Modeling the response of the north-western Black Sea ecosystem to changes in nutrient delivery by the Danube river after its damming in 1972
Lancelot, C.; Staneva, J.; Beckers, J.-M.; Stanev, E. (2000). Modeling the response of the north-western Black Sea ecosystem to changes in nutrient delivery by the Danube river after its damming in 1972, in: Balopoulos, E.T. et al. (Ed.) International conference. Oceanography of the eastern Mediterranean and Black Sea. Similarities and differences of two interconnected basins, Zappeion international conference Centre, Athens, Greece, 23 to 26 February 1999. Research in Enclosed Seas Series, 8: pp. 232-233
In: Balopoulos, E.T. et al. (2000). International conference - Oceanography of the eastern Mediterranean and Black Sea: Similarities and differences of two interconnected basins, Zappeion international conference Centre, Athens, Greece, 23 to 26 February 1999. Research in Enclosed Seas Series, 8. European Commission: Brussel. ISBN 92-828-9019-8. 494 pp., more
In: Research in Enclosed Seas Series. Office for Official Publications of the European Communities: Luxemburg, more
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Document type: Conference paper
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Marine/Coastal; Brackish water; Fresh water |
| Authors | | Top |
- Lancelot, C., more
- Staneva, J.
- Beckers, J.-M., more
- Stanev, E.
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| Abstract |
The Black Sea, with its unique features, such as being the largest enclosed catchment basin receiving freshwater and sediments inputs from rivers draining half of Europe and parts of Asia, is very sensitive to cultural eutrophication. The north-westem Black Sea ecosystem has indeed experienced several changes during the last decades driven by human perturbations occurring almost synchronously in the coastal ecosystem itself and in the drainage basins of the rivers: (i) manipulation of hydrologic regimes of out-flowing rivers, in particular the damming in 1972 of the Danube river by the 'Iron Gates' at approximately 1 000 km upstream, (ii) urban and industrial expansion and the intensive use of agricultural fertilisers, (iii) introduction of exotic species such as the gelatinous Ctenophore Mnemiopsis sp. , (iv) selective and excessive fishing. In less than 30 years the Black Sea ecosystem has been evolving from a highly biodiverse ecosystem characterized by rich biological resources to a low biodiversity ecosystem dominated by a dead-end gelatinous food-chain. During the late 1980-early 1990's there was an almost total collapse of fisheries, which coincided with an unprecedented increase of the jellyfish Aurelia but also of the exotic combjelly Mnemiopsis unintended introduced in the Black Sea in the mid-1980's. At the time, it was thought that these carnivores- lacking predators and feeding zooplankton, fish eggs and larvae -were drammaticaIly reducing recruitment of fish to the adult stocks. Since 1994, however, some positive signs of coastal Black Sea ecosystem recovering are to be observed. Phosphorus and nitrogen loads have considerably decreased. Some planktonic and benthic species considered to be extinct or very rare nowadays in the Black Sea are again very common. The abundance of jellyfish has levelled out and the number of anchovy eggs and larvae has increased. Incidentally these events are corresponding with the economical collapse which recently occurred in the Eastern aid Central-Europe countries. To which extent the present-day catastrophic socio-economical situation induces such a rapid response of the coastal ecosystem cannot be assessed by the simple correlation's between historical and ecological events. Mechanistic models, which describe carbon and nutrients cycling in coastal ecosystems over seasons and years as a function of meteorological and human forcing provide a powerful tool which encompass this complexity. As a first step in this direction, the complex ecological model BIOGEN describing the carbon, nitrogen, phosphorus and silicon cycling throughout different aggregated chemical and biological compartments of the planktonic and benthic marine systems has been implemented in the north-western Black Sea to assess the response of this semi-enclosed marine ecosystem to cultural eutrophication by the Danube river. The Danube river receiving the effluents from eight European countries is affecting the north-western Black Sea ecosystem and represents the most significant source of river-bome nutrients flowing in the Black Sea. The trophic resolution of BlOGEN has been chosen to simulate the major ecological changes reported in this highly vulnerable coastal area since the 1960's. Particular attention has been paid to establish the link between quantitative and qualitative changes in nutrients, phytoplankton composition and the related foodweb structure. The BlOGEN numerical code structure thus includes 34 state variables assembIed in five interactive modules describing the dynamics of (i) phytoplankton composed of three distinct groups each with a different trophic fate (diatoms, nanophytoflagellates, non-silicified opportunistic species); (ii) meso- and micro-zooplankton; (iii) gelatinous organisms composed of three distinct groups each deprived of predators (the omnivorous Noctiluca and the camivores Aurelia and the alien Mnemiopsis) and of organic matter degradation and associated nutrient regeneration processes by (iv) planktonic and (v) benthic bacteria. The latter benthic module calculates nutrient exchanges at the sediment-water interface. The capability of the BIOGEN model to simulate the recent historical changes ofthe trophic structures in response to quantitative and qualitative nutrient modifications by the Danube is demonstrated by running the model for three contrasting years of the period 1985-1995. For this purpose, the numerical code of BIOGEN has been implemented in an aggregated and simplified representation of the north-westem Black Sea hydrodynamics. The numerical frame consists in coupling a OD 'Lagrangian-Iike' BIOGEN box model subjected to Danube with a 1D-BIOGEN representing the open-sea boundary conditions. The volume of the coastal box and the exchanges flux es between box es and adjacent areas were estimated from the 3-D GHER general circulation of the Black Sea submitted to climatic forcing. The Danube nutrients inputs were calculated from reported monthly nutrient concentrations at the Danube out-flow in combination with climatic Danube freshwater discharges. Model results clearly show that the recent coastal eutrophicated-related problems of the Black Sea were not only driven by the quantity of nutrients discharged by the Danube but the balance between them was important as well. BIOGEN predictions clearly demonstrate that phosphate rather than silicate was the limiting nutrient driving the structure of the phytoplankton community affecting in turn the structure and functioning of the planktonic food-web. In particular it is shown that a well-balanced N:P:Si nutrient enrichment with respect to phytoplankton needs as for instances that observed in 1991 had a positive effect on the linear food-chain diatoms-copepods while the regenerated-based microbial food chain remained at its basal metabolism. When present in the system, the gelatinous camivores as wel1 were taking benefit of this enrichment throughout their feeding on copepods. Furthermore a significant synergetic effect of fishing pressure andcultural eutrophication could be demonstrated for this period by acting indirectly on the fishing pressure by tuning the mortality coefficient of copepods. On the other hand, model runs for the years 1985 and 1995 indicate that nitrogen (1985) or phosphate (1995) deficiency has been driving the structure of the planktonic food-web towards the dominance of an active microbial food-web in which bacteria and microzooplankton were playing a key role, the former as nutrient regenerator and the latter as a trophic path to the copepods and hence to the camivorous. Under such conditions, however, the model predicts a significant reduction of the gelatinous organisms invasion, in perfect agreement with present-days records. From these model scenarios it is concluded that the observed positive signs of Black Sea ecosystem recovering are well related to the reduction of nutrient loads by the Danube, in particular phosphate. Furthermore the observed and predicted rapid response of the north-westem Black Sea to the significant interannual and seasonal changes of the Danube nutrients signature -both in quantity and quality- indicates that this semi-enclosed coastal area represents an excellent pilot site where the effect of nutrient reduction on the quality of a given ecosystem can be tested. |
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