Chemical variability in the Black Sea: implications of continuous vertical profiles that penetrated the oxic/anoxic interface

Abstract

During the 1988 Black Sea Expedition, we employed a pumping system to generate continuous vertical profiles of nutrients, sulfide, temperature, salinity and light transmission to depths as great as 360 m and to collect oxygen samples from discrete depths. The results can be summarized as follows: (1) In contrast to most prior investigators, we did not find regions where both oxygen and sulfide occurred at concentrations >~1 μm. (2) The profiles demonstrated the existence of a double phosphate maxima in the central Black Sea which disappears at the margins. (3) Our results in combination with historical data strongly suggest that maximum nitrate concentrations in the upper layers of the Black Sea have risen significantly in recent years. The maximum nitrate value of ~7.5 μM was similar in all pump profiles and found at a similar σ t, even though regional variation in the depth of the maximum was ~100 m. Our data also suggest an increase in the nitrate inventory of the Black Sea. (4) We found undulating patterns of chemical variables (particularly sulfide and ammonium) in the vicinity of the Bosporus, arising from the high salinity inflow. The effect of this process was still evident in one profile located 100 km to the northeast of the Bosporus. (5) In general, ammonium, nitrate and nitrite disappeared within the suboxic region, suggesting that this zone was a sink for all three species. This implies that the deeper strata are not supplying combined nitrogen to the photic zone at least during the spring and summer. (6) Small-scale vertical variability in chemical distributions in and near the oxic/anoxic interface was usually correlated with temperature and salinity fine structure. (7) Our data support the view that the oxic/anoxic interface is rising.