Temporal Fluctuations of Zooplankton Communities in Varna Bay (Western Black Sea) and Sevastopol Bay (Northern Black Sea): A Comparative Study

Abstract

Human activities are mainly concentrated in the coastal regions which appeared less capable to assimilate the adverse effects of those threats. The consequences of foremost pressures to European coastal areas, respectively to the Black Sea such as water pollution, eutrophication, loss of biological diversity, introduction of non-indigenous species, over fishing, land use and landscape deterioration, and coastal erosion have been already well documented (Moncheva et al., 2001, Prodanov et al., 2001, Gubanova et al., 2002, Kamburska, 2004, Kideys et al., 2005, Oguz, 2005, Kamburska et al., 2006). Insufficiency of comparable time series data across the coastal areas of the Black Sea is still a key problem for a comprehensive assessment of the marine environment. The present study is focused on the comparison of the zooplankton quality and quantity parameters in two vulnerable coastal areas Sevastopol Bay (Northern Black Sea) and Varna Bay (Western Black Sea). It aims to assess the current state and long-term trends of zooplankton communities of the two regions of the Black Sea and the response to anthropogenic and environmental shifts. The objectives are: 1) to reveal the structure of zooplankton community in both areas; 2) to contrast temporal variability of the plankton fauna structure in the coastal ecosystems of Northern and Western Black Sea. Observations are based on the long-term data for mesozooplankton abundance, key species and taxonomic groups, temperature and salinity collected at monitoring stations in Varna and Sevastopol Bays during the period 1967 - 2005 (Varna Bay) and 1976-2005 (Sevastopol Bay). The results of inter-annual dynamics of mesozooplankton quantity revealed similarities in the timing of maximum abundance during the 1980s in both areas. The structure of the community significantly shifted over the decades and the reorganization was mainly with respect to dominant groups and species. Thus Oithona nana maintained high density in the period 1976-1980, while Acartia clausi (dominant over the year) and Pleopis polyphemoides were constant components of the plankton fauna with similar dynamics mode in the study regions. In spite of the observed similarities, zooplankton communities in Varna and Sevastopol Bays manifested peculiar features. The total abundance in Varna Bay ranged from 1083 ind.m-3 to 52 978 ind.m-3, while in Sevastopol Bay it varied from only 276 ind.m-3 to 14501 ind.m-3. During the period 1976-1980, the zooplankton amount in Sevastopol Bay was from 2 to 12 fold higher than in Varna Bay. Since the 1980 the total mesozooplankton abundance increased in Varna Bay in contrast to Sevastopol Bay. Noctiluca scintillans was regularly presented in Sevastopol Bay, but with lower numerical abundance compared to Varna Bay where it often reached ¿blooming¿ concentrations. The alterations in zooplankton assemblages could be further attributed to the impacts of climate and anthropogenic activities in both regions.