Modelling benthic denitrification processes over a whole drainage network

Abstract

Denitrification is an important process of nitrogen cycling in river ecosystems as it can regulate nitrogen availability, and therefore primary production, controlling the degree of eutrophication . The strength of denitrification within a river network can alternate very rapidly in space and time and is essentially driven by the interactions between surface water, river geomorphology and microbial process rates. In this study, benthic denitrification was quantified over an entire drainage network by linking a deterministic sediment module [Thouvenot M., Billen G., Garnier J., 2007. Modelling nutrient exchange at the sediment–water interface of river systems. J. Hydrol. 341(1–2), 55–78] to a hydrological/biogeochemical model (Riverstrahler). The benthic module included the calculation of nutrient exchanges across the sediment–water interface as a result of the sedimentation flux of organic material provided by Riverstrahler. Along the Seine, the coupled model was able to reproduce nutrients concentrations in the water and the impact of pollution from point sources in terms of fluxes across the sediment–water interface. Over the entire drainage network of the Seine river system the model simulated the observed increase of organic matter content in the sediment with stream order and its subsequent increase of oxygen and nutrients fluxes at the sediment–water interface. Moreover, due to the high variability of denitrification in space and time, Riverstrahler is a better tool to approximate denitrification over an entire year and drainage network than a calculation based on sparse direct measurements of benthic denitrification. Finally, the nitrogen budgets of two different hydrological years (wet and dry) showed that although for both years riparian denitrification was more important than benthic denitrification, the latter could not be neglected during dry years as it contributed to up to 10% of the losses from river inputs.