Simple SummaryIn order to reliably record pollution from microplastics within animals, indicators for a monitoring program must be found. For this purpose, we collected invertebrates, fish, and sediment cores at 10 sites along the Wadden Sea coast of Lower Saxony, Germany; determined the amount of microplastics; and made recommendations for potential bioindicator species, based on the sampling conditions and results. The species studied included lugworm (Arenicola marina), common periwinkle (Littorina littorea),blue mussel (Mytilus edulis),and European flounder (Platichthys flesus). In total, microplastics were detected in 88% of the specimens and in each sediment core sample. Regarding the polymer composition, eight different types of microplastic were identified. Based on the results, the species blue mussel and European flounder are recommended for microplastic monitoring in biota.AbstractMonitoring strategies are becoming increasingly important as microplastic contamination increases. To find potentially suitable organisms and sites for biota monitoring in the German Wadden Sea, we collected invertebrates (n= 1585), fish (n= 310), and sediment cores (n= 12) at 10 sites along the coast of Lower Saxony between 2018 and 2020. For sample processing of biota, the soft tissue was digested and the sediment samples additionally underwent a subsequent density separation step. Microplastic particles were identified using Nile red and fluorescence microscopy, followed by polymer composition analysis of a subset of particles via µRaman spectroscopy. All investigated species, sediment cores, and sites contained microplastics, predominantly in the morphology class of fragments. Microplastics were found in 92% ofArenicola marina, 94% ofLittorina littorea,85% ofMytilus edulis, and 79% ofPlatichthys flesus, ranging from 0 to 248.1 items/g. Sediment core samples contained MPs ranging from 0 to 8128 part/kg dry weight of sediment. In total, eight polymers were identified, predominantly consisting of polyethylene, polyvinylchloride, and polyethylene terephthalate. Considering the sampling, processing, and results, the speciesMytilus edulisandPlatichthys flesusare suitable species for future microplastic monitoring in biota.