Atmospheric Exchange of Carbon Dioxide in a Low-Wind Oligotrophic Lake Measured by the Addition of SF6

• Published in: Limnology and oceanography 1998-06, Vol.43 (4), p.647-656
• Main Authors: Cole, Jonathan J., Caraco, Nina F.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Carbon dioxide; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fresh water ecosystems; Fundamental and applied biological sciences. Psychology; Geochemistry; Groundwater; Lakes; Lentic systems; Methane; Mineralogy; Precipitation; Rain; Silicates; Surface water; Synecology; Water geochemistry; Water temperature; Wind velocity
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.1998.43.4.0647

Many freshwater lakes are supersaturated in CO2with respect to the atmosphere. This concentration gradient implies a net flux of CO2from the water to the air. The actual rate of gas exchange is governed by both this concentration gradient and the gas transfer coefficient, K. To directly measure k, we added the chemically and biologically inert gas, sulfur hexaflouride (SF6), to the epilimnion of Mirror Lake, New Hampshire, a small (15 ha), low-wind softwater lake. K was independent of wind speed over the 50-d summer stratification period and averaged 2.65 ± 0.12 cm h-1 (95% CI; normalized to a Schmidt number of 600); K800was better correlated to precipitation events than it was to wind speed. Our data support the idea that gas excahnge across the air-water interface is largely independent of wind at low wind speeds. The surface water o fMirror Lake was persistently supersaturated in CO2with respect to the atmosphere. During a 3.5-year period the partial pressure of CO2in the surface waters of the lake averaged 726$\pm 39 \muatm$(95% CI) and showed substantial seasonal variation (360-2,000$\muatm$). Diel and day-to -day variation in CO2were very small compared to the CO2to estimate CO2gas exchange in the lake. Mirror Lake released from 26 to 50 g C m-2 to the atmosphere each year, depending on the method of calculating k. Atmospheric CO2exchange is a large term in the C economy of the lake-the most conservative gas flux estimate is about four times as large as outflow plus seepage of toal dissolved inorganic carbon and 1.5 times as large as the export of dissolved organic C from the lake.