VARIATIONS OF PARTICULATE ORGANIC-CARBON IN THE CENTRAL NORTH-SEA

Particulate organic carbon (POC) was measured in the central North Sea following a drifter system over periods of .apprx. 2 wk in May, July and Sept. 1981. The water depth in the area was 45-50 m. Average concentrations of POC were between 50-250 mg .cntdot. m-3. A diurnal cycle of POC with a magnitude of 15-90 mg .cntdot. m-3, which must have been caused by net production during the day and net mineralization during the night, was found in the upper part of the water column. In May, before the summer thermocline was established, relatively high concentrations of POC were found in deeper water which indicates that at that time of the year an important input of organic matter to the benthic community occurs. After formation of the summer thermocline POC concentrations in the surface layer were generally larger than in the bottom layer; in July a maximum occurred in the thermocline. A straight relation was found between POC and primary production; there is an inert fraction of POC of .apprx. 40 .mu.g .cntdot. 1-1 which was present even when there was no production at all. The average age of the recently produced labile POC was calculated at .apprx. 5 days. A provisional budget of production and mineralization in the water column, based also on O2 consumption data, indicates that .apprx. 30% of daily production was mineralized below the thermocline, of which 15% on the sea bottom.