Comparative effects of urea, ammonium, and nitrate on phytoplankton abundance, community composition, and toxicity in hypereutrophic freshwaters

Dissolved nitrogen (N) as urea ([NH2](2)CO), nitrate (NO3-), and ammonium (NH4+) was added to naturally phosphorus (P)-rich lake water (up to 175 mu g 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 mu g P L-1 and dissolved N: P was < 20 : 1 by mass. In particular, NH4+ and urea favored non-heterocystous cyanobacteria and chlorophytes and NO3-, 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.