Cyanobacterial harmful algal blooms (cHABs) have occurred regularly in Lake Erie for the past two decades. The blooms are commonly dominated by Microcystis aeruginosa, a cyanobacterium with the ability to produce a group of hepatotoxins known as microcystins (MC). Current forecasting of the blooms predicts the spatiotemporal biovolume based on chlorophyll-a concentrations collected via remote sensors and known drivers of cyanobacteria cell growth. Forecasting however, cannot predict MC concentration of a bloom. The focus of this research was to examine MC production and loss rates to provide information that would be useful to develop forecasting of MC concentration for Microcystis blooms in the western basin of Lake Erie.
Throughout 2018 and 2019 summers, 26 microcosm experiments were conducted at a near river mouth and off-shore location to quantify rates of MC production and loss. Eleven trials of production experiments with nutrient amendments were conducted each year during the annual cHAB. Nutrient additions of phosphorous with different nitrogen sources were added to 2 L of whole lake water at the beginning of the experiment to understand the effects of nitrogen on MC production. Bottles were incubated in-situ at each site for 72 hours with daily sample collection. Four trials of MC loss experiments were conducted monthly in 2019 to understand if loss of MC was due to biotic factors. Triplicate two liters of whole (biotic treatment) and 0.2µm filtered lake water (filtered control) were spiked with 1µg/L of 15N-labeled dissolved microcystin LR to quantify loss rates of this congener under ambient incubation. Subsamples were collected every 3 hours for the first 24 hours, then at hour 36 and 48 for measurement of microcystins.
Production studies with nutrient amendments indicated various nitrogen sources did not significantly impact the production of MC during 2018 and 2019. Ambient rates of MC production were higher earlier in the bloom season and decreased over time. Across sites the near river mouth had higher rates of production. MC loss was concluded to be significantly due to biotic factors at both sites at all stages of the bloom with rates highest at the near river mouth location.