Wisconsin Lake Water Quality

1,283 of the 3,133 Wisconsin lakes in this index carry a recent clarity, phosphorus or algae reading, spread over 72 counties. Those readings come from the Wisconsin Department of Natural Resources, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, with no recent nutrient or algae sampling behind them. The Wisconsin DNR runs a Citizen Lake Monitoring Network that contributes a large share of the actual measurements. Coverage skews toward the lake-rich northern counties, Vilas, Oneida, and Sawyer, where lake associations have done decades of consistent Secchi-disk sampling. Most measured Wisconsin lakes are productive, often shallow, and frequently algal in late summer; each lake page shows its own phosphorus and chlorophyll-a readings.

By surface area the largest lake here is Lake Winnebago, at about 131,995 acres, and 264 of the lakes in this index cover more than 500 acres. Size is worth knowing before you go: a large deep basin buffers nutrient inputs by sheer volume, while a shallow one warms quickly and can turn over its whole water column in a single summer storm. Mesotrophic is the dominant trophic class among the measured lakes, a workable middle ground between the pristine oligotrophic deep lakes and the algal-pressured eutrophic shallow ones. The county with the lowest nutrient and algae readings on average is Outagamie. The gap between counties is large enough that a single figure for Wisconsin lake water quality hides real local variation.