Science for Water Policy (SWAP) - Norwich, September 2th-4th, 2002.
Oral presentation : Implementation of the Water Framework Directive to the ecological assessment of lakes using macrozoobenthos assemblages. Garcia XF, Pusch M, Walz N.
Keywords : typology, ecological status, lakes, macrozoobenthos fauna, Water Framework Directive
Introduction. - The Water Framework Directive (WFD) provides a legal framework for the sustainable management of inland and costal waters in Europe. The first task imposed by the WFD is to assess the current ecological status of the aquatic ecosystems. The WFD proposes an assessment approach which compares the actual ecological conditions with type-specific reference conditions. The types have to be previously defined on geographical and morphological characteristics.
However, the present heavy modification of many aquatic ecosystems in Europe renders this method difficult to apply. Especially, it has become a true challenge to find undisturbed type-specific reference systems. In this study, we implemented the approach proposed in the WFD to a number of lakes in order to test it. We draw some conclusions concerning the strong points and the shortcomings of the approach, and suggest some addition.
Method. - The 10 studied lakes are a part of a larger project funded by the regional water agency, which aims to assess the ecological status of 30 lakes from Brandenburg (northern Germany). In each lake macrozoobenthos was sampled in 6 sectors with 6 replicates from the infraprofundal to the littoriprofundal zone (1.5 to 6 m), using an Eckman-Birge grab sampler. A multivariate ordination method (correspondence analysis, Benzecri 1976) was used to ordinate and identify type-reference assemblages from the species database.
Results. - The WFD proposes two systems (A and B) to define the type of each lake. System A is based on 4 abiotic characteristics (altitude, mean depth, surface area and geology). System B includes these 4 variables as "obligatory factors", and a list of additional abiotic characteristics as "facultative factors". In our study, the use of system A resulted in the classification of the 10 studied lakes into 3 types, mainly differentiated by the mean depth. Undisturbed type-specific reference lakes were only found for types I (Stechlinsee) and II (Gr. Wummsee), lakes from type III being all highly impacted and polytrophic.
According to the WFD, the definition of type-specific biological reference conditions for the "benthic invertebrate fauna" includes taxonomic composition, abundance and diversity of the fauna, as well as the ratio of disturbance sensitive taxa to insensitive taxa.
The ordination resulting from the correspondence analysis reflects both the trophic status of the lakes (F1 axis) and the typology based on System A (F2 axis). Hence, the typology based on macroinvertebrates is in agreement with the abiotic typology, except for Stolpsee and Sacrower See. Type-specific reference biocenoses are deduced from the correspondence analysis. For two lake types, the reference levels of the faunal diversity has also be defined. The comparison of the findings to the type-specific biological reference conditions enabled the classification of the 10 lakes into the 5 levels of modification defined by the European Commission in the WFD.
Conclusion. - Using the approach given by the WFD, it was possible to achieve an ecological assessment of the 10 studied lakes. However, the test of the approach also showed two shortcomings.
1. First, the use of the only System A appears inadequate to reach a precise typology belonging to specific biological characteristics on the national level. In system B, the list of the facultative factors has to be completed in order to cover all the types met in Europe. For instance, the possibility of the connection of a lake to a navigation waterway, which produces mechanical disturbance and immigration of neozoans, is still lacking.
2. The second point concerns the assessment of the level of modification of lakes. The WFD approach assumes that the modification of the ecological conditions leads to the disappearance of reference species. However, lake modification is also accompanied by the colonisation of disturbance insensitive species, including a significant proportion of species independent of the type. This aspect is especially important for lake types with no reference lakes left, so that the invertebrate assemblages cannot be reconstructed. In these impacted lakes the ecological status cannot be assessed by the WFD approach based on reference assemblages.
In spite, for these modification levels a different approach has to be used. Here, a list of disturbance insensitive indicator species is useful, which can statistically be derived from databases. The case of the four polytrophic lakes Beetzsee, Breitlingsee, Mellensee and Plauer See forms a good example for the difficulties in the application of the WFD approach described above.
Reference. - Benzecri J.P. 1983. L'analyse de l'inertie intra-classe par l'analyse d'un tableau de correspondances. Cahier de l'Analyse des Données, 8 : 351-358.