Disentangling the anthropogenic and environmental correlates of microplastic pollution in the coastal waters of a metropolis in Southern China

Microplastics are widely recognized as significant marine pollutants with health implications, yet their abundance patterns, sources, and environmental determinants remain unknown in many regions. In this study, we surveyed 31 sites to assess the status of microplastic pollution in the surface waters of Hong Kong, one of the most densely populated cities in Southern China. Microplastics (20 μm-5 mm) were detected at all survey sites, with abundances ranging from 29 to 2.7 items/L (mean: 14.4 ± 8.6 items/L) along an estuary-to-ocean gradient. The highest abundances were observed in the western estuarine sites, which receive substantial river runoff, and within Victoria Harbour, which receives effluents from a major sewage treatment plant. Low-density polymers such as polypropylene (PP) and polyethylene (PE) were predominant, while high-density polymers like polyethylene terephthalate (PET) and polyvinyl chloride (PVