Caring for Green Lake

Green Lake is a treasured lake for Wisconsin and the Midwest. Measuring 236 feet at its greatest depth, it is the deepest natural inland lake in Wisconsin. Like many lakes, it faces the challenges posed by dense urban landscapes and increased agricultural pressures that can accelerate erosion and phosphorus loading.

For Green Lake, the visible result has been increased weed and algae growth. In 2014, the Wisconsin Department of Natural Resources determined that Green Lake’s phosphorus levels were high enough to cause low oxygen at certain lake depths, as high phosphorus levels can create dead zones. At some levels, Green Lake’s oxygen levels are below acceptable thresholds for fish and aquatic life based on national Clean Water Act guidelines.

The Green Lake Association (GLA) is proactively working to reverse these trends. Their goal is to not only improve water quality for Green Lake, but to develop a framework for cleaner lakes throughout Wisconsin and the Midwest where similar conditions exist.

A grant from the Community Impact Fund within the Oshkosh Area Community Foundation has supported the hiring of a full-time project manager to help GLA implement lake research and a phosphorus prioritization plan, encourage voluntary agricultural conservation practices, and develop an online watershed dashboard.

Using Citizen Scientists for data collection

“I’m very concerned about Green Lake’s water quality. Over 25 years, you see a lot of changes, including water clarity, frequency of algae blooms and fish habits. With a body of water this large, change is gradual – both for the good and the bad,” noted Citizen Scientist Wayne Nowicki who bought a house on Green Lake 25 years ago with his wife Geri.

Citizen Scientists, like Wayne, measure and record water clarity once a month throughout the summer using a black and white device called a Secchi disk. With the disk, volunteers can determine water transparency by lowering the disk into the water and measuring the depth at which the disk is no longer visible from the surface. Water clarity is an indicator of lake health and signifies the amount of algae in the water. High algae concentrations and poor clarity can negatively affect plant, insect, fish and wildlife communities.

Long-term water clarity monitoring can indicate trends in lake health, for better or for worse. Citizen Scientist data are used in combination with water clarity readings collected by the United States Geological Survey (USGS). These data allow tracking of the long-term lake health and seasonal fluctuations in algal blooms.

Digging deep into conservation practices

Over 60% of Green Lake’s drainage area is agricultural so farmers play an important role in helping to achieve a cleaner, healthier lake. Cover crops, retention ponds, grassed waterways, restored streams and buffers are examples of best management practices (BMPs) sprinkled throughout the Green Lake watershed. These conservation practices are quietly paving the way towards a cleaner, healthier Big Green Lake.

“We’re trying not to over apply nitrogen, phosphorus and [potassium] because it costs too much money and it has a really good chance of winding up [in nearby streams],” said Dave Wilke, local farmer and past president of the Green Lake County Farm Bureau.

By keeping soil in place, these practices are actively making a difference for Big Green’s waterways. Between 2012 and 2017, Green Lake BMPs have resulted in an estimated 3,900 pounds of phosphorus diverted from Big Green Lake. In terms of visible improvements, those BMPs prevented 1,950,000 pounds of weeds and algae from growing in the lake and tangling toes.

Measuring and communicating challenges and progress

The GLA has partnered with the Nelson Institute for Environmental Studies, an interdisciplinary center within the University of Wisconsin- Madison, to develop website that serves as a data and research clearinghouse. The dashboard web site compiles more than a century of work to assist researchers, inform the public and boost conservation efforts on behalf of Green Lake. View these hundreds of resources in one convenient spot at

Be Part of the Change

The Green Lake Legacy Initiative is just one of many projects supported by the Oshkosh Area Community Foundation’s Community Impact Fund, which combines dollars from generous donors who believe in giving in ways that address ever-changing community needs together. The fund generates grants to preserve the quality of life in Winnebago County, Waushara County, Green Lake County or the city of Ripon. You can be a part of making our community a great place to live for now and forever by donating cash, securities, IRA assets, real estate or other assets to help make more grant dollars available.


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