BIOMANIPULATION OF LAKE-221 IN THE EXPERIMENTAL LAKES AREA (ELA) - EFFECTS ON PHYTOPLANKTON AND NUTRIENTS

Lake 221 was dominated by omnivorous perch (Perca flavescens); the add ition of northern pike (Esox lucius) resulted in trophic changes that eventually affected the phytoplankton community. Two years following t he introduction of pike, perch were greatly reduced. Subsequently, Cha oborus abundance increased, rotifer and cladoceran abundance and bioma ss decreased, and the large-bodied cladoceran Daphnia catawba increase d. The phytoplankton community shifted from chlorophyte to cyanophyte codominance with dinoflagellates. Phytoplankton biomass and phosphorus (P) increased because of nutrient recycling and excretion by pike and zooplankton. In years three and four, algal biomass and the ratio of suspended to dissolved P decreased because a larger portion of dissolv ed P was bound in an increased bacterial population. Phytoplankton cel l size and production decreased, but the production:biomass ratio incr eased. In year five, chrysophytes dominated as phytoplankton biomass a nd production increased and bacterial abundance declined. Phytoplankto n responses were primarily an indirect result of the introduction of p iscivorous fish, which altered internal nutrient recycling.