Shagawa Lake in northeastern Minnesota is an anomalously productive lake in a region dominated by many very fresh and unproductive lakes. It receives treated sewage from the town of Ely. Mine wastes once entered the lake.
A 1-m core of lake sediments was studied to ascertain the prehistoric trophic status of this lake and its response to mining, lumbering, and intensive human settlement in its watershed.
Fossil diatoms, Cladocera, and some detrital minerals were identified and counted at 10-cm intervals throughout the core. A pollen stratigraphy of the upper 30 cm enabled the lacustrine changes to be related to vegetational changes during the last 100 yrs.
The core contains three distinct zones. The lower zone (100 to 75 cm) is dominated by benthic and littoral diatoms and by a diverse assemblage of Cladocera. These features suggest that the littoral zone was more extensive or more productive or both than when the upper zones were deposited. The middle zone (75 to 25 cm) contains a planktonic diatom flora characterized by Cyclotella comta, a common form in the unpolluted lakes of this latitude. The upper zone (25 to 0 cm) contains abundant remains of Stephanodiscus minutus, Fragilaria crotonensis, and F. capucina, which are planktonic diatoms living today in the lake and in many highly enriched lakes to the south. This zone coincides with the appearance of hematite and limonite mineral grains that entered the lake when mining began in the area about 1890 and with an increase in ragweed (Ambrosia) pollen that marks land clearance and the beginning of European settlement. The cladoceran stratigraphy of the upper zone indicates that Chydorus sphaericus became much more abundant than Bosmina longirostris. This change was probably induced by the increasing severity and duration of blue-green algal blooms.
The Stratigraphic changes that occurred at the time of European settlement indicate that Shagawa Lake responded rapidly to increased nutrient levels caused by a variety of human activities in and around the lake.