Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere

The fluxes of CH4 and CO2 to the atmosphere, and the relative contributions of ebullition and molecular diffusion, were determined for a small hypertrophic freshwater lake (Priest Pot, UK) over the period May to October 1997. The average total flux of CH4 and CO2 (estimated from 7 sites on the lake) was approximately 52 mmol m(-2) d(-1) and was apportioned 12 and 40 mmol m(-2) d(-1) to CH4 and CO2 respectively. Diffusion across the air-water interface accounted for the loss of 0.4 and 40 mmol m(-2) d(-1) of CH4 and CO2 respectively whilst the corresponding figures for ebullition losses were 12.0 (CH4) and 0.23 (CO2) mmol m(-2) d(-1). Most CH4 (96%) was lost by ebullition, and most CO2 (99%) by diffusive processes. The ebullition of gas, measured at weekly intervals along a transect of the lake, showed high spatial and temporal variation. The CH4 content of the trapped gas varied between 44 and 88% (by volume) and was highest at the deepest points. Pulses of gas ebullition were detected during periods of rapidly falling barometric pressure. The relevance of the measurements to global estimates of carbon emission from freshwaters are discussed.