Macrophytes : limitations of using them to assess reservoir status according to the water framework directive

Macrophytes are among the major groups of organisms that the Water Framework Directive (WFD) recommends should be used in assessing the status of reservoirs. The use of macrophytes in reservoir monitoring is still fairly limited and studies are needed on their inter-calibration and sources of variation. Many status assessment methods based on macrophyte communities have been defined for lakes, Nevertheless few of them have been tested for reservoirs. The purpose of the study is to highlight the limitations of using macrophytes to assess reservoir status according to the Water Framework Directive.
An assessment of macrophyte composition and abundance and of water quality was carried out in seven Walloon reservoirs between 2010 and 2012. The main limitations of the macrophyte approach for assessing these reservoirs were: i) geographical features, ii) physical variables, iii) water chemistry and iiii) lack of ecological information on the various macrophytic groups, such as helophytes, algae and mosses. Few of the taxa occurring in the Walloon reservoirs have an indicator score for status assessment based on macrophyte communities. We used two assessment methods to test the relevance of using macrophyte communities to assess reservoir status: the Flanders (Belgium) method (BE-FL method) and the United Kingdom method (UK-method). The relationships between macrophyte metrics for both methods and nutrient enrichment were examined. Highly significant correlations were found between the chemical variables (O-P [mg P/l]); P tot (μg P/l); total hardness (°F), conductivity (μS/cm) and the trophic UK-metric (LMNI); and between it and the abundance of ‘Bl-FL disturbance indicators’ metric (V). The ‘UK group of macrophyte species’ metric does not provide direct information on the quality or degree of degradation of the environment from which the sample was taken.