MICROELECTRODE STUDIES OF SEASONAL OXYGEN-UPTAKE IN A COASTAL SEDIMENT - ROLE OF MOLECULAR-DIFFUSION

Oxygen uptake rates and microdistributions of O2 in sediment of Aarhus Bay, Denmark, were studied over a year. Oxygen uptake was determined in the laboratory by 3 methods: (a) total O2 uptake in closed cores, (b) O2 flux through the diffusive boundary layer and (c) O2 consumption within the oxic surface zone. The O2 consumption rates in the oxic surface layer, calculated from (b) and (c) using molecular diffusion coefficients of O2, closely agreed and accounted for 70 % of the total O2 uptake. The O2 uptake rates strongly depended on in situ O2 concentrations in the overlying seawater, which in the bay varied from 100 % air saturation during winter to 23 % during summer. The potentially higher O2 respiration during the summer months was counteracted by a lower O2 concentration in the bottom water, which led to a small seasonal variation in O2 uptake. Depth of the oxic sediment zone reached 5.1 mm during winter, narrowed down rapidly to a few mm upon settling of a spring phytoplankton bloom, and was only 1.2 mm during summer. Modeling of O2 consumption from O2 microprofiles showed zero-order kinetics, i.e. constant O2 consumption rates throughout the oxic zone during winter. Enhanced O2 consumption was found at the oxic-anoxic interface during summer, presumably due to reducing solutes which diffused up from anoxic layers. Sediment O2 uptake was impeded by 3 to 5 % during winter and by 12 to 16 % during summer due to transport resistance through the 300-mu-m thick diffusive boundary layer. Results demonstrate the importance of the boundary layer for sediment O2 uptake and its regulation.