Journal
LIMNOLOGY AND OCEANOGRAPHY
Volume 55, Issue 3, Pages 1213-1230Publisher
WILEY
DOI: 10.4319/lo.2010.55.3.1213
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Funding
- Natural Sciences and Engineering Research Council of Canada
- Canada Research Chair Program
- Canada Foundation for Innovation
- Province of Saskatchewan
- University of Regina
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Urea is the most abundant nitrogen (N) fertilizer used on agricultural soils, yet its effects on adjacent aquatic ecosystems are largely unknown. Here 21-d, 3000-liter mesocosm experiments were conducted monthly in a hypereutrophic lake during July-September 2007 to quantify how addition of urea might affect phytoplankton abundance, gross community composition, and algal toxicity in a phosphorus (P) -rich lake. Repeated measures analysis of variance demonstrated that addition of sufficient urea to increase ratios of soluble N : P from similar to 15 : 1 to > 24 : 1 (by mass) also increased algal biomass (as Chlorophyll a) and microcystin concentrations 200-400%, as non-N-2-fixing but toxic cyanobacteria (Microcystis, Planktothrix) and less harmful chlorophytes (Micractinium, Oocystis) replaced colonial N-2-fixing cyanobacteria (Anabaena, Aphanizomenon). No significant effects of urea amendment were recorded for trials in which N: P ratios were elevated at the start of the experiment, or in which ambient light levels were reduced to 25 mu mol quanta m(-2) s(-1), although preliminary evidence suggests that urea addition stimulated growth of heterotrophic bacteria irrespective of light regime. Development of toxic non-N-2-fixing cyanobacteria by N pollution of P-rich lakes is consistent with findings from whole-lake experiments and paleolimnological studies of deep lakes, and suggests that the fertilization needed to feed 3 billion more people by 2050 may create conditions in which future water quality in P-replete regions is degraded further by urea export from farms and cities.
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