Past and present biochemical data, starting from:RN J. Elliott Pillsbury August-1965 cruise till RN Bilim April-1994 cruise, were compared in terms of density dependent profiles for the dynamically different regions of the Black Sea. Examining of these data sets revealed that the suboxic zone has expanded since late 60's. An unusual event in July-1992, during which an intense blooming of planktonic organisms was observed and concurrent dissimilatory nitrate reduction occurred within the offshore waters of the western Black Sea, has been evaluated in terms of biochemical and physical properties. During this unusual period, the nitracline was eroded down to the sigma(theta) = 15.4 density surface where conventional nitrate peak has been observed during the last decade. In the meantime, the dissolved oxygen concentration of 25 mu M, which was expected to be attained at the 15.40 density surface was observed to shift to the sigma(theta) = 14.9-15.0 surfaces in July, 1992. In a similar manner, the onset of the phosphate gradient layer for the cyclonic regions of the Black Sea appeared to have deepened from sigma(theta) = 14.5 density surface in September-1991 to 14.9-15.0 surfaces in July-1992, whereas the position of sulfidic layer (H2S > 1 mu M) has remained unchanged, between the density surfaces of 16.15-16.20 during recent decades.