The Great Lakes, interconnected freshwater seas, are a significant natural resource. These five lakes—Superior, Michigan, Huron, Erie, and Ontario—contain approximately 20% of the world’s surface fresh water and about 84% of North America’s surface fresh water. They span over 500 miles and serve as a vital source of drinking water for over 40 million people across the United States and Canada. They also support a diverse ecosystem and a substantial economy related to shipping, fisheries, tourism, and recreation.
Understanding Great Lakes Water Quality
Assessing the “cleanliness” of the Great Lakes is a complex evaluation based on various scientific indicators. Water clarity, for instance, is measured using a Secchi disk, lowered until it is no longer visible, providing an indication of light penetration and suspended particles. Dissolved oxygen levels are crucial, as fish and other aquatic organisms depend on oxygen for survival. Monitoring water temperature is another important physical parameter, influencing processes within the lakes.
Scientists also analyze concentrations of various pollutants, including nutrients like phosphorus and nitrogen, which can lead to excessive algal growth. Heavy metals, industrial chemicals, pharmaceuticals, and plastic waste are routinely monitored due to their persistent and toxic nature. The presence and impact of invasive species also factor into water quality assessments, as they can significantly alter aquatic ecosystems and biodiversity. Different metrics can yield varying results, making a singular cleanliness ranking challenging. A comprehensive approach provides a clearer picture of overall lake health.
The Cleanest Great Lake: A Ranking
Lake Superior is considered the cleanest of the five Great Lakes. Its immense volume, depth, and colder temperatures contribute to its superior water quality. Lake Superior is the largest freshwater lake globally by surface area and the deepest of the Great Lakes, with an average depth of 483 feet and a maximum depth of 1,333 feet. Its vastness and depth allow for greater dilution of pollutants, and its cold waters limit biological activity and algae proliferation.
The lake also has an exceptionally long water retention time, averaging 191 years, meaning water remains in the lake for nearly two centuries. This long retention time, combined with its volume, helps dilute contaminants, maintaining its pristine conditions. The other Great Lakes generally exhibit lower water quality. Lake Huron and Lake Michigan are often considered relatively cleaner than Lake Erie and Lake Ontario, which are impacted by more significant human activities and shallower depths.
Influences on Lake Health
The varied water quality across the Great Lakes stems from a combination of natural characteristics and human-related activities. Natural factors include each lake’s unique physical attributes, such as volume, depth, and water retention time. In contrast, Lake Erie is the shallowest of the Great Lakes, with an average depth of 60 feet, and has the shortest retention time, only 2.6 years. This makes Lake Erie particularly susceptible to rapid changes in water quality from incoming pollutants. Lake Huron has a retention time of 22 years, Lake Michigan 99 years, and Lake Ontario 6 years.
Human activities exert substantial pressure on the lakes’ health. Agricultural runoff, laden with excess fertilizers and manure, fuels harmful algal blooms, especially in Lake Erie’s western basin. Industrial discharges and legacy pollution, including heavy metals and industrial chemicals, have accumulated in sediments and continue to pose threats to water quality and aquatic life. Urban runoff and inadequately treated wastewater also contribute pollutants, leading to public health concerns. Invasive species, such as zebra and quagga mussels, have altered the food web and nutrient cycling, increasing water clarity but also contributing to nuisance algae. Climate change presents additional challenges, with warming temperatures, altered precipitation patterns, and reduced ice cover impacting water levels, increasing the frequency and intensity of harmful algal blooms, and making the lakes more hospitable for invasive species.
Protecting the Great Lakes’ Future
Efforts are underway to protect and restore the water quality of the Great Lakes, involving collaboration between various stakeholders. The United States and Canada formalized their commitment with the Great Lakes Water Quality Agreement (GLWQA), first signed in 1972 and updated several times, most recently in 2012. This agreement serves as a framework for both nations to restore and maintain the integrity of the Great Lakes ecosystem. The International Joint Commission (IJC) plays a key role in monitoring progress and advising governments on achieving the agreement’s objectives.
Specific programs, such as the Great Lakes Restoration Initiative (GLRI) in the United States, provide funding and support for projects aimed at addressing pollution, combating invasive species, and restoring habitats. These initiatives focus on reducing nutrient loads from agricultural and urban runoff, managing toxic contaminants, and preventing the introduction and spread of new invasive species. Continued stewardship, monitoring, and public engagement are essential for the long-term health of this irreplaceable freshwater resource.