Understanding the importance of terrestrial organic carbon subsidies in aquatic food webs is necessary for the integration of these spatially distinct ecosystems. Terrestrial carbon may play a particularly unique role in reservoirs in the eastern United States, where detritivorous gizzard shad (Dorosoma cepedianum) are often extremely abundant and have strong effects on food webs and nutrient cycling. Gizzard shad are especially dominant in reservoirs with agricultural watersheds, which receive large inputs of terrestrial detritus and inorganic nutrients. In these heavily subsidized, highly productive reservoirs, the extent to which gizzard shad benefit from direct allochthonous detrital subsidies versus increased phytodetritus production is unknown. Using natural abundances of deuterium, a stable isotope of hydrogen, we quantified the contribution of terrestrial carbon to gizzard shad tissue in a eutrophic reservoir that resides in a predominantly agricultural watershed.

Results/Conclusions

Aquatic primary producers (phytoplankton) were far more depleted in deuterium than the dominant terrestrial vegetation (Zea mays, Glycine max, Acer rubrum). Phytoplankton mean δD was -200.9‰, while the mean δD for terrestrial vegetation, weighted by dominance in the watershed, was -95.96‰. The mean value for gizzard shad dorsal muscle tissue was -156.9‰. We analyzed the data using a simple two source mixing model, and found that approximately 42% of gizzard shad body carbon is allochthonous. Thus, allochthonous carbon apparently represents a significant proportion of the diets of these detritivorous fish, even in systems with high in situ productivity. Our results suggest that allochthonous subsidies contribute to the strong effects of these fish on reservoir ecosystems.