BENTHIC MINERALIZATION RATES AT 2 LOCATIONS IN THE SOUTHERN NORTH-SEA

Benthic oxygen uptake, sulphate reduction and benthic bacterial produc tion were measured at two contrasting locations in the southern North Sea: the shallow and turbulent Broad Fourteens area in the Southern Ei ght, and the deeper Oyster Grounds, a deposition area, where thermohal ine stratification occurs during summer, Oxygen uptake and sulphate re duction showed a clear seasonal pattern in the Broad Fourteens area, i ndicating a supply of carbon to the benthic system that is closely rel ated to the standing stock of carbon in the water column. This close b enthic-pelagic coupling is probably due to the influence of the tide i n this part of the North Sea, which keeps the water column permanently mixed, At the Oyster Grounds, no seasonal pattern was observed. Peaks in oxygen uptake and sulphate reduction were found in winter, Irregul arly occurring events, such as storms and fishery-related activities, are likely to affect the benthic mineralization patterns in this area, Annual benthic carbon mineralization rates estimated from oxygen upta ke rates were 44 gC . m(-2) at the Broad Fourteens, and 131 gC . m(-2) at the Oyster Grounds, of which 26 and 28%, respectively, could be at tributed to sulphate reduction (assuming an annual sulphide reoxidatio n rate of 100%). Although sulphate reduction rates in the southern Nor th Sea are higher than previously suggested, aerobic respiration is th e most important pathway for benthic carbon mineralization at the stat ions visited, Production rates of benthic bacterial carbon measured wi th labelled leucine were much higher than carbon mineralization rates based on oxygen uptake or sulphate reduction. This may either imply a very high bacterial carbon conversion efficiency, or point to shortcom ings in the accuracy of the techniques. A critical evaluation of the t echniques is recommended.