The primary environmental concern for the Great Lakes is eutrophication, which results from excessive nutrients causing algal blooms and ecological imbalance.

One serious environmental problem threatening the Great Lakes is eutrophication. It happens when too many nutrients, especially nitrogen and phosphorus, get into the water system, usually via industrial discharges, wastewater, and agricultural runoff. These nutrients promote the fast growth of algae, which results in toxic algal blooms that damage fish, lower oxygen levels, and disturb aquatic ecosystems. The natural balance of life in the lakes may be impacted by the dense algae's ability to block sunlight, which would kill underwater plants. Furthermore, certain algae create poisons that are dangerous to both people and animals. The health and sustainability of the Great Lakes are seriously threatened by eutrophication because of its effects on biodiversity and water quality.