Land–Ocean Connectivity Through Subsidies of Terrestrially Derived Organic Matter to a Nearshore Marine Consumer

Abstract

Land–ocean coupling in the form of riverine inputs of terrestrial matter can constitute an energetic subsidy to food webs in nearshore coastal areas. In regions with distinctly seasonal rainfall patterns, the strength and spatial footprint of any terrestrial signal in receiving marine food webs is predicted to mirror seasonal changes in fluvial forcing. Here, we test this prediction in a subtropical bay by isotopically (δ13C and δ15N) characterizing the main primary producers and reconstructing (using a Bayesian stable isotope mixing model) their contributions to the diet of thinstripe hermit crabs (Clibanarius vittatus). Seasonal rainfall flushed terrestrial carbon out of coastal watersheds, and this material made a sizable (up to 28%) contribution to the diet of marine consumers, in addition to mangroves, seagrass and algae. Our isotope model indicates that inputs of terrestrial grasses and other littoral vegetation were 15% greater as a result of increased fluvial forcing. In addition, the spatial footprint of the terrestrial signal in marine consumers propagated more widely throughout the bay during high-rainfall periods. Given the widespread conversion of natural watershed habitats for agriculture and urban development, understanding the nature, temporal dynamics and strength of such land–ocean coupling will become increasingly important.