Biomanipulation of Lake 221 in the Experimental Lakes Area (ELA): Effects on Phytoplankton and Nutrients

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Abstract
Lake 221 was dominated by omnivorous perch (Perca flavescens); the addition of northern pike (Esox lucius) resulted in trophic changes that eventually affected the phytoplankton community. Two years following the introduction of pike, perch were greatly reduced. Subsequently, Chaoborus abundance increased, rotifer and cladoceran abundance and biomass decreased, and the large-bodied cladoceran Daphnia catawba increased. 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 dissolved P was bound in an increased bacterial population. Phytoplankton cell size and production decreased, but the production:biomass ratio increased. In year five, chrysophytes dominated as phytoplankton biomass and production increased and bacterial abundance declined. Phytoplankton responses were primarily an indirect result of the introduction of piscivorous fish, which altered internal nutrient recycling.