The effects of rivers and manmade impoundments on freshwater phytoplankton communities in northern lower Michigan.

Abstract

We looked similarities in sets of phytoplankton species between Douglas Lake and Lake Kathleen in Cheboygan County, MI, connected by the Maple River. We also compared changes in density and diversity of phytoplankton flowing through a natural lake opening (between Douglas Lake and the upper Maple River) with changes in phytoplankton flowing across the Maple River Dam (separating Lake Kathleen from the lower Maple River). We took plankton tows and water samples at four sites: in Douglas Lake, downriver from Douglas Lake, in Lake Kathleen, and downriver from Lake Kathleen. By identifying species of plankton found in the tows and counting the number of cell/ml in our water samples using Palmer-Maloney cells, we obtained lists of species, their absolute densities, and overall diversity for each site. These data were compared between sites. The two lakes were found to support smiilar collections of species, likely indicating a communication of species via the Maple River. We found an increase in the density of plankters as they moved through the natural lake opening, wich was significant for diatoms (t=3.284, df=6, p=0.017) but not for soft algae. This may have been due to the colonization of the lake but not the river by zebra mussels. All plankton other than diatoms and Dinobryon sertularia droppped sharply in density as they crossed the dam (t=8.545, df=6, p<0.001), making for a considerable drop in diversity. We concluded that the turbidity of the dam-crossing likely eliminated many plankters from the river, while diatoms and D. sertularia were unaffected.