Coexistence of nonnative and Black Sea species in phytoplankton of north-eastern part of the Black Sea: discussion of invasion hypotheses.

In spring and early summer 2005, a new Mediterranean diatom species, Chaetoceros trondsenii, was noted in phytoplankton of the northeastern Black Sea. The parameters of the created phytoplankton species dynamics model were determined in experiments with nitrogen and phosphorus supply. Computational experiments with the model gave the following results: for Chaetoceros trondsenii establishment in an ecosystem, an elevated stability of the water column (water rate exchange in the upper mixed layer should be lower than 0.1 day−1), a relatively high initial number of this species, and low values of nitrogen and phosphorus concentration at the boundary of the seasonal thermocline are necessary. The hypothesis of the introduction of the species into the Black Sea by the raised intrusion of Mediterranean water in winter is more preferable than the hypothesis of releases of the species with ballast water.