Toxic cyanobacterial blooms threaten freshwaters worldwide but have proven difficult to predict because the mechanisms of bloom formation and toxin production are unknown, especially on weekly time scales. Water quality management continues to focus on aggregated metrics, such as chlorophyll and total nutrients, and these measures may not be sufficient to explain complex community changes and functions such as toxin production. For example, nitrogen (N) speciation and cycling play an important role in shaping cyanobacterial communities on daily time scales based on function because declining N has been shown to select for N2 fixers. Addition of N pulses from N2 fixation (termed new N) may stimulate and sustain contemporary phytoplankton communities, including those that are potentially toxic (Fig. 4).
We described how rapid early summer declines in N followed by bursts of N2 fixation have shaped cyanobacterial communities in a eutrophic lake (Lake Mendota, Wisconsin, USA), possibly driving toxic Microcystis blooms throughout the growing season. On weekly time scales in 2010 and 2011, we monitored the cyanobacterial community using the phycocyanin intergenic spacer (PC-IGS) region to determine population dynamics. In parallel, we measured microcystin concentrations, N2 fixation rates, and potential environmental drivers (such as nutrient concentrations, light levels, and lake stability metrics) that contribute to structuring the community. We defined a time period called the “toxic phase” when toxins were significantly above the World Health Organization level for safe drinking water (1 µg L-1).
In both years, cyanobacterial community change was strongly correlated with dissolved inorganic nitrogen (DIN) concentrations, and Aphanizomenon and Microcystis alternated dominance throughout the pre-toxic, toxic, and post-toxic phases of the lake. Microcystin concentrations increased within a few days after the first N2 fixation rates were observed. We attribute this to a response in N stress by the non-N2 fixing, Microcystis, and subsequent growth due to new N inputs, because following large early summer N2 fixation events, Microcystis increased and became most abundant. Maximum microcystin concentrations coincided with Microcystis dominance. In both years, DIN concentrations dropped again in late summer, and N2 fixation rates and Aphanizomenon abundance increased once again before the lake mixed in the fall. Estimated N inputs from N2 fixation were large enough to supplement, if not support, the toxic Microcystis blooms.