Seasonal dynamics of benthic O2 uptake in a semienclosed bay: Importance of diffusion and faunal activity

The benthic O2 uptake and the O2 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 O2 penetration depth, the diffusive O2 uptake, and the volumespecific O2 consumption rate strongly depended on the seasonal changes in bottom water O2 concentration and the sedimentation of organic carbon. The in situ O2 penetration depth varied between 0.5 mm in summer and 4.5 mm in winter. The diffusive O2 uptake varied between 8 and 30 mmol m22 d21, whereas the volume-specific O2 consumption rate varied by a factor of 13. The O2 distribution was very sensitive to environmental controls, and microprofiles obtained in the laboratory tended to overestimate the in situ O2 penetration depths and underestimate the in situ diffusive O2 uptake. Three-dimensional O2 flux calculations based on in situ microtopographic mapping showed that the actual diffusive exchange rate was ;10% higher than the simple one-dimensional, microprofilederived diffusive O2 exchange. The total O2 uptake measured in the laboratory showed less distinct seasonal variation, but on the average, it was ;20% higher than the diffusive O2 uptake. The difference reflected the microtopography of the sediment surface and the contribution from benthic macrofauna. In situ total O2 uptake was generally twice as high as laboratory rates, reflecting a higher fauna-related O2 consumption in the larger enclosures incubated in situ. Annually, the in situ three-dimensional diffusive O2 consumption was 6.2 mol O2 m22, whereas the additional benthos-mediated O2 uptake was 3.9 mol O2 m22. Thus, 40% of the total O2 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. The oxygen uptake rate of the seafloor is the most widely used measure of benthic mineralization (e.g., Thamdrup and Canfield 2000). The total O2 uptake (TOU) is usually quantified from the O2 disappearance rate in sediment enclosures, and the TOU thereby represents an integrated measure of the diffusive, advective and fauna-mediated O2 consumption. The enclosure technique, however, does not provide information on the O2 penetration depth or the vertical distribution of O2 consumption rate. The introduction of O2 microelectrodes in aquatic biology in the early 1980s allowed the benthic O2 dynamics to be studied in more detail (Revsbech et al. 1980). Early microprofile measurements revealed that the oxic zone extends only a few millimeters into fine-grained coastal and shelf sediments (e.g., Revsbech et al. 1980). The oxic