The urban footprint on aquatic ecosystems: comparison of microplastic dynamics in water bodies with divergent usage patterns

Abstract Microplastic pollution has become a pervasive threat to aquatic ecosystems, affecting water bodies regardless of their protection status or proximity to urban pressures. This study investigates the impact of urbanization pressure on microplastic (MP) pollution in aquatic ecosystems by comparing two water bodies in the megacity Istanbul: Küçükçekmece Lagoon and the protected freshwater reservoir of Büyükçekmece Lake. Seasonal sampling of surface waters and the eastern sand smelt fish Atherina pontica was conducted in both systems. All samples were (i) digested using hydrogen peroxide (H 2 O 2 ), (ii) followed by visual examination under a stereo microscope and (iii) heat treatment test to identify potential microplastics (MPs). Subsequently, Raman spectroscopy was employed to characterize their polymer compositions. Microplastics obtained were analysed using stereo microscopy and Raman spectroscopy to determine their type, size, colour and polymer composition. The findings revealed that both the MP concentration in surface waters (0.17 items/L) and the prevalence of MPs in fish digestive tracts (38.0%) were significantly higher in Küçükçekmece Lagoon compared with Büyükçekmece Lake (0.05 items/L; 23.0%). Fibre-type MPs dominated both environments, and polymer analyses identified low-density polyethylene (LDPE), polypropylene (PP), polyethylene (PE) and polyethylene terephthalate (PET) as the major polymer types. These results highlight the differences between urban and protected reservoirs, suggesting that anthropogenic factors – potentially including population density, irregular urban development and associated management practices – may influence MP inputs, while protected areas may be more influenced by transport via meteorological processes, surface run-off and recreational activities. The findings also support the suitability of Atherina pontica as an effective bioindicator for assessing MP loads, particularly in lentic water bodies. In conclusion, these observed differences between urban and protected reservoirs suggest that environmental management strategies focusing on potential anthropogenic and meteorological inputs, such as wastewater infrastructure and surface run-off, may be important for reducing the impacts of MPs on aquatic life.