Identifying linkages between the nitrogen cycle, cyanobacterial community structure, and cyanotoxin production in a eutrophic lake

Background/Question/Methods

Lake Mendota, Madison, WI harbors an abundant and diverse array of bloom-forming cyanobacteria. The cyanobacterial community composition (CCC) can be highly variable on a daily scale, contain numerous nitrogen fixing and non-nitrogen fixing genera, and have multiple genotypes capable of forming ephemeral, potentially toxic blooms. Nutrients play an integral role in the CCC, but it is unclear how phosphorus, nitrogen, and trace metal limitation might directly influence community dynamics and cyanotoxin production. The process of biological nitrogen fixation gives some cyanobacteria a clear advantage over other phytoplankton during times of nitrogen stress. However, the release of dissolved organic nitrogen and ammonium during nitrogen fixation may stimulate a subsequent non-nitrogen fixing bloom and/or cyanotoxin production. We hypothesized that nitrogen fixation may act as a potential driver of the CCC and cyanotoxin production and that this shift in community structure is directly linked to nitrogen cycling. To assess this link, we sampled several locations in Lake Mendota during the summer of 2010 measuring the CCC by automated phycocyanin intergenic spacer analysis (APISA), in situ nitrogen fixation, cyanotoxins, and several other biological, chemical, and physical parameters that may be related to structuring the CCC.

Results/Conclusions

Shortly after lake stratification, nitrate concentrations dropped drastically, Aphanizomenon and Gloeotrichia bloomed, and nitrogen fixation rates increased significantly. After this initial bloom, nitrate concentrations increased followed by a rapid rise in Microcystis and Chroococcus abundance. This regime lasted for almost a month until nitrate was drawn down to detection. At this time, Aphanizomenon bloomed again concurrent with nitrogen fixation rates. Analysis of similarity (ANOSIM) showed that each month contained a significantly distinct CCC with total dissolved nitrogen, total nitrogen, and nitrate concentrations explaining the majority of variability observed (canonical correspondence analysis). Total nitrogen to total phosphorus ratios never dropped below the Redfield ratio. However, nitrogen fixation significantly increased when nitrate concentrations were below 20 µg L^-1. Together, these results have large implications for the role of nitrogen fixation in the CCC, the formation of potentially toxic blooms, and the protection and sustainability of recreational waters.