Water splash from something dropping into lake

The quality and clarity of lake water can considerably impact how enjoyable it can be to live on a lake. Check out our lake pages to see how the DNR rates local lakes for clarity and quality.

Water Quality

The DNR classifies a lake’s water quality by its score on the trophic state index, which is calculated based on water clarity, chlorophyll concentration, and phosphorus concentration. Water quality classifications include oligotrophic (good), mesotrophic (fair), and eutrophic (poor).

Drawing of oligotrophic lake

Oligotrophic lakes are usually considered “good” for human enjoyment because the water is generally clear and low in nutrients, while the lake bottoms are generally firm and sandy. These conditions are great for swimming, diving, pleasure boating, clean shorelines, and supporting bigger (but fewer) fish. However, these same conditions might be labeled “poor” for sustaining large populations of fish, insects, and plant life, which may benefit fishing and biodiversity.

Drawing of eutrophic lake

Eutrophic lakes, on the other hand, are usually murky, contain more algae, have soft and mucky bottoms, and have more plants growing along the shorelines. These conditions are “bad” for human activities like swimming, diving, and pleasure boating. However, they do support more biodiversity and larger populations of smaller fish.

Drawing of mesotrophic lake

Mesotrophic lakes have characteristics that fall between oligotrophic lakes and eutrophic lakes. They comprise the majority of lakes in our area.

Water Clarity

The DNR measures a lake’s water clarity by the number of feet a Secchi disc can be lowered into the lake while remaining visible to someone on the surface.

Secchi disc being lowered into a lake to measure water clarity

Locally, water clarity varies from only one foot (Silver Lake in the Town of Farmington) to a whopping twenty-four feet (Pearl Lake in the Town of Leon). Water clarity is one factor used to determine if a lake is classified as oligotrophic, mesotrophic, or eutrophic.

Surface Water Quality, Wisconsin DNR
Understanding Lake Data