| Estuaries and coastal areas are able to trap significant quantities of material (both solid and dissolved), and thus act as filters between land and ocean. From a management point of view it is essential to quantify the filtering capacity of estuaries and the processes that influence the filtering capacity. In this way the effectiveness of management measures, such as nutrient reduction and the restoration of brackish-water zones, to reduce the total nutrient load to the ocean can be quantified. Traditionally, the filtering efficiency is calculated by the ratio between the nutrient removal rates within the estuary and the input of nutrients by the river. However, a large fraction of the nutrients present in an estuary comes from the ocean, and is transported into the estuary by means of tidal currents. Since the removal processes within the estuary do not distinguish between river-based and ocean-based nutrients, the filtering efficiency will be overestimated by this method. In this study, a new method has been developed to quantify the filtering capacity of a estuary for nutrients by means of a nutrient-tracer method. With this method the nutrients from a particular source (e.g. river) are labelled, and their transport and processes are dynamically modelled. Spatial patterns of the labelled nutrients can be produced at different times of the year. With this nutrient-tracer method, the yearly averaged removal of nitrogen and phosphorus, discharged by the Schelde river into the Westerschelde is calculated. Within the Westerschelde a total of 2.5x109 g nitrogen/yr and 6x107 g phosphorus/yr is removed by processes such as burial, grazing and denitrification. With an annual load by the river Schelde of 1.4x1010 g nitrogen and 1x109 g phosphorus, this results in a filtering capacity of 18 % for nitrogen and 6 % for phosphorus within the Westerschelde estuary. Financed by Ministry of Transport, Public Works and Water Management |
