Air-water gas exchange and the carbon cycle of Green Bay, Lake Michigan

Abstract

The purpose of this study was to constrain estimates of the kinetics of gas transfer across the air-water interface as well as quantify the net flux of carbon between southern Green Bay (1635 km2) and the atmosphere. In 1994 and 1995, over 3500 measurements of surface water CH4 and CO2 were made using a continuous sample disk equilibrator. Estimates of CH4 flux from southern Green Bay to the atmosphere based on air-water concentration gradients, shear corrected wind speeds and the U/K (wind speed/transfer coefficient) relationship of Broecker et al. (1978) agreed to within ~10% of the estimate of CH4 influx from sediments and rivers (Klump and Fitzgerald (1998) and this study). Corrections for wind shear based on air-water temperature differences resulted in flux estimates that were ~30% higher than those based on a neutral drag coefficient of 1.3 x 10-3. The implied support for the U/K relationship of Broecker et al. (1978) suggests that the kinetics of air-water gas exchange are ~2.2 times higher than that predicted by the frequently used U/K relationship of Liss and Merlivat (1986). Southern Green Bay exported 13 x 107 moles CH4 yr-1 in 1994 and 16 x 107 moles CH4 yr-1 in 1995. Inter-annual differences in CH4 flux were shown to be largely due to dramatic differences in wind direction--which altered the hydrodynamics of the bay and ultimately, sediment temperatures. In Sturgeon Bay (a shallow, isolated section of the study site), spatially weight averaged CH4 concentrations rose by a factor of 2.1 for every 10°C increase in water temperature (r2 = 0.82); CH4 flux to the atmosphere increased by a factor of 1.8 (r2 = 0.46). Southern Green Bay exported 180 x 107 moles of CO2 to the atmosphere in 1994 and 240 x 107 moles of CO2 in 1995. However, the spatial and temporal direction and magnitude of flux were far from uniform. Using published rates of primary productivity, the ratio of a real primary productivity to heterotrophic respiration as a function of distance from the Fox River is presented along with a preliminary budget for allochthonous carbon inputs.