Simulation of annual plankton productivity cycle in the Black Sea by a one-dimensional physical-biological model

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

Seasonal variability of wind and thermohaline-driven circulation in the black sea: Modeling studies

The seasonal variability of the Black Sea circulation is studied using an eddy-resolving primitive equation model. A series of numerical experiments is carried out to determine the relative importance of wind stress, air-sea thermohaline fluxes, and river-induced lateral buoyancy forcing in driving the circulation on the monthly and seasonal timescales. A synthesis is made of the results with those obtained under yearly climatological conditions by Oguz et al. [1995] to assess whether the major circulation features are a response to the yearly forcings or are dominated by the seasonal cycle. The model experiments indicate that under all forcing mechanisms, the overall basin circulation is characterized by a very strong seasonal cycle dominating the yearly signal described by Oguz et al. [1995]. The purely wind-driven circulation reveals most of the observed circulation features including a well-defined meandering boundary current system and subbasin scale cyclonic gyres forming the interior flow structure of the basin. Topography obviously remains a crucial factor in controlling the pattern of the persistent rim current system all year long. The dynamical instabilities of the rim current produce strong meandering and mesoscale eddies which often modulate the basin and subbasin scale structures of the circulation. The surface thermohaline fluxes generate simpler circulation patterns with a comparable strength but mostly in the opposite direction to the wind-driven circulation. Two important by-products emerge from the present work. First is the necessity of reanalyzing the heat flux climatology. The existing surface thermohaline fluxes, even though not affecting critically the general characteristics of the surface circulation patterns, may induce rather unrealistic horizontal temperature distributions and water mass properties in the surface layer. Second, the role of the northwestern shelf in the cold intermediate water (CIW) mass formation process is shown to be secondary during moderate-to-high winter discharge conditions from the northwestern rivers. In these conditions the freshwater outflow reduces the density of the cold water formed on the shelf by about 1 kg/m(3) as compared with that of the basin interior, which is the major reservoir for the formation of the winter CIW