Terrestrial organic carbon subsidies to detritivorous fish in reservoirs with contrasting watershed land use as revealed by hydrogen isotopes

Terrestrial organic carbon inputs to lakes may play a substantial role in subsidizing food webs and structuring ecosystems. Their role may be unique in reservoirs in the eastern USA, where an abundant fish (gizzard shad, Dorosoma cepedianum) links benthic and pelagic habitats. Gizzard shad are detritivores, and are especially abundant in reservoirs with agricultural watersheds, where inputs of detritus and inorganic nutrients are high. The extent to which gizzard shad consume settled autochthonous production versus detritus from terrestrial (allochthonous) sources is unknown, as is the effect of watershed land use on allochthonous support of gizzard shad.

We used deuterium to quantify the contribution of allochthonous detritus to gizzard shad diets across 11 reservoirs in Ohio, USA with land use ranging from 11 to 91% agricultural. Deuterium signals for phytoplankton varied among reservoirs (-252.2‰ to -152.1‰) but all were depleted relative to terrestrial vegetation (-111.6‰). Using a modified two-source mixing model, we found that the degree of allochthony varied among lakes of contrasting productivity and land use. Even in hypereutrophic Acton Lake, gizzard shad diets were nearly 20% terrestrial. Our results suggest that terrestrial detritus subsidizes gizzard shad populations, thereby contributing to the strong effects of this fish on reservoir ecosystems.