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.