Experimental evidence that pollution with urea can degrade water quality in phosphorus-rich lakes of the Northern Great Plains
Limnol. Oceanogr., 55(3), 2010, 1213-1230 | DOI: 10.4319/lo.2010.55.3.1213
ABSTRACT: 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 ~ 15 : 1 to > 24 : 1 (by mass) also increased algal biomass (as Chlorophyll a) and microcystin concentrations 200-400%, as non-N2-fixing but toxic cyanobacteria (Microcystis, Planktothrix) and less harmful chlorophytes (Micractinium, Oocystis) replaced colonial N2-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 µmol 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-N2-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.