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¹ TÜBİTAK Marmara Research Centre Gebze/Kocaeli Türkiye
² İstanbul University Institute of Marine Sciences and Management Vefa/İstanbul Türkiye DOI : 10.4194/TRJFAS28976 Viewed : 299 - Downloaded : 276 This study aimed to investigate the spatio-temporal distribution of phytoplankton communities and potentially harmful species in the northeastern Sea of Marmara, including İzmit Bay, from 2015 to 2018. Seawater samples were collected at six stations during the study to assess the species composition and abundance of the phytoplankton community, as well as the current environmental variables. A total of 146 taxa belonging to twelve taxonomic classes were identified in the phytoplankton community. Seasonal patterns revealed that diatoms were dominant in winter and dinoflagellates were more common in other seasons. Phytoplankton abundance decreased from inner part of the İzmit Bay to outer regions and showed strong relationships with temperature and inorganic nutrient levels. The diatom Skeletonema spp. formed a bloom in the winter of 2018. A total of twenty-two potentially harmful taxa were detected, of which fourteen are known to be toxic or ichthyotoxic, two are known to cause fish kills, and six are known to cause water discoloration during bloom events. These findings reveal pronounced seasonal and spatial changes in phytoplankton community structure and their environmental drivers, indicating the importance of long-term monitoring for detecting HAB-related risks and assessing ecosystem health in eutrophic marine environment. Keywords : Phytoplankton, Harmful species, Ecological index, İzmit Bay, Sea of Marmara