Green Lake (New York) - Biogeochemistry

Biogeochemistry

Fayetteville Green Lake (FGL) is located in the Oswego River-Lake Ontario drainage system; and receives surface water from Round Lake. The water column is chemically stratified into an oxygenated upper portion (mixolimnion) and a euxinic deeper portion (monimolimnion). The stratification is maintained through the large input of ground water rich in dissolved solids to the deeper portion of the lake. The difference in the dissolved solids (Ca, Mg, SO₄) between the water input to the surface and water input to the deep creates the permanent stratification observed. FGL has a sulfate concentration around 13.5 mmol/L (about half that of modern seawater) which is derived from groundwater input to the deep portion of the lake. A well-developed chemocline (a separation in a body of water due to differing chemistries) is present between 18–20 meters depth. The water below the chemocline is anoxic and sulfidic, and at the chemocline there is a dense layer of purple sulfur bacteria which makes the water appear pink. The purple sulfur bacteria thrive in the anoxic waters and utilize the sulfide from the deeper water in their chemosynthesis. The color of the lake is created by the large amount of total dissolved solids (from the groundwater input) which disperse the sunlight creating a greenish color. Annual whiting events occur in FGL, due to the large Synechococcus population in the lake. Calcite precipitates in microenvironments surrounding the Synechococcus and this is believed to dominate the whiting events. Calcite precipitation is common year round at FGL, and this leads to the carbonate crust that surrounds most of the lake, and covers branches and other material that falls below the surface. Dead Man's Point is a notable example of this, and is classified as a thrombolitic bioherm.