The Water Framework Directive (WFD), implemented in 2000, is the major policy instrument for water quality in the European Union. Its main aim is to homogenise and standardise water quality assessments in all Member States, stimulate water quality improvement and contribute to the management of transboundary water problems. Surface water quality descriptions are a crucial part of the WFD. The first WFD assessment was presented in 2009 and in 2014 a second set of results were available, allowing comparison of the status between both years. The main issue for policymakers is whether the quality has changed over the past years. In this study, two methods are evaluated to examine the differences in surface water quality.
The WFD method uses the differences in the percentage of water bodies with a good quality for comparing between both assessments. The time-series method uses the WFD metrics to calculate water quality with all available monitoring data, aggregated to grid cells. The conclusion concerning the WFD method is that the two assessment reports of 2009 and 2014 are too dissimilar in method and standards and therefore not suitable to evaluate changes over time. The time-series method showed a small improvement for phytoplankton and macrophytes and no improvement for benthic invertebrates.