SEASONAL-VARIATION IN NITRIFICATION AND DENITRIFICATION IN ESTUARINE SEDIMENT COLONIZED BY BENTHIC MICROALGAE AND BIOTURBATING INFAUNA

Measurements of seasonal variation in oxygen fluxes, nutrient fluxes, and denitrification were obtained in an estuarine sediment inhabited b y benthic microalgae and bioturbating infauna. Oxygen dynamics in the upper sediment strata were found to be controlled by the microalgae an d there was a net flux of O-2 out of the sediment during spring and au tumn. High assimilation by the microalgae reduced the efflux of NH4+ a nd PO43- from the sediment to the water column during daytime. Denitri fication based on NO3- from the water column (D-w) only occurred in wi nter and spring, when NO3- was present in the water column, and activi ty was proportional to the water column NO3- concentration. The rate o f D-w was reduced during daytime when the upper oxic zone of the sedim ent increased due to O-2 production by benthic microalgae. Coupled nit rification-denitrification (D-n) in the sediment was stimulated by the O-2 Production during winter and spring, at which times NO3- and NH4 were present in the water column in high concentrations. In contrast, during summer, when the concentration of NO3- and NH4+ in the water c olumn was low, benthic microalgae inhibited D-n by competing with nitr ifying bacteria for NH4+. D-w accounted for 80% of the total denitrifi cation during winter, while on an annual basis, D-w and D-n each accou nted for 50% of the total denitrification activity. Benthic infauna, s uch as Corophium spp., Hydrobia spp., and Nereis spp., occurred in den sities of up to several thousand ind. m(-2) from May to October. Oxyge n consumption, D-w and D-n were linearly correlated with the density o f the amphipod Corophium spp., all the processes studied being stimula ted by the pumping of O-2- and NO3--rich water through the burrows in the upper 2 to 6 cm of the sediment. During summer, the D-n activity w as, therefore, the net result of the inhibitory effect by benthic micr oalgae and the stimulatory effect of the benthic infauna. However, as the concentration of inorganic nitrogen in the overlying water and the sediment nitrification potential are both low in shallow coastal wate rs during summer, when benthic infauna density is high, we conclude th at the stimulatory effect of bioturbating infauna on both D-w and D-n is of minor importance to the annual denitrification budget.