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Dissolved nitrogen (N) as urea ([NH₂]₂CO), nitrate (NO^‐₃), and ammonium (NH⁺₄) was added to naturally phosphorus (P)‐rich lake water (up to 175 µg P L^‐1) to test the hypotheses that pollution of hypereutrophic lakes with N increases total algal abundance, alters community composition, and favors toxic cyanobacteria that do not fix atmospheric N₂. Monthly experiments were conducted in triplicate in polymictic Wascana Lake, Saskatchewan, Canada, during July, August, and September 2008 using large (> 3140 liters) enclosures. Addition of all forms of N added at 6 mg N L^‐1 increased total algal abundance (as chlorophyll a) by up to 350% relative to controls during August and September, when soluble reactive P (SRP) was > 50 µg P L^‐1 and dissolved N : P was < 20 : 1 by mass. In particular, NH⁺₄ and urea favored non‐heterocystous cyanobacteria and chlorophytes and NO^‐₃, urea promoted chlorophytes …