Abstract. Consumers in aquatic food webs can be supported by terrestrial and aquatic primary production. However, we know little about how allochthony, i.e., the proportion of consumer biomass derived from terrestrial organic matter, varies along environmental gradients. Using hydrogen isotopes (deuterium), we quantified allochthony of an ecologically dominant detritivorous fish species (gizzard shad, Dorosoma cepedianum) in reservoir ecosystems, along a gradient of watershed land use (agriculture vs. forest). We predicted that allochthony would decline with an increase in the proportion of watershed land composed of agriculture (% agriculture). This is based on the supposition that as % agriculture increases, so does the export of dissolved inorganic nutrients to lakes, stimulating algal production and reducing the importance of terrestrial organic carbon subsidies. Allochthony accounted for ;34% of gizzard shad production (mean of 11 lakes), although this fraction varied greatly (0-68%) among lakes and isotope mixing model assumptions. Contrary to our hypothesis, we found no relationship between allochthony and % agriculture. However, allochthony was inversely related to total watershed area, as well as the absolute area of the watershed (rather than the percentage) composed of agriculture. We speculate that watershed area and allochthony are negatively correlated because watershed area exerts a strong control on the relative subsidies of dissolved inorganic nutrients vs. particulate organic carbon. Gizzard shad biomass was positively related to phytoplankton primary production but negatively related to allochthony, suggesting that phytodetritus is a higher quality resource than terrestrial detritus. Overall, our results show that both autochthonous and allochthonous carbon fuel the production of this ecologically important detritivore, the relative importance of allochthony decreases with increasing watershed size, and variation in gizzard shad production is closely tied to variation in autochthonous primary production.

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