Salinity in the deep sea varies mostly only slightly and ranges between 34.69 % in the Pacific and 35 % in the northern Atlantic. Only in the Red Sea and Mediterranean salinity is significantly higher with about 39 %. This influences also the adjacent seas, as the heavy surface water flows in and sinks to deeper levels. This may affect also the species composition in the NE Atlantic.
The oxygen content of the deep sea ranges usually between 3.6 ml/L in the Pacific and 7 ml/L in the Atlantic. But there are also areas in the Eastern Pacific, northern Indian Ocean, and in the Black Sea, with much less oxygen saturation. For instance, along the Californian coast at 600 to 1100 m depth, hypoxic (low levels of oxygen) conditions can be encountered with a minimal oxygen concentration of 0.2. ml/L. The fish species encountered in this area showed a particularly reduced metabolism rate as an adaptive response to hypoxic conditions. Anoxy (lack of oxygen) prevails at 200 to 2000 m depth in the Black Sea, in the 1800 m deep Cariaco trench, and in the immediate vicinity of hydrothermal vents.
By Franz Uiblein, a MAR-ECO scientist