The use of pressure-response relationships between nutrients and biological quality elements: A method for establishing nutrient supporting element boundary values for the Water Framework Directive
The Water Framework Directive requires nutrient boundary concentrations to be established as part of the assessment of ecological status. In this report we use data and relationships developed during the intercalibration exercise for lakes and national monitoring data for rivers to determine ranges of potential nutrient (N & P) boundary concentrations at the intercalibrated boundaries for high/good and good/moderate biological status.
Where data were available we compared the use of different regression models, including multivariate (N+P), and both type I and type II univariate (N or P) models. We suggest that the most appropriate statistical approach is to use either multivariate ordinary least squares (OLS) regression with both N and P as predictor variables or, for univariate relationships, to use type II regression, as the slope of a conventional ordinary least squares regression is likely to be underestimated unless model uncertainty is low, resulting in incorrect predicted boundary values.
We also used two categorical methods to determine boundary values. Firstly, by calculating the distribution of mean nutrient concentrations for water bodies categorised by biological status. Secondly, we developed a method to determine the nutrient concentration at which the mis-match between biological and nutrient status was minimised. Both methods produced boundary values that were similar to those from regression models.
We compiled the results from all of these approaches, together with uncertainty estimates, to provide ranges for the “most likely” and “possible” ranges of boundary values for intercalibration and broad water body types.
For many relationships, particularly in rivers, uncertainty was relatively high, with nutrient concentration typically accounting for only 35-45% of variability. As a result of this uncertainty the range of boundary values that might be predicted if a different, but similar (for example water body type) data set were used was relatively high.
We were only able to use data or published relationships from a limited range of lake and river intercalibration types, but comparing the resulting boundary values to those currently being used by Member States, we demonstrate that in most cases the majority of national boundary values fall within the range of predicted values if uncertainty is taken into consideration.
Given the high degree of variability in the relationships between nutrients and biological status we suggest that further discussion and guidance is needed on how they can be used to support the objectives of the WFD as it is clear that even for well-defined water body types a range of values occur in water bodies that are considered to be in good status according to the most sensitive biological quality element.
PHILLIPS Geoff;
BIRK Sebastian;
BOHMER Jurgen;
KELLY Martyn;
WILLBY Nigel;
POIKANE Sandra;
2019-01-03
Publications Office of the European Union
JRC114381
978-92-79-98199-9 (online),
1831-9424 (online),
EUR 29499 EN,
OP KJ-NA-29499-EN-N (online),
https://publications.jrc.ec.europa.eu/repository/handle/JRC114381,
10.2760/226649 (online),
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