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Great Lakes Monitoring

Water Indicators
Lake Erie Phosphorus

Except in shallow bays and shoreline marshes, the Great Lakes were cool and clear before European settlement and industrialization. The lakes received small amounts of fertilizers such as phosphorus and nitrogen from decomposing organic material in runoff from forested lands. Small amounts of nitrogen and phosphorus also came in from the atmosphere. Following the arrival of European settlers, the Great Lakes ecosystem experienced drastic changes, as waves of immigrants logged, farmed and fished commercially in the region. As agriculture intensified after the turn of the 20th century and more people moved into urban areas around the lakes, the Lakes started showing signs of distress. In the mid 1900s, the combination of synthetic fertilizers, existing sources of nutrient-rich organic pollutants, such as untreated human wastes from cities, and phosphate detergents caused an acceleration of biological production in the lakes. In the 1950s, Lake Erie developed massive algal blooms and an area of depleted oxygen in the Central Basin of the lake, a "dead zone," where levels of oxygen in the bottom waters were too low for fish and other organisms to stay alive.

R/V Peter Wise Lake Guardian

Phosphorus is an essential element for all organisms and is often the limiting factor for aquatic plant growth in the Great Lakes. Although phosphorus is found naturally in tributaries and run-off waters, the historical problems caused by elevated levels have predominately originated from man made sources. Sewage treatment plant effluent, agricultural runoff and industrial processes have released large amounts of phosphorus into the Lakes.

Strong efforts begun in the 1970’s to reduce phosphorous loadings have been successful in also reducing nutrient concentrations in the Lakes, although high concentrations still occur locally in some embayments and harbors. Phosphorus loads have decreased in part due to the removal of phosphorus from detergents, changes in agricultural practices (e.g., conservation tillage and integrated crop management), and improvements made to sewage treatment plants and sewer systems. 

Lake Erie is the smallest of the lakes in volume and is exposed to the greatest effects from urbanization and agriculture. As mentioned before, Lake Erie was the first Great Lake to show evidence of lake-wide eutrophic imbalance with massive algal blooms and depletion of oxygen.

Lake Erie can be divided into three distinct basins based upon depth: a very shallow Western Basin, a deeper Central Basin, and a still deeper Eastern Basin.  Because of its shape and depth the Central Basin often experiences depletion of oxygen at its lower depths during the summer months. This depletion of oxygen is unacceptable for aquatic creatures which need oxygen in the water to live. In addition, phosphorus concentrations have been elevated in the Central Basin during the 1990's and cannot be solely explained by external total phosphorus loadings to the lake. Although the surface water quality goal is currently being met, the depletion of oxygen at its lower depths and potentially rising phosphorus concentrations may result in problems for the Central Basin of Lake Erie.

The results of the annual monitoring program carried out by the USEPA's Great Lakes National Program Office are shown below. Phosphorus levels decreased in the 1980's in response to phosphorus control measures, but began an upward trend in the 1990's which has continued to the present day. USEPA is leading an international study to determine the causes and solutions to this disturbing trend.

Disruptions of the food web by nuisance species can also affect surface water quality in the Great Lakes even without changes in external loadings of phosphorus. For example, the relationship between zebra mussels and phosphorus cycling is not fully understood, but in the vicinity of zebra mussel infestations, both food web dynamics and nutrient cycling have been greatly altered. Nuisance infestations of cladophora (coincident with the occurence of zebra mussels) occur in Lake Michigan and Lake Erie. Severe blue-green algae blooms in Lake Erie in recent summers may be related to zebra mussels interrupting normal biological and chemical processes.


International Joint Commission. Indicators for Evaluation Task Force. Indicators to Evaluate Progress under the Great Lakes Water Quality Agreement. Windsor, Ontario, April 1996, 82 pp.

International Joint Commission. Phosphorus Management Strategies Task Force. Phosphorus Management for the Great Lakes. Final Report to the Great Lakes Water Quality Board and Great Lakes Science Advisory Board. Windsor, Ontario, July 1980, 129 pp.

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