SHORT-TERM VARIABILITY IN PELAGIC-BENTHIC EXCHANGE OF PHYTOPIGMENTS AND THEIR RELATIONS TO BENTHIC BACTERIAL VARIABLES IN THE NORTH-SEA

The short-term variability in pelagic-benthic exchange of phytopigment s and the response of benthic bacteria to changes in phytopigment inpu t in the bottom were studied in a temperature stratified water column including the sediment surface layer in the Oyster Grounds, North Sea, from 5 to 19 July 1994. The exchange of inorganic nutrients and phyto pigments across the thermocline was limited during the cruise, with ex ception of a single short-lasting event on 14 July. This event resulte d in enhancement of pigments throughout the water column and affected the pigment content in the surface mixed layer (SML) more and longer t han in the benthic mixed layer (BML). The event did not result in a me asurable change of sedimentation iii the BML. The temporal variability of phytopigments in the SML was not reflected in the BML, where varia tions in phytopigments were determined by variations in tidal advectio n, sedimentation and resuspension. Variations in phytopigment fluxes t o the bottom were demonstrated, with pigment sedimentation during slac k tide exceeding sedimentation during full tide. The bulk of the phyto detrital material which settled during slack tide was resuspended duri ng full tide. Nevertheless, there was net sedimentation of phytopigmen ts, but insufficient to compensate for the decrease of the pigments in the sediment. The benthic phytopigment inventory gradually decreased during the cruise. Spatial variations in benthic phytopigments were si gnificantly reflected by the spatial variations in bacterial variables , however the temporal decrease in algal pigments in the sediment was not reflected by the temporal variations in the benthic bacterial vari ables. Spatial relations between primary phytopigments and benthic bac terial production were decoupled in time implying that chlorophyll a a nd fucoxanthin are poor indicators of the amount of bacterial substrat e in the absence of fresh algal input (directly utilizable organic mat ter). The spatial relations between benthic bacterial variables and be nthic phytopigments however indicate that the dominant source of food for bacteria in the Oyster Grounds is phylodetritus.