Cross-ecosystem trophic subsidies to sandy beaches support surf zone fish

Food webs in ecotones linking adjacent ecosystems may depend on cross-ecosystem subsidies. In surf zones of temperate sandy beaches, higher-level consumers often rely on intertidal prey that utilize allochthonous primary production. We evaluated the importance of phytoplankton and kelp-based prey, as well as physical characteristics of beaches, to diet of a surf zone fish, barred surfperch (Amphistichus argenteus), through stomach content and stable isotope analyses. Our results suggested that barred surfperch rely on prey from both phytoplankton and kelp-based subsidies, but their relative contribution to diet varied widely across beaches. Sand crabs (Emerita analoga), which depend on phytoplankton, were abundant at every beach, but their contribution to diet in stomach contents varied from 2 to 87% among sites. At the majority of beaches, δ¹³C values of fish muscle tissue, which reflects diet integrated over time, were within 0.5 ‰ of sand crab values, suggesting a reliance on phytoplankton-based prey. However, kelp-dependent prey associated either with beach wrack or subtidal reefs was also present in surfperch stomachs from all beaches (up to 41–72%). The notable enrichment in ¹³C of juvenile surfperch at two beaches and adults at one beach relative to sand crabs suggested a longer-term contribution of kelp-based prey to fish diet. The detection of kelp-based prey in surfperch diets also indicates the potential for reciprocal subsidies in these ecotones. Our results suggest trophic connectivity between surf zones and kelp forests and sandy beaches is spatially variable and that opportunistic higher-level consumers can shift their diet in response to the availability of phytoplankton and kelp-based food resources.