Sources and effects of oxygen deficiencies in the south-eastern North Sea

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Abstract
Oxygen concentrations, oxygen comsumption and its possible sources were investigated in the southeastern North Sea (east of 6°N20′E, south of 56°50′N) in 1983–1985. 1983 was a year with extremely low, 1984 with high oxygen concentrations in the bottom water, but even in 1984 only 40–70 % oxygen saturation was found in some areas in the subthermocline waters. BOD values suggest that the oxygen content in the deep water could be used up within 20–30 days, given a calm weather period. This oxygen consumption, extrapolated from the BOD measurements, was fastest well north of areas with highest organic seston stocks. This supports the assumption of a northward transport of seston clouds with the residual currents. The theoretical oxygen demand for the degradation of measured organic seston stocks of the water column was as high as about half of the saturation oxygen content of the subthermocline water. Thus, even in years with average plankton and other organic seston stocks the oxygen balance of the bottom water would be endangered quickly, should a prolonged vertical density stratification develop. Mean oxygen consumption for macrobenthos inhabited sediment was 18 mg · m−2 · h−1. Average benthic community oxygen consumption amounted to 14 % of total biological oxygen demand of subthermocline water. Possible impact of oxygen depletion on benthic communities was investigated. Cluster analysis, diversity index and evenness index showed that benthic community structures were different in years with and without oxygen deficiency in the bottom water, suggesting effects of low bottom water oxygen levels on benthos.