Planned phosphorus reduction may make Lake Erie more toxic, model predicts

The plans to curb algal blooms in Lake Erie by reducing phosphorus runoff could have unintended consequences, potentially making the lake more toxic, according to a new study. The analysis, which relies on a type of model more commonly used in sociology and economics, suggests that both nitro­gen and phosphorus need to be reduced in Lake Erie to control algal blooms. Situated between major agricultural areas in the United States and Canada, Lake Erie experiences runoff that introduces high levels of nitrogen and phosphorus into the water system, resulting in massive blooms of photosynthetic organisms. Some of these algal blooms contain cyanobacteria known to produce powerful toxins that can harm aquatic animals and compromise drinking water. One such cyanobacterium, Microcystis, produces microcystin, a potent liver toxin implicated in the 2014 Toledo drinking water crisis. As an effort to curb excessive algal growth in Lake Erie, U.S. and Canadian government agencies have focused on reducing phosphorus loading. In fresh waters, phytoplankton growth is often limited by phosphorus availability. However, according to Ferdi Hellweger et al., this simple assumption does not address nor explain the ecology of toxigenic strains or their production of toxins, where nitrogen, temperature and stress from reactive oxygen species are important factors. Using data from a meta-analysis encompassing 103 studies on the biology of Microcystis, Hellweger et al. developed an agent-based model of cyanobacterial metabolism. The authors used this model to probe how toxin production changed across a range of nutrient and environmental conditions. The model predicts that phosphorus reduction alone could have harmful effects. While the reduction would likely lower total biomass, it would make nitrogen and light more available, boosting toxin production, promoting toxigenic strains, and increasing toxin concentrations. According to the authors, this mechanism may partly be responsible for the observed resurgence of cyanobacteria after historical phosphorus load reduction to Lake Erie and many other systems. “Based on the results reported by Hellweger et al., and on the increasing body of literature, the need to also consider nitrogen reduction should be assessed in the control of algal blooms,” write Irina Ofiţeru and Cristian Picioreanu in a related Perspective.