Cyanobacteria phenology and toxicity across six Minnesota temperate lakes

Abstract

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 relationships among cyanobacteria abundances and composition, toxin concentrations, and nutrient conditions, we monitored six temperate Minnesota lakes with differing watershed land uses and lake morphometric characteristics across a latitudinal gradient. This project combined limnological approaches, comprehensive phytoplankton community analyses using taxonomic approaches, and advanced analytical characterization of toxin molecules to determine mechanisms leading to bloom formation and toxicity. Findings show that our study lakes had differing bloom phenologies influenced by different community assemblages and nutrient limitation states. The southern lakes had contrastingly different watershed land uses, such that Peltier Lake was predominantly urbanized, and Carrie Like was highly agricultural, which led differing nutrient growth conditions. Peltier (low N:P) experienced chronic surface blooms, whereas Carrie (extremely high N:P) did not. In Peltier Lake, the dominant cyanobacteria taxa present switched midsummer from nitrogen fixers (Dolichospermum spp.) to non-nitrogen fixers (Microcystis spp.) which tracked with decreasing nitrogen to phosphorus ratios. This community shift was counterintuitive based on changes in nutrient deficient growth conditions, suggesting that other drivers were likely impacting the shift in dominant cyanobacteria genera. Random Forest Models predicted major drivers of cyanotoxins in the bloom dominated lake, Peltier, to be in-lake growth conditions including dissolved organic carbon, soluble reactive phosphorus, and total phosphorus concentrations. Cyanotoxins were surprisingly detected in all lakes, including our least productive systems such as a northernly located lake, White Iron, and we observed a gradient of microcystin congeners present among our sample lakes. Cyanotoxin production can vary at the species (strain) level, therefore, it is essential to determine abiotic drivers of cHABs for various strains in differing lake types to properly inform management and mitigation of future system specific HABs.