The short timescale variability of the oxygen inventory in the NE Black Sea slope water

Abstract

Warm winters have recently become common over the Black Sea, leading to the risk of shoaling oxygen penetration. The insufficient supply of oxygen to the near-bottom layer may result in a decrease in faunal abundance. However, there is a lack of data on the temporal variations in oxygen throughout the water column over short timescales of hours to weeks. In this paper, new observations over the upper part of the NE Black Sea continental slope are presented. Regular, frequent measurements were performed using a moored profiler from January to early March 2016. The profiling allowed for direct observations of the temperature in the cold intermediate layer (CIL), pycnocline structure, current velocity, and oxygen stratification and in particular, the depths of hypoxia onset. The average local oxygen inventory below a depth of 30 m was 24.9 mol m-2. Relatively high/low oxygen inventory cases were related to the thin/thick main pycnocline, which was associated with the onshore/offshore location of the Black Sea Rim Current. The pycnocline hindered the vertical transport of oxygenated water to the CIL. The vertical displacements of the hypoxia onset depth ranged from 97 to 170 m, while the shelf edge depth in this region usually ranged from 90 to 100 m. Intermittently, the hypoxia boundary depth fluctuated on two timescales: ~17 h due to inertial oscillations and 3-7 days due to current meanders and eddies.