Biogeochemistry 49: 1–19, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere PETER CASPER 1 , STEPHEN C. MABERLY 2 , GRAHAME H. HALL 2 & BLAND J. FINLAY 2 1 Institute of Freshwater Ecology and Inland Fisheries, Department of Limnology of Stratified Lakes, Alte Fischerhütte 2, D – 16775 Neuglobsow, Germany; 2 Institute of Freshwater Ecology, Windermere Laboratory, Far Sawrey, Cumbria LA22 0LP, U.K. Received 21 August 1998; accepted 2 May 1999 Key words: carbon dioxide, diffusion, ebullition, emission, freshwater, hypertrophic lakes, methane Abstract. The fluxes of CH 4 and CO 2 to the atmosphere, and the relative contributions of ebullition and molecular diffusion, were determined for a small hypertrophic freshwater lake (Priest Pot, U.K.) over the period May to October 1997. The average total flux of CH 4 and CO 2 (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 CH 4 and CO 2 respectively. Diffusion across the air-water interface accounted for the loss of 0.4 and 40 mmol m −2 d −1 of CH 4 and CO 2 respectively whilst the corresponding figures for ebullition losses were 12.0 (CH 4 ) and 0.23 (CO 2 ) mmol m −2 d −1 . Most CH 4 (96%) was lost by ebullition, and most CO 2 (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 CH 4 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. Introduction Increasing atmospheric concentrations of the radiatively active gases CH 4 and CO 2 have stimulated research on their emission from terrestrial and aquatic environments (Conrad 1996; Segers 1998). On a global scale freshwater envi- ronments contribute more than 20% of the total CH 4 flux to the atmosphere (Khalil & Shearer 1993) and, because of their large area, wetlands are con- sidered to be the most important source. However, estimates of the emission of CH 4 and CO 2 from freshwater lakes (Rudd & Hamilton 1978; Miller & Oremland 1988; Kling et al. 1992; Smith & Lewis 1992; Cole et al. 1994;