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This study aimed at the development of an multi-metric assessment system for rivers using phytoplankton
with respect to the EU Water Framework Directive (EU WFD). The study was restricted to the planktonrich
German river types. In a first step the commensurability of river types referring to their potential to
build up phytoplankton biomass was analysed. Therefore a correlation analysis of seasonal Chlorophyll a
mean values and total phosphorous (TP) concentrations was run with a large data set. Since the observed
biomasses for a given TP concentration differed among but clustered within the predefined river types,
new sub-types were defined, which were partly described by the catchment-specific run-off value of
10 l s-1 km-2 (lowland and mountain streams) or simply by the catchment size (medium-sized lowland
rivers). As TP is regarded to be the most important triggering factor for eutrophication, class boundaries
were redrawn in compliance with the EU WFD by extrapolation from the river type-specific geogenic
background concentration. The TP class boundaries aided the reconstruction of chlorophyll a reference
concentration. Both TP and chlorophyll a were used to define the trophic spectrum of each river type. The
final multimetric assessment system uses several metrics, such as algal biomass, algal classes, and
indicator taxa. The system is also capable of assessing the impact of anthropogenic salinity, which is
reflected by the “Phytoplankton-Halobienindex”.
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