In this study, algal assemblages, phytoplankton biomass, chlorophyll-a and selected physico-chemical parameters were investigated in a drinking water reservoir in Omerli, Istanbul from October 1999 to November 2000. Water samples were collected from three sampling stations at the surface, a depth of 1 m and at 5 m depth intervals to the bottom (approximately 20 m). Particular attention was given to the ecology of steady state phytoplankton assemblages. When the euphotic depth (z(eu)) was less then 1.52 m and the ratio of the euphotic to mixing zone (z(eu)/z(mix)) was 0.2 m ( during October 1999 and September and October 2000), Microcystis aeruginosa Kutz. dominanted, accounting for 85-95% of the total phytoplankton biomass. A shift to Fragilaria crotonensis Kitton and Asterionella formosa Hass. occurred during April and early May 2000 when z(eu)>4.1 m. Other dominant species were Coelastrum microporum Nag. Pediatrum boryanum (Turp.) Menegh. and P. duplex (May to mid June 2000); C. microporum, Staurastrum spp. and Cryptomonas spp. (late June to mid July 2000); and Scenedesmus spp., S. gracile and Nitzschia holsatica Hust. (late July to late August 2000). Below 10 meters, a high biomass of Mougeotia sp. was recorded at all sites. However, in October and early November 2000, this species appeared in the surface water. During late May to late August 2000, the z(eu) depth changed between 2.7 and 3.38 m. pH and total organic carbon (TOC) concentrations were found to be relatively high in the surface waters, whereas total phosphorus (TP) and Soluble Reactive Silicate (SRSi) concentrations were high below the 5 meters. The total nitrogen (TN) concentrations were high either in the surface waters or in deep layers. It is suggested that the observed increase in cell biomass of some species was controlled by the nutrient concentrations. The changes in the phytoplankton assemblages resulted in a reduction of the euphotic depth.