Oxygen uptake, bacterial distribution, and carbon-nitrogen-sulfur cycling in sediments from the baltic sea - North sea transition

Abstract

Chemical zonations and bacterial distribution and metabolism were studied at fourteen sediment stations in the Belt Sea, the Kattegat, and the Skagerrak. Water depths were 14-200 m and sediment types ranged from clay and silt to fine sand. The oxic zone as determined by microelectrodes was only 1.3-5.6 mmdeep while the oxidized zone (Eh > +100mV) was one to several em. Roughly 90% of the oxidized layer was thus anoxic. A pH minimum occurredjust below the oxic-anoxic interface. Subsurface nitrate maxima extended to depths of 1.5-5 cm. Total bacterial densities at the sediment surface were 0.25-10⁸ × 108 cm^-3. Bacterial distributions showed no correlation with the measured rates of aerobic or anaerobic metabolism. Oxygen uptake and sulfate reduction were positively correlated but in some sediments most sulfate reduction took place below the analyzed sediment layer of 0-15 em. The respiration rates were compared to rates of acetate and ammonia turnover measured by ¹⁴C and ¹⁵N tracer experiments, respectively. Acetate turnover rates highly exceeded sulfate reduction, probably due to problems of identifying the bacterially available acetate pool and its specific radioactivity. Ammonia turnover was positively correlated to oxygen uptake rates with a mean molar O²:N ratio of 2.3:1. A rough carbon budget for the Belt Sea and Kattegat areas showed that 44% of the total primary production in the water column reached the sea floor by sedimentation. Only 10% of the deposited organic matter was permanently buried into the sediment while 90% was remineralized.