The Haptophyte Phaeocystis forms dense phytoplankton blooms in many nutrient-rhich ocean areas. In these blooms, Phaeocystis occurs almost exclusively as up to several mm large, spherical colonies, although it can also occur as ca. 6 µm small flagellates, which grow at least as fast as the cells within the colonies. Thus, the organisation within the colonies obviously protects the Phaeocystis cells efficiently against grazing or infection. As construction and properties of the Phaeocystis colony were so far insufficiently described, the mode of action of this protection was unknown. Therefore, structure, mechanical properties and permeabilities of Phaeocystis-colonies were studied in this thesis. Specific staining techniques revealed a concentration of polysaccharides and aminogroups at the periphery of the colonies. In contrast, lipids were not discerned in the extracellular colony material. Using the micropipette-aspiration technique, it was shown that the colonies possess a thin, yet mechanically very stable, plastic skin. Transfer of colonies in different buffers whose osmolarities were increased by molecule with different sizes revealed that the colony skin posesses pores with a size of 1-4.4 nm. Based on these properties, the colony may be regarded as an efficient mechanical protection against a large spectrum of organisms (especially virusses, protozoa, and small copepods) which are known to efficiently decimate single Phaeocystis cells through grazing or infection. This result confirms that the dominance of colonies in Phaeocystis-blooms is indeed a result of reduced grazing pressure. The colony skin agrees remarkably in its composition, structure, and functioning with primary cell walls of higher plants, which should be taken into account as a basis for further studies on structure, function, and functioning of Phaeocystis colonies.Organic matter sinks from phytoplankton blooms with different efficiencies. Most of the material sinks in form of aggregates or fecal material. While the sedimentation efficiency of phytoplankton with mechanically stable, mineralized cell walls, such as diatoms, can be relatively well assessed using microscopic studies on sediment trap material, this possibility is very limited for purely organic, rapidly degradable phytoplankton such as Phaeocystis. Consequently, fewer studies report sedimentation of Phaeocystis than sedimentation of diatoms. Still, it has been suspected that Phaeocystis-derived biomass, might sinkt efficiently as amorpous material, e.g. in fecal strings of krill. Using specific biomarkers, we studied the impact of krill grazing on sedimentation of organic matter derived from a phytoplankton bloom dominated by Phaeocystis pochetii in Balsfjorden, Northern Norway. In contrast to previous studies, we found that Phaeocystis contained a high percentage of polyunsaturated fatty acids. The euphausids Thysanoessa raschii and inermis fed on Phaeocystis colonies and caused sedimentation of Phaeocystis-derived organic matter by excreting fast-sinking fecal strings. This process describes the most efficient of the so far known sedimentation mechanisms of organic mater derived from Phaeocystis cells, and is consistent with our concept of a mechanical protection, which is inefficient against larger organisms such as krill.Lab experiments have shown that the larger copepods which are abundant in the North Sea (e.g. Temora longicornis), are able to efficiently ingest Phaeocystis colonies. In contrast, grazing pressure exerted by zooplankton during Phaeocystis blooms in the North Sea has been estimated to be very low. Phaeocystis cell numbers have been shown to decline rapidly due to cell lysis. The further fate of the organic matter derived from the cells is largely unclear. To assess degradation of cell biomass and if the zooplankton community feeds on Phaeocystis colonies, fatty acid compositions of the phytoplankton bloom, diverse organic fractions, and the copepod community was assessed. Like in Balsfjorden, the percentage of polyunsatuated fatty acids (especially C18) was high. As crustaceans accumulate specific fatty acids of their phytopolankton diet, the accumulation of Phaeocystis-specific C18-fatty acids in the copepod community and decapod larvae implied that they fed on Phaeocystis. In aggregates, sea foam, and a surface slick, Phaeocystis-typical fatty acids were largely degraded, though colonies were still discernible. As lipids occur in Phaeocystis colonies exclusively within the cells, this indicated effective degradation of cell biomass.The results from this study imply that ecology and biogeochemistry of Phaeocystis colonies are decisively influenced by their specific, mechanical defence strategy of a thin, organic skin. Thereby, they seem to deviate significantly from the ecology and biogeochemistry of other phytoplankters. In their strategy of a mechanical protection which is characterized by their large size combined with a high mechanical stability, Phaeocystis colonies differ significantly from other small flagellates, e.g. those of Phaeocystis or other Prymnesiophyceans and resemble large diatoms such as Coscinodiscus. In spite of this analogy, and in spite of the observed efficient sedimentation of Phaeocystis-derived organic matter in krill fecal strings, sinking of Phaeocystis-derived organic matter is probably much lower than that of diatoms.