CARBON-DIOXIDE FLUXES ACROSS THE AIR-WATER-INTERFACE AND ITS IMPACT ON CARBON AVAILABILITY IN AQUATIC SYSTEMS

Diffusion of CO2 across the air-water interface was analyzed with a mo del that simulates both transport and reaction of CO2 in a stagnant bo undary layer. The atmospheric C influx was determined in relation to s everal environmental variables: pH, total dissolved inorganic C, tempe rature, and the thickness of the stagnant boundary layer in relation t o ambient windspeed. We used the model to calculate the atmospheric CO 2 influx into six experimental ditches for a period of 6 or 8 months, starting in early spring. Three of the six ditches were dominated by a quatic macrophytes and three by benthic algae. Each series received th ree levels of external N and P input. A comparison with net C assimila tion during the same period, as estimated from continuous oxygen measu rements, showed that, especially in the ditches dominated by submersed macrophytes, a sizable fraction of the C requirements during this per iod could have been obtained from atmospheric CO2. In the ditches domi nated by benthic algae, this fraction was considerably less, but nonet heless substantial, and was related to the level of N and P loading. I ncreased primary production due to enhanced external N and P loading i ncreased the atmospheric C input due to the resultant higher pH values . The trophic state with respect to N and P and the availability of C are therefore interrelated.