Modelling the spatial distribution of phytoplankton and inorganic nitrogen in Narva Bay, southeastern Gulf of Finland, in the biologically active period

The mean spatial distributions of phytoplankton and inorganic nitrogen (IN) in Narva Bay (southeastern Gulf of Finland) during a single biologically active period were studied by means of numerical modelling. The model implemented is three-dimensional ecohydrodynamic model MIKE 3. Model validation gave reasonable justification for studying the mean spatial distributions of IN and phytoplankton in Narva Bay. Results showed that mean inorganic nitrogen concentration in Narva Bay was highest near the river mouth (0.015 to about 0.03 gN/m3), while phytoplankton concentration was relatively low there. Another area of elevated IN concentration was found at the southern coast of Narva Bay some 30 km to the west from the river mouth, where high phytoplankton concentration persisted during summer. In general, coastal waters enriched with inorganic nitrogen (IN > 0.006 gN/m3) formed a narrow belt of approximately 3–8 km wide that extended from the middle of the eastern coast and along the entire southern coast of Narva Bay. According to phytoplankton distribution the coastal waters can be characterized with concentrations exceeding 0.45 gC/m3. The results allow to the separation of the Narva Bay coastal area into zones with different trophic status and nutrients origin: (a) the river mouth area with high IN and low phytoplankton concentration due to intensive hydrodynamic processes, (b) the area to the right of the river mouth with high phytoplankton concentration due to river IN discharge, (c) the southwestern coastal part of the bay enriched with nutrients due to upwelling, and (d) the area between upwelling zone and the river mouth influenced by both upwelling and river discharge.