The paper outlines the mathematical structure of the numerical mathematical model of water eutrophication. The model is based on the numerical non-stationary hydrothermodynamic model MECCA (Model for Estuarine and Coastal Circulation Assessment) supplemented with a chemical-biological modular unit designed in accordance with the principles of preparation of the water quality model RCA-HydroQual with some modifications introduced by the author. The chemical-biological unit of the model includes a description of the dynamics of the following hydroecological variables at a local point of space: biomass of phytoplankton, mineralization-resistant organic phosphorus in suspended (detrital) and dissolved fractions, labile organic phosphorus in suspended and dissolved fractions, dissolved mineral phosphorus, stable organic nitrogen in suspended and dissolved fractions, labile organic nitrogen in suspended and dissolved fractions, ammonium and nitrate nitrogen, suspended and dissolved fractions of organic carbon resistant to biochemical oxidation, suspended and dissolved fractions of labile organic carbon and water-dissolved oxygen. The paper presents results of calibration and verification of 1D version of the model for the case of the Tyligulskiy Liman (Estuary) in the northwestern part of the Black Sea. A conclusion is drawn that the model makes it possible to display the main features of the annual dynamics of hydroecological characteristics of the liman during phytoplankton vegetation season, in particular, the conditions determining the processes of primary production and biochemical oxidation of organic substance, regeneration of mineral forms of biogenic elements. Application of the model in order to assess the impact of deepening the ‘liman-sea’ connecting canal on the hydroecological characteristics of the Tyligulskiy Liman proved that intensification of water exchange with the sea through the canal will help to reduce phytoplankton production, concentration of organic substance in the water of the liman and, ultimately, will lead to improvement of its trophic status.
The model is expected to be further used to assess the effectiveness of various scenarios of managing the hydroecological regime of the limans of the north-western Black Sea region considering the changes of climatic conditions.
Introduction. The regularities of the formation and dynamics of the oxygen regime in inland seas – the Black Sea and Caspian, under the influence of natural and anthropogenic factors are estimated.
Purpose. One of the features of the Black Sea is the absence of dissolved oxygen in the water below 200 meters depth. A comparison is made between the hydrological conditions of the Black Sea and the Caspian, where the depths are also quite large, but the intensity of vertical exchange is different. In addition, it is necessary to distinguish the reasons for the formation of oxygen deficient in the seas. They can be natural and human-made origin as well. The paper presents an analysis of the cause-and-effect patterns of the development of pelagial hypoxia as the examples of the Caspian and the Black Sea.
Methods. The work is the review of the problem literature that allows an assessment of the current state of gas exchange of pelagic and abyssal marine basins.
Results. Three main formation factors of the features of the hydrological structure and processes responsible for the intensity of gas exchange in the pelagic zone are identified. So, in the Black Sea, saline waters come with the Lower Bosporus Current and flow down the slope, filling the deep layers of the Black Sea basin, forming a stable vertical stratification density, that limiting vertical gas exchange. The second factor is contributing to oxygen saturation of the lower layers during the process of winter vertical circulation that is mainly expressed in the northwestern part.
The third factor is the regime shift of 1976-1978 in the Black Sea as a decrease of the winter temperature and salinity in the surface layer that led to increasing of static stability. According to actively developed convection processes, covering the entire body of the Caspian Sea, in contrast to the Black Sea, there is an intensive exchange between the upper and the deep layers and the intensity of convection depends on the temperature regime of the year. In the Caspian Sea, the regime shift of 1976-1978 led to a twofold increase in the static stability of water below 100 m, the almost complete cessation of the ventilation of the deep waters of the (the process the slope cascading) and the extreme decrease in the concentrations of dissolved oxygen.
Conclusion. In the Black Sea, the formation of a stable of the density vertical stratification is due to the intrusion of saline dense waters of the Lower Bosporus Current, and the winter vertical circulation is expressed only in the northwestern part of the sea, which generally limits vertical gas exchange with the deep water. In the Caspian Sea, convective mixing plays a main role in the formation of the hydrological structure of water and the ventilation of the bottom layers. In cold winters in the northern Caspian, strong cooling, as well as salinity during ice formation, creates the conditions for the formation of waters with a density that allows them to flow down the slopes of the bottom along isopycnic surfaces and aerating the deep layers of the sea.
Assessments of changes in the intra-annual spatio-temporal variability of the hydrological characteristics of the Kuialnytskyi Lyman lagoon under various runoff volumes of the Velykyi Kuialnyk River were obtained from the results of calculations, using the predictive three dimensional hydrothermodynamic model Delft3D-FLOW.
Scenarios of increasing the river runoff inflow up to 25% and 75% of the monthly natural river runoff under conditions of 2015 were modelled, as well as under different by hydraulicity typical years of the modern climatic period of the 21st century (1990-2030), determined according to the most appropriate for the Kuialnytskyi Lyman lagoon region climatic scenario from the ENSEMBLES database, which corresponds to the global A1B scenario, calculated by the MPI-REMO model. Monthly values of the natural runoff of the Velykyi Kuialnyk River, calculated with the use of “climate-runoff” model, were used during the simulation.
Implementation of water management within the catchment area of the Velykyi Kuialnyk River and realization of various engineering and technical measures aimed at the increase of river inflow to the Kuialnytskyi Lyman lagoon, are identified to significantly affect the hydroecological regime of the lagoon only in case of supplying at least 75% of the natural river runoff volume into the lagoon.
The increase of the natural runoff of the Velykyi Kuialnyk River is incapable to provide the stabilization of the hydroecological regime of the Kuialnytskyi Lyman lagoon independently, without periodical replenishment of the lagoon with seawater from the Odessa Gulf.
Under the absence of replenishment of the lagoon with sea water and deficiency in the runoff of other small water streams flowing into the lagoon, the increase of the runoff of the Velykyi Kuialnyk River even up to 75% of the volume of its natural runoff could provide the stabilization of the annual cycle of water level and salinity variability in the lagoon only during the high-water years.