Atmospheric exchange of carbon dioxide in a low‐wind oligotrophic lake measured by the addition of SF₆

Abstract

Many freshwater lakes are supersaturated in CO, with respect to the atmosphere. This concentration gradient implies a net flux of CO, from 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 (SF,), 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-I (95% CI; normalized to a Schmidt number of 600); k,, was better correlated to precipitation events than it was to wind speed. Our data support the idea that gas exchange across the air-water interface is largely independent of wind at low wind speeds. The surface water of Mirror Lake was persistently supersaturated in CO, with respect to the atmosphere. During a 3.5-year period the partial pressure of CO, in the surface waters of the lake averaged 726 + 39 patm (95% CI) and showed substantial seasonal variation (360-2,000 patm). Diel and day-to-day variation in CO, were very small compared to the CO, pool. We combined our estimates of k with weekly measurements of the partial pressure of CO, to estimate CO, gas exchange in the lake. Mirror Lake released from 26 to 50 g C m-* to the atmosphere each year, depending on the method of calculating k. Atmospheric CO2 exchange 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 total dissolved inorganic carbon and 1.5 times as large as the export of dissolved organic C from the lake.

Carbon source/sink function of a subtropical, eutrophic lake determined from an overall mass balance and a gas exchange and carbon burial balance

“…Carbon budget studies using a gas exchange and carbon burial balance showed the ratio of gaseous carbon exchange across the watereair interface of most northern temperate lakes to be greater than 1, similar to the average global ratio for lakes which is also greater than 1 indicating that most lakes are a carbon source (Algesten et al, 2004;Cole and Caraco, 1998;Dillon and Molot, 1997;Hanson et al, 2004) (Table 3). In our study, a net carbon budget of 9955 t and a ratio of gas emission to carbon burial of only 0.08 clearly indicate that subtropical, eutrophic Lake Donghu is a great carbon sink.…”

“…Up until now, mass balance carbon budget studies have been heavily biased towards north temperate lakes (Charlton, 1977;Eadie and Robertson, 1976;Maier and Swain, 1978;Mcconnaughey et al, 1994;Quay et al, 1986) where gaseous carbon (CO 2 and CH 4 ) fluxes, in particular CH 4 , were usually not taken into account. However, CO 2 and CH 4 fluxes across the watereair interface are relatively high (Cole et al, 1994;Xing et al, 2005) and are important parts of whole-lake carbon budgets (Quay et al, 1986;Cole and Caraco, 1998).…”

Diurnal sampling reveals significant variation in CO2 emission from a tropical productive lake

“…Direct measurements of pCO 2 were taken by headspace equilibration according to Cole and Caraco (1998) with modification. Three 30 mL glass bottles (triplicates) were filled with 20 mL of lake surface water (0.5 m depth) and immediately capped and sealed with rubber and metal caps.…”

“…CO 2 fluxes across air-water were estimated using the following equation according to Cole and Caraco (1998) …”

Diffusive emission of methane and carbon dioxide from two hydropower reservoirs in Brazil

“…Furthermore, several authors suggest that different environmental variables are related to greenhouse gas emission from a reservoir, such as input of carbon species by rivers and streams (Del Giorgio et al, 1999;Tranvik et al, 2009), meteorological factors (Striegl and Michmerhuizen, 1998;Cole and Caraco, 1998), and biological influences (Dumestre et al, 1999(Dumestre et al, , 2002.…”