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¹ Istanbul University, Institute of Marine Sciences and Management, 34134, Istanbul, Turkey DOI : 10.4194/1303-2712-v11_4_03 Viewed : 4379 - Downloaded : 3966 The potentially harmful and bloom-forming dinoflagellate Prorocentrum minimum (Pavillard, 1916) Schiller, 1933 was followed monthly during four years in the Golden Horn Estuary (GHE). Simultaneously measured some environmental variables including salinity, temperature, dissolved oxygen, inorganic nutrient and chlorophyll a concentrations were evaluated together with P. minimum data. The density of this species increased mostly in late spring and summer during three years and caused two significant blooms in July 2000 and July 2001. Environmental conditions which triggered these bloom events were considered during whole study period. Blooms have mostly occurred in middle and upper estuary where water movements are insufficient (salinity 15.2-18.5 psu, temperature 19.0-24.2°C). The cell density of P. minimum reached to 5.5 × 10⁶ cells L^-1 at the early stage of the first bloom, and after two weeks, it reached to highest level of 70.0 × 10⁶ cells L^-1. Cell density was calculated as 36.0 × 10⁶ cells L^-1 at the second bloom in July 2001. Dissolved oxygen value reached to super saturation (29.0 mg L^-1) and chlorophyll a concentration was measured as 200 μg L^-1 in July 2000. Inorganic nutrient concentrations were lower than before bloom periods. The results appeared that P. minimum could well develop at the high temperature and moderate salinity under the calm weather in nutrient-rich estuarine environment. Keywords : Phytoplankton, Prorocentrum minimum, harmful algae, eutrophication, Golden Horn