Lake 227, located in the Experimental Lakes Area of northwestern Ontario, stands as the world’s longest-running whole-lake experiment, providing definitive proof that controlling phosphorus is central to managing freshwater eutrophication. Beginning in 1969, researchers subjected the five-hectare lake to controlled nutrient additions. Despite radical shifts in nitrogen inputs over decades—including halting nitrogen additions entirely in 1990—the lake remained eutrophic, demonstrating that nitrogen-fixing cyanobacteria compensate for missing nitrogen while phosphorus continues to drive sustained algal blooms.
Experimental Design and Lake Characteristics in Northwestern Ontario
The Experimental Lakes Area allows scientists to study aquatic ecosystems at a whole-lake scale rather than relying on laboratory tanks or water bottles. Lake 227 covers approximately five hectares, has an average depth of 4.4 meters, and reaches a maximum depth of about 10 meters. This scale captures complex nutrient movements between water, sediments, plants, algae, microbes, fish, and the atmosphere. Researchers began fertilizing the lake weekly during the ice-free season in June 1969 to test how nitrogen and phosphorus affect algal growth and whether carbon limits productivity. By adding known quantities of nutrients rather than studying accidental pollution, scientists observed exact ecosystem responses over multiple seasons.
Understanding Eutrophication and Nutrient Dynamics
Eutrophication occurs when a lake receives excessive nutrients, such as nitrogen and phosphorus, from fertiliser runoff, sewage, or stormwater, stimulating unusually high plant and algal growth. While these essential nutrients support aquatic life in balanced amounts, over-enrichment causes algae to multiply rapidly and cloud the water. As dense algal blooms die, their decomposition consumes oxygen, creating low-oxygen or oxygen-free conditions in deeper water that threaten fish and other organisms. Certain blooms contain cyanobacteria, or blue-green algae, which produce toxins and form unpleasant surface scums that disrupt drinking-water supplies, recreation, and fisheries.
Early Fertilization Years and Carbon Limitation Tests
From 1969 to 1974, researchers added nitrogen and phosphorus at a weight ratio of roughly 12 to 1 to ensure adequate supplies while studying carbon limitation. Phytoplankton blooms immediately increased in direct proportion to the added phosphorus. These early results challenged prevailing assumptions about what limited algal growth. Although short-term tests indicated carbon could limit photosynthesis during parts of the summer, total algal biomass continued to rise in response to phosphorus inputs. The experiment proved that a nutrient can limit a process temporarily without controlling the final amount of biomass produced over a full season or several years.
Nitrogen Reduction and Cyanobacteria Compensation
In 1975, researchers lowered the nitrogen-to-phosphorus ratio in the Lake 227 fertilizer to approximately 4 to 1, and eventually stopped adding nitrogen entirely in 1990. This adjustment favored nitrogen-fixing cyanobacteria, organisms capable of converting atmospheric nitrogen gas into biologically available forms. This nitrogen fixation replaced the direct nutrient additions, maintaining the supply required for continued algal growth. A peer-reviewed study published in the Proceedings of the National Academy of Sciences confirmed that these cyanobacteria compensated for the missing nitrogen, validating the decades-long lesson that controlling phosphorus remains the primary requirement for controlling freshwater eutrophication.