The Black Sea deep waters constitutes the world's largest reservoir of toxic hydrogen sulfide. 100 m of ventilated surface waters are all that separate this reservoir from the atmosphere. This situation results from the permanent halocline that separates the surface layer (of low salinity due to river inflow) from the deeper layer (of high salinity due to inflowing Mediterranean seawater), restraining ventilation to the upper layer (Fig. ).

In the lower part of the halocline, a permanent suboxic layer separates the Black Sea surface oxygenated waters ([O2] > 20 µM) from the deep sulfidic waters ([H2S] > 20 µM) . More precisely, considered a threshold of 10 µM of oxygen because they analyzed high-quality oxygen data. The threshold of 20 µM of oxygen was applied later to analyze historical oxygen data of lower quality. The upper (O2 disappearance) and lower (H2S onset) interfaces of this suboxic layer are controlled by different biogeochemical and physical processes , and undergo uncorrelated vertical migrations . Sinking organic matter is mainly respirated aerobically within the oxycline: the lower part of the oxygenated layer where oxygen concentration decreases downwards to 20 µM. Increasing flux of organic matter, induced by a period of high nutrient load from the 1970s to the late 1880s, resulted in higher oxygen consumption above the suboxic layer and a shoaling of the upper suboxic interface .

After reduction of nutrient inputs around 1990 , the Black Sea was described as a recovering ecosystem . This perspective was supported by improved eutrophication indices in the open sea as well as the stabilization of the upper suboxic interface in the 1990s . However, the timescale of the expected recovery (i.e., the timescale associated with the chain of biogeochemical mechanisms relating oxycline penetration depth to riverine nutrient loads) is not quantitatively understood. Several processes cause the oxycline depth to respond with a time lag to the reduction of riverine nutrient inputs. First, nutrients are mainly delivered to the northwestern shelf, where the accumulation of organic matter in the sediments buffers the riverine inputs, with slow diagenetic processes controlling and delaying the nutrient outflow across the seaward boundary . Second, the intermediate oxidation–reduction cycling of nitrogen, sulfur, manganese, iron and phosphorus that separates oxygen from hydrogen sulfide can delay the response of the lower suboxic interface to changing nutrient fluxes by several years .

In addition to these biogeochemical factors, the dynamics of the upper and lower interfaces of the suboxic layer are controlled by physical processes . In the Black Sea, dense waters formed by winter cooling and mixing do not sink to the deepest layer, as in the Mediterranean Sea, but accumulate on top of the permanent halocline (Fig. ). The resulting cold intermediate layer (CIL) is a major feature of the Black Sea vertical structure. Cold intermediate water formation and advection by the cyclonic basin-wide Rim Current ventilate the oxycline and thereby influence variability in the depth of the upper suboxic interface . Recently, atmospheric warming was shown to reduce the ventilation of the lower oxic layer . At deeper levels, the dense sinking plume formed by the Mediterranean inflow through the Bosporus, which entrains water from the overlying CIL, injects fingers of oxygenated water directly into the deeper part of the suboxic layer and upper sulfidic layer and thus acts to control the depth of the lower suboxic interface .

Previous long-term analyses of the vertical migration of the suboxic interfaces either ended (1955–1995; ) or started (1985–2015; ) with the eutrophication period, excluding the large–scale overview required to grasp the interactions of eutrophication and climate factors. Those analyses lacked a comprehensive consideration of the natural spatial and seasonal variability of the vertical distribution of oxygen.

In the presence of large gradients, uneven data distribution may induce artificial signals when interannual trends are assessed from direct annual averages. In the stratified Black Sea, properties expressed in terms of depth coordinates (m) present a high spatial variability due to mesoscale features and to the general curvature of Black Sea isopycnals . As an alternative, using density (isopycnal levels, σθ) as vertical coordinate is generally considered a stable solution to assess the vertical migration of the chemocline on a decadal scale . However, the spatial confinement of the lateral oxygen injections associated with the Bosporus plume, as well as the spatial variability of diapycnal ventilating processes , imposes a horizontal structure to the oxygen penetration depth when expressed in terms of density . As this spatial gradient might scale with the temporal variations (a range of 0.17 kgm-3 was observed during the Knorr 2003 campaign, ), it has to be considered when deriving interannual trends.

The present study describes the application of the DIVA (Data-Interpolating Variational Analysis) detrending procedure to untangle the temporal and spatial variability of three indices related to the Black Sea oxygenation status: the depth and density level of oxygen penetration and the oxygen inventory. These values were diagnosed from a composite historical data set of oxygen vertical profiles. We review the evolution of those indices through the past 60 years and discuss the respective controls of eutrophication and climate factors.

The spatial analysis of oxygen penetration depth showed that the use of density coordinates does not eliminate the sampling error associated with uneven spatial coverage (Fig. ). Deeper oxygen penetration (on a density scale) in the Bosporus area were expected, in relation with the intermediate lateral injections associated with the Bosporus plume. In addition, deeper oxygen penetration (on a density scale) in the southern and eastern periphery suggests the occurrence of diapycnal ventilation along the steep bathymetry . The aggregation of the most recent ship-based profiles in the Bosporus area and in the southeastern region (Fig. ), might have led to an overestimation of the basin-average oxygen penetration depth in the last decade, hence to an underestimation of the shoaling trend of the Black Sea oxic layer.

