Nanoparticles are produced and used in huge amounts increasing their probability to end up in surface waters. There, they are subject to environmentally driven modification processes. Consequently, aquatic life may be exposed to different nanoparticle agglomerate sizes, while after sedimentation benthic organisms are more likely to be affected. However, most ecotoxicity studies with nanoparticles exclusively investigated implications of their characteristics (e.g. size) on pelagic organisms, ignoring environmentally modified nanoparticles. Therefore, a systematic assessment of factors triggering the fate and toxicity of nanoparticles under environmentally relevant conditions is needed. The present thesis, therefore, investigates the implications of nanoparticle related factors (i.e., inherent material-properties and nanoparticle characteristics) as well as environmental conditions towards the pelagic living organism Daphnia magna and the benthic species Gammarus fossarum. In detail, inert titanium dioxide (nTiO2) and ion-releasing silver nanoparticles (nAg), both of varying particle characteristics (e.g. initial size), were tested for their toxicity under different environmental conditions (e.g. ultraviolet-light (UV-light)). The results indicate that the toxicity of nTiO2 and nAg is mainly determined by: their adsorption potential onto biota, and their fate in terms of reactive oxygen species or Ag+ ion release. Thus, inherent material-properties, nanoparticle characteristics and environmental conditions promoting or inhibiting these aspects revealed significant implications in the toxicity of nTiO2 and nAg towards daphnids. Furthermore, the presence of ambient UV-light, for example, adversely affected gammarids at 0.20 mg nTiO2/L, while under darkness no effects occurred even at 5.00 mg nTiO2/L. Hence, the currently associated risk of nanoparticles might be underestimated if disregarding their interaction with environmental parameters