Abstract This study was conducted to understand the seasonal and vertical variations in the composition and abundance of phytoplankton assemblages in the Marine Park of Gökçeada Island and to evaluate the relations of phytoplankton community structure with the environmental parameters. Water samples were taken from the water column at surface water, 10 m, 20 m and 28 m depths in Yıldız Bay seasonally between May 2009 and January 2010. A total of 28 species belonging to Bacillariophyceae (61%) and 18 species belonging to Dinophyceae (39%) were identified. Cerataulina pelagica, Cylindroteca closterium, Leptocylindrus minumus from Bacillariophyceae were present in all seasons and reached high individual numbers that these species have a wide ecological tolerance to environmental factors. Prorocentrum micans from Dinophyceae was present throughout the seasons in small numbers. The maximum density of total phytoplankton (67987 cells L⁻¹) was observed in the winter period. Accordingly the water became turbid by mucilage clouds due to the excessive growth of the species known as producing mucilage, Skeletonema costatum, Cylindrotheca closterium and Gonyaulax fragilis. The ratio of nitrogen to phosphorus in the water column was recorded high (50.9) in this period. Lowest diversity and similarity between seasons were recorded in the summer period due to the excessive growth of Proboscia alata from Bacillariophyceae. Consequently it can be assumed that since Yıldız Bay is exposed to touristic activities in summer months, environmental disturbance causes unnecessary growth of opportunistic species and these species reduce the species diversity in the region by pressuring the growth of other species.
Key words: Phytoplankton, Marine Park of Gökçeada, North Aegean Sea, Eastern Mediterranean