Algal Blooms


  • The Great Lakes have warmed faster than nearby air temperatures, leading to longer warm seasons and prolonged stratification.
  • More total and intense precipitation is increasing runoff and combined sewer discharge, leading to greater nutrient loads in the lakes.
  • Warmer temperatures, prolonged stratification, and increased nutrient loading are leading to increased occurrence of harmful algal blooms.
  • Hypoxic “dead zones” can result when algal blooms sink, decompose, and reduce dissolved oxygen concentrations. A greater risk of algal blooms may increase the incidence of hypoxia and fish kills.

Climate Change and the Contributing Causes of Algal Blooms

Harmful algal blooms produce dangerous conditions for humans, fish, and wildlife that can lead to fish kills and beach closures. Rising carbon dioxide concentrations, warming lake temperatures, a longer stratified-lake season, increasing extreme precipitation, and an abundance of nutrients all combine to increase the risk of harmful algal blooms, particularly on Lake Erie. 1 2 3 4

The contributing factors that lead to algal blooms. These factors may amplify algal bloom occurrences due to a changing climate.

Lake Temperature and Lake Stratification

Lake stratification is a phenomenon that occurs from late spring to early fall in temperate climates. The phenomenon arises largely due to the dependence of the density of water on temperature. Warming of the water’s surface by the sun causes distinct thermal layers to form: cooler, denser water settles to the bottom of the lake, while warmer, lighter water forms a layer on top. Lake stratification depends on many factors such as lake depth, shape, and size. For example, shallower lakes may not experience stratification because wind allows mixing, while the combination of depth and geography of Lake Erie regularly produce stratification. Stratification can exacerbate the negative impacts of harmful algal blooms. As hypoxic “dead zones” form beneath the lake’s surface due to the decomposition of algae, lack of mixing allows these zones to persist for long periods of time.

Climate change will exacerbate these effects even further, and it is already affecting temperature and stratification of both the Great Lakes and inland lakes in the region.

Lake Ontario and Lake Huron saw summer surface water temperatures increase by 2.9°F (1.6°C) and 5.2°F (2.9⁰C), respectively, between 1968 and 2002. 5 In general, these trends are expected to continue or accelerate, though the rate of warming may slow following significant declines in lake ice cover. Future lake temperatures are projected to warm faster in the spring, reach higher temperatures in the summer, and cool more slowly in the fall. 6

Increases in lake surface temperature could result in longer summer stratification of the water in the lakes. 7 8 9 10 Inland lake stratification occurred earlier in 2007 by more than five days compared to 1961 due to the lake surface temperatures, thermal storage, and length of the stratified season. 11

Visual of the stratification processes for lakes.

Fish and Hypoxia

Under normal conditions, algae are critical to the health of the Great Lakes ecosystem, providing the main source of energy that sustains many species of marine life. However, when algal blooms grow uncontrollably, they can create low-oxygen, hypoxic conditions. When excessive organic matter from algal blooms sinks into bottom waters, it decomposes and reduces dissolved oxygen concentrations. Warmer temperatures and stratified lake temperatures, that reduce vertical mixing, create the conditions for algae to grow out of control and cause oxygen-depleted “dead zones.” Massive fish kills result from these organisms being trapped in warm-temperature waters that are oxygen-depleted.

Warm-temperature waters are oxygen depleted which lead to many fish deaths. Photo provided by Michigan Sea Grant.

Nutrient Loading and Extreme Precipitation

Inadequate farming and community land use practices increase nutrient loading, in particular phosphorus loading. Overusing fertilizers, keeping livestock near water supplies, sewage discharges, and run-off are all common contributors to high nutrient loading. Heavy precipitation increases levels of contaminated runoff and nutrient loading in lakes and major waterways. Current vulnerabilities to extreme and sustained precipitation that encourage algal blooms, such as agricultural runoff and combined sewage overflows, will be amplified with more total and extreme precipitation. Models project that the observed increases in the frequency and intensity of heavy downpours in the region will continue in the future. 12 13 14 15 16 17

Lake Erie Algal Blooms

While algal blooms are a growing concern for all the Great Lakes and in many inland lakes throughout the region, Lake Erie is particularly impacted because of its shallow depth, warm surface temperatures, and proximity to agricultural land for nutrient runoff. Following the occurrence of algal blooms and other environmental concerns on Lake Erie in the 1960s, the U.S. and Canada signed the 1972 Great Lakes Water Quality Agreement, and this agreement enforced dramatic reductions in the amount of phosphorus entering the Great Lakes. 18 Lower phosphorus loads reduced algae growth, including an 89% drop in toxic blue-green algae. 19 20

Toxic algal blooms have become common once again in the Western Lake Erie Basin since the 1990s, with significant blooms in 2003-2005, 2011, and 2015. 21 22 23 Blooms typically form throughout the late summer and can persist into the fall. Surface scum that washes ashore often results in foul-smelling, decaying, mats of algae. In such events, beaches and recreational boating areas can become unusable and fishing economies are severely affected. 

Satellite imagery of the October 2011 Lake Erie algal bloom. Image provided by the NASA Moderate Resolution Imaging Spectroradiometer (MODIS).

Recent Significant Algal Blooms and Economic Impacts

The most recent significant algal blooms in Lake Erie occurred in 2011, 2014, and 2015. The 2011 bloom was the largest harmful algal bloom on record in Lake Erie (severity index of 10) until it was eclipsed in 2015 (severity index of 10.5). While the 2015 bloom remains the largest on record, algal blooms in 2017 (severity index of 8) and 2019 (severity index of 7.3) also rated as relatively severe. The severity index as defined by NOAA is based on the amount of a bloom’s biomass over a sustained period. 24 According to NOAA, harmful algal blooms cause approximately $82 million in economic losses each year from losses in fishing and tourism in the region. 25 A recent study of the Canadian Lake Erie basin estimates that harmful algal blooms will cause annual losses of $272 million (in CAD) in 2015 prices over a 30-year period if action is not taken. 26 

An analysis of the 2011 bloom found that long-term trends in agricultural practices and spring storm activity increased phosphorus loading to the western basin of the lake and produced record-breaking nutrient loads. The combination of these factors are consistent with expected future conditions.. Without scientifically guided management plans, an increasing risk of algal blooms in the coming years is anticipated. 27

The bloom severity index for 2002-2019, and the index is based on the amount of biomass over the peak 30-days. Image provided by NOAA National Centers for Coastal Ocean Science (NCCOS).

Useful Resources on Algal Blooms

From the Center of Excellence in Great Lakes and Human Health:

An experimental harmful algal bloom bulletin has been developed to provide a weekly forecast for Microcystis blooms in western Lake Erie. When a harmful bloom is detected by the experimental system, scientists will issue the forecast bulletin below. The bulletin depicts the bloom’s current location and future movement, as well as categorizes its intensity on a weekly basis. Click here to view the bulletin.

For information regarding safety precautions during harmful algal blooms, click here.


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