Abstract: The fate of a Phaeocystis globosa bloom in the Southern North Sea off Belgium, the Netherlands and Germany in Mai 1995 was investigated during a cruise with RV Belgica. We used fatty acids as biomarkers to follow the fate of Phaeocystis-derived biomass of a Phaeocystis dominated spring bloom. The Phaeocystis bloom showed a fatty acid composition with a characteristic dominance of polyunsaturated C18-fatty acids, which increased in concentration with number of double bonds up to 18:5 (n-3), and high concentrations of 20:5 (n-3) and 22:6 (n-3). In contrast to most previous studies, fatty acid analysis of the mesozooplankton community (mainly calanoid copepods) and meroplankton (Carcinus maenas megalope) demonstrated that Phaeocystis was a major component in the diet of these organisms. Massive accumulations of amorphous grey aggregates, in which Phaeocystis colonies were major components, were dominated by saturated fatty acids and only few of the polyunsaturated C18-fatty acids. A hydrophobic surface slick that covered the water surface during the bloom showed very similar patterns. Foam patches contained few Phaeocystis-typical fatty acids, but increased amounts of diatom-typical compounds such as 16:1 (n-7) and 20:5 (n-3), and 38% fatty alcohols, indicating that wax esters dominated the lipid fraction in the foam with ca. 76% (w/w). The fatty acid compositions of surface sediment showed that no sedimentation of fresh Phaeocystis occurred during the study. The results indicate that diverse processes degrade Phaeocystis -derived organic matter while floating or in suspension, and little evidence for a massive sedimentation of the Phaeocystis bloom.