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ABSTRACT Sandy beaches are iconic interfaces that functionally link the ocean with the landviathe flow of organic matter from the sea. These cross‐ecosystem fluxes often comprise uprooted seagrass and dislodged macroalgae that can form substantial accumulations of detritus, termed ‘wrack’, on sandy beaches. In addition, the tissue of the carcasses of marine animals that regularly wash up on beaches form a rich food source (‘carrion’) for a diversity of scavenging animals. Here, we provide a global review of how wrack and carrion provide spatial subsidies that shape the structure and functioning of sandy‐beach ecosystems (sandy beaches and adjacent surf zones), which typically have littlein situprimary production. We also examine the spatial scaling of the influence of these processes across the broader land‐ and seascape, and identify key gaps in our knowledge to guide future research directions and priorities. Large quantities of detrital kelp and seagrass can flow into sandy‐beach ecosystems, where microbial decomposers and animals process it. The rates of wrack supply and its retention are influenced by the oceanographic processes that transport it, the geomorphology and landscape context of the recipient beaches, and the condition, life history and morphological characteristics of the macrophyte taxa that are the ultimate source of wrack. When retained in beach ecosystems, wrack often creates hotspots of microbial metabolism, secondary productivity, biodiversity, and nutrient remineralization. Nutrients are produced during wrack breakdown, and these can return to coastal waters in surface flows (swash) and aquifers discharging into the subtidal surf. Beach‐cast kelp often plays a key trophic role, being an abundant and preferred food source for mobile, semi‐aquatic invertebrates that channel imported algal matter to predatory invertebrates, fish, and birds. The role of beach‐cast marine carrion is likely to be underestimated, as it can be consumed rapidly by highly mobile scavengers (e.g. foxes, coyotes, raptors, vultures). These consumers become important vectors in transferring marine productivity inland, thereby linking marine and terrestrial ecosystems. Whilst deposits of organic matter on sandy‐beach ecosystems underpin a range of ecosystem functions and services, they can be at variance with aesthetic perceptions resulting in widespread activities, such as ‘beach cleaning and grooming’. This practice diminishes the energetic base of food webs, intertidal fauna, and biodiversity. Global declines in seagrass beds and kelp forests (linked to global warming) are predicted to cause substantial reductions in the amounts of marine organic matter reaching many beach ecosystems, likely causing flow‐on effects for food webs and biodiversity. Similarly, future sea‐level rise and increased storm frequency are likely to alter profoundly the physical attributes of beaches, which in turn can change the rates at which beaches retain and process the influxes of wrack and animal carcasses. Conservation of the multi‐faceted ecosystem services that sandy beaches provide will increasingly need to encompass a greater societal appreciation and the safeguarding of ecological functions reliant on beach‐cast organic matter on innumerable ocean shores worldwide.
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This content will become publicly available on February 1, 2026
Coupling between landward and seaward fringes of sandy beaches: algal deposits on the upper beach influence biogeochemistry and faunal assemblages in the swash zone
Abstract Energy subsidies from the sea typically underpin ocean-exposed sandy beach ecosystems. Strandings of detached macroalgae – ‘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 effects 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 has the potential to create a spatial asymmetry between the location of subsidies (concentrated at the landwards fringe) and the location of some of the putative recipients distributed at the ocean fringe. Here, we tested whether the fauna of the swash zone can benefit from wrack subsidies, by 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. Filter feeders were an important compartment in the food web of the swash macrofauna; we hypothesize that wrack deposits in the upper beach enhance food availability for this trophic guild via increasing nutrients in the water column, stimulating primary production and providing particulate organic matter derived from algal decay. Besides, several polychaetes and peracarid species function as secondary consumers and detritus feeders that plausibly benefit, directly or indirectly, on the particulate organic matter that accumulates on the sediment surface or percolates into the interstitial environment. 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 favourably change the nutrient regime across all the beach face and thereby couple the swash zone with the upper strandline.
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- Award ID(s):
- 2022138
- PAR ID:
- 10630662
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Marine Biology
- Volume:
- 172
- Issue:
- 2
- ISSN:
- 0025-3162
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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