Seasonal changes in phytoplankton composition and phytoplankton abundance and physico-chemical factors affecting these parameters were investigated in this study. Quantitative and qualitative phytoplankton and nutrient analysis were carried out between 1998 and 1999 at 12 sampling stations located from inshore to offshore of Karataş, situated in the north-eastern Mediterranean. A total of 135 taxa were determined, belonging to four algae classes: Cyanophyceae, Bacillariophyceae, Dinophyceae and Dictyochophyceae. Bacillariophyceae appeared to be the dominant group in terms of total species number and cell numbers during the research period. The number of phytoplankton species was high in winter and quite low in summer. The highest phytoplankton abundance determined in summer was due to the increase in the numerical abundance of Hemiaulus membranaceus Cleve from the diatoms. Phytoplankton diversity declined to the lowest level in the summer. Nutrient concentrations were high in winter but low in summer. The lowest and highest concentrations of NO3+NO2-N, PO4-P and silicate were 0.20 and 8.09 µg-at l-1, 0.04 and 0.57 µg-at l-1 and 0.33 and 8.20 µg-at l-1 respectively.

Seasonal changes in phytoplankton composition and phytoplankton abundance and physico-chemical factors affecting these parameters were investigated in this study. Quantitative and qualitative phytoplankton and nutrient analysis were carried out between 1998 and 1999 at 12 sampling stations located from inshore to offshore of Karataş, situated in the north-eastern Mediterranean. A total of 135 taxa were determined, belonging to four algae classes: Cyanophyceae, Bacillariophyceae, Dinophyceae and Dictyochophyceae. Bacillariophyceae appeared to be the dominant group in terms of total species number and cell numbers during the research period. The number of phytoplankton species was high in winter and quite low in summer. The highest phytoplankton abundance determined in summer was due to the increase in the numerical abundance of Hemiaulus membranaceus Cleve from the diatoms. Phytoplankton diversity declined to the lowest level in the summer. Nutrient concentrations were high in winter but low in summer. The lowest and highest concentrations of NO3+NO2-N, PO4-P and silicate were 0.20 and 8.09 µg-at l-1, 0.04 and 0.57 µg-at l-1 and 0.33 and 8.20 µg-at l-1 respectively.