Lake 227 Experiment: Cutting Nitrogen Did Not Stop Algal Blooms, Phosphorus Remained the Driver

Scientists fertilized Lake 227 in Ontario starting in 1969 with both nitrogen and phosphorus. They cut nitrogen inputs in the 1970s and stopped adding nitrogen entirely in 1990, but they kept adding phosphorus. The lake stayed highly eutrophic and produced algal blooms every year. Nitrogen-fixing cyanobacteria replaced the missing nitrogen, so the added phosphorus kept driving algal growth. The experiment, part of the Experimental Lakes Area, is the longest running whole lake nutrient study in the world. It shows that reducing nitrogen alone may not control freshwater eutrophication when phosphorus remains available. For lake management, the practical takeaway is to prioritize phosphorus reduction in freshwater systems, while remembering that coastal and other ecosystems may respond differently.