| The growth and abundance of phytoplankton in freshwater lakes has long been attributed to the concentration of phosphorus (P), and this idea of P-limitation has been a paradigm accepted by limnologists. Hence, lake managers have relied on the strategy of reducing P to restore water quality of eutrophic lakes. Recently however, several researchers have proposed that nitrogen (N) is equally important as P, and have stated that the P paradigm has eroded. These researchers suggest that both P and N inputs need to be constrained. In spite of the evidence that suggests N-limitation, there are still several researchers that hold onto the paradigm that only P regulates phytoplankton biomass. Limnologists need more data to solve this hotly debate topic. The goal of this dissertation is to provide insights into the dual nutrient management strategy controversy by studying how western Lake Erie cyanobacteria responded to low concentrations of N and P. In western Lake Erie nitrate concentrations decrease throughout the growing season to very low levels. Nutrient enrichment bioassays conducted monthly during the summers of 2010 and 2011 indicated that N (and not P) constrains cyanobacterial growth during August and September when nitrate concentrations are very low. Experiments conducted during 2012 showed that N-limited cyanobacterial blooms are able to utilize many forms of N. However, nutrient dilution assays indicated that N-limitation could not be induced during early summer when P is the primary limiting nutrient. Following N-limitation, the cyanobacterial bloom shifted from Microcystis to the N-fixing Anabaena . Furthermore, during 2011, the concentration of the cyanotoxin microcystin was highly correlated with Anabaena biovolume. Genetic diversity of the Microcystis population was assessed during 2011 and showed that diversity was very similar spatially and temporally in spite of the wide range of N, indicating that Lake Erie Microcystis can survive in low N waters. Finally, long-term data sets show that annual summer nitrate concentrations in western Lake Erie have been declining since 1995, and yet cyanobacterial blooms are prevalent. Overall the results suggest that additional N inputs will likely exacerbate cyanobacterial blooms, however, reducing N inputs will not be effective in ameliorating eutrophication. |
