Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 4 2008 10.5194/bg-5-981-2008 http://www.biogeosciences.net/5/981/2008/ http://www.biogeosciences.net/5/981/2008/bg-5-981-2008.html http://www.biogeosciences.net/5/981/2008/bg-5-981-2008.pdf 981 1006 2008-07-04 Present nitrogen and carbon dynamics in the Scheldt estuary using a novel 1-D model A. F. Hofmann a.hofmann@nioo.knaw.nl K. Soetaert J. J. Middelburg Netherlands Institute of Ecology (NIOO-KNAW), Centre for Estuarine and Marine Ecology – P.O. Box 140, 4400 AC Yerseke, The Netherlands A 1-D, pelagic, reactive-transport model of a completely mixed, turbid, heterotrophic estuary – the Scheldt estuary – is presented. The model resolves major carbon and nitrogen species and oxygen, as well as pH. The model features two organic matter degradation pathways, oxic mineralisation and denitrification, and includes primary production as well as nitrification. Apart from advective-dispersive transport along the length axis, the model also describes O₂, CO₂, and N₂ air-water exchange. The aim of this study is to present a model which is as simple as possible but still fits the data well enough to determine the fate and turnover of nutrients entering the estuary and their spatial patterns in the years 2000 to 2004. Nitrification is identified as one of the most important processes in the estuary, consuming a comparable amount of oxygen as oxic mineralisation (1.7 Gmol O₂ y^−1 vs. 2.7 Gmol O₂ y^−1). About 10% of the 2.5 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. 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