A 1-D, pelagic, reactive-transport model of a completely mixed, turbid, heterotrophic estuary - the Scheldt estuary - is presented. The model contains major carbon and nitrogen species and oxygen, as well as pH. The model features three organic matter degradation pathways, oxic mineralisation, denitrification and sulfate reduction, and includes nitrification and sulfide re-oxidation. Apart from advective-dispersive transportalong the length axis, the model also describes O2, CO2, NH3, and N2 air-water exchange. The aim of this model exercise is to determine the fate and turnover of nutrients entering the estuary and their spatial patterns at the beginning of the 21st century. Nitrification is identified as one of the most important processes in the estuary,consuming with 1.7 Gmol O2 y-1 more oxygen than oxic mineralisation (1.4 Gmol O2 y-1). About 8% of the 2.4 Gmol of nitrogen entering the estuary per year is lost within the estuary due to denitrification. Nitrogen and carbon budgets are compared to budgets from the seventies and eighties, showing that nitrification activity has peaked in the eighties, while denitrification steadily declined. Our model estimates an average of 3.6 Gmol y-1 of CO2 export to the atmosphere in the years 2001 to 2004, which is a comparatively low estimate in the context of previous estimates of CO2 export from the Scheldt estuary. |
