Unraveling the interplay between seasonal variations in macroinvertebrates and microplastics in urban stream sediments.

This study presents the findings of a case study investigating the seasonal dynamics (spring vs. autumn) of macroinvertebrates, microplastic particle abundance, and sediment composition in the benthos of the Vrapčak urban stream in Zagreb, Croatia, at two sites: upstream (V1) and downstream (V2). The abundance of macroinvertebrates remained constant between seasons, but taxa such as Chironomidae, Ephemeroptera, Diptera pupae and Simuliidae showed significant seasonal variation. The dominance of Chironomidae and Oligochaeta supports the urban stream syndrome hypothesis, where tolerant species increase and sensitive species decrease. Sediment analysis revealed a uniform mineral composition in both seasons with an average carbonate content of 52.8%. The analysis of microplastics revealed that the number and mass of particles at site V1 were almost twice as high as at site V2. In spring, site V1 recorded 12 microplastic particles ranging from 0.5 to 5 mm in size, while site V2 recorded 13 particles. In autumn, V1 had 13 particles, whereas V2 had 19 particles, with V1 showing overall higher concentrations. These variations between sites and seasons may be influenced by site-specific environmental factors, such as differences in flow conditions. The predominant microplastic types were Polyethylene (32%), Polystyrene (28%) and Ethylene Vinyl Acetate (17%). It is noteworthy that 12% of the particles could not be classified, which is likely due to environmental degradation and illustrates the difficulties in identifying microplastics. While no significant differences were found between sites in physico-chemical parameters, seasonal variations were observed in water temperature, flow velocity and nutrient concentrations. This case study highlights the need for standardized microplastic sampling methods and continuous monitoring to manage the dynamic and pollutant-prone urban stream ecosystems. The integrated approach (e.g., combining FTIR microplastics analyses with ecological methods) is promising to understand and mitigate the impacts of urbanization on freshwater environments. [ABSTRACT FROM AUTHOR]