Seasonal dynamics of benthic O-2 uptake in a semienclosed bay: Importance of diffusion and faunal activity

The benthic O-2 uptake and the O-2 microdistribution in a coastal sediment of Aarhus Bay, Denmark, were investigated during a seasonal study. Measurements were performed in situ by a profiling lander and a flux chamber lander, as well as on recovered sediment cores. The O-2 penetration depth, the diffusive O-2 uptake, and the volume-specific O-2 consumption rate strongly depended on the seasonal changes in bottom water O-2 concentration and the sedimentation of organic carbon. The in situ O-2 penetration depth varied between 0.5 mm in summer and 4.5 turn in winter. The diffusive O-2 uptake varied between 8 and 30 mmol m(-2) d(-1), whereas the volume-specific O-2 consumption rate varied by a factor of 13. The O-2 distribution was very sensitive to environmental controls, and microprofiles obtained in the laboratory tended to overestimate the in situ O-2 penetration depths and underestimate the in situ diffusive O-2 uptake. Three-dimensional O-2 flux calculations based on in situ microtopographic mapping showed that the actual diffusive exchange rate was similar to10% higher than the simple one-dimensional, microprofile-derived diffusive O-2 exchange. The total O-2 Uptake measured in the laboratory showed less distinct seasonal variation, but on the average, it was similar to20% higher than the diffusive O-2 uptake. The difference reflected the microtopography of the sediment surface and the contribution from benthic macrofauna. In situ total O-2 uptake was generally twice as high as laboratory rates, reflecting a higher fauna-related O-2 consumption in the larger enclosures incubated in situ. Annually, the in situ three-dimensional diffusive O-2 consumption was 6.2 mol O-2 m(-2), whereas the additional benthos-mediated O-2 uptake was 3.9 Mol O-2 m(-2). Thus, 40% of the total O-2 uptake was due to faunal activity and respiration. The present study demonstrates the importance of realistic faunal representation during sediment incubations in order to obtain correct benthic mineralization rates.