Above and in the underground: Linking microplastic patterns in cave and surface crustaceans along a karst river stretch.

Although microplastic contamination is intensively studied in aquatic ecosystems, subterranean habitats are rarely considered especially with respect to the exposure of biota. We conducted a pilot sampling campaign to investigate differences in microplastic contamination in surface and cave dwelling organisms from the same functional feeding guild. Samples from a pair of amphipods (surface- Synurella ambulans, cave- Niphargus stygius) and a pair of isopods (surface and cave population Asellus aquaticus) were obtained from the sinking river Pivka in the Slovenian karst region. We used a state-of-the-art analytical pipeline that included plastic-conserving purification, rigorous chemical detection and automated analysis including quality assurance/quality control. Microplastic particles were found in both surface and cave crustaceans. Most of the plastic particles detected were fragments, with a small proportion of fibres and no spheres. The largest proportion of particles found in the organisms fell within the size range of 20-50 μm, followed by the range of 50-100 μm. However, a few fibres were larger (> 500 μm) but were in the size range of food particles previously found in crustaceans. The total number of particles (reaching up to 70 particles/pool of specimens) seem not to be related to the type of habitat where the animals were collected (surface-subterranean), but rather to the taxon and feeding strategy of the animals. These preliminary results on a limited number of samples imply a taxon-specific risk of microplastic ingestion suggesting that different taxa should be monitored to obtain a holistic overview of microplastic contamination of groundwater biota. This short communication emphasizes the need for more extensive sampling campaigns focusing on groundwater biota and covering a larger number of organisms. Future studies should also include monitoring water and sediments to establish a better link between the presence of microplastics in the environment and their accumulation in organisms.