Surprise Lake

Surprise Lake earns an A — water clarity, phosphorus, and chlorophyll-a all rate among the best monitored waters in Oconto County. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

A TSI value of 36 puts Surprise Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. A maximum depth of 20 ft puts Surprise Lake in the middle of Wisconsin's depth distribution. Surprise Lake is small — 66 acres alongside partial shoreline records — which makes it sensitive to even modest changes in watershed runoff or recreational pressure. Surprise Lake sits at rank 43 of 66 in Oconto County, in the lower half of the local distribution.

An invasive species record — Purple Loosestrife — has been logged at Surprise Lake; Wisconsin DNR maintains the official infested-waters list. Walleye are documented at Surprise Lake, one of 4 fish species on record for the lake. Public access is available — the lake is on the Wisconsin DNR public-access list. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.