With increasing global plastics production, the amounts of fragmenting, microscopic plastic debris (microplastics, MPs) are anticipated to rise in aquatic environments. The ecological consequences of this pollution are currently unknown. Studies are being conducted at present to assess these risks but many have been shown to be uninformative from a risk assessment perspective due to flawed and environmentally unrealistic experimental designs; the main problem being confounding effects of food dilution due to the use of particle free controls. Natural particles, such as clay are in the microplastic-size range and ubiquitous in the environment. Hence, to counteract and improve on the poor experimental design to test microplastic effects, the aim of the study was to develop a simple, high throughput screening method which accounts for naturally occurring microparticles like suspended clay in mixtures with MP. Lethal and sub lethal effects of MP alone and in mixtures at different ratios were investigated. Single particle exposures revealed a significant and up to a hundred times higher LC50 for clay compared to the plastics. Among the plastics, weathered plastics were four to five times more toxic than the pristine forms. The mixtures indicated interaction effects of all particles and revealed toxic microplastic-specific effects. A mean decrease of 19 % in the protein content across all particle types was observed after 96 h, however, one plastic type showed a higher reduction in the protein content. Testing MPs effects in mixtures with natural particulates is important due to various interaction effects but more comparative studies with environmental relevant concentrations are required in future. / <p>Master thesis is used for further publication.</p> / WEATHER-MIC, irPLAST, MICROPOLL