Izmit Bay is the northeast extent of the Sea of Marmara, consisting of two different water bodies. Vertical mixing primarily occurs during winter and thermal stratification takes place in summer. The bay is generally exposed to considerable anthropogenic inputs and industrial discharges. The picophytoplankton (P-Phyto) communities in Izmit Bay and their correlations with physicochemical parameters were studied monthly at mid-section stations and seasonally at coastal stations in 2012 within the context of the present study. During these studies, P-Phyto abundances were relatively low during the winter and the spring. At the same time, they were significantly high in the summer and the autumn. The significant statistical relationships between water temperature and cell abundance showed the discriminative impact of the temperature on population dynamics. Generally, the dominant group of P-Phyto was Picocyanobacteria (P-Cyan), especially in the warm periods. (C) 2021 Elsevier B.V. All rights reserved.