Algae are microscopic organisms that naturally occur in aquatic environments, forming the base of many food webs. When conditions allow for excessive growth, these organisms can form harmful algal blooms that threaten ecosystems, human health, and local economies. Lake Erie, the shallowest and warmest of the Great Lakes, has experienced annual toxic algal blooms for over a decade. The western basin is particularly affected due to its shallow depth and proximity to nutrient inputs.
Nutrient Availability
Nutrient loading is a primary factor driving excessive algal growth in Lake Erie, with phosphorus being the key nutrient. Phosphorus acts as a limiting nutrient, meaning its availability often controls the extent of algal growth. When excess phosphorus enters the lake, it fuels rapid algal reproduction, leading to dense blooms. The timing of phosphorus delivery is also important, with spring runoff from the Maumee River being a strong predictor of bloom severity in the western basin.
Major sources of phosphorus include agricultural runoff, urban stormwater, and wastewater treatment plant discharges. Agricultural activities, such as applying fertilizers and manure, contribute significantly to non-point source pollution in the Lake Erie basin. Rainfall washes these phosphorus-laden materials into streams and rivers, which then carry them into the lake. About 85% of the phosphorus entering Lake Erie comes from agriculture, particularly dissolved reactive phosphorus.
Urban stormwater runoff also carries phosphorus from various sources like lawns and golf courses into the lake. While discharges from municipal wastewater treatment facilities are considered point sources that have seen reductions in recent decades, non-point source pollution from both urban and rural landscapes remains a substantial challenge. Nitrogen also contributes to bloom toxicity, though phosphorus is the primary focus of reduction efforts. Efforts are underway to reduce phosphorus inputs, with targets set by Canada and the U.S. to cut levels by 40% from 2008 levels.
Physical Water Characteristics
The physical properties of Lake Erie’s water play a significant role in influencing algal growth. Water temperature is a major contributor, as warmer temperatures promote faster growth rates for cyanobacteria, the blue-green algae forming harmful blooms in the lake. These blooms typically appear in July, peak in mid-August to early September, and can persist through mid-October as water temperatures remain elevated. The shallow western basin of Lake Erie heats up from surface to bottom, providing an expansive environment for cyanobacteria to thrive.
Light availability is another factor, as sunlight is important for photosynthesis by algae. Turbidity, or the cloudiness of the water caused by suspended sediment, can influence light penetration. However, certain cyanobacteria like Microcystis can float to the surface in calm water, allowing them to access sunlight even with increased sediment runoff. This surface accumulation can also concentrate toxins and intensify the green appearance of the bloom.
Water stratification, where layers of water with different temperatures form, can trap nutrients and create conditions favorable for certain types of algae. In stratified conditions, the warmer surface layer can become nutrient-rich, while deeper waters may experience oxygen depletion (hypoxia) due to the decomposition of dead algae. This lack of oxygen, sometimes called a “dead zone,” can negatively affect fish and other aquatic organisms. Calm winds also allow algal toxins to concentrate at the surface, increasing their potential harm.
Biological Interactions
Other living organisms within the Lake Erie ecosystem significantly influence algal growth, particularly invasive species like zebra and quagga mussels. These mussels, introduced in the early 1990s, have significantly altered the lake’s food web and nutrient cycling. They are highly efficient filter feeders, consuming phytoplankton and other particles from the water column. This filtering action increases water clarity and light penetration, which can favor certain types of algae that thrive in clearer conditions, such as the benthic alga Cladophora.
Beyond clarifying the water, these mussels also alter nutrient cycling by excreting nutrients in a form more readily available to harmful algal blooms. They shunt phosphorus and other nutrients from deeper offshore waters to the nearshore environment, where they become concentrated. This “nearshore nutrient shunt” makes it more challenging to control harmful algal blooms by recycling nutrients in bloom-prone areas. The waste produced by adult mussels can also be used as food by cyanobacteria, creating a feedback loop that exacerbates bloom conditions.
The presence of zebra mussels has been linked to an increase in the biomass of harmful cyanobacteria like Microcystis aeruginosa and the concentration of its toxins, even when nutrient levels are low to moderate. The overall impact of these invasive species has been to contribute to severe and persistent algal blooms in Lake Erie. Their ability to sequester and then release phosphorus nearer to the shore enhances conditions for harmful algae to flourish.
Climate Trends
Broader climate trends are exacerbating the conditions that promote algal growth in Lake Erie. Rising air and water temperatures are a significant factor, as cyanobacteria, the primary component of Lake Erie’s harmful algal blooms, grow faster in warmer waters. The surface temperature in the western basin of Lake Erie can reach 26 degrees Celsius (78.8 degrees Fahrenheit) or higher, extending the growing season for algae. This has led to blooms starting earlier in the year, sometimes as early as April, and lasting longer into the fall, even into November.
Increased frequency and intensity of extreme precipitation events also contribute to the problem. Heavy rainfall washes more nutrients, particularly phosphorus, from land into the lake via rivers and streams. This increased runoff provides a greater supply of the nutrients that fuel algal blooms. Climate models predict that a wetter spring and summer will result in even more phosphorus runoff, intensifying the problem.
Changes in ice cover patterns also influence water temperature and mixing. A warmer fall can also sustain warm water-loving cyanobacteria for longer periods. While a dry summer following spring nutrient input can lead to stable, stagnant water columns that allow cyanobacteria to thrive, overall warming trends and altered precipitation patterns are making Lake Erie more susceptible to prolonged and severe algal blooms.