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.