The increase in human activities along with climate changes has increased the risk of the eutrophication of coastal waters. The effects of these adverse events occur first in the structure of phytoplankton and then in other links of the food chains. The purpose of this study is to determine the water quality based on the chlorophyll-a and phytoplankton data in the Iranian coastal of the
Caspian Sea during ۲۰۱۷-۲۰۱۸. According to references of the used method, for the water quality determination, the data which are more than the annual median is important as well as their minimum frequency at ۲۰% out of the collected samples. The result showed that the mean of chlorophyll-a, abundance, and biomass of phytoplankton were ۱.۸۴ (mg/m۳), ۱۳۸ (million cells/m۳), and ۲۴۲ (mg/m۳), respectively. The frequency of samples with an average more than the annual median of chlorophyll was more than ۲۰% in all seasons except in spring. However, the frequency of samples with an average more than the annual median of abundance and biomass was more than ۲۰% in all seasons except in summer. So, the highest water quality based on chlorophyll-a, abundance, and biomass were observed in spring, summer, and summer, respectively. Overall, taking into account all three parameters (chlorophyll-a, abundance, and biomass), good-medium quality (oligo-mesotrophic) water was obtained in warm seasons (spring and summer) and medium quality (mesotrophic) in cold seasons (autumn and winter). As the details of the phytoplankton community results also indicated the high abundance and frequency of the harmful and toxic species Pseudonitzschia seriata in the winter season and the increase in the percentage abundance and diversity of edible, native, and resident species (Cyclotella meneghiniana, Thalassionema nitzschioides, and Binuclearia lauterbornii) in most seasons except in winter. The results of this study provided an overview of the quality of the ecosystem, which makes it easy to understand the data obtained from phytoplankton analysis. The result is applicable for exploitation planning and executive activities (such as fish cage culture). Meanwhile, the study shows the importance of using regional scales for the correct and logical understanding of the ecosystem state.