Impact of Parameterization of Vertical Turbulent Diffusion on the Results of Simulating the Phytoplankton Biomass Dynamics in the Deep Part of the Black Sea

Purpose. The paper is purposed at studying the impact of vertical mixing processes in the Black Sea on distribution and dynamics of the basic components of marine ecosystem based on numerical simulation.Methods and Results. Two variants of the lower trophic level model of the Black Sea ecosystem differing in the way of parameterizing the processes of vertical turbulent diffusion were used. In the first variant, the diffusion coefficients are represented as the functions depending on depth and time. At that, the time dependence is of seasonal character. In the second variant, in order to describe the vertical exchange processes, the turbulent model was added to the circulation one. In both versions, the biogeochemical parts of the models consisting of 15 compartments include the same equations, coefficients, and functions describing the interactions between different ecosystem components in the upper 200-meter layer of the sea. The calculations for 12 years (1998-2009) were done for both versions of the ecosystem model, and the results were compared. The results of modeling the nitrates distribution were compared with the in situ measurements in the deep part of the Black Sea taken from the interdisciplinary oceanographic database. Besides, the results of simulating the chlorophyll surface concentration were also compared with the analogous satellite-derived measurement results.Conclusions. As for the above-noted calculations, seasonal variability of the basic ecosystem parameters is insignificantly different, at that the parameterization of vertical turbulent diffusion produces a certain effect upon the vertical distribution of ecosystem parameters. Interannual variability in both calculations is characterized by a biomass decrease in the euphotic zone of the sea deep part resulted from a negative trend in the amount of nutrients inflowing with the river waters. The annual average concentrations in the sea upper layer corresponding to the first calculation are higher than those obtained in the calculation by the turbulent model. This is related to the fact that the vertical circulation cell formed due to the wind field cyclonic vorticity over the Black Sea results to be more intense for the first calculation. The nitrates entering into the euphotic zone from the underlying layer is provided mainly by advection rather than turbulent diffusion.