Establishing supporting element standards : a revised approach and applications

The Water Framework Directive requires member states to determine thresholds for a range of supporting chemical quality elements that will support good ecological status. This is a fundamental step in maintaining natural biodiversity as well as ensuring ecosystem service provision. All countries have defined thresholds, which have been used to report status in river basin planning cycles. The original intention of the work was to determine from empirical analysis a likely range of boundary values using pan-European data, which could be used to help inform any revision of current national boundary values. However, the analysis revealed relatively high levels of uncertainty, emphasising the challenge in establishing these boundary values. This highlighted the need for a more robust method. Binary logistic regression presented advantages over other methods, especially for complex data. However, a probability value must be selected objectively to derive the nutrient threshold. This can be achieved by comparing the proportions of matching and mismatching status classifications of nutrients and a BQE using a confusion matrix. The method is applied to selected supporting elements for rivers and lakes.