Lake Poygan

On the LakeGrade rubric Lake Poygan fails the cutoffs — a TSI of 64 and persistent algal pressure put it in the bottom tier. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

At a TSI of 64, Lake Poygan reads as eutrophic — nutrient-rich enough that summer algal growth and reduced clarity are expected, not unusual. A maximum depth of just 11 ft means sediment-bound phosphorus releases back into the water column whenever wind stirs the bottom — a classic shallow-lake dynamic. Lake Poygan covers 14,024 acres alongside partial shoreline records, large enough that conditions can vary meaningfully between bays. Within the 5 graded lakes of Winnebago County, Lake Poygan sits at rank 4, near the bottom of the county list.

Lake Poygan is on the Wisconsin infested-waters list for zebra mussels — boaters should follow clean-drain-dry protocols when moving gear to or from the lake. Walleye are documented at Lake Poygan, one of 8 fish species on record for the lake. A documented public access point at Lake Poygan makes the lake usable for shore fishing, paddle craft, and trailered boats. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.