The spatial and temporal distribution of suspended particulate matter (SPM) concentration in Izmit Bay (Sea of Marmara) were studied in order to examine the prevailing transport mechanisms under varying seasonal hydrographical conditions. The prevailing current system of the Bay occurs in two opposite directions and changes seasonally, flowing into the Bay as a lower layer and flowing out to the Sea of Marmara as an upper layer in autumn and winter. SPM concentrations and distribution pattern of both upper and lower water layers display variations throughout the year, reflecting a seasonal cycle and variability of the circulation and stratification of two water layers seems to control the vertical and spatial distribution of SPM in Izmit Bay. The SPM concentration of the lower layer is higher than that of the upper layer with an increase in intensity of the pycnocline in summer. Homogeneous vertical distribution of SPM during February and March is supported by the weak stratification of the two layers. SPM transportation from Izmit Bay to the open sea occurs in winter, whereas it is carried to interior of the Bay from central to eastern basins in summer. Both the grain size distribution of the bottom sediments and the temporal-spatial variation of SPM content of the water column indicate that the transportation and depositional conditions are different in the three basins. Predominantly fine-grained-sized bottom sediments are deposited in the central and eastern basins due to the reduced current velocities in the lower layer. The western basin is subjected to deposition of sediments with various size, from both the central basin and the Sea of Marmara. (C) 1999 Academic Press.