Understanding the oxygen budget and related ecological processes in the river Mosel: the RIVERSTRAHLER approach

The oxygen budget in the transboundary river Mosel was analyzed with the he lp of the RIVERSTRAHLER model. The model, developed for the river Seine, wa s used after minor modifications of the kinetics of microbial processes. Th e impact of benthic filter feeders (Dreissena polymorpha) was introduced in to the model to better explain phytoplankton decline in the canalized secto r of the river. Hydro-geomorphology, meteorology and point and non-point so urces of nutrients were analyzed as required by the model, according to the stream order concept at the scale of the whole drainage network of the mai n tributaries and along the main branch of the river Mosel, from Millery to Koblenz. The model was validated on water quality data (phytoplankton biom ass -Chl a-, nutrients, oxygen) collected at half-monthly intervals during the period 1993-1995. A reasonable agreement was found at both the seasonal and spatial scales. The validated model was used to calculate the oxygen b udget that shows variations in the contributions of biological processes (n et primary production, bacterial and benthic respiration, nitrification) al ong successive stretches of the main river branch. Bacterial respiration do minates in sectors particularly affected by effluent inputs. Benthic filter feeders colonising these canalized sectors contribute to increases in resp iration and oxygen deficit through their own respiration and their impact o n phytoplankton. Several possible management scenarios, aimed at improving oxygenation, were tested with the model. An 80% reduction of both phosphate s (to reduce eutrophication) and organic matter (to decrease bacterial acti vity) restores autotrophic conditions in the 7(th) order sector.