- 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 conspiring to increase the 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 Conspiring 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 are conspiring to increase the risk of harmful algal blooms, particularly on Lake Erie.1 2 3 4
Lake Temperature and Lake Stratification
The density of water is dependent on temperature and varies with depth in the water column. Surface waters are usually warmer and less dense compared to bottom waters that are usually colder and denser. During summer months, the warming of surface waters reduces the likelihood that wind and wave action will be able to mix surface water with deeper water. This thermal stratification restricts the vertical movement of oxygen and nutrients. With greater frequency, strength and duration of stratification, the likelihood of expanded dead zones increase.
Climate change is already affecting temperature and stratification of both the Great Lakes and inland lakes in the region.
Lake Ontario and Lake Huron saw temperatures increase by 2.8°F (1.6°C) and 5.2°F (2.9⁰C), respectively, between 1968 and 2002.5 In Lake Superior, summer surface water temperatures increased by about 4.5°F (2.5°C) between 1979 and 2006.6 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 warm faster in the spring, reach higher temperatures in the summer, and cool more slowly in the fall.7
Increases in lake surface temperature could result in longer summer stratification of the water in the lakes.6 7 8 9 In 2007, stratification in inland lakes occurred earlier by more than five days than it did in 1961, which was related to lake surface temperatures and thermal storage, as well as the length of the stratified season.10 In Lake Michigan, the average stratification season between 1961 and 1990 lasted 135 days, but is projected to last 225 days by 2090. The longer stratification season would decrease the amount of winter-spring mixing and increase the chances of more low-oxygen, hypoxic conditions for lakes throughout the region.11 12
Nutrient Loading and Extreme Precipitation
Nutrient loading, and particularly phosphorous loading, are increased with inadequate farming and community land use practices. Overusing fertilizers, keeping livestock near water supplies, sewage discharges and run-off are all common contributors to high nutrient loading.
Heavy precipitation increases 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.13 14 15 16 17 18
Lake Erie Algal Blooms
While algal blooms are a growing concern on all the Great Lakes and in many inland lakes throughout the region, Lake Erie is particularly impacted. 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 dramatically reduced the amount phosphorus entering the Great Lakes.19. Lower phosphorus loads reduced algae growth, including an 89% drop in toxic blue-green algae.20 21 3 4
Toxic algal blooms have become common once again in the Western Lake Erie Basin since the 1990s, with significant blooms in 2003, 2004, 2005, and 2011.22 23 24 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.
2011 Algal Bloom
In 2011, Lake Erie experienced the largest harmful algal bloom on record. Economic damage was significant, with an estimated $2.4 million dollar loss in Ohio's recreational fishing industry alone and a $1.3 million dollar loss to Maumee Bay State Park due to lack of visitors.22 At it's peak, it was three times more intense than any previously observed bloom, 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 like the 2011 bloom are anticipated.25
Fish Kills and Hypoxia
Under normal conditions, algae are critical to the health of the ecosystem, providing the main source of energy that sustains many species of marine life. When algal blooms grow uncontrollably, however, low-oxygen, hypoxic conditions can be the result. When excessive organic matter from algal blooms sinks into bottom waters, it decomposes and reduces dissolved oxygen concentrations. With warmer temperatures and stratified lake temperatures that reduce vertical mixing, oxygen-depleted "dead zones" are created. When organisms are trapped in waters of suitable temperature that are oxygen-depleted, they die, sometimes resulting in massive fish kills.
Useful Algal Bloom Links
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 frequently asked questions regarding safety precautions during harmful algal blooms, click here.
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