Work Performed
1.1 Model development
The regional Black Sea ecosystem model (BSEM) has been applied for the first time for biogeochemical simula¬tions of the Black Sea ecosystem.
One of the key modification of the existing models is the introduction of two new components - the carnivore predators Mnemiopsis and Noctiluca shunt.
The model is coupled to the General Estuarine Transport Model (GETM). It is forced with fluxes, obtained from realistic meteorological conditions and tuned for the Black Sea ecosystem in particular.
Main results of the hydrodynamic model development are published.
1.2 Testing and model validation
– Coupled BSEM and JRC hydrodynamic model has been rigorously and precisely verified and validated against measured data and independent calculations
– The quality of the forcing data affecting our simulations has been analysed and reported
1.3 Assessing and developing the scenarios
It has been concluded that the model has the potential to simulate realistically evolution in the Black Sea ecosystem at seasonal and inter-annual scales:
the growth in the phytoplankton/zooplankton biomass and changes in seasonal cycles of the main ecosystem components
Scenarios with nutrient load and without nutrient load from the rivers have been performed.
Final Results and Impact
The coupled model is able to capture basic physical and biological processes affecting phytoplankton variability and presents a good agreement with available data and model studies. Areas near the Danube plume and along the western Black sea coast and shelf are characterised by relatively high production throughout the year due to the continuous supply of nutrients by river discharge (primarily by Danube and also by Dniepr and Dniestr) and additionally due to the increased stratification caused by the low salinity zone. The Danube influenced area is clearly identified in the satellite chlorophyll im-ages and the simulated phytoplankton especially during summer when nutrient supply from the deep waters is limited and Danube provides the main nutrient supply. In the absence of any physical mechanism to efficiently supply these inorganic nutrients into the euphotic zone, eutrophication could not play effective role in bıological production throughout the entire basin, except coastal regions.
Despite that the North-Western Shelf area is rich in nutrients, this contribution is not the main source of nutrients in the deep sea area. Nitrate load from rivers is mainly spreading and circulating along the Rim current. The dynamical processes necessary to supply nutrients into the surface layer are generated by the strong wind stress forcing and intense cooling in the region, which subsequently lead to strong convective mixing in the water column to bring the nutrients from its subsurface pool. Therefore, the changing climatic forcing introduced an efficient mechanism, which intensified the anthropogenically-driven biogeochemical processes during the 80s. It is found that the vertical flux of nutrients from the pycnocline in winter is a key factor for phytoplankton growth in the sea interior and supports new production. Conversely, without appreciable subsurface nutrient accumulation in the absence of the eutrophication, strong vertical advective and convective processes in the water column might not be so effective for generating strong biological production in the basin. Consequently, the ecosystem conditions observed in the Black Sea in the recent 3-4 decades could be interpreted as the result of the joint process of anthropogenic and natural climatic forcing.
