Phenology of cyanobacteria bloom development and maintenance in nutrient-rich Minnesota lakes differing in nutrient limitation status

Cyanobacteria harmful algal blooms (cHABs) represent both chronic and emerging water quality threats in lakes globally and are the consequence of complex, interacting stressors. While we know that water temperature, nutrient loading and availability, and water column mixing conditions are important drivers of cHABs, the combination of abiotic conditions leading to bloom development, maintenance, and toxicity remain poorly understood across different lake types. To better understand the correlation of biovolume and toxin production to nutrient conditions, we monitored two southern Minnesota lakes of differing nutrient statuses. This project combined limnological approaches and cutting-edge molecular methods to determine mechanisms leading to bloom formation and toxicity. Findings show that our study lakes had differing bloom phenologies, along with the nitrogen limited lake having greater cyanobacteria biovolumes. The dominant cyanobacteria taxa present switched midsummer from nitrogen fixers (Dolichospermum sp.) to non-nitrogen fixers (Microcystis sp.) which tracked with decreasing nitrogen to phosphorus ratios. Shotgun metagenomic analyses showed microcystin genes were present among various genomes throughout the sampling season. Microcystin concentrations corresponded to the cyanobacteria biovolume and toxin gene presence. Cyanotoxin production can vary at the species level, therefore, it is essential to determine abiotic drivers of cHABs of differing lake types to properly inform management and mitigation of future system specific HABs.