Primary production in estuarine oxic/anoxic interfaces: contribution of microbial dark CO2 fixation in the Ebro River Salt Wedge Estuary

The present study assesses the contribution of dark carbon fixation to the primary production in the oxic/anoxic interface of a shallow estuarine envi ronment that develops a salt-water wedge with the presence of sulfide. Prim ary production was partitioned into oxygenic photosynthesis, anoxygenic pho tosynthesis and dark fixation. The results show the importance of dark fixa tion in the oxic/anoxic interface with values higher than 5 mg carbon fixed per cubic meter and per hour in some cases. The average rate of primary pr oduction in the dark during the anoxic season for the Ebro River salt wedge resulted in 42 mg C m(-2) d(-1) in the interface. This represents at least twice the contribution of oxygenic photosynthesis to the primary productio n in such interface. Because this process is probably important in other sa lt-wedge or highly stratified estuaries with oxic/anoxic interfaces contain ing sulfide, the estimates of carbon fixation made so far for these systems may have been underestimated, and should therefore be revised taking into account the contribution of dark processes.