Estimated CO2 emissions have been too low

Posted on March 28, 2013

Despite their small relative area, inland waters play an important role in the global carbon cycle by transporting, storing and transforming carbon from the terrestrial environment and releasing it to the atmosphere.  However, patterns of seasonal variation in the partial pressure of dissolved CO2  (and thus CO2 emissions from fresh waters) aren’t well understood.  For example, it’s unclear whether CO2 concentrations are influenced more by photo- and microbial-based transformations of aquatic carbon (which occur faster in sunny summer conditions), or by inputs of organic carbon from the catchment (which are low in summer because rain events are less common).  Seasonal fluctuations in the partial pressure of CO2 have been examined by reference to data on water chemistry and other environmental conditions collected from 851 lakes and 64 streams in Sweden and Finland between 1966 and 2010.   CO2 pressures in both lakes and streams were greatest in winter, when catchment inputs and rates water mixing were high.  At that time CO2 pressures were higher in lakes than in streams, but in summer the pattern was reversed and CO2  pressures were significantly lower in lakes than in streams.  This seasonal change was explained by the thermal stratification of lakes in summer, which inhibits water mixing and traps accumulated CO2 in the lower water layers.  In autumn, the stratification breaks down, water mixing is resumed and more CO2 is available near the surface to escape to the atmosphere.  Because most of the data available for carbon flux modeling have been collected in the summer months, it seems that previous estimates of the global flux of carbon from fresh waters have been too low.  

Reference:  Weyhenmeyer, G.A. et al.  2012.  Carbon dioxide in boreal surface waters: a comparison of lakes and streams.  Ecosystems 15, 1295–1307.