Fossilized pigments as stratigraphic indicators of cultural eutrophication in Shagawa Lake, northeastern Minnesota

The recent cultural eutrophication of Shagawa Lake is reflected in the content of fossilized pigments in organic matter from a core of profundal sediment. Concentrations of chlorophyll derivatives and carotenoids in the organic matter of recently deposited sediment (1 to 10 cm deep) are three times as high as in the organic matter of sediment from the presettlement period (34 to 149 cm). Concentrations of the fossilized pigments prior to settlement also indicate that at that time Shagawa Lake was not among the more oligotrophic lakes on noncalcareous glacial drift in northeastern Minnesota. Owing to cultural eutrophication, it now appears to be the most eutrophic of the northeastern lakes and is probably somewhat more eutrophic than the average lake in Minnesota, although the ratio of sedimentary chlorophyll derivatives to sedimentary carotenoids remains characteristic of its original condition.