Oxygen budgets calculated from in situ oxygen microprofiles for Northern Adriatic sediments

Oxygen budgets are presented for sediments from the Northern Adriatic Sea. Six shallow stations (25-45 m water depth) along the mud-belt which marks the discharge route of the river Po, were compared to a sandy, off-shore station at 39 m water depth, and to a southern station at 251 m water depth. Budgets were calculated from in situ oxygen microprofiles, recorded with an oxygen profiling lander in March 1992 and August 1993. Four one-dimensional diffusion-reaction models were tested to fit the experimental profiles including the diffusive boundary layer. The model assuming a two-layered oxic zone with discrete reactivities in each sublayer proved superior in describing the oxygen profiles from both the shallow and the deep sediments. In March, areal respiration was equal to the predicted benthic fluxes which increased with increasing distance from the Po delta from 6.4 to 17.1 mmol O-2 m(-2) day(-1). These values are equivalent to the oxidation of 58-156 mg C m(-2) day(-1), or 11-26% of the daily averaged primary production. In contrast to March, the oxygen microprofiles for shallow sediments in August could only be described by assuming benthic oxygenic photosynthesis. Areal benthic photosynthesis ranged from 2 to 23 mmol O-2 m(-2) day(-1). All the oxygen produced by benthic photosynthesis was consumed within the sediment. Predicted oxygen fluxes at the shallow stations along the coastal belt ranged from 10.2 to 20.2 mmol O-2 m(-2) day(-1). Due to benthic photosynthesis, areal respiration at the photic stations exceeded predicted fluxes by 60% on average. Areal respiration for the coastal belt stations was 16.3-30 mmol O-2 m(-2) day(-1), which represents 150-277 mg C m(-2) day(-1), or 27-50% of the daily averaged pelagic primary production. For the sandy station, the predicted oxygen fluxes and areal respiration fall within the ranges for the coastal belt stations. Areal respiration at the deep station amounted to 3.5 mmol O-2 m(-2) day(-1), equivalent to 31.9 mg C m(-2) day(-1), or 13-16% of the daily averaged primary production for the central Adriatic basin. A highly reactive similar to 3 mm surface layer was observed at this station, which accounted for 60% of the areal respiration. The distribution of organic carbon along the coastal belt reflects the decreasing input rates in southward direction. Terrestrial carbon is deposited near the Po delta as indicated by the high C:N ratio of the bulk organic carbon. The organic carbon contents of the sandy and the deep station are discussed in relation to respiration, deposition and protective adsorption to sediment particles.