SULFUR AND CARBON CYCLING IN THE MONIMOLIMNION OF MEROMICTIC GREEN LAKE, NY

Fayetteville Green Lake (FGL), NY is a well-studied meromictic lake. The 54m depth of this glacially carved basin and groundwater influx from distinct levels have likely sustained the separation of the mixolimnion and monimolimnion since its formation. One of the most interesting features is the presence of an 18m deep mat made up of photosynthetic sulfur bacteria that accounts for most of the lake productivity. Both purple and green sulfur bacteria are present and result in a very distinct purple water horizon at times. These bacteria utilize H₂S as an electron donor instead of H₂O and produce elemental sulfur instead of O₂. The production of H₂S has been attributed to SO₄ reduction taking place in the anoxic monimolimnion.

In June 2015, we sampled FGL to place more constraints on the sulfur and carbon cycles. Samples were taken every 3m and sampled for SO₄, H₂S, DIC, DOC, and CH₄. The measured δ³⁴S of SO₄ and H₂S is nearly identical to measurements taken by Deevey et al. 1963 and by Zerkle et al. 2010 suggesting a very stable sulfur cycle exists in the lake. δ³⁴S_SO4 increased with depth from 24.8‰ to 33.2‰. The δ³⁴S of H₂S also increased with depth from -29.8‰ at 22m to -22.9‰ at 52m and showed an apparent sulfur isotopic fractionation of 57‰ between SO₄ and H₂S, much larger than the 48‰ microbial fractionation expected from laboratory experiments. δ¹⁸O_SO4 also increased with depth from 11.3‰ at 4m to 13.9‰ at 52m. To our knowledge the oxygen isotopic values of SO₄ in FGL have not been reported before. There is a strong linear relationship between δ³⁴S_SO4, δ¹⁸O_SO4, and depth suggesting an increasing importance of microbial sulfate reduction with depth. δ¹³C of DIC was likewise nearly identical to that reported in Deevey et al. 1963, suggesting a very stable carbon cycle in the monimolimnion of FGL. δ¹³C_DIC was nearly constant at -9‰ throughout the mixolimnion and then decreased to value of -19.3‰ at 52m. DIC concentrations increased with depth as was previously observed by Deevey et al. 1963, Takahashi et al. 1968, and Brunskill 1969. We propose a simple model where sulfur is reduced in the sediment rather than in the water column followed by diffusion of H₂S and HCO₃ up to the chemocline. Photosynthetic sulfur bacteria then convert H₂S and HCO₃ to elemental sulfur, pyrite, and biomass which eventually settle back to the sediment to be recycled.