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 reductions in nutrient loads were successful in decreasing high concentrations of nutrients that were contributing to the formation of algal blooms, nuisance algae, and hypoxic (low dissolved oxygen) areas in the Great Lakes. There has been a resurgence of nutrient-related impairments resulting from impacts of invasive species, land use changes, rising surface water temperatures, phosphorus levels, 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 Poor to Fair and the 10-year trend is Unchanging.
Only Lake Superior’s offshore phosphorus concentrations are considered Good. The offshore waters of lakes Michigan, Huron and Ontario continue to have phosphorus concentrations below objectives. Phosphorus concentrations remain above objectives in Lake Erie’s western and central basins and are elevated in some nearshore regions and embayments of each of the other lakes. Elevated nutrient concentrations can lead to algal blooms. Cyanobacteria algal blooms can produce toxins such as microcystin. These toxins can impact drinking water safety and are harmful to people, wildlife and pets when present at high levels. Decomposition of large amounts of algae from blooms can lead to the formation of 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 frequent and largest extent of cyanobacteria algal blooms in the Great Lakes with the bloom extent being highly variable from year-to-year. The percentage of the Lake Erie shallow-water area experiencing cyanobacteria algal blooms has not changed significantly from 2012 to 2023. 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, cyanobacteria algal blooms have also adversely impacted ecosystem health and recreational activities. The extent of those blooms has not changed in the past decade. Localized, short-lived blooms have also been reported in Lake Superior in recent years. These blooms tend to be isolated events not typically observable via the remote sensing method used for this assessment. Based on the spatial extent of algal blooms, conditions are considered Poor for Lake Erie, Fair in lakes Michigan and Huron, and Good in Lake Ontario. Status and trends in Lake Superior are Undetermined due to current limitations of the remote sensing method used to estimate bloom extent in the lake.
Cladophora is a native benthic 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. Conditions in those lakes are assessed as Poor. Excessive Cladophora poses problems including beach and shoreline fouling, clogging of municipal water intakes, and can also impact tourism and recreational fishing. Cladophora buildup 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 promoted nutrient retention in the nearshore and has contributed to increased water clarity in many areas of the Great Lakes, promoting increased Cladophora growth. Additionally, large mats of Cladophora can persist in areas with low nutrient concentrations in the surrounding water, which is further complicating the management of Cladophora.
Nutrients and algae levels are influenced by the timing and magnitude of nutrient inputs, effects of invasive species on nutrient cycling, extreme storm events and warming water temperatures. Changes to one or more of these factors can impact areas of the Great Lakes experiencing nuisance algal growth, algal blooms, and hypoxia. Actions are being taken to address excess inputs of nutrients and to prevent further introductions and spread of invasive species. Warming water temperatures and increased frequency of extreme storm events have the potential to impact algal bloom and nuisance algae dynamics, and these conditions and their impacts are being actively investigated by scientists.
Total phosphorus concentrations are highest in Lake Erie and other nearshore and embayment areas of the Great Lakes
Sub-indicators supporting the Nutrients and Algae assessment
Sub-Indicator | Lake Superior | Lake Michigan | Lake Huron | Lake Erie | Lake Ontario |
Nutrients in Lakes | Good & Unchanging | Fair & Unchanging | Fair & Unchanging | Poor & No Trend | Fair & Unchanging |
Algal Bloom Extent | Undetermined & Undetermined | Fair & Unchanging | Fair & No Trend | Poor & Unchanging | Good & Unchanging |
Cladophora | Good & Unchanging | Poor & No Trend | Fair & Undetermined | Poor & No Trend | Poor & No Trend |
Status
