Modelling the Transfer and Retention of Nutrients in the Drainage Network of the Danube River

Publication TypeJournal Article
Year of Publication2002
AuthorsGarnier, J, Billen, G, Hannon, E, Fonbonne, S, Videnina, Y, Soulie, M
JournalEstuarine, Coastal and Shelf Science
Start Page285
Date Published03/2002
Mots-clésDanube Basin, modelling, nutrient retention, nutrient transfer and delivery
The Danube catchment basin (817 000 km2, 76×106 inhabitants) is the major freshwater contributor to the Black Sea (6300 m3 s-1, 80% of the annual river discharge into the north-western Black Sea). The aim of the modelling approach developed for the Danube River, is to establish how land use and management of the whole watershed are linked to nutrient (N, P, Si) delivery and retention by the river. The approach uses an adaptation of the RIVERSTRAHLER model, which is based on a schematic representation of the drainage network deduced from geomorphological analysis by stream orders. The whole catchment was divided into 10 sub-basins and one branch, to provide a description satisfying both the need to take into account the heterogeneity of the system and the availability of constraints and validation data. On the basis of this description, a hydrological model was developed, which adequately simulated the seasonal variations of the discharge measured at the outlet of the basin. The model itself resulted from the coupling of the hydrological model with a biogeochemical model (RIVE), which takes into account the main ecological processes. It established a link between microscopic processes, their controlling factors and their macroscopic manifestations in terms of nutrient cycling and ecological functioning at the scale of the whole drainage network. The model was validated for the period from 1988 to 1991 on the basis of available observations of the major water-quality variables involved in the eutrophication processes (inorganic nutrients, phytoplankton biomass, dissolved oxygen, etc.). A reasonable agreement was found between the simulations of the model and the observations. Nutrient fluxes to the Black Sea, calculated for our reference period, are in the same range as those obtained via other approaches. Si/P and N/P ratios suggest silicon, rather than phosphorus, limitation for diatoms and phosphorus, rather than nitrogen, limitation for overall phytoplankton in the coastal zone of the Black Sea. The sharp drop in N and P delivery to the Black Sea, observed since 1991, was simulated with a scenario constructed to reproduce new constraints based on documented modifications of human activity in the watershed. Due to the scarcity of data, there is a need for further validation of the model. Nevertheless, the structure of the model allows the specificity of each sub-basin to be taken into account in future management plans.