The 2012 Great Lakes Water Quality Agreement states that “the Waters of the Great Lakes should be free from nutrients that directly or indirectly enter the water as a result of human activity, in amounts that promote growth of algae and cyanobacteria that interfere with aquatic ecosystem health, or human use of the ecosystem.”
Assessment highlights
In the 1980s and early 1990s, basin‐wide restoration efforts were successful in reducing high-levels of nutrients that were contributing to the formation of algal blooms, nuisance algae, and hypoxic (low oxygen) areas in the Great Lakes. There has been a resurgence of nutrient-related impairments due to impacts from invasive species, land use changes, climate change and other factors. Although nutrients and algal conditions for Lake Superior are generally Good, conditions remain Poor in Lake Erie and are Fair in lakes Michigan, Huron and Ontario. Overall, the Nutrients and Algae indicator is assessed as Fair and the trend is Unchanging.
Differing levels of nutrients are required to maintain each of the Great Lakes at their desired food web status. Only Lake Superior’s offshore phosphorus concentrations are considered Good. The offshore waters of lakes Michigan, Huron and Ontario have phosphorus concentrations below objectives for nutrient concentrations and food web status. Phosphorus concentrations remain above objectives in Lake Erie’s western and central basins and in some nearshore regions and embayments of each of the other lakes.
Elevated nutrient concentrations may lead to the formation of harmful algal blooms (HABs). HABs are made up of cyanobacteria, which sometimes produce toxins such as microcystin. These toxins can impact drinking water safety and may be harmful to people, wildlife and pets when present at high levels. Decomposition of large amounts of algae can also lead to hypoxic zones (such as in the central basin of Lake Erie), which can suffocate aquatic organisms and degrade habitat. The western basin of Lake Erie experiences the most consistent and widespread HABs in the Great Lakes and conditions are highly variable from year-to-year. However, the percent of Lake Erie nearshore area (areas less than 16 meters in depth) experiencing blooms declined between 2012 and 2020. In other areas of the Great Lakes such as Green Bay (Lake Michigan), Saginaw Bay (Lake Huron), Lake St. Clair, and in embayments such as Hamilton Harbour and the Bay of Quinte in Lake Ontario, HABs have also adversely impacted ecosystem health and recreational activities. Localized, short-lived blooms have also been observed in Lake Superior in recent years, but these blooms have been primarily confined to the immediate nearshore. Based on the geographical extent of blooms, conditions are considered Poor for Lake Erie, while nearshore conditions are considered Fair in lakes Michigan and Huron, and Good in lakes Superior and Ontario.
Cladophora is a native alga that provides food and shelter for invertebrates and small fish but that sometimes reaches nuisance levels over broad areas in the nearshore regions of lakes Erie, Ontario, and Michigan. Cladophora conditions in those lakes are considered Poor. Excessive Cladophora poses many problems including beach and shoreline fouling, clogging of municipal water intakes and can also impact tourism and recreational fishing. Cladophora washed up on shorelines may also harbor pathogens and create an environment conducive to the development of botulism outbreaks, which pose a risk mainly for fish and birds. The introduction of invasive dreissenid mussels has changed nutrient dynamics and contributed to increased water clarity in many areas of the Great Lakes promoting increased Cladophora growth. However, it has also been observed that large mats of Cladophora can persist despite low nutrient concentrations in the surrounding water which is further complicating the understanding and management of Cladophora.
Nutrients and algae levels are influenced by the timing and magnitude of nutrient inputs, effects of invasive species on nutrient cycling, weather, and climate change. A change to one or more of these influences has resulted in certain areas of the Great Lakes experiencing nuisance algal growth, HABs and hypoxia. Actions are being taken to address excess inputs of nutrients and to prevent further introductions and spread of invasive species. However, in some geographic areas of the Great Lakes, regular and routine monitoring does not occur, which limits our ability to determine the extent of the problem and track changes over time. Climate change in the form of warming water temperatures and increased frequency of extreme storm events may cause nuisance algae and HABs to form in areas where they have not typically occurred such as western Lake Superior.
Harmful Algal Blooms are frequently observed in Western Lake Erie, Green Bay, and Saginaw Bay but bloom extents are variable from year to year
Sub-indicators supporting the Nutrients and Algae assessment
Sub-Indicator | Lake Superior | Lake Michigan | Lake Huron | Lake Erie | Lake Ontario |
Good and Unchanging | Fair and Unchanging | Fair and Unchanging | Poor and Unchanging | Fair and Unchanging | |
Good and Undetermined | Fair and Unchanging | Fair and Unchanging | Poor and Improving | Good and Unchanging | |
Good and Unchanging | Poor and Unchanging | Fair and Undetermined | Poor and Unchanging | Poor and Undetermined |
Status
