Carbon Dioxide in Boreal Surface Waters: A Comparison of Lakes and Streams

The quantity of carbon dioxide (CO 2 ) emissions from inland waters into the atmosphere varies, depending on spatial and temporal variations in the partial pressure of CO 2 (pCO 2 ) in waters. Using 22,664 water samples from 851 boreal lakes and 64 boreal streams, taken from different water depths and during different months we found large spatial and temporal variations in pCO 2 , ranging from below atmospheric equilibrium to values greater than 20,000 μatm with a median value of 1048 μatm for lakes (n = 11,538 samples) and 1176 μatm for streams (n = 11,126). During the spring water mixing period in April/May, distributions of pCO 2 were not significantly different between stream and lake ecosystems (P > 0.05), suggesting that pCO 2 in spring is determined by processes that are common to lakes and streams. During other seasons of the year, however, pCO 2 differed significantly between lake and stream ecosystems (P < 0.0001). The variable that best explained the differences in seasonal pCO 2 variations between lakes and streams was the temperature difference between bottom and surface waters. Even small temperature differences resulted in a decline of pCO 2 in lake surface waters. Minimum pCO 2 values in lake surface waters were reached in July. Towards autumn pCO 2 strongly increased again in lake surface waters reaching values close to the ones found in stream surface waters. Although pCO 2 strongly increased in the upper water column towards autumn, pCO 2 in lake bottom waters still exceeded the pCO 2 in surface waters of lakes and streams. We conclude that throughout the year CO 2 is concentrated in bottom waters of boreal lakes, although these lakes are typically shallow with short water retention times. Highly varying amounts of this CO 2 reaches surface waters and evades to the atmosphere. Our findings have important implications for up-scaling CO 2 fluxes from single lake and stream measurements to regional and global annual fluxes.