INTERACTIONS BETWEEN URBAN LAKE HYDROLOGY AND SEDIMENTS; AN EXAMPLE FROM SUMMIT LAKE, AKRON, OHIO

Summit Lake is an urban lake located in Akron, Ohio. Once used by industry, Summit Lake is currently being revitalized to provide urban recreation. It is important to study the lake’s overall health through chemical and physical processes to make sure it is suitable for increased recreational use. Summit Lake is dimitic, meaning the water column gets mixed twice a year. From May to October the lake is thermally stratified and the hypolimnion becomes anoxic. The anoxia results in the release of orthophosphate as phosphorus from the lake sediment into the water column. Throughout the warm months, orthophosphate as phosphorus increases in the hypolimnion reaching an average value of 1100 μg/L by October. As Akron works to reduce the number of combined sewage overflows and nutrients entering surface waters, it is important to characterize the internal nutrient loading of orthophosphate as phosphorus into the anoxic hypolimnion. During the sunny spring/summer months, as algal productivity and water temperature both increase, there is a drawdown of CO₂ in the epilimnion. These conditions result in the precipitation of calcite grains referred to as a whiting. Sediment cores from Summit Lake show rhythmic layering of white and brown mud. XRD, SEM, and optical microscopy reveals the white layers to be composed of ellipsoidal to rhombohedral, calcite grains about 5 μm long. The white layers are interpreted as deposition from the spring and summer whiting, and the brown layers are interpreted as deposition of organic matter through the remainder of the year. Thus, a white and brown sediment couplet is a varve deposited in one year. Because the layering has not been bioturbated, the varves provide a high-resolution chronology of the sediment deposit. When recently collected cores were correlated to year 2003 cores, there were 18 sediment couplets present in the 18 years between the two coring times thus supporting the varve interpretation. The varve chronology, resulting from the seasonal water cycle, allows sediment records of heavy metal pollution to be interpreted with great temporal resolution.