Abstract The objective of this synthesis is to\ud present the key messages and draw the main conclusions\ud from the work on lakes in the REBECCA\ud project, pointing out their links to theoretical ecology\ud and their applicability for the WFD implementation.\ud Type-specific results were obtained from analyses of\ud large pan-European datasets for phytoplankton, macrophytes,\ud macroinvertebrates and fish, and indicators\ud and relationships showing the impact of eutrophication\ud or acidification on these biological elements\ud were constructed. The thresholds identified in many\ud of the response curves are well suited for setting\ud ecological status class boundaries and can be applied\ud in the intercalibration of classification systems. Good\ud indicators for phytoplankton (chrysophytes, cyanobacteria)\ud and macrophytes (isoetids and charaphytes)\ud responses to eutrophication were identified, and the\ud level of eutrophication pressure needed to reach the\ud thresholds for these indicators was quantified. Several\ud existing metrics developed for macrophytes had low\ud comparability and need further harmonisation to be\ud useful for intercalibration of classification systems.\ud For macroinvertebrates, a number of metrics developed\ud for rivers turned out to be less useful to describe\ud lake responses to eutrophication and acidification,\ud whereas other species based indicators were more promising. All the biological elements showed different responses in different lake types according to alkalinity and humic substances, and also partly\ud according to depth. Better harmonisation of monitoring\ud methods is needed to achieve better precision in\ud the dose–response curves. Future research should\ud include impacts of hydromorphological pressures and\ud climate change, as well as predictions of timelags\ud involved in responses to reduction of pressures
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