Supply and Exchange of Water and Nutrients in the Grådyb Tidal Area, Denmark

Supply of nutrients to estuaries and coastal waters is an increasing problem because it can contribute to eutrophication of these environments. In the worst cases it may lead to oxygen depletion of the bottom water. For a micro-tidal area in western Denmark a water exchange model based on conservation of salt and water has been constructed. It is demonstrated how this model can be used for predicting the concentration of total N in the tidal inlet as a function of supplied quantities. The significance of the different nutrient sources are evaluated and by use of the model a net budget for total N is set up. Finally, it is suggested that this net budget can be used for a rough estimation of the annual mean net flux of total N from the sediment to the water. The result obtained compares well with mass balance studies of fine-grained sediment carried out in the same area. It is concluded that total P cannot be treated in the same way as total N because a substantial part of P in the water is adsorped to clay and silt particles suspended in the water leading to a non-conservative behaviour.