Long term assessment of nitrogen cycling and harmful algal blooms in Lake Mendota, WI, USA

Anthropogenic eutrophication of freshwaters has accelerated the occurrence of cyanobacterial blooms leading to altered ecosystem function and structure, water quality degradation, economic loss, and threatened animal and human health. However, toxic cyanobacterial blooms have proven difficult to predict because the mechanisms of toxin production are unknown. While phosphorus is often considered the nutrient that limits primary production over long time scales (e.g. years to decades) in freshwaters, nitrogen availability may ultimately structure cyanobacterial population dynamics and stimulate toxin production on daily to weekly time scales. We aimed at describing the role of nitrogen in structuring cyanobacterial communities in a eutrophic lake using two long-term datasets from Lake Mendota, WI. Cyanobacterial community change occurred rapidly following late spring declines in dissolved inorganic nitrogen concentrations. In every year, nitrogen-fixing cyanobacteria made up the majority of the cyanobacterial community in early summer, but gave way to potentially toxic, non-nitrogen fixers during the peak of summer. This would suggest a logical series of events where nitrogen limitation selects for nitrogen fixers and that new nitrogen inputs from nitrogen fixation may stimulate and support contemporary algal blooms. To address this mechanism, we monitored nitrogen fixation rates and toxin production from 2010 and 2011 in congruence with the long-term data. In both years, nitrogen fixation rates increased significantly following a rapid decline in nitrate, which coincided with an Aphanizomenon-dominated community. Shortly thereafter, Microcystis, and concentrations of the cyanotoxin microcystin, increased linearly until mid-summer. However, this correlated to only one Microcystis genotype confounding analyses that normally lack species-level population dynamics. In order to assess the role of nitrogen and phosphorus loading on freshwater lakes, more field-based studies need to be conducted on the mechanisms leading to cyanobacterial toxin production.