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The annual cycle of the plankton dynamics in the central Black Sea is studied by a one-dimensional vertically resolved physical-biological upper ocean model, coupled with the Mellor-Yamada level 2.5 turbulence closure scheme. The biological model involves interactions between the inorganic nitrogen (nitrate, ammonium), phytoplankton and herbivorous zooplankton biomasses, and detritus. Given a knowledge of physical forcing, the model simulates main observed seasonal and vertical characteristic features, in particular, formation of the cold intermediate water mass and yearly evolution of the upper layer stratification, the annual cycle of production with the fall and the spring blooms, and the subsurface phytoplankton maximum layer in summer, as well as realistic patterns of particulate organic carbon and nitrogen. The computed seasonal cycles of the chlorophyll and primary production distributions over the euphotic layer compare reasonably well with the data. Initiation of the spring bloom is shown to be critically dependent on the water column stability. It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface waters begins to develop. It is followed by a weaker phytoplankton production at the time of establishment of the seasonal thermocline in April. While summer nutrient concentrations in the mixed layer are low enough to limit production, the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrient to support subsurface phytoplankton development. The autumn bloom takes place some time between October and December depending on environmental conditions. In the case of weaker grazing pressure to control the growth rate, the autumn bloom shifts to December-January and emerges as the winter bloom or, in some cases, is connected with the spring bloom to form one unified continuous bloom structure during the January-March period. These bloom structures are similar to the year-to-year variabilities present in the data. isms due to the hypoxia/anoxia have currently become common phenomena [Zaitsev, 1992]. The effect of pollution is, however, relatively less severe in the central Black Sea because of its isolation from coastal and shelf waters by the Rim Current frontal zone [Sapozhnikov, 1991;Oguz et al., 1994].In this study we examine the basic physical and biological processes controlling the seasonal cycle of the plankton productivity in the Black Sea. Using a series of numerical experiments, our aim is specifically to explore the conditions and the biological processes which may account for the observed bloom structures (see section 3). The model is restricted to two dimensions (time and depth) and is applied for the conditions appropriate to the central Black Sea. As compared with the northwestern shelf and the Rim Current frontal zone around the basin, horizontal variabilities and contributions of antropogenic inputs from rivers are less important, and this simplified approach might be justifiable for the interio...

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Quasi-synoptic hydrographic data and satellite imagery are used to describe the circulation and the structural variability of the Black Sea with particular emphasis on the Turkish coast. The circulation is indicated to involve a variable cyclonic circulation with no apparent central locus and a well-defined cyclonic "Rim Current" containing meanders and interacting eddy fields confined to the shelf slope. Interspersed between the coastal eddies are filaments and intense jets, often with dipole eddies at their termina. The extension of these features across the shelf-slope into the central basin offshore waters implies important dynamical processes related to the shelf-deep basin exchanges. These features are often steered by the topography and evolve continuously through the mixed baroclinic-barotropic instability of the Rim Current.