Oguz, T. (2002) "The role of physical processes controlling the Oxycline and Suboxic Layer structures in the Black Sea". Global Biogeochem. Cycles, 16(2), 101029-101042.
Oguz, T., H.W. Ducklow, J.E. Purcell, P. Malanotte-Rizzoli (2001) "Simulation of recent changes in the Black Sea pelagic food web structure due to top-down control by gelatinous carnivores". J. Geophys. Res., 106, 4543-4564.
Oguz, T., J.W. Murray and A. Callahan (2001) "Modeling redox cycling across the suboxic- anoxic interface zone in the Black Sea". Deep Sea Research I, 48, 761-787.
Oguz, T., H. W. Ducklow, P. Malanotte-Rizzoli (2000) "Modeling distinct vertical biogeochemical structure of the Black Sea: Dynamical coupling of the oxic, suboxic and anoxic layers". Global Biogeochemical Cycles, 14(4), 1331-1352.
The vertical biogeochemical structure of the Black Sea comprises four distinct layers. The euphotic zone extends from the free surface to the depth of the 1% light level and has a maximum thickness of ~50 m. This is the layer of active aerobic planktonic processes, and is also characterized by high oxygen concentrations of the order of 300 micro moles. The uppermost 20-30 m of the aphotic zone is called the oxycline/upper nitracline zone in which oxygen concentration reduces to about 10 micro mole limit whereas nitrate concentration increases to around 6-8 micro moles. In the subsequent oxygen deficient layer of about 30 m, known as the Suboxic Layer (SOL), nitrate concentrations undergo a sharp decrease to trace values. The SOL is followed by a deep anoxic layer consisting of hydrogen sulphide and ammonium pools. The suboxic-anoxic interface zone involves a series of complicated bacterially-mediated redox reactions. These control the downward transport of nitrate and the upward transports of ammonium and sulfide near the interface zone. The suboxic layer most commonly exists in very thin layers within steep chemical gradients in sediments of rivers and other eutrophic systems, but also exists in the major hypoxic ocean basins. However, it is most easily studied in the Black Sea owing to the great physical stability of the water column, which enabled accurate resolution of the oxygen and sulfide gradient structure. Thus, they seem to maintain a well-defined redox structure which constitutes one of the unique characteristics of the Black Sea biogeochemical system.
The model which provided a unified representation of the biogeochemical pump involves the pelagic food web component, efficient remineralization-nitrification-denitrification cycle, and redox reactions near the unoxic interface. The current version of the pelagic food web structure involves four groups (or dominant species) of phytoplankton. They are diatoms, dinoflagellates, small phytoplankton, Emiliania huxleyi. The second trophic level is represented by mesozooplankton and microzooplankton groups. The top predators are two particular gelatinous carnivores: the jellyfish Aurelia aurita and ctenophore Mnemiopsis leidyi. The opportunistic species Noctiluca scintillans and bacterioplankton further accompany these species groups.
The Black sea is nitrogen-limited system with N/P ratios varying around 5-to-10. The model involves a sophisticated nitrogen cycle which incorporates the dissolved organic nitrogen pool, ammonia, nitrate. The particulate matter are first decomposed by dissolved oxygen. Once oxygen is depleted, nitrate is consumed as an oxidizer in the system. Nitrate also oxidizes ammonium, and dissolved manganese present near the anoxic layer. The particulate manganese generated as a byproduct then oxidizes hydrogen sulfide. This set of reactions maintains efficiently stability of the system and prevents upward rising of sulfidic waters towards the surface layer.
Observed and simulated are shown in the figưure for the annual distributions of total phytoplankton, mesozooplankton, Noctiluca, and Aurelia biomass for the eutrophic, Aurelia-dominated (pre-Mnemiopsis) phase of the ecosystem. The data for phytoplankton and mesozooplankton biomass are taken from measurements carried out at 2-4 weeks intervals during January-December 1978 at a station, off Gelendzhik along the Caucasian coast. The Noctiluca and Aurelia biomass data are taken from measurements on the Romanian shelf and the interior basin, respectively, during the late 1970s and early 1980s (after Oguz et al., 2001a). The major bloom event of the year took place during late winter to early spring as a consequence of nutrient accumulation in the surface waters at the end of the winter mixing season. This was followed by two successive and longer events during spring-early summer, and autumn. The early spring bloom was followed first by a mesozooplankton bloom of comparable intensity, which reduced the phytoplankton stock to a relatively low level, and then by an Aurelia bloom that similarly grazed down the mesozooplankton. The phytoplankton recovered and produced a weaker late spring bloom, which triggered a steady increase in Noctiluca biomass during the mid-summer. As the Aurelia population decreased in August, first mesozooplankton and then phytoplankton, Noctiluca and Aurelia gave rise to successive blooms during September-November period.
Observed, and simulated profiles of dissolved oxygen, hydrogen sulphide, nitrate and ammonium within the Suboxic Layer and onset of the sulfide zone. Even with a highly simplified representation of the redox processes, the model provided a quasi-steady state suboxic-anoxic interface zone structure similar to observations. It was able to give quantitative evidence for the presence of an oxygen depleted and non-sulfidic suboxic. This model pointed out the crucial role of the downward supply of nitrate from the overlying nitracline zone and the upward transport of dissolved manganese from the anoxic pool below for maintenance of the suboxic layer.
It is commonly accepted up to now that variations in the chemical properties in the Black Sea always comform on the fixed density surfaces which are independent of the circulation characteristics, even though they may occur at different depths in different parts of the basin. For example, the upper and lower boundaries of the SOL are suggested to correspond to the fixed density surfaces of sigma_t~15.6 and ~16.2 kg/m3, respectively.
Re-analysis of the available data together with a one-dimensional coupled physical-biogeochemical model simulations, on the other hand, provided a new perspective and interpretation for this approach. While the lower boundary of the Suboxic zone appears to be stable at sigma_t~16.2 kg/m3, the upper boundary is found not to be isopycnally uniform as asserted previously. It is found to vary depending on the intensity of vertical diffusive and advective oxygen fluxes across the oxycline. Anticyclones, with downwelling and downward diffusion (thus, with stronger net downward supply of oxygen), attain a thinner SOL at a deeper part of the water column relative to cyclones, which are characterized by weaker net downward oxygen supply. The upper boundary position of the SOL changes from sigma_t~15.6 in cyclonic to 15.9 kg/m3 in anticyclonic regions, whereas its position in the peripheral Rim Current transition zone occurs at intermediate density values. Thus, interpretation of vertical oxygen variations in terms of density will be misleading without taking into account physical characteristics of the water column.
Horizontal distributions of (a) sigma_t (in kg/m3) at the depth of 10 micro mole oxygen concentration, (b) the depth of 10 micro mole oxygen concentration, and (c) the thickness of the SOL obtained from the measurements of the R.V. Bilim 1991 survey in the southeastern part of the Black Sea. The SOL thickness is defined by the difference between the depths of sigma_t~16.2 kg/m3 (lower boundary) and of the 10 micro mole oxygen concentration (upper boundary). The station locations are shown by solid circles. The actual measured values are also indicated.