Carbon dioxide fluxes across the air‐water interface and its impact on carbon availability in aquatic systems

Diffusion of CO, across the air-water interface was analyzed with a model that simulates both transport and reaction of CO, in a stagnant boundary layer. The atmospheric C influx was determined in relation to several environmental variables: pH, total dissolved inorganic C, temperature, and the thickness of the stagnant boundary layer in relation to ambient windspeed. We used the model to calculate the atmospheric CO, 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 aquatic macrophytes and three by benthic algae. Each series received three levels of external N and P input. A comparison with net C assimilation during the same period, as estimated from continuous oxygen measurements, showed that, especially in the ditches dominated by submersed macrophytes, a sizable fraction of the C requirements during this period could have been obtained from atmospheric CO,. In the ditches dominated by benthic algae, this fraction was considerably less, but nonetheless substantial, and was related to the level of N and P loading. Increased primary production due to enhanced external N and P loading increased 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 interrelate& In this paper we discuss the transport of CO2 across the air-water interface. Our aims are to quantify the cnhancement of the C influx due to chemical reactions and to establish the extent to which atmospheric C influx can contribute to the carbon needs of aquatic communities in relation to their trophic state. We also examine the ecological significance. The rate of photosynthesis by aquatic plants in surface waters can be limited by the availability of inorganic C, inorganic nutrients (N, P, and, in case of diatoms, Si), micronutrients, and light irradiance (Kirk 1983; SandJensen 1989). Eutrophication of surface waters can enhance primary production by removing P or N limitation. During periods of high primary production and u’nder saturating light conditions, the availability of dissolved inorganic C (DIC) may become rate limiting. Adams et al. (1978) found that photosynthesis of