BIOGEOCHEMISTRY OF CARBON AND SULFUR ISOTOPES IN CRYSTAL LAKE, WEST-CENTRAL OHIO

Crystal Lakes are a series of four interconnected mesotrophic, moulin-induced glacial lakes in west-central Ohio. The study site, Main Lake (a.k.a. Crystal Lake), is the largest and deepest lake among them. It is about 12.5 acres with a maximum depth of 11.9 meters and a mean depth of 3.8 meters. Thermal stratification develops during the warmer months.

Photosynthesis is the primary pathway for carbon and sulfur isotope fractionation in natural waters. Photosynthesizers present at Crystal Lake include green algae, diatoms, cyanobacteria, and purple sulfur bacteria (PSB). PSB utilize sulfide as an electron donor instead of water. A layer of concentrated PSB population exists between oxic and anoxic water in lakes where sufficient light and sulfide are present.

Field parameters collected in warmer months show turbidity and chlorophyll peaks around 6 m with variations caused by temperature, light, and nutrient availability. The dissolved oxygen minimum and the redox and sulfate maxima generally correspond with the turbidity and chlorophyll peaks, indicating the presence of a PSB layer. This layer occurs at the boundary between the metalimnion and hypolimnion. δ¹³C-DIC values on May 24, 2013 ranged from -4.88 ‰ near the surface to -10.25 ‰ at 11 m. δ¹³C-DOC had a narrow range of -26.47 ‰ to -28.20 ‰, indicating algae is likely the primary source of DOC throughout the water column. The estimated carbon isotope fractionation factor between algae and DIC during photosynthesis, α, is about 0.978. δ¹³C-DIC values are highest in the epilimnion and metalimnion, with a slight decrease from the surface to the boundary of the metalimnion and hypolimnion. A heavier peak at 6 m corresponds with the layer of PSB. This peak may be due to the influence of PSB on carbon isotope fractionation. δ¹³C-DIC sharply decreases below the peak indicating addition of light δ¹³C from the decay of organic debris.

The isotopic composition of different sulfur species, SO₄^2- , S⁰ and S^2-, and seasonal variation are being analyzed. Currently, no sulfur isotopic composition measurements of PSB or systematic studies of the impact of fractionation between sulfate and sulfide by PSB exist in the literature. The potential role of PSB on sulfur isotopic composition and their effect on the sulfur isotopic distribution in the lake system are currently in progress.