The elongated Bay of Izmit is one of the most heavily polluted semienclosed basins in Turkey. From systematic data obtained during field studies between 1984 and 1988, it is clear that two distinct water masses are present throughout the year. The brackish water which originated in the Black Sea forms a 10 to 15 m thick surface layer. Its salinity and temperature vary from 22-24 ppt and 20-24 degrees C in summer, to 26 ppt and 7 degrees C, in winter. Marine water, which arrived from the Mediterranean, underlies the permanent pycnocline and has much more stable properties: salinity and temperature ranges are 37.5-38.5 ppt and 14-15 degrees C, respectively.