Abstract
Dissolved nitrogen (N) as urea ([NH 2 ] 2 CO), 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 N 2 . 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 + 4 and urea favored non‐heterocystous cyanobacteria and chlorophytes and NO ‐ 3 , urea promoted chlorophytes, some cyanobacteria, and transient blooms of siliceous algae, whereas N 2 ‐fixing cyanobacteria and dinoflagellates exhibited little response to added N. Added N also increased microcystin production by up to 13‐fold in August and September, although the magnitude of response varied with N species and predominant algal taxon ( Planktothrix agardhii , Microcystis spp.). These findings demonstrate that pollution with N intensifies eutrophication and algal toxicity in lakes with elevated concentrations of SRP and low N : P, and that the magnitude of these effects depends on the chemical form, and hence source, of N.