A series of red tides were observed during 2015 in the Izmit Bay (the Marmara Sea) which is located in the most industrialized and populated region of Turkey. Six samplings were carried out in this area following the red tides. Nitrite-N, nitrate-N, ammonia, silica and orthophosphate concentrations were analyzed spectrophotometrically. Physicochemical conditions were measured by CTD probe. Plankton quantification was performed using counting chambers under microscopes. Prorocentrum micans was the most abundant species, except on May 14, 2015, when Noctiluca scintillans was dominant. The abundance of P. micans reached average 18x10(6) ind./L on May 3, 2015 in the Karamursel station, simultaneously with elevated levels of NH3 and o-PO43-. The sample was also abundant in dead amphipods ((72 +/- 12) ind./L) that had been covered by mucilage aggregates produced by P. micans. The highest biomass (calculated by carbon) was recorded as (268 +/- 26.0) mg/L on May 14 in the Hereke station. Beside the anthropogenic wastewater discharges, unknown sources and resuspensions caused increases in nutrient levels. After long term northeaster gusts (35 km/h for 5 d) an upwelling occurred on November 6, 2015 after wind-induced sediment resuspension. Although nutrient discharges remarkably decreased over 30 years through established wastewater treatment plants, harmful phytoplankton blooms still occur. Comparing the present results with other studies in nearby Mediterranean seas reveals that the most intense harmful dinoflagellate bloom in recent years occurred in the Izmit Bay. Therefore, additional protection measures necessary for a cleaner Izmit Bay. These incidents also demonstrate that contaminants, accumulated in sediment, may have long-lasting effects on enclosed marine ecosystems.