Vertical variations in the principle chemical properties of the Black Sea in the autumn of 1991

Abstract Basin-wide principal hydrochemical data for the upper 500 m of the Black Sea were obtained at a total of 197 stations during a multi-ship, multi-institutional, Hydroblack -91 cruise in September 1991. Only offshore stations, 38 from the 1991 joint cruise, 5 from R/V Knorr (1988), 16 from R/V Atlantis-II (1969), and 9 from R/V Bilim (1990) cruises, are considered in conjunction with the present and past data as a function of potential density rather than depth. The oxycline, coinciding with the upper boundary of the permanent pycnocline ( σ Θ = 14.2–14.3), extended down to the σ Θ = 15.3–15.4 surfaces during warm periods (June–September) and to the σ Θ = 15.75 surface during spring periods. The lower boundary of the oxycline (O 2 ⩽ 20 μ M) has remained constant within the last two decades when past and present spring data are considered, but has risen by 0.30 density units when late summer-autumn data are considered. Independent of the geographical location, the base of the transition layer was established at the σ Θ = 16.15–16.20 surfaces where H 2 S ⩾ 5 μM. Nitrate and phosphate reached their maxima at σ Θ = 15.3–15.4 surfaces and then decreased steadily with different slopes until the σ Θ = 15.85 –15.95 surfaces. At the anoxic interface, phosphate concentration increases about 10 fold from ∼ 0.02 –0.5 μ M at the σ Θ = 15.85 –15.90 surface to 4 –6 μM at the σ Θ = 16.15 –16.20 surface. Comparison with the data from the 1970's suggests that both the maximum values and the depth integrated mass of nitrate (g NO 3 —N/m 2 ) between the σ Θ = 14.2 and 16.0 surfaces have increased 2–3 fold, and the density surface where the NO 3 maximum is established has shifted by 0.30 units in the late 1980's.