| Summary: | Microplankton in an oligotrophic arctic lake were assessed by direct counts for one summer prior to nutrient additions and three summers during which inorganic nitrogen and phosphorus were added to the lake at approximately ten times ambient loading rates. Protozoa increased significantly in both number and biomass following fertilization, and community structure changed from dominance by oligotrichs prior to fertilization to dominance by the bacterivorous peritrich Epistylis rotans in the second and third years of fertilization. Rotifer abundance and biomass was not significantly different among summers, although one species, Conochilus natans that had not been seen previously, was present during the second and third year of fertilization. By the third year of fertilization both protozoan and rotifer biomass had declined from peak levels, while crustacean zooplankton nauplius abundance had increased suggesting the emergence of top-down regulatory controls as the lake became eutrophic.
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