"Acquisition of rapid, high resolution photosynthesis (i.e. carbon fixation) rates is a major challenge in aquatic research. One way to measure primary productivity in situ at an unprecedented temporal and spatial resolution is by active fluorometry (mainly Fast Repetition Rate techniques). Implementation of this technique, however, requires that the electron transfer rates (ETRs) measured by FRR fluorometry can be easily converted to carbon fixation rates via the electron requirement for carbon fixation (ER). Such conversions require detailed knowledge and a mechanistic understanding of the variation of the ER, which has been shown to exhibit considerable inter- and intraspecific variability. The objective of this study is, therefore, to assess the individual and combined effects of environmental condition and phytoplankton community composition on the ER. A combination of state-of-the-art mesocosms with cultured phytoplankton and bioassays with natural phytoplankton communities subjected to different kinds of nutrient stress will be used to tease apart the individual and combined effects of environmental conditions (nutrient availability) and phytoplankton community composition on the ER. This knowledge will be critical for successful conversion of ETRs to carbon fixation-based primary productivity rates Only if generic relationships between the ER and environmental-taxonomic conditions can be established, can active fluorometry become a universally applied tool for primary productivity measurements in freshwater. The proposed project will lay the foundation for the widespread implementation of automated, routine primary productivity estimates and ecological assessments of freshwater habitats, as required by the European Water Framework Directive in an effort to restore adversely affected freshwater systems to ""good ecological health""."