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Microplastics have been shown to be able to adversely affect aquatic organisms by themselves (physical effects) but are also discussed to act as vector for chemical pollutants, which are associated with microplastics such as additives or contaminants sorbed to the microplastic material (chemical effects). The aim of this thesis was to systematically analyse how microplastics potentially adversely affect freshwater organisms by themselves and in combination with chemical pollutants. The presented results were obtained from an experimental laboratory approach, which was based on established ecotoxicological methods. Juvenile water fleas (Daphnia magna), as representative for limnic zooplankton, and tadpoles of the African clawed frog (Xenopus laevis), as representative for amphibians, were first exposed to pristine microplastics alone, before combining them with selected chemical pollutants in the next step. High concentrations of both, microplastics and chemical pollutants, were used for exposures in order to get a better mechanistic understanding of potential adverse impacts and to identify critical concentrations of microplastics for freshwater organisms. The results of this thesis provide evidence that small-sized microplastics in the range of a few micrometres at high concentrations in the unit range of mg L-1 are most likely to pose a risk for freshwater organisms, in this case freshwater zooplankton. Removal of a chemical contaminant from the aqueous phase can lead to reduced effect rates of a chemical contaminant in organisms in an equilibrated system when water is the most important uptake pathway, as shown in daphnids for bisphenol A. The vector effect of microplastics may most likely play a role for contaminants which affect organisms at very low concentrations such as endocrine disruptors, as indicated by a trend for enhanced exposure to ethinylestradiol in the presence of microplastics in tadpoles. Both, physical and chemical effects of microplastics in this study were observed only at high concentrations which are presumably above present environmental concentrations. Breakdown of bigger plastic items and ongoing emission of microplastics, though, can be expected to increase the abundance of microplastics in rivers and lakes. The results presented in this study can be compared to recent and future environmental exposure scenarios and can help to estimate the effects of microplastics on freshwater organisms.