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

Abstract

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 volumespecific 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 mm 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 ~10% higher than the simple one‐dimensional, microprofilederived diffusive O 2 exchange. The total O 2 uptake measured in the laboratory showed less distinct seasonal variation, but on the average, it was ~20% 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.