How does a phosphorus reduction affect microcystin concentrations? Evidence from field observations

Cyanobacterial harmful algae blooms and their toxins (e.g. microcystins) threaten the safe use of lakes for drinking and recreation and this hazard is expected to increase in a warming climate. This is an urgent environmental problem, but there is no scientific consensus about the best management approach. Reducing inputs of the growth-limiting nutrient phosphorus can be successful in decreasing cyanobacterial biomass, but the effect on toxin concentrations is unclear. Evidence from laboratory data and models suggests toxin concentrations may even increase. What has been missing is field data to support or refute this hypothesis. Here, we present two analyses of the National Lakes Assessment (NLA) dataset. The first focuses on changes in microcystin concentration within individual lakes over time. In the second analysis, we applied Generalized Additive Models (GAMs) across all lakes and time points. Both analyses show that when phosphorus only is reduced, microcystin concentration increases. A dual reduction of nitrogen and phosphorus, however, results in a decrease of biomass and toxins. This supports a management approach of dual nutrient reduction.