Systematic measurements of optical properties, concentrations of major and minor chemicals and primary production throughout 1989-1992 enabled us to identify the oligotrophic state of Sapanca lake, Marmara region, Turkey. Lake waters which have low concentrations of major anions and cations (total = 49 mu M) overturn every February-March, ventilating the bottom waters and enriching the surface waters with nutrients. Surface waters cool down to 6.5 degrees C by late winter and then warm steadily to 26 degrees C by late summer, while temperatures in deep waters range between 6.5 and 10.0 degrees C throughout the year. When the seasonal thermocline develops, the dissolved oxygen profiles exhibit a subsurface maximum in the thermocline, while in the hypolimnion water, the content varies seasonally from 11.5-12.0 ppm (350-375 CIM) in March to 0.5-1.7 ppm (16-56 mu M) in late autumn. Surface nitrate concentrations vary markedly with season, from < 0.15 mu M in summer to 5.7 mu M in early March, whilst the bottom water concentrations range from 13.5-14.0 mu M in late autumn to 5.7 mu M after the winter overturn. Phosphate concentrations are always less than 0.1 mu M throughout the entire water column. Subsurface chlorophyll-a maximum descends in late summer to 20-25 m depths, where the Light intensity is less than 1% of the surface value. Below 10-15 m depths, corresponding to the upper thermocline, primary productivity is very low. The range was from 35 to 93 mg C/m(2) day(-1) during 1989-1991, consistent with the values in other oligotrophic lakes.