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

The quantity of carbon dioxide (CO2) emissions from inland waters into the atmosphere varies, depending on spatial and temporal variations in the partial pressure of CO2 (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 mu atm with a median value of 1048 mu atm for lakes (n = 11,538 samples) and 1176 mu atm 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 CO2 is concentrated in bottom waters of boreal lakes, although these lakes are typically shallow with short water retention times. Highly varying amounts of this CO2 reaches surface waters and evades to the atmosphere. Our findings have important implications for up-scaling CO2 fluxes from single lake and stream measurements to regional and global annual fluxes.