Abstract. The time series of vertical profiles of hydrophysical parameters and dissolved oxygen are critical for identifying trends in water column stratification, mixing, and ventilation in the Black Sea. Due to global warming, warm winters have recently become common over the Black Sea, and the temperature in the cold intermediate layer (CIL) is increasing. Regular measurements that are as frequent as every 2 h were performed using a moored profiler near the shelf break at a depth of 220 m in the northeastern Black Sea from January to early March 2016 to assess the temporal dynamics of the oxygen inventory over the upper part of the continental slope on timescales from hours to months. The moored profiler was equipped with a sensor suite that included the CTD 52-MP CTD with the fast oxygen sensor SBE43F and the acoustic Doppler current meter Nortek Aquadopp 2 MHz, which allowed for direct observations of the CIL temperature, the pycnocline structure, the current velocity and the oxygen stratification, in particular, the depths of the hypoxia onset. The average 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 that was associated with the inshore/offshore location of the Black Sea Rim Current. The pycnocline hindered the vertical transport of oxygenated water to the CIL. The new CIL emerged by horizontal advection above the pycnocline only at the end of the observational survey. The vertical displacements of the hypoxia onset depth ranged from 97–170 m, while the shelf edge depth in this region usually ranges from 90–100 m. Intermittently, the hypoxia boundary depth fluctuated on two-time scales: ~ 17 h due to the inertial oscillations and approximately 5 days due to the current meanders and eddies. Further efforts are urgently needed for monitoring the rise of hypoxia onset depth above the shelf break in the Black Sea.
Gelatinous macroplankton of the open pelagial and shelf of the Black Sea: Distribution in autumn 2016 and interannual changes in biomass and abundance
