Selective particle ingestion by a filter-feeding fish and its impact on phytoplankton community structure

The ingestion rates of filter-feeding gizzard shad for different sizes of suspended particles were measured using mixtures of microspheres and zooplankton. Ingestion rate increases as a function of particle size, leveling off at 60 pm. The particle-size-dependent ingestion rates were consistent with a model of filtering efficiency based on the cumulative frequency of interraker distances of gizzard shad gill rakers. Comparison of ponds containing gizzard shad with control ponds without fish showed that gizzard shad suppressed Ceratium, the only phytoplankton species large enough to be ingested at a maximum rate. Gizzard shad did not have a significant effect on populations of Synedra, Peri-dinium, Navicula, Kirchneriella, Cyclotella, and Chlamydomonas. Populations of Ankistrodesmus, Cryptomonas, Cosmarium, Rhodomonas, and algae and bacteria from 2-4 pm were enhanced by gizzard shad. Most studies of grazer impact on fresh-water phytoplankton communities have fo-cused on zooplankton. Herbivorous zoo-plankton usually feed on nannoplankton ~50 pm in diameter (Porter 1977; Briand and McCauley 1978; Geller and Miiller 198 1). Because of their unavailability to zooplankton, some limnologists have as-sumed that net phytoplankton are dead ends in the planktonic food webs of temperate zone lakes (Porter 1977). This food web concept ignores field studies showing that several species of North American fish con-sume filamentous and colonial algae (Mur