Long-term dynamics of mesozooplankton by the example of the Copepoda community in Sevastopol Bay (1976-1996)

The results of the regular year-round studies of mesozooplankton in Sevastopol Bay carried out in 1976, 1979-1980, 1989-1990, and from 1995 to 1996 are presented. The emphasis is put on the Copepoda community. It is shown that a pronounced transformation of the Copepoda species composition took place as early as in 1976. During the 1980s, the abundance of eutrophic species increased and the Copepoda community became more unstable. During the period from 1989 to 1990, the sharpest and the most essential changes in the community and in the individual populations of copepods proceeded. The number of species decreased almost two times, and the forms that dominated before, namely, Acartia clausi (the small form) and Oithona nana, disappeared, whereas A. clausi and Pseudocalanus elongatus became the dominant species. In addition, the total abundance of the Copepoda community decreased by more than one order of magnitude and their share in the total mesozooplankton abundance dropped from 62 to 15%. During that period, pronounced changes in the principal characteristics that determine the community structure took place. The changes in the structure of the Copepoda community and in the seasonal dynamics over the twenty-year-long period are considered. The possible reasons for the community degradation are discussed, namely, deterioration of the environmental quality for marine organisms due to the anthropogenic activity, the long-term climatic fluctuations, and the impact of the invader ctenophore Mnemiopsis leidyi.