Paleolimnology of Douglas Lake, Michigan, with special emphasis on hypolimnetic oxygen conditions.

Abstract

The history of hypolimnetic oxygen conditions in three basins of Douglas Lake, Michigan was investigated using paleolimnological techniques. Douglas Lake is a multidepression lake of glacial origin in the northern lower peninsula of Michigan. Several investigations of hypolimnetic oxygen depletion rates indicated that depletion rates and the severity of eutrophication increased in the lake during the 20$\sp{\rm th}$ century. Cores were raised from three basins of the lake in 1989, dated using $\sp{210}$Pb activity and pollen stratigraphy, and analyzed for stratigraphic signals that are redox-sensitive. The solubility of Fe and Mn are redox dependent and the preservation of plant pigments is enhanced in reducing conditions. In addition, the remains of larval Chironomidae (Diptera) were investigated as potential paleoindicators of hypolimnetic oxygen conditions. The chemical profiles support the conclusions of other researchers that oxygen depletion rates are increasing. Fe and Mn accumulation rates increase and there is enhanced preservation of chlorophyll in the most recent sediments of two of the basins, representing the time interval following World War II. Development of housing around the lake also increased during this time. In the third basin, Fe and Mn accumulation and chlorophyll preservation has increased steadily since the region was deforested about 1880, suggesting increasing productivity and oxygen depletion rates throughout the 20$\sp{\rm th}$ century. Profundal chironomid fauna do not show much change during the time period following deforestation. In the third basin however, there is a distinct change from oxygen-requiring taxa to those tolerant of low oxygen at the deforestation horizon. It is concluded that chironomid assemblages have utility in cases where there is a complete change from oxygenated to anoxic waters, but are not as useful as indicators of gradual change in oxygen levels. Substantial differences in chemical and biological stratigraphy of the three cores supports the contention of early limnologists that the basins of Douglas Lake, whose hypolimnia are completely separated from each other by shoals, act almost as separate lakes.