ON THE TROPHIC FATE OF PHAEOCYSTIS-POUCHETII (HARIOT) .6. SIGNIFICANCE OF PHAEOCYSTIS-DERIVED MUCUS FOR VERTICAL FLUX

The development and decline of a phytoplankton spring bloom dominated by the prymnesiophyte Phaeocystis pouchetii were studied in Balsfjord, northern Norway between 30 March and 27 May 1992. At a fixed station, the concentration and composition of suspended particulate matter was monitored and compared to the particulate matter collected in sedimen t traps at six different depths. Direct sedimentation of phytoplankton contributed a minor fraction to particle flux and was confined to a f ew diatom genera. No evidence was found for pronounced aggregation of Phaeocystis colonies during bloom decline or direct sedimentation of e ither Phaeocystis colonies or single cells, Particle flux was dominate d by faecal-pellet sedimentation during most of the study period, sugg esting zooplankton grazing to be a main loss factor. Despite an abrupt decrease in faecal-pellet sedimentation after the decline of the bloo m, particulate-carbon sedimentation rates remained high. High post-blo om sedimentation rates were characterized by elevated C/N and C/Chl a ratios of largely amorphous sedimented material. Post-bloom sedimentat ion coincided with a decrease in transparent exopolymeric particles (T EP) in the surface layer, suggesting that this change resulted from ag gregation and sedimentation of carbon-rich exopolymeric material accum ulated in the surface layer in the course of the bloom. While organic- carbon accumulation indicates the significance of disintegration of Ph aeocystis colonies, post-bloom mucilage sedimentation could be a secon dary pathway for the vertical flux of Phaeocystis-derived organic matt er.