Seasonal Dynamics of CO 2 Flux Across the Surface of Shallow Temperate Lakes Dennis Trolle, 1,2 * Peter A. Staehr, 3 Thomas A. Davidson, 1 Rikke Bjerring, 1 Torben L. Lauridsen, 1,2 Martin Søndergaard, 1 and Erik Jeppesen 1,2,4 1 Department of Bioscience, Aarhus University, Vejlsøvej 25, 8600 Silkeborg, Denmark; 2 Sino-Danish Centre for Education and Research (SDC), Beijing, China; 3 Department of Bioscience, Aarhus University, Frederiksborgvej 399, 4000 Roskilde, Denmark; 4 Greenland Climate Research Centre (GCRC), Greenland Institute of Natural Resources, Kivioq 2, 3900 Nuuk, Greenland ABSTRACT We used data collected from 1989 to 2009 from 151 shallow (mean depth < 3 m) temperate lakes in Denmark to explore the influence of lake trophic status, surface area and catchment size on the seasonal dynamics of the air–water flux of CO 2 . Monthly CO 2 fluxes were derived from measure- ments of acid neutralizing capacity (ANC), pH, ionic strength, temperature, and wind speed. CO 2 fluxes exhibited large seasonal variability, in par- ticular in oligo-mesotrophic lakes. Most of the lakes emitted CO 2 during winter (median rates ranging 300–1,900 mg C m -2 day -1 ), and less CO 2 during summer or, in the case of some of the highly eutrophic lakes, retained CO 2 during summer. We found that seasonal CO 2 fluxes were strongly negatively correlated with pH (r = -0.65, P < 0.01), which in turn was correlated with chloro- phyll a concentrations (r = 0.48, P < 0.01). Our analysis suggests that lake trophic status (a proxy for pelagic production) interacts with the lake ANC to drive the seasonal dynamics of CO 2 fluxes, lar- gely by changing pH and thereby the equilibrium of the free CO 2 and bicarbonate relation. Long-term observations from four lakes, which have all undergone a period of oligotrophication during the past two decades, provide further evidence that CO 2 efflux generally increases as trophic status decreases, as a consequence of decreased pH. Across these four lakes, the annual average CO 2 emission has increased by 32% during the past two decades, thus, demonstrating the strong link between lake trophic status and CO 2 flux. Key words: shallow lakes; air–water CO 2 flux; lake trophic status; recovery; eutrophication. INTRODUCTION Lakes can be viewed as sentinels tracking changes in their catchments due to their intimate contact with the surrounding landscape (Williamson and others 2008), and they are believed to function as hotspots for organic carbon storage and carbon dioxide (CO 2 ) exchange with the atmosphere (Cole and others 2007; Tranvik and others 2009). The CO 2 concen- tration in lakes is rarely in equilibrium with the overlying atmosphere and often exhibits super- saturated CO 2 levels (for example, Kling and others 1992; Cole and others 1994). Consequently, most lakes are net heterotrophic, which is due to the considerable supply and substantial degradation of Received 12 September 2011; accepted 22 November 2011; published online 20 December 2011 Author Contributions: DT and EJ conceived and designed the study; DT analyzed the data; DT wrote the paper together with EJ, PAS, and TAD; RB, TLL, and MS contributed to data analysis and commented on the manuscript. *Corresponding author; e-mail: dtr@dmu.dk Ecosystems (2012) 15: 336–347 DOI: 10.1007/s10021-011-9513-z Ó 2011 Springer Science+Business Media, LLC 336