The presence of microplastics (MP) in environment is considered a global problem. Most research is focused on marine ecosystems, although freshwaters are also burdened with MP, so proposed project covers riverine system (Krka) and the Adriatic Sea at the mouth of the Krka River to assess MP occurrence in water, sediment and aquatic organisms (fish and mussels), considering wild and farmed populations. The project objectives are to assess relation of MP occurrence with effects and biological changes in wild and exposed aquatic organisms and point out environmental risks and novel biomarkers. Biological effects of MP on organisms will be studied in anthropogenically affected areas (wastewater, agriculture, tourism, nautical marinas), control site (river spring) and farming sites of brown trout, gilthead seabream and mussels. Intestine, gills and muscle in fish and digestive gland, gills and mantle in mussels represent target organs. In addition to wild populations, impacted by mixture of contaminants, organisms will be exposed to certain types, sizes, concentrations and shapes of MP and their combination with other polymer types and pollutants (metals). Measured biological changes upon exposure to MP (fish and mussels from environment, mussels from experiments) will include physiological changes, genotoxicity, efficiency of respiration, induced oxidative stress, neurotoxicity, general stress. The fate of MP in soft tissues and possible mechanism of incorporation into the shell will be analysed in exposed mussels, a well-known sentinel organism. Acute toxicity of MP will be assessed in daphnids, representatives of primary consumers. Fish intestinal parasites, acanthocephalans, will serve as a new model for assessing mechanism of possible co-exposure and MP uptake in parasites. Results on MP impact in environmental conditions, aquaculture, experimental exposures, will provide input data for environmental risk assessment and novel biomarkers of MP exposure monitoring.