Lake Ontario, the smallest in surface area but one of the deepest of the Great Lakes, is a massive freshwater reservoir shared by the United States and Canada. It serves as a source of drinking water for millions and a major hub for commerce and recreation. Determining if Lake Ontario is “clean” requires a detailed examination of various water quality measures, as the answer is not a simple yes or no. Conditions vary significantly between the deep, open waters and the nearshore areas close to human development.
Defining Water Quality in a Major Freshwater System
Water quality in Lake Ontario is scientifically assessed using physical, chemical, and biological indicators. Physical indicators include water clarity, often measured with a Secchi disk, and temperature profiles. Clarity in the offshore waters has increased in recent decades, partly due to the filtering action of invasive mussels.
The chemical profile focuses on parameters like dissolved oxygen, necessary to support aquatic life, and the concentration of nutrients such as phosphorus and nitrogen. Monitoring these levels helps scientists understand the risk of excessive plant growth and low-oxygen “dead zones.” Biological indicators track the health of the ecosystem, examining factors such as chlorophyll a (a measure of algae biomass) and the presence of fecal indicator bacteria like E. coli. This comprehensive approach establishes that “clean” is a relative term dependent on the intended use, such as drinking, swimming, or supporting a fishery.
Safety for Recreational Use
The primary concern for the public is the safety of direct contact with the water for activities like swimming and boating. Recreational safety is monitored by testing for fecal indicator bacteria; elevated levels of E. coli lead to beach closures. In Ontario, the recreational water quality guideline is a maximum of 200 E. coli colony-forming units per 100 mL of water.
Localized issues, particularly near urban centers and after heavy rainfall that causes runoff, are the main reason for specific advisories. Monitored U.S. beaches were safe for swimming approximately 95% of the time between 2018 and 2019, with Canadian beaches safe about 91% of the time. Harmful Algal Blooms (HABs) are another recreational concern, though they are less widespread in Lake Ontario than in other Great Lakes like Lake Erie.
HABs are limited primarily to specific, warmer embayments like Hamilton Harbour and the Bay of Quinte. These blooms, which can produce toxins, are less frequent and extensive compared to other Great Lakes. When present, they can cause skin irritation or gastrointestinal illness if the water is accidentally ingested.
Current Status of Consumption Safety
Drinking Water
Municipal drinking water drawn from Lake Ontario is safe for consumption due to rigorous treatment protocols. Water treatment plants employ a multi-barrier approach, including coagulation, filtration through materials like sand and activated carbon, and disinfection. This process removes particulates, chemical impurities, and microorganisms, ensuring the finished water meets strict quality standards.
The final step involves primary disinfection, typically with chlorine, followed by secondary disinfection using a residual, such as ammonia-stabilized chlorine, to keep the water safe through the distribution system. This system is highly effective; reports indicate that over 99.9% of tests on municipal residential drinking water in Ontario meet quality standards. Untreated raw water from the lake is not safe to drink directly.
Fish Consumption Advisories
The safety of eating fish from Lake Ontario is regulated through consumption advisories due to the bioaccumulation of contaminants in fish tissue. Polychlorinated biphenyls (PCBs) and mercury are the primary legacy contaminants driving these warnings. Though PCB levels have declined dramatically (by as much as 90% for some species since the 1970s), they remain above the benchmarks for unrestricted consumption.
Advisories are highly specific, varying based on the species, size, and location where the fish was caught. Mercury levels have also decreased by half over the past four decades, but they remain a factor in advisories. Emerging contaminants, such as per- and polyfluoroalkyl substances (PFAS), are now being monitored and can prompt new consumption restrictions.
Special guidance is issued for sensitive populations, including children under 15 and women who are or may become pregnant. This guidance recommends eating less or avoiding certain species entirely due to the potential for contaminants to affect development and health.
Primary Environmental Stressors Affecting Cleanliness
The main factors preventing Lake Ontario from achieving a pristine state are nutrient inputs, invasive species, and historical contamination. Nutrient loading, specifically phosphorus and nitrogen, primarily comes from agricultural runoff and wastewater discharges. This excess nutrient flow causes Harmful Algal Blooms (HABs) in nearshore areas and embayments.
Invasive species, such as zebra and quagga mussels, have fundamentally altered the lake’s ecosystem. These mussels filter the water, increasing clarity in offshore regions, but they also redistribute nutrients, leading to increased localized issues near the shore. Legacy contaminants, including the PCBs and mercury that led to the fish advisories, persist in the lake’s sediments from decades of industrial activity. Although banned, these contaminants continue to cycle through the food web, stressing the ecosystem.