MODELING THE FINE-STRUCTURE OF THE PHYTOPLANKTON CONCENTRATION IN A STABLY STRATIFIED SEA

The paper presents a one-dimensional mathematical model describing the vertical distribution of chlorophyll a and nutrients in a stratified sea. The model takes into account the most important physical, chemica l and biological processes controlling the evolution of this function. This model was tested with data obtained in situ during the Baltic re search expedition of R/V Prof. Siedlecki in 1980. The vertical distrib utions of temperature field, chlorophyll a and nutrient concentrations were taken as initial conditions for various simulations. The results of numerical simulations explained the influence of biological proces ses on the variability of phytoplankton concentration in time and spac e under various dynamic conditions. The variable dynamic conditions we re simulated by the change of intensity of the layer-mixing processes, achieved by superimposing on a constant vertical shear a variable str ucture of the vertical distribution of the Brunt-Vaisala frequency. Th is study reveals that the Richardson number values decrease and the va lues of the turbulent mass exchange coefficient increase with the incr ease of the vertical velocity gradient. For values of vertical shear g reater than 0.1 ms-1 m-1, microscale interbeddings are completely mixe d. Under the same hydrodynamic conditions various chlorophyll a concen tration distributions may be obtained, depending on the initial concen tration of nutrients. Hydrodynamic conditions, initial distributions o f chlorophyll a and nutrient concentrations play a basic role where th e microstructure of these functions is concerned.