Duluth Lake

Duluth Lake earns a D — measurements through 2025 show persistent nutrient pressure that limits clarity through the summer. Secchi readings are the weakest of the three parameters, suggesting suspended sediment or algal biomass dominates the optical signal.

A TSI in the eutrophic range signals a lake doing what nutrient-loaded waters do — supporting an active algal community at the cost of clear water. The lake's maximum depth is not yet documented in state morphometric records — context for its physical structure remains limited. The lake's surface area is not consistently recorded across state datasets — physical context remains partial. Within Douglas County's 44 graded lakes, Duluth Lake ranks 31 — below the local median, though not at the bottom.

An invasive species record — Ruffe — has been logged at Duluth Lake; Wisconsin DNR maintains the official infested-waters list. The state fisheries records do not list documented species for Duluth Lake, which usually reflects a lack of formal fisheries survey work rather than an empty lake. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.