The present study was conducted during August 1997-August 99 to investigate the Eastern Harbour (eutrophic marine basin) environments and the corresponding phytoplankton communi ty structure, species composition and blooming of different causative organisms that are known to be inherently continuously variable. Anthropogenic eutrophication and geographical dispersion from neighbouring areas into the harbour strongly influenced such variability, and hinder any particular seasonal trends. Yet, the pronounced late winter-spring increasing population and other blooming pulses in summer and autumn caused by less than a dozen species (mainly of diatoms, eutrophication symptom) were predominant characteristics of the production cycle in the harbour, Discharge water into the harbour created rich spectra for algal growth, not necessarily for its nutrient content, but ultimately for the stabilisation of the water column, salinity fluctuation was a crucial factor limiting the phytoplankton variability. Pyramimonas and Micromonas species culminated at maximum abundance in spring with the initiation of thermo-haline stratification of the water column, and it also seems sensitive to the inter-annual variations in temperature. Dinoflagellates predominated at intermittent periods in summer well stabilized the water column, with low nitrate, and relatively high phosphate concentrations. The species-specific patterns include aspects as; the cyclic abundance of the centric diatom Skeletonema costatum', the episodic blooms of mdegenous species that normally do not show blooming in the harbour {Cyclotella narta, Leptocylindrus minimus and Lithodesmfum unduhtum); the reappearance of endogenous dinoflagellate species that were not so far recorded during the last decades (Gyrnnodinum sanguineum and Gonyaulax spinifera); and the seasonal shift in predominance and trends (increasing-decreasing abundance and disappearance) of the major dinoflagellate species {Prorocenirum iriestinum, Gymnodinium caienatum and Scrippsiella trochoidea).