SEASONAL BENTHIC MICROBIAL ACTIVITY IN THE SOUTHERN NORTH-SEA - OXYGEN-UPTAKE AND SULFATE REDUCTION

From an initial cruise, 6 sites were selected from the 41 sampled to be representative of the major types of sediment in the southern North Sea. The 6 sites in the mixed, frontal and stratified areas of the southern half of the North Sea exhibited a wide range of environmental conditions, particularly sediment type, water depth and organic matter content. Benthic microbial activity, and its seasonal variability, were determined at bimonthly intervals from September 1988 to September 1989. The significance of aerobic and anaerobic respiration in the degradation of organic carbon was investigated by measuring the rate of oxygen uptake and sulphate reduction for each sediment. All sites showed significant rates of oxygen uptake (range 5.3 to 27.8 mmol 02 M-2 d-1) and sulphate reduction (range 0.05 to 11.8 mmol SO4 m-2 d-1), and hence of organic matter mineralization. Aerobic respiration accounted for between 47 and 89 % of annual organic matter degradation depending on the site, the balance being due to anaerobic sulphate reduction. Benthic mineralisation rates were greatest at a station near the Dogger Bank and one near the Dutch/Belgian coast. However, benthic mineralisation as a proportion of the net annual primary production in the water column was higher at stratified stations in the central North Sea (average 47 %) than in the more southerly stations with mixed water columns (average 26 %). Estimates of benthic organic mineralisation were used to calculate the magnitude of total benthic mineralisation in the southern half of the North Sea and compared to estimates of net primary production for the same area. On the average, for the whole of the southern North Sea, benthic mineralisation was equivalent to 17-45 % of the total net primary production. These results demonstrate that the bottom sediments are important sites of organic degradation in the North Sea system.