Stable isotope evidence for trophic subsidy of coastal benthic fisheries by river discharge plumes off small estuaries

Abstract

Major rivers produce large plumes which subsidize benthic marine food webs. Because most plumes are smaller, we tested whether these also can link marine food webs with riverine discharges. We used stable isotopes to detect assimilation of terrestrial organic matter by fish, crustaceans and cephalopods harvested from plume areas off two small estuaries in eastern Australia, contrasted with values from marine reference sites. A terrestrial signal was evident in most marine consumers as shifts in carbon and nitrogen isotope ratios. The strongest signal for terrestrial carbon uptake was found in two species harvested commercially, the portunid crab, Portunus sanguinolentus, and the flounder, Pseudorhombus arsius, demonstrating a link between river discharge and fisheries productivity in coastal seas. Against a backdrop of the general presence of a trophic signal imparted by small plumes, absolute contributions of these subsidies were, however, smaller than in larger systems. Also, for the species occurring in both coastal and estuarine waters (sand whiting, Sillago ciliata), isotopic variation was considerably smaller in marine waters than across the estuarine gradient. Overall, small plumes can make contributions to the energy requirements of coastal fisheries species, but their ephemeral nature and small physical dimensions set limits to the degree of land–water ecotonal coupling.