A comparison of lake classifications based on aquatic macrophytes and physical and chemical water body descriptors

Abstract

The Water Framework Directive (WFD) requires that surface water bodies within a river basin be differentiated according to type, defined according to prescribed geographical or physico-chemical water body descriptors. Type-specific biological reference conditions, representing high ecological status, must be established for each derived water body type. A reference network of 20 lakes in Northern Ireland, representative of a WFD environmental typology, was sampled for physical and chemical variables and the lakes were classified on the basis of their aquatic macrophytes. A comparison was made between the efficacy of a WFD based multimetric approach and a multivariate approach at partitioning variation in lacustrine macrophyte communities. It was demonstrated by canonical correspondence analysis that a multivariate model explained more biological variation than a WFD multimetric classification. The predictive power of a set of environmental variables was tested using multiple discriminant analysis and canonical analysis of principal coordinates. These statistical methods were used to test how well the variables discriminated between groups in both models. The continuous variables effectively discriminated the a priori macrophyte classification groups; poor classification cross-validation rates were obtained using the WFD-based environmental classification lake groups. It was concluded that the WFD-based environmental classification did not adequately describe the ecological continuum that was evident from a classification based on aquatic macrophytes. It is implied from the findings that type specific reference conditions derived from large scale environmental classifications are inadequate as they do not sufficiently describe the ecological variation in lake macrophyte communities.