BIOGEOCHEMISTRY OF FOUR OHIO LAKES AND ITS IMPACT ON CARBON ISOTOPE GEOCHEMISTRY

Freshwater lake eutrophication has become a major environmental problem for many countries around the world. This study examined four freshwater lakes of varying trophic status in southwestern Ohio. They are Grand Lake St. Marys, Indian Lake, Kiser Lake, and the Main Lake of Crystal Lakes. The Main Lake is a small yet deep natural lake; all others are shallow reservoirs. The primary interest of our investigation was the comparison of different levels of eutrophication and its impact on carbon isotope fractionation. Since lighter ¹²C is preferential used by algae during photosynthesis, the residual dissolved inorganic carbon (DIC) is progressively heavier in carbon isotope composition. The question being, could carbon isotope composition of dissolved inorganic carbon be used as a proxy for trophic status in freshwater lakes? In answering this, the biogeochemistry and isotope composition of each lake was carefully evaluated during August of 2010. It was found that Grand Lake St. Marys had the highest δ¹³C (DIC) value and total phosphate concentration, but lowest nitrate concentration, among the four lakes. Grand Lake St. Marys was the most eutrophic lake among them and has suffered severe algal blooms in the past two years. The δ¹³C (DIC) values of the other three lakes did not show a clear trend in δ¹³C (DIC)–total P-Nitrate relationship. It was concluded that although carbon isotope composition of DIC is clearly related to primary production, it was not effective for assessing trophic status.