Rates of microcystin production and biodegradation in the western basin of Lake Erie

Here we studied microcystin (MC) production and degradation during two growth seasons (2018 and 2019) in two western Lake Erie sites, one site in Maumee Bay and the second in the center of the western basin. MC production and biodegradation rates were quantified using the microcosm approach. MC production rates were greater with elevated nutrients than under ambient conditions and were highest near shore during the initial phases of the bloom, and production rates were lower in later bloom phases. We examined biodegradation rates of MC-LR in natural waters by the addition of extracellular ¹⁵N₁₀-MC-LR (1 µg L^-1). Extracellular MC-LR remained stable in the abiotic treatment, and adsorption onto sediments was minimal, while extracellular MC-LR decreased in all biotic treatments, suggesting biodegradation. Greatest rates of extracellular MC-LR biodegradation (-8.76 d^-1) were observed during peak bloom conditions, while lower rates were observed with lower cyanobacteria biomass. Cell-specific incorporation of ¹⁵N₁₀-MC-LR measurements by NanoSIMS showed that a small percentage of the heterotrophic bacterial community actively degraded MC-LR. Our measured bulk rates of MC production and biodegradation, combined with the microscale analysis of MC incorporation by single cells, suggest that MC predictive models could be improved by incorporating MC production and biodegradation rates.