Experimental evidence that pollution with urea can degrade water quality in phosphorus‐rich lakes of the Northern Great Plains

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 ά ) 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 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–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.