Terrestrial inputs may play a substantial role in subsidizing aquatic food webs. The detritivorous gizzard shad (Dorosoma cepedianum) is a dominant fish in the eastern USA, particularly where terrestrial inputs of detritus and inorganic nutrients are high, as from agricultural watersheds. The extent to which gizzard shad consume settled phytodetritus versus detritus from terrestrial sources is unknown.
This study used deuterium to quantify the contribution of terrestrial detritus to gizzard shad across 11 reservoirs in Ohio, USA with land use ranging from 6 to 86% agricultural. Deuterium values for gizzard shad were similar among all 11 reservoirs, with means ranging from -145.9‰ to -179.4‰. Allochthony estimates ranged from 27 to 48%. We found that allochthony was related to both land use and watershed area. Our results suggest that gizzard shad rely heavily on phytodetritus, while several factors may influence the availability and quality of terrestrial subsidies.