Major modifications of the Black Sea benthic and planktonic biota in the last three decades

Abstract

During the last three decades, increasing eutrophication and other man-made activities have considerably changed the structure and functioning of the Black Sea ecosystems, mainly in its NW corner, affecting both the qualitative and the quantitative state of the benthic and planktonic communities. Marked changes have been registered especially in the structure of littoral ecosystems. The macrophytobenthos has shown a gradual but continuous decline, approximately since 1945-1950, due to both natural (e.g. occasional massive frosts) and anthropogenic factors (silting of the rocky bottom, increased turbidity, diminution of light penetration). The previously large belts of Cystoseira barbata, a perennial brown alga, along the western coast have practically disappeared as have numerous other associated and/or epiphytic algal and animal species. The present algal flora, which displays a reduced species diversity, is uniform. Generally it is dominated by Enteromorpha intestinalis, E. linza, Ceramium elegans and C. arborescens. These newly dominant species with short and nearly seasonal life cycles show considerable production but, nevertheless, they do not reach the levels attained by Cystoseira during previous decades. The evolution of zoobenthos communities is marked by a qualitative impoverishment, expressed by a 50 to 60% reduction of species number since the 1960s, and by the decline in density of the numerous populations formerly omnipresent. Thus species which were prevailing and characteristic formerly (Spio filicornis, Corbula mediterranea, Syndesmia fragilis, Spisula subtruncata, Mytilus galloprovincialis) have given place to fewer opportunistic species (Neanthes succinea, Polydora limicola, Melinna palmata, Mya arenaria, Scapharca inaequivalis) which have proliferated. The resulting population increases did not compensate however for the reduction of general biomass and density, which are lower by 35-84% than those measured 25 years ago. While zoobenthic communities have become more and more homogeneous as a result of mass proliferation of a few species, their structure is now more unstable owing to the permanent disturbance generated by blooms are related benthic mass mortalities. Due to intense eutrophication phytoplankton biomass surpasses that of the past. Some essential structural changes have occurred and new quantitative and qualitative characteristics have been recorded. The increase of the amplitude and frequency of algal blooms is a significant ecological consequence of the accumulation of nutrients in sea water. Since 1970, blooms are no longer exceptional phenomena. For example, in the 1980's alone, 46 blooms due to 15 algal species were recorded in the Romanian littoral waters. Besides the bloom-producing species, other numerous mass species have remarkably developed. During the 1980's, 79 species recorded densities larger than 100,000 cells l −1 , as compared to 57 species in the 1970's and only 38 species in the 1960's. Numerical density of the main species increased from the levels of the 1960's, for 66% of the species in the 1970's, and for 78% of the species in the 1980's. From the 1960's to the 1980's the proportion of non-diatoms in the numerical density of phytoplankton increased from 8% to 62% of the total. Changes in the quantitative proportions of the main algal groups are due to the changes in the quantitative proportions of the main algal groups are due to the changes in the nutritive basis. These changes include the diminution of the ratios Si:N and Si:P (which is detrimental to diatoms), and the increase of organic matter (which favors the phytoplankters with mixotrophic affinities belonging to Dinoflagellata, Euglenida, and Chrysophyta). Between 1983 and 1990 the average biomass of phytoplankton in Romanian coastal waters was more than 8 times higher than that assessed between 1959 and 1963