The north-western part of the Black Sea Ukrainian coast is characterized by the presence of 12 marine lagoons which do not presently have permanent natural connections with the sea. Because of regional climate change, these lagoons have experienced a significant deficit of annual freshwater balance during the last decades and, consequently, an increase in salinity and eutrophication of their waters. One way to stabilize the hydroecological regime of lagoons is to maintain their regular connection with the sea via artificial connecting channels. The deepest and most prolonged Tyligulskyi Liman lagoon is used as an example to determine the morphometric characteristics (width, depth) of the artificial connecting channel which ensures bidirectional water exchange of the lagoon with the sea, i. e. its partial flushing. A numerical hydrodynamic model is used to estimate how the morphometric characteristics of the connecting channel influence the intensity of water exchange between the lagoon and the sea and of water renewal by seawater for the various parts of the lagoon. The dynamics of sea water volume concentration in reference points in the lagoon is used as an indicator of seawater intrusion into the lagoon and their degree of water renewal in its various zones. The proposed methodology could be used for other lagoons of the same type in the north-western part of the Black Sea.

Biological wastewater treatment plants (the BTPs) of city of Odesa (Northern and Southern Plants) are the most powerful permanent sources of nutrient pollution of coastal waters of Odesa Region of the north-western part of the Black Sea (the NWPBS) and the Khadzhybei Liman. The article includes a comparative analysis of changes, taking place since the beginning of the 21st century, related to the qualitative composition of return water of the Northern and Southern BTPs and the amount of nutrients reaching the marine environment together with such water, as well as the analysis of possible influence of such changes on eutrophication of coastal waters of Odesa Region of the NWPBS and the Khadzhybei Liman. It was established that despite significant decrease (by 1.7 - 2 times) of return water discharged from the BTPs the amount of nitrates and nitrites reaching the marine environment together with such water increased by 4-6 times. At the same time the reduced inflow of organic matter, ammonium nitrogen and phosphates can be observed. Treatment facilities of Odesa were put into operation back in the 1970s. They use a then typical technology of biological cleaning of wastewater from nutrient compounds in aeration tanks providing aerobic conditions. The technology is considered as incomplete in terms of nitrogen compounds treatment since its final product includes nitrate nitrogen in large quantities. When reaching the marine environment it is absorbed by algae at the stage of primary production of organic matter and thus is transformed in organic nitrogen again. It was noted that since the coastal waters of Odesa Region of the NWPBS and the Khadzhybei Lyman have a high level of trophicity and currently there is a disbalance between mineral nitrogen and phosphorus concentrations in the water towards insufficiency of mineral nitrogen reserves (as compared with the standard Redfield stoichiometric ratio), additional inflow of nitrates and nitrites in the marine environment together with return water from the BTPs would, in certain circumstances, lead to algal bloom and aggravation of negative eutrophication-related consequences. The research allowed making a conclusion that a modern flow sheet of advanced biological cleaning (treatment) of wastewater from nitrogen compounds should be implemented at the Northern and Southern BTPs in order to reduce the inflow of nitrates in the marine environment. Such flow sheet should ensure both nitrification and denitrification processes.

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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.