, Ascher J, Corti G, Ceccherini MT, Nannipieri P, Pietramellara G (2004) Distribution of microbial communities in a forest soil profile investigated by microbial biomass, soil respiration and DGGE of total and extracellular DNA. 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Trends Ecol Evol 26: 129-135. doi.org/10.1016/j.tree.2010.12.011\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"marine-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mabi","sideBox":"Learn more about [Marine Biology](https://www.springer.com/journal/227)","snPcode":"227","submissionUrl":"https://submission.nature.com/new-submission/227/3","title":"Marine Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"wrack decay, nutrients, biodiversity, sediment, swash zone","lastPublishedDoi":"10.21203/rs.3.rs-4744352/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4744352/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEnergy subsidies from the sea typically underpin ocean-exposed sandy beach ecosystems. Strandings of detached macro-algae \u0026ndash; \u0026lsquo;wrack\u0026rsquo; \u0026ndash; can be a spectacular form of such cross-ecosystem transfers of organic matter that sustain consumers in the recipient shore system; this has given rise to a model of wrack promoting the diversity and abundance of invertebrates, with scaling effect on upper trophic levels.\u003c/p\u003e \u003cp\u003eHowever, most wrack is often wave-cast to the upper beach, whereas a distinct part of the shore fauna is limited to the ocean fringe of beaches \u0026ndash; the \u0026lsquo;swash zone\u0026rsquo;. This creates a spatial asymmetry between the location of subsidies (landwards fringe) and the location of the putative recipients (ocean fringe).\u003c/p\u003e \u003cp\u003eHere, we tested whether the fauna of the swash zone can benefit from wrack subsidies, sampling fauna and algal deposits on a range of beaches in NW Spain. We also measured the potential functional link between algal wrack and nutrients released from wrack during decay.\u003c/p\u003e \u003cp\u003eWrack decay increased nutrient concentrations, and it is the combination of wrack cover, nutrient levels, and sediment coarseness that jointly drove variation in the assemblage structure of the swash fauna among beaches. Similarly, the density of the swash fauna and species richness increased markedly at higher nutrient levels and wrack cover.\u003c/p\u003e \u003cp\u003eThese findings expand the \u0026lsquo;wrack enhancement\u0026rsquo; model to include the promotion of consumers at the ocean edge of sandy shores; it also contains a cross-shore linkage via decomposition processes that favourable change the nutrient regime across all the beach face and thereby couple the swash zone with the upper strandline.\u003c/p\u003e","manuscriptTitle":"Coupling between landward and seaward fringes of sandy beaches: algal deposits on the upper beach influence biogeochemistry and faunal assemblages in the swash zone.","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-12 09:19:22","doi":"10.21203/rs.3.rs-4744352/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Acceptable after minor revision","date":"2024-07-31T21:54:14+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-07-18T03:54:47+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-07-18T02:35:42+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-07-16T14:19:59+00:00","index":"","fulltext":""},{"type":"submitted","content":"Marine Biology","date":"2024-07-15T08:47:17+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"marine-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mabi","sideBox":"Learn more about [Marine Biology](https://www.springer.com/journal/227)","snPcode":"227","submissionUrl":"https://submission.nature.com/new-submission/227/3","title":"Marine Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c70ef592-ba32-49a8-ba7f-8d321abbb010","owner":[],"postedDate":"August 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-02-10T16:00:54+00:00","versionOfRecord":{"articleIdentity":"rs-4744352","link":"https://doi.org/10.1007/s00227-024-04524-0","journal":{"identity":"marine-biology","isVorOnly":false,"title":"Marine Biology"},"publishedOn":"2025-02-03 15:57:17","publishedOnDateReadable":"February 3rd, 2025"},"versionCreatedAt":"2024-08-12 09:19:22","video":"","vorDoi":"10.1007/s00227-024-04524-0","vorDoiUrl":"https://doi.org/10.1007/s00227-024-04524-0","workflowStages":[]},"version":"v1","identity":"rs-4744352","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4744352","identity":"rs-4744352","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}