Ecologische normen voor vermesting, verzuring en verdroging. Aanzet tot een risicobenadering

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In this report a strategy for ecological standard setting is proposed for environmental variables in the field of acidification, eutrophication and desiccation, using a risk assessment. Risks at the species level and ecosystem level are specified. At the species level the risks are assessed on the base of the ecological amplitude of a species. The ecological amplitude describes the occurrence probability of a species as a function of environmental variables. Values of the environmental variable are assessed that correspond with a specified reduced occurrence probability. Reduced occurrence probability may be due to intoxication (too much) and limitation (too little). These values are quantified for each species as the uppervalue and lowervalue of the environmental variable that envisage 90% of the observations of a species (5% percentile and 95% percentile values). At the ecosystem level risks are specified as the concentration that protects a specified percentages of all species. In the dutch environmental policy for pollutants this percentages is set at 95%. The ecological standard is assessed by combining the risks at the species level and the ecosystem level. The number of species, for which the 5% and 95% percentiles are not exceeded, are plotted as a function of the environmental variable. From this relation the maximum environmental level can be assessed that protects 95% of all species. This strategy for standards setting is illustrated with a case study in the Province of South-Holland. Ecological standards are calculated for nitrogen load and changes in groundwaterlevel using data on 285 plant species. This report will be issued in english.