Lake toxin concentrations increase when phosphorus is reduced

Abstract Harmful algal blooms are an urgent global societal problem threatening safe use of lakes for drinking and recreation, whereas the management solutions are hotly debated. Reducing phosphorous loading can be successful in decreasing cyanobacterial biomass but the effect on toxin concentration is unclear. The underlying mechanisms, laboratory data, limited circumstantial evidence from field observations and models suggest toxin concentrations may actually increase. What has been missing is field data to support or refute this. We analyzed timeseries data from hundreds of lakes (US National Lakes Assessment) and show that when phosphorus only is reduced, microcystin (the toxin produced by the cyanobacteria Microcystis) concentration increases. However, when nitrogen is reduced together with phosphorus, microcystin concentration decreases along with biomass.