Exploring long-term trends in microcystin toxin values associated with persistent harmful algal blooms in Grand Lake St Marys

•Grand Lake St Marys has experienced some of the most extensive and toxic HABs in North America with microcystin levels ranging from non-detect to well over 185 µg/L over the past decade.•This study used a Bayesian model to test relationships between environmental parameters and microcystin levels, finding week to week variations in toxin are primarily driven by in-lake conditions (internal nutrients and temperature).•In contrast, monthly variations in toxin are primarily driven by external nutrient loading in winter/spring months and internal loading in summer/fall months.•Week to week, there is little that can be done on short time scales to reduce toxin concentrations in summer as biomass and toxins are so high that even exorbitant nutrient reductions have low probability of an impact.•Long term, this study demonstrates that watershed management to reduce nutrient loading can slowly begin to improve water quality.