Due to recent floodings of the Dutch rivers, new policy was developed in order to prevent repetition of these events. One of the proposed measure is the excavation of floodplains. The sediments of floodplains however is polluted with among others trace metals. In This thesis the bioavailability of trace metals in wet floodplain systems (lakes) is described. The focus was on the role of binding of trace metals by sulfides, present under anaerobic conditions. These sulfide concentrations appear strongly temporally and spatially variable depending on local conditions in the lakes (depth, organic matter, total sulfur pool) and external factors (inundations, season). Extractable metals in combination with sulfides provide a better explanation for benthic species composition than total metal content. Effects on the species level (bioaccumulation, growth, survival) are also related to metals bound by sulfides. The risk assessment of sediments can be improved by taking the studied process into account, if its variability is recognized.