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The state of knowledge on harmful algal blooms of cyanobacteria in the Great Lakes

Cyanobacteria bloom in July 2020 at Wenona Beach on Saginaw Bay, Lake Huron.(As part of Lakes Appreciation Month, MI Environment features a story on harmful algal blooms by Michelle Selzer, lake coordinator with the Michigan Department of Environment, Great Lakes, and Energy’s Water Resource Division, from the State of the Great Lakes report.)

Harmful Algal Blooms (HABs) of cyanobacteria in freshwater systems like the Great Lakes are a growing threat to human and ecological health. The term HAB generally describes accumulations of cyanobacteria in amounts that are aesthetically unappealing and capable of producing algal toxins. While not all cyanobacteria produce toxins, significant blooms can still pose risks to human and ecosystem health.

Cyanobacteria, which live in both freshwater and saltwater environments, are among the oldest life forms on Earth. Since the mid-1990s, some waterbodies in the Great Lakes Basin have experienced an increase in the size, duration and frequency of toxic cyanobacteria HABs. In the case of the Western Basin of Lake Erie and Green Bay in Lake Michigan, these blooms can last all summer long. Beyond these well-known locations, HABs also occur in other areas, including Saginaw Bay and Georgian Bay (Canada) in Lake Huron; Sodus Bay (New York) and the Bay of Quinte (Canada) in Lake Ontario; the Canadian side of Lake St. Clair in the St. Clair Detroit River System; and in Thunder Bay (Canada) and near the Apostle Islands (Wisconsin) in Lake Superior. HABs are also common phenomena, unfortunately, in many inland lakes and ponds, particularly those with developed shorelines and watersheds.

The presence of cyanobacteria blooms in Lake Superior in recent years has been a surprising development because the lake is a cold, nutrient-poor Great Lake, relative to the other lakes where HABs frequently occur. These conditions are considered limiting for cyanobacteria. Research on Lake Superior points to the role of severe storms that cause flooded rivers to supply unusually high pulses of nutrients to the lake. Rivers may also play a critical role in “seeding” areas of the lake with cyanobacteria that can form a bloom under the right conditions. Research is ongoing to better understand conditions that may facilitate the development of these unusual blooms in Lake Superior, including changes in light penetration, temperature fluctuations, shifts in nutrient concentrations, and dynamic internal and external nutrient loading sources to the lake.

The drivers of HAB occurrences in the Great Lakes Basin are complex and may or may not be synergistic depending on factors such as the trophic state (i.e., the amount of biological productivity) of the lake. Known drivers of HABs of cyanobacteria in the Great Lakes include changes in agricultural management practices in the watersheds, extreme weather events occurring in the spring and drought conditions in the summer. Increased air temperatures warm the lakes, especially in the shallow nearshore areas, and may cause reduced ice cover in the winter months. The invasion of zebra and quagga mussels has resulted in the trapping of nutrients closer to shore. These nearshore areas are also often prone to significant wave action and sediment resuspension, which can release accumulated nutrients from the sediment and further stimulate algal growth.

The Great Lakes scientific community anticipates that climate change will increase air and water temperatures as well as modify precipitation patterns leading to more frequent extreme weather events in the Great Lakes region. This may lead to more intense and widespread HABs of cyanobacteria, although warmer winters could also produce less snow and reduce the loads of nutrients that are carried down by rivers during spring snowmelt. The interaction of these complex environmental factors and the importance of increased nutrient availability continue to be areas of valuable research, especially under increasingly unpredictable future climate scenarios. In addition to climate change factors, the fishery managers in the Great Lakes are beginning to research the long-term biological impacts of mussels on nearshore eutrophication and offshore nutrient delivery that supports the food web and the primary productivity of the Great Lakes fishery.

Although the understanding of HABs of cyanobacteria has increased considerably in recent years, additional ecological research and expanded environmental monitoring technologies are needed at different spatial and temporal scales to improve the understanding of climate change impacts and the role of internal and external nutrient sources fueling HAB occurrences in the Great Lakes. Expanding ecological forecasting, monitoring, and modeling for HABs across the lakes will also be critical to improve predictions of when and where HABs may occur, including their duration, severity, and toxicity.

As our knowledge improves, this information can be used to inform binational, federal, state and local resource management decision-making to address controllable nutrient sources and better inform the public about risks associated with HABs. Managers are taking a more holistic and adaptive approach that is designed to incorporate new research and lessons that have been learned about the impacts of previous management actions. This will be a key in our Great Lakes community’s ability to restore both the short- and long-term ecosystem health of the Great Lakes region and protect against future HAB occurrences in places like Lake Superior.


Caption: Cyanobacteria bloom in July 2020 at Wenona Beach on Saginaw Bay, Lake Huron.