On typology and reference conditions for phytoplankton in rivers and lakes in Germany
Phytoplankton characteristics, such as biomass, blooms and composition, reflect the degradati- on of trophic state in a broad range and the pretictive power is limited (Vollenweider 1968, Reynolds 1997 etc.). The EU-WFD gives the great chance to specify this reaction for different water body types, determined by hydrological and geomorphological processes. The biological descriptions of specific water body types will give us a deeper inside of the response of biologi- cal groups to disturbances and the opportunity to define more specific management tools to achieve a higher ecological status. The water body types must represent typical habitats, which are inhabited by specific as- semblages of organisms. Therefore, the types would fit the best with biological characteristics,if they are defined as ecotopes (Platteeuw and van der Molen 2001).On the other hand, it is almost unsolvable to define characteristic habitats, which are meaningful for all four biological groups (fish, macrophytes/phytobenthos, phytoplankton, macroinvertebrates), and to reduce the number of types to a realistic and practicable level (10 to 20 types). This is because Germany´s landscape cover three ecoregions (alps; highlands; flatland) and a lot of different geomorphotypes. This impossible task given by the WFD results in a typology of lakes or rivers, which are adequately only to those biological groups, for which the typology was primarily developed. Development of assessment method for macroinvertebrates are the most advanced ones in Germany. Thus, the authorised typology of rivers is dominated by criteria important for macroinvertebrates. The typology of lakes is a result of former national assessment criteria and of criteria suggested by EU-WFD and is derived from a data set of 319 lakes larger than 0.5 km 2 (Mathes et al. 2002). We have to arrange our biotic data within this given typology, since it is unproductive to develop specific typology for each biological group. We have to verify the typologys applicability for phyto- plankton and propose additional criteria, when they are absolutely necessary. Concerning the classification of river phytoplankton, only few of all types must be evaluated by phytoplankton. Our first preliminary results reveal that even within one degradation state and one water type the reaction of phytoplankton on trophic disturbances is still very broad. Therefore, we recom- mend the description of alternative states within one degradation status. Describing alternative states also for reference conditions, which are likely in shallow lakes and rivers with a alternati- ve dominance of macrophytes or phytoplankton, is useful for considering the interaction of the different biological groups.
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