Compositional change of FGs is expected to follow longitudinal processes in rivers, and also better indicate ecological status than other systems based on taxa composition or species richness. In order to prove the reliability of FGs in rivers, their compositional changes are related to natural gradients like geography, as well as to physical and chemical anthropogenic pressures, occurring along the Loire. Major findings of the Thesis are the followings: (i) Different lentic phytoplankton functional approaches are able to identify reliable river zones along the Loire based on FG composition of potamoplankton. The level of understanding provided, however, depends on the taxonomical and ecological resolution of approaches. In rivers, no satisfactory water quality management can be built without fine resolution of benthic and planktonic diatoms, as well as of cyanobacterial taxa. (ii) The FG classification is able to delimit natural and human-mediated changes in the potamoplankton composition along the River Loire. These changes can be described by the Q(r) composition index, which successfully indicates local morphological alternations like damming, or regional scale differences in nutrient availability according to land use practices. (iii) Both the German PhytoFluss index based on taxa-level resolution and the Hungarian HRP index based on FGs provide comparable and reliable water quality indications along the River Loire. In case of adequate implementation of indices according to site-specific river typology, these national potamoplankton assessments can be successfully compared or implemented at international level, as also required by the European Water Framework Directive. (iv) FG classification of potamoplankton displays similar river zonation in each year along the River Loire. Besides quantitative differences, major FGs and the subdominant accessory FGs all follow similar longitudinal trends, while further FGs were identified to reflect specific annual hydrological regimes. (v) In the River Loire, ecosystem functioning (as the biomass:TP ratio) displayed different relationships with taxa and FG richness. The highest potamoplankton richness was identified as a consequence of physically mixed habitats from either natural or humanmediated sources, while the best functioning occurred at low taxa number and at medium FG richness. Accordingly, high richness/diversity in large rivers might not provide automatic evidence for better ecosystem functioning, and cannot be a general objective without understanding its functional properties and controlling factors at different scales.