MEPS 126:111-121 (1995)  -  doi:10.3354/meps126111

Seasonal variation in nitrification and denitrification in estuarine sediment colonized by benthic microalgae and bioturbating infauna

Rysgaard S, Christensen PB, Nielsen LP

Measurements of seasonal variation in oxygen fluxes, nutrient fluxes, and denitrification were obtained in an estuarine sediment inhabited by benthic microalgae and bioturbating infauna. Oxygen dynamics in the upper sediment strata were found to be controlled by the microalgae and there was a net flux of O2 out of the sediment during spring and autumn. High assimilation by the microalgae reduced the efflux of NH4+ and PO43- from the sediment to the water column during daytime. Denitrification based on NO3- from the water column (Dw) only occurred in winter and spring, when NO3- was present in the water column, and activity was proportional to the water column NO3- concentration. The rate of Dw was reduced during daytime when the upper oxic zone of the sediment increased due to O2 production by benthic microalgae. Coupled nitrification-denitrification (Dn) in the sediment was stimulated by the O2 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 column was low, benthic microalgae inhibited Dn by competing with nitrifying bacteria for NH4+. Dw accounted for 80% of the total denitrification during winter, while on an annual basis, Dw and Dn each accounted for 50% of the total denitrification activity. Benthic infauna, such as Corophium spp., Hydrobia spp., and Nereis spp., occurred in densities of up to several thousand ind. m-2 from May to October. Oxygen consumption, Dw and Dn were linearly correlated with the density of the amphipod Corophium spp., all the processes studied being stimulated by the pumping of O2- and NO3--rich water through the burrows in the upper 2 to 6 cm of the sediment. During summer, the Dn activity was, therefore, the net result of the inhibitory effect by benthic microalgae 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 waters during summer, when benthic infauna density is high, we conclude that the stimulatory effect of bioturbating infauna on both Dw and Dn is of minor importance to the annual denitrification budget.

Denitrification . Nitrification . Microalgae . Bioturbation . ISN regulation

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