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.

Annual cycle of sound-scattering mesoplankton in the oxycline and hypoxic zone in the northeastern Black Sea

“…Compared with the open ocean conditions, there are only two migrating copepods in the Black Sea, and their diel vertical migration depths are rather shallow, less than 120 m (Morozov et al, 2020). In the winter, the Black Sea oxic zone is usually ventilated due to surface cooling, turbulent mixing, and deep convection (e.g., Ostrovskii et al, 2018). The vertical gradients of dissolved oxygen vary greatly in the winter.…”

“…Oxygen stratification is sometimes characterized by the fall from approximately 300 to 10 μM across a 40 m thick layer (Ostrovskii et al, 2018). Such conditions would be a substantial barrier for the vertical migration of zooplankton.…”

“…In such a way, the seasonal march of the daytime and diapause SSLs is described in great detail. It is important that in the Black Sea, the dissolved oxygen profile tightly hinges on the density stratification since both are basically due to vertical mixing processes (e.g., Ostrovskii et al, 2018). Hence, displacements of the SSLs with regard to the oxy-isolines are much smaller than those versus the depths.…”

“…The SeaBird 52MP CTD with SBE 43F was regularly calibrated at the facility of the Southern Branch of Shirshov Institute of Oceanology, Gelendzhik. The dissolved oxygen measurements by using the Aanderaa 4330F and SBE 43F sensors at the profiler were originally described in (Ostrovskii, Zatsepin, 2016) and later in the companion paper (Ostrovskii et al, 2018).…”

Using Argo Floats to Characterize Altimetry Products: A Study of Eddy-Induced Subsurface Oxygen Anomalies in the Black Sea

“…To date, the characterization of Black Sea coastal eddies thus remains a technical challenge which, in particular, limits our capacity to characterize their contribution in the Black Sea biogeochemical cycles. Indeed, novel high-frequency observations obtained by tethered (Ostrovskii and Zatsepin, 2011;Ostrovskii et al, 2018) and free-floating profilers (Stanev et al, 2013;Akpinar et al, 2017) revealed that mesoscale eddies and their interaction with the shelf-slope may substantially influence the biogeochemical balance of the Black Sea by inducing diapycnal mixing events. As vertical oxygen transport is limited in the Black Sea by a sharp pycnocline, diapycnal mixing in the basin periphery may act as a net flux of oxygen to intermediate layers, thereby enabling horizontal diffusion (i.e., along isopycnals) towards the basin interior (Ostrovskii and Zatsepin, 2016).…”