A dynamics model for nutrients was coupled with a one-dimensional physical and biological upper layer model to simulate the temporal changes not only in distributions of a nutrient but then of phytoplankton and zooplankton and study the role of these biological characteristics in the dynamics of the Gdańsk Gulf ecosystem. The 1D model consists of three submodels: a meteorological submodel for the physics of the upper layer and a biological submodel, which also is driven by output from the physical submodel. The biological upper layer model {nutrient-phytoplankton-zooplankton-detritus} incorporates formulations of the primary production mechanism and of the remineralization mechanisms within the mixed layer, in the lower layers and at the bottom as well as of the daily migration of zooplankton mechanism. The model is based on total inorganic nitrogen (NO₃+NO₂+NH₄) and phosphate (PO₄).
The calculations were made for 180 days (from March to August) for two stations at Gdańsk Gulf (at station A, near the Vistula river mouth, and at station B, located further to the open sea). The results of the numerical investigations described here were compared with the mean observed values of characteristics investigated for 10 years, 1990-2000. Comparison of computed and measured values shows the model reproduces the time-vertical structure of characteristics investigated in accordance with the in situ observations. The numerical simulations shown that the differences between the simulated and mean observed values of nutrient in the upper layer are c. 1 mmol m^-3 for total inorganic nitrogen and 0.1 mmol m^-3 for phosphate. The slight differences between the calculated and observed values of surface chlorophyll-a and microzooplankton biomass are ca. 5-20%, depending on the location of the hydrographic station and the month for which the calculations were made. The model can be used to describe the temporal patterns for nutrients distributions and phytoplankton and zooplankton biomass.