Urea as a Nitrogen Source for <i>Microcystis aeruginosa</i>

Over the last decade, Lake Erie has experienced annual harmful algal blooms events dominated by the toxic cyanobacterium Microcystis aeruginosa. It is still unclear what causes Microcystis blooms to occur, but there is broad agreement that eutrophication of freshwater systems from anthropogenic sources (urban, industrial, etc.), has led to their proliferation. In particular, the organic compound urea has been implicated as an important source of anthropogenic nitrogen, due to its increased use in agricultural practices. Currently, urea constitutes more than 50% of the nitrogen used for agricultural fertilizer globally, and its usage has increased more than a 100-fold over the past four decades. To determine the effects of urea on Lake Erie phytoplankton, environmental surveys were conducted for urease enzymatic activity, phytoplankton biomass, nutrient concentrations, and phytoplankton community composition. 48-hour in situ microcosm experiments, spiked with various nitrogen species (nitrate, ammonium, and urea), were also performed to identify if the species of nitrogen influences phytoplankton biomass in Lake Erie. Results from this study confirm the presence of urea as a bioavailable form of nitrogen in Lake Erie, and indicated that as the bloom prolongs in duration the primary nutrient limiting phytoplankton biomass shifts from phosphorus to nitrogen. These results reinforce the importance of understanding the role of both nitrogen and phosphorous in driving harmful algal blooms in freshwater systems, and present the new idea that seasonal nitrogen limitation of plankton growth may be crucial in determining the size and extent of Microcystis blooms in Lake Erie.