ONTOGENIC AND TEMPORAL SHIFTS IN THE DIET OF THE AMPHIPOD GAMMARUS-FASCIATUS, IN THE OHIO RIVER

A field study was conducted to determine if ontogenetic or temporal sh ifts occur in the diet of the amphipod Gammarus fasciatus Say in the O hio River. Amphipods were collected monthly from cobble and snag habit ats in the Ohio River for 1 yr. Gut contents of collected amphipods we re analyzed microscopically for the presence of detritus, filamentous algae, diatoms and animal matter. Gammarus fasciatus consumed each foo d type in different amounts, depending on amphipod size and month coll ected. Detritus was the most common food item found in amphipod guts ( 100% of microscope fields in guts examined), followed in order by fila mentous algae, diatoms and animal matter (the ranges of each food type in the guts of G. fesciatus were: 0.036-0.287, 0.061-0.281, 0.002-0.0 72, respectively). Food use shifted ontogenetically: small G. fasciatu s were limited to a diet consisting mainly of detritus, whereas larger animals consumed significant amounts of filamentous algae, diatoms an d animal matter. There were also monthly differences in foraging, pres umably due to differences in seasonal abundances of food types and oth er environmental factors. We suggest that the abundance of, and abilit y to use filamentous algae, diatoms and animal matter allow population s of G. fasciatus to persist and maintain significant year-round popul ations in the Ohio River.