Considering spatial variability revealed a clear shoaling trend for oxygen penetration depth. This shoaling can be seen on both depth and density scales (Fig. a, b). This confirms the hypothesis that the shoaling of oxygen penetration depth is not due to a general shoaling of the main halocline, but is associated with a shifted biogeochemical balance in the oxygen budget .

Using σθ coordinates depicts clearer temporal variations (Figs.  and ). The shoaling rate varies in time and was more intense during 1970–1985 and from 1996 onwards. Argo diagnostics using different oxygen threshold show a larger discrepancy in the case of pycnal coordinates. The co-occurrence of density and oxygen gradients (Fig. ) results in a higher sensitivity to the sensor accuracy for the σθ diagnostic for oxygen penetration. However, even a systematic underestimation by 10 µM of oxygen concentration by Argo profilers does not invalidate our results.

The positive correlations between CIL cold content and oxygen inventory observed for all the periods illustrate the ventilation of intermediate layers by CIL formation and advection (Fig. b). In the early 1990s, the transient recovery of the three oxygenation diagnostics (Figs. a, b, c, a) provided arguments supporting the stability of the oxic interface . This stabilization matched the convenient perception of a general recovery of the Black Sea ecosystem after the reduction of nutrient load around 1990 . However, Fig.  indicates that the oxygenation diagnostics obtained for the period 1986–1998 were associated with much higher ventilation rates (i.e., higher CIL cold content) than during the previous periods. If, in response to nutrient reduction, the biogeochemical oxygen consumption terms had been lower during the period 1986–1998 than previously, the increased ventilation during that period would have resulted in higher oxygen inventories. Instead, oxygen inventories observed during 1986–1998 are lower than those observed in the previous decade for similar levels of CIL cold content. We conclude that high CIL formation rates during this period provided enough ventilation to mask ongoing high oxygen consumption.

The fact that the relationship between oxygen inventories and CIL content for the last period 1999–2015 is similar to that of 1986–1998 indicates a stabilization in the biogeochemical oxygen consumption terms. Higher air temperature in this last period , by limiting winter convective ventilation events led to the lowest oxygen inventories ever recorded for the Black Sea (Fig. c).

Forecasted global warming, without excluding transient high ventilation periods, will limit CIL water formation and reduce the oxygenation of the Black Sea intermediate layers. At the same time, uncertainties remain regarding the capacity of re-flourishing economies of the lower Danube watershed to recover their productivity in a more sustainable, less polluting form. Economic development in the Danube Basin could reverse the improving situation of eutrophication if nutrients are not managed properly . Under these conditions, there is no reason to expect that the oxycline shoaling observed over the past 60 years will stabilize.

There are reasons to worry about a rising oxycline in the Black Sea. First, biological activity is distributed vertically on the whole oxygenated layer, as indicated by zooplankton dial migration . The reduction of the oxygenated volume described in this study could therefore have impacted on Black Sea living stocks by reducing carrying capacity and increasing predation encounter rates. It would be timely to estimate now the impact that a further shoaling of the oxic interfaces would bear on the Black Sea resources for the fishing industry.

Second, under present conditions, a massive atmospheric release of hydrogen sulfide caused by a sudden outcropping of anoxic waters remains unlikely, due to the stability of the Black Sea pycnal structure. Such outcropping event of sulfidic waters would have dramatic ecological and economical consequences . On 27 September 2005, an anomalous quasi-tropical cyclone was observed over the western Black Sea that led, in a few days, to the outcropping of waters initially located at 30 m depth . Two years earlier, sulfide was measured in the same area (western central gyre) at around 80 m . Because global warming is expected to increase the occurrence of extreme meteorological events , every meter of oxycline shoaling would bring the Black Sea chemocline excursion events closer to the realm of possibility.

The present study evidenced the decline of the Black Sea oxygen inventory during the second half of the 20th century and first decade of the 21st, and highlighted the threat that further atmospheric warming casts upon the vertical stability of the Black Sea oxygenated layer.

Further works are urgently required to assess how actual nutrient emission policies adequately prevent, in the context of forecasted warming, the ecological and economical damages that would arise from a further shoaling of the oxic interface.

Spatially resolved biogeochemical models are needed to integrate explicitly the interacting processes affecting the Black Sea oxycline.

It is also essential (1) to determine to which extent the shoaling of the oxygen penetration depth entrains a shoaling of the sulfidic onset depth, (2) to set up a continuous monitoring of the Black Sea oxygen inventory and the intensity of winter convective ventilation (through CIL cold content), and (3) to clarify and quantify the interplay of diapycnal and isopycnal ventilation mechanisms and, in particular, the role played by the peripheral permanent/semi-permanent mesoscale structures and how this relates to the intensity of the Rim Current . We propose that these objectives might be answered by maintaining in the Black Sea a minimum population of both moored and drifting autonomous profilers equipped with oxygen and sulfidic sensors.