The amount of nitrogen and phosphorus in surface water has been reduced through purification of wastewater. Nonetheless, toxic algae blooms still flourish frequently. Because the purification plants are better at removing phosphorus than nitrogen, the balance between these two elements is shifting on a global scale. This results in a comparatively high level of nitrogen in the purified water, which causes abundant growth of of algae with a preference for nitrogen. Thus concludes an international team of researchers, which includes Wageningen University & Research, in a publication in PNAS.