Long-term trends of nutrients and trophic response variables for the Great Lakes

Based primarily on data collected over the past four decades by Environment Canada, long-term trends of eutrophication-related variables are developed for the offshore waters of the Laurentian Great Lakes. Trends of spring concentration are reported for the major nutrient species: phosphorus [total phosphorus (TP), and soluble reactive phosphorus (SRP)]; nitrogen [total oxidized nitrogen (NO3 + NO2), and ammonia nitrogen (NH3)]; and silica [soluble reactive silica (SiO2)]. Summer trends of surface chlorophyll a and Secchi depth are developed as indicators of lake trophic response. The results show that phosphorus has declined significantly in all the lakes, whereas nitrogen and silica have both increased. Along with documenting the impacts of the 1978 Great Lakes Water Quality Agreement phosphorus controls and the introduction of dreissenids, the results demonstrate conclusively that the offshore waters are now overwhelmingly phosphorus limited, which supports the conclusion that controlling phosphorus remains the only viable option for managing the trophic status of the Great Lakes offshore waters.