On Interannual Variability of Chemical Characteristics of Redox Layer and Cold Intermediate Layer of the Black Sea

Based on the measurements on more than 1,700 stations during last three decades in the northeastern Black Sea, analysis of seasonal and interannual variability of chemical characteristics of Cold Intermediate Layer (CIL) and redox layer is given. Studied species include main nutrients, dissolved oxygen, hydrogen sulfide, and carbon system elements. Our studies showed that surface layers ventilation with dissolved oxygen down to the depth of the CIL occurs in winter. The intensity of ventilation is determined by climate forcing, which may be regulated by large-scale climate formations like the NAO. This ventilation sets the upper boundary conditions for the downward transport of O₂. The Black Sea hydrogen sulfide boundary oscillates in the density field with an amplitude of σ _θ = 0.05–0.15 kg m⁻³ depending on the climate variability, which is well related to the NAO index. The position of the sulfide boundary indicates on the volume of the oxic layer in the Black Sea and plays a major role in the functioning of the Sea ecosystem. No clear trend of pH decrease in the Black Sea CIL, testifying to progressive acidification, was revealed. The CIL pH variability was significantly correlated with the CIL oxygen changes, which were logically consistent with the interannual variability of the winter vertical mixing intensity.