Cross-ecosystem subsidies support surf zone fish on island beaches

Many ecosystems function as open systems where cross-ecosystem exchanges influence community and population dynamics. This is particularly true for ecosystems that rely on subsidies, such as temperate sandy beaches where food webs are primarily supported by two types of marine subsidies, phytoplankton and macroalgal wrack. We hypothesized that the relative abundance of these subsidies affect prey availability and the diet of higher trophic level consumers as exemplified by fish in surf zones. We evaluated the influence of subsidies on the diet of a common surf zone fish, barred surfperch (Amphistichus argenteus), using surveys of intertidal macroinvertebrates and wrack and the results of stomach content and stable isotope analyses across beaches of the California Channel Islands, USA. Estimates of coastal productivity and the abundance and diversity of macroinvertebrates varied considerably among sites. Canopy biomass of nearby kelp forests and beach orientation strongly influenced wrack abundance which in turn affected species richness (1-23 species) and abundance (339-66524 ind. m^-1) of wrack-associated macroinvertebrates. For suspension-feeding sand crabs, abundance varied over an order of magnitude (862-18767 ind. m^-1). The diet of barred surfperch reflected observed variation in prey composition among beaches. Sand crabs were a major component of barred surfperch diet, present in 77% of stomachs. Wrack-associated species, unreported for barred surfperch in previous diet analyses, were a smaller component of barred surfperch diet, present in 19% of stomachs, but made up a greater percentage of their diet on beaches with abundant wrack (65% of total prey items on beach with highest abundance of wrack). Our results suggest that trophic connectivity between kelp forests and sandy beaches extends back into the surf zone and has implications for the transfer of energy and nutrients in coastal food webs.