Applying the critical load concept to the nitrogen load of the river Rhine to the Dutch coastal zone

To reduce negative impacts on the aquatic environment, policymakers in various countries have agreed to reduce the emission load. The goal is a nearly zero emission for man-made synthetic substances and near background value emission for natural occurring substances. Remarkable is the fact that this policy is much stricter and pronounced than the international policy for acidification. To manage the acidification problem the critical load concept was developed. This concept is not aimed at a nearly zero reduction, but is based on the idea that a certain load is acceptable. It takes into account the buffer capacity of bedrocks, soils and freshwater and the no-effect level on organisms of acidifying substances. Based on quality targets (background values and objectives) for nitrogen (in fresh and sea water) and policy measures (e.g. 50% reduction in the load between the years 1985 and 1995) the critical load concept is applied and discussed for the river Rhine and the associated Dutch coast. The annual average critical load for nitrogen in the river Rhine, based on the ecological objectives in the coastal zone, appeared to be smaller than the critical load calculated on the quality objective for the river Rhine: respectively, 79 and 97 ktonnes N a −1. The average near background load for riverine nitrogen to the coastal zone is 26 ktonnes N a −1. It is concluded that when the critical load concept is applied to the riverine loads of hazardous substances there must be first of all a harmonization in ecological objectives in the whole catchment continuum and the associated coastal zone.