The northeastern Black Sea redox zone: Hydrochemical structure and its temporal variability

Detailed studies of the vertical structure of hydrochemical parameters in the northeastern Black Sea near Gelendzhik are presented and discussed. This work is based on a set of systematic observations carried out in this region during R/V “Akvanavt” and R/V “Ashamba” cruises from 1997 to 2005 and also on data from previous studies from 1984 to 1997. The Black Sea region near Gelendzhik is far from the influences of the Bosporus input and Danube River inflow. Therefore, the vertical structure in this region is more stable compared to the western Black Sea and reflects “integrated,” rather than local, changes of the Black Sea. Seasonal variations in the distribution of chemical parameters at the redox interface are connected to seasonal variations in hydrophysical processes and organic matter production. In winter, maxima of organic phosphorus and urea were absent in the vicinity of the onset of hydrogen sulfide. The concentrations of nitrate were lower in winter than in summer. Winter mixing in the anticyclone eddies led to reduced vertical gradients in the redox layer and to the disappearance of an upper phosphate minimum. Study of the interannual dynamics of oxygen concentrations in the Cold Intermediate Layer and anoxic waters boundary in the density field position revealed climatic-scale changes that may be connected with changes in the North Atlantic Oscillation (NAO) index.