Chemical contamination is still a likely reason for EU waterbodies not to reach a “ood ecological status” However, risk assessment of contaminated waterbodies is mainly focused on a substanceby- substance basis, while mixed contamination is the rule rather than the exception. This is definitively the case for toxic trace metals such as zinc, nickel and copper, which often occur close to their legally allowed environmental limits. Chronic ecotoxicity data, many of which generated and analyzed by the applicants in a recently concluded joint FWO project, have shown significant mixture effects in individual species. Even in cases where each individual metal was harmless, the metals caused significant harm when dosed in combination. This is called the “omething from nothing”phenomenon. Indirect evidence and standard risk modeling suggest that this phenomenon could even be more pronounced in certain multi-species communities, for instance because synergies could emerge via the foodweb, but there is hardly any experimental data to evaluate this. The general objective of this project is therefore to quantify and increase our understanding of metal mixture effects on planktonic freshwater community structure and function. We will combine experimental approaches with ecological modelling to identify conditions leading to “omething from nothing”effects on communities. Our research will ultimately find its application in improving metal mixture risk assessment.