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

Abstract

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, some cyanobacteria, and transient blooms of siliceous algae, whereas N2‐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.