| Summary: | In this thesis we develop a one-dimensional ecosystem model of the Scheldt estuary and investigate the estuarine filter function with respect to nitrogen and carbon. Only 10% of the total imported nitrogen is lost from the estuary to the atmosphere, which is in contrast to the seventies and eigties where this loss amounted to 40% and 20% respectively. Whilst the estuary remains a significant source of CO₂ to the atmosphere, our results suggest, that there is a downward trend in CO₂ degassing from the nineties to our model time period. Furthermore, we structure, unify and connect existing pH modelling approaches and develop a methodology to quantify the influences of biogeochemical and physical processes on the pH. This methodology is illustrated via three theoretical scenarios applied to a simple one-box model of the upper Scheldt estuary: halving the organic matter load entering the model area and two ship spill scenarios of ammonium nitrate fertilizer and ammonia. By applying this methodology to our one-dimensional model of the Scheldt estuary we shed light on dominant processes governing the yearly averaged longitudinal pH profile along the estuary, as well as driving forces for interannual changes in mean estuarine pH. Nitrification is identified as the main process governing the pH profile along the estuary, while CO₂ degassing accounts for the largest total proton turnover per year. The main driver for changes in mean estuarine pH from 2001 to 2004 was a changing freshwater flow which influenced pH directly via alkalinity and dissolved inorganic carbon but also, to a significant amount, indirectly via total ammonium concentration and nitrification rate. We then further elaborate our pH modelling methodology and add chemical interpretation through explicitly connecting reaction stoichiometry and influences of processes on pH. The total rate of change of protons can be decomposed into a linear combination of the biogeochemical process rates and the coefficients in this expression quantify a sensitivity of ...
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