Paleolimnological Investigation of Kabetogama Lake Cyanobacteria Blooms and Other Indications of Increased Trophic Status

Kling, H

This report documents the pre- and post-impoundment quantitative variation in cyanobacteria akinetes and other discernable microscopic fossils over a period of approximately 150 years in a short sediment core from Kabetogama Lake, Minnesota. The changes appear to be related to higher than normal water level fluctuations, climate change, and other anthropogenic activities that increased nutrient levels and resulted in a corresponding increase in numbers of eutrophic indicator taxa depositied since the 1960s in the Kabetogama Lake core. Both non siliceous and siliceous microfossils analyzed from an archived short core taken in 2001 from Kabetogama Lake of the Namakan Reservoir indicate that the lake, which has been under artificial lake level control for the past 100 years, has undergone some significant change over its recent history. The changes in non siliceous and siliceous microfossils indicate that the lake has undergone a shift in trophic status from what appears to have been a productive system with periods of summer nitrogen limitation alleviated historically by the presence of Anabaena (nitrogen-fixing bluegreens) prior to impoundment. Rather soon after impoundment, the maximum numbers of bluegreen remains (akinetes), especially those of Anabaena, occur in the core. An initial response to reservoir creation was also seen in the increased accumulation of cladoceran, protozoan, and thecate amoeba remains, from which an increase in food supply for these zooplankton organisms can be inferred. There seemed to be a lag period in the timing of increases in most of the other algal microfossils. Many of the dominant planktonic diatom taxa indicative of a relatively productive shallow lake system were present throughout the core but in lower abundance post-impoundment. Other taxa, particularly the araphid planktonic diatoms, historically found in the core in low numbers, have increased dramatically in the last 20 years, indicating a rapid change in trophic status during that period. Major increases in terrestrial remains, such as remains of higher plants, pollen, and charcoal, all of which are related to land clearing activity, fires, and flooding events at the turn of the century, correspond to the timing of early settlement in the area. The considerable variation in these types of terrestrial remains in the core post-impoundment may be a result of water level variation.