Plastic physical properties and season influence macroplastic litter colonization by freshwater biota.

Plastic debris in inland waters is less studied than in marine systems, particularly regarding its interactions with biota. Despite being problematic pollutants, plastics may serve as habitats for freshwater organisms. A research gap exists regarding how plastic physical properties and season influence colonization. We addressed this by investigating macroplastics colonization by aquatic biota in a German reservoir over a full year. Plastics were categorized by type (hard, foam, soft, and film) and surface texture (porous vs. smooth), and polymer composition was determined by near-infrared spectroscopy. We hypothesized that warmer seasons and rougher textures promote higher organism abundance and diversity. Both plastic and natural items were colonized throughout, with higher abundance and diversity in spring and summer. Rotifera, Nematoda, and Ciliophora were the most abundant colonizing animal groups, with remarkable material-specific differences: nematodes predominantly colonized Styrofoam, which supported the highest animal abundances, possibly due to its high surface area. Biodiversity did not differ between wood and plastic, although Styrofoam hosted a greater taxon richness than wood. In conclusion, both material physical properties and seasonality affected the colonization of plastic debris. As plastics-attached organisms may contribute significantly to sessile freshwater communities, clean-up actions in late autumn or early winter may be a relevant consideration for minimizing ecological effects.