Chemical oceanographic understanding of the southern Black Sea has been improved by recent measurements of the optical transparency, phytoplankton biomass (in terms of chlorophyll-a and particulate organic matter) and primary productivity. During the spring-autumn period of 1995-1996, light generally penetrated only into the upper 15-40 m, with an attenuation coefficient varying between 0.125 and 0.350 m(-1). The average chlorophyll-a (Chl-a) concentrations for the euphotic zone ranged from 0.1 to 1.5 mu g 1(-1). Coherent sub-surface Chi-a maxima were formed near the base of the euphotic zone only in summer. Production rate varied between 247 and 1925 in the spring and between 405 and 687 mgC m(-2) d(-1) in the summer-autumn period. The average POM concentrations in the euphotic zone varied regionally and seasonally between 3.8 and 28.6 mu m for POC, 0.5 and 3.1 mu m for PON and 0.02 and 0.1 mu m for PP. Atomic ratios of C/N, C/P and N/P, derived from the regressions of POM data, ranged between 7.5 and 9.6, 109 and 165, and 11.2 and 16.6, respectively. In the suboxic/anoxic interface, the elemental ratios change substantially due to an accumulation of PP cohering to Fe and Mn oxides. The chemocline boundaries and the distinct chemical features of the oxic/anoxic transition layer (the so-called suboxic zone) are all located at specific density surfaces; however, they exhibit remarkable spatial and temporal variations both in their position and in their magnitude, which permit the definition of long-term changes in the biochemical properties of the Black Sea upper layer.