Eutrophication of lakes cannot be controlled by reducing nitrogen input: Results of a 37-year whole-ecosystem experiment

TLDR

Lake 227 has been fertilized for 37 years with constant phosphorus and decreasing nitrogen to test whether limiting nitrogen can control eutrophication. The study aims to show that reducing phosphorus inputs, rather than nitrogen, is necessary to control eutrophication. In the last 16 years, the lake received only phosphorus fertilization. Reducing nitrogen inputs stimulated nitrogen‑fixing cyanobacteria, allowing biomass to continue in proportion to phosphorus and keeping the lake highly eutrophic despite seasonal nitrogen limitation.

Abstract

Lake 227, a small lake in the Precambrian Shield at the Experimental Lakes Area (ELA), has been fertilized for 37 years with constant annual inputs of phosphorus and decreasing inputs of nitrogen to test the theory that controlling nitrogen inputs can control eutrophication. For the final 16 years (1990–2005), the lake was fertilized with phosphorus alone. Reducing nitrogen inputs increasingly favored nitrogen-fixing cyanobacteria as a response by the phytoplankton community to extreme seasonal nitrogen limitation. Nitrogen fixation was sufficient to allow biomass to continue to be produced in proportion to phosphorus, and the lake remained highly eutrophic, despite showing indications of extreme nitrogen limitation seasonally. To reduce eutrophication, the focus of management must be on decreasing inputs of phosphorus.