Caloric needs of detritivorous gizzard shad Dorosoma cepedianum are met with sediment bacterial and algal biomass

The food source of sediment-ingesting animals is a subject of controversy, centering on the notion that the microbial biomass in sediments is insufficient to support caloric need. Gizzard shad Dorosoma cepedianum are omnivorous, sediment-ingesting fish that frequently impact the trophic structure of temperate lake and reservoir food webs. Prior gut content analyses of this fish have typically determined the bulk of ingested material to be amorphous matter. In the present study, lipid-based assays were used to quantify microbial biomass in sediment and to determine the food source undergoing digestion within the gut of gizzard shad. Microbial biomass was not uniformly distributed throughout the sediment; rather, the low-density fraction of sediment was enriched in microbial biomass nearly 7-fold, compared to whole sediment. Gizzard shad ingesta contained 8 times more low-density sediment than whole sediment, suggesting selective feeding. On average, the ceca-lined segment of the gut contained 4.9 mg dissolved lipid ml(-1) gut fluid. Dissolved fatty acids were from bacteria and phototrophic microeukaryotes. Estimates of calories gained by the fish were in good agreement with a previous estimate of gizzard shad caloric requirements and indicated that bacteria and microeukaryotes were the food source of the fish. These findings indicate that (1) sediment microeukaryotes and bacteria serve as important food sources for gizzard shad, (2) selective feeding on a sediment fraction enriched in living microbial biomass meets fish caloric requirements, and (3) detritivory is the mechanism by which gizzard shad link benthic and pelagic food webs.