Lake toxin concentrations increase when phosphorus is reduced

Abstract Cyanobacterial harmful algae blooms (e.g. those of Microcystis) and their toxins (e.g. microcystins - MCs) threaten the safe use of lakes for drinking and recreation and this hazard is anticipated to worsen due to a warming climate. Present management approaches aim at lowering biomass by reducing the input of the growth-limiting nutrient, typically phosphorus. It is assumed that phytoplankton stoichiometry (MC per biomass) remains constant and that cyanotoxins decrease proportionally. However, lower biomass due to phosphorus reduction means an increase in other resources, like nitrogen and light, which stimulate MC production (we show here a comprehensive summary of published laboratory results). High MC cell quotas benefit toxin-producing strains within the community, causing the toxigenic fraction to increase. While reducing phosphorus may increase MC concentrations in lakes, there is currently insufficient evidence from natural field conditions to support this effect. Here, we present two analyses leveraging the National Lakes Assessment (NLA) dataset. The first focuses on changes in MC concentration within individual lakes across time. In the second analysis, we applied Generalized Additive Models (GAMs) across all lakes and time points. Both analyses indicate that lower phosphorus levels reduce biomass but increase available inorganic nitrogen, available light the MC/biomass ratio, which, in turn, can lead to peaking MC concentrations. Hence, field data from hundreds of lakes confirm that a reduction in phosphorus increases MC concentrations. However, when nitrogen is reduced together with phosphorus, both biomass and MC concentrations decrease. As such, there is a surging need to focus on dual reduction management.