Spatial and temporal variability of the Black Sea suboxic zone

We coupled an in situ electrochemical analyzer to a CTD to conduct high-resolution, real-time profiling of redox species across the oxic–anoxic transition zone of the Black Sea water column. Voltammetry was performed using gold–amalgam working electrodes to measure simultaneously soluble oxygen and sulfur species (H2S/HS−, Sx2-, S8) at a resolution of greater than one measurement per meter. In situ data agreed with measurements made in an on-deck voltammetry flow cell coupled to a pump profiling system, and from water samples collected with conventional CTD rosette bottle casts. In situ voltammetric analyses provided rapid redox information, thus enabling more accurate targeting of specific geochemical features by the CTD rosette package. We observed much less lateral oxygen injection from the Bosphorus in 2003 (less than 95 km from Bosphorus) than in 2001 (up to 150 km). This difference can be attributed to variability in physical processes including seasonal temperature and wind variations between winter conditions (2003) and early summer conditions (2001). Furthermore, suboxic zone thickness varied basin-wide, exhibiting changes in the depth of oxygen extinction (minimum detection limit=3 μM) and sulfide onset (minimum detection limit=30 nM). The density surface for oxygen extinction was more variable than the density for the onset of sulfide. Vertical shifts in oxygen extinction and sulfide onset also were observed at the western central gyre station for seven profiles measured over 21 days in 2003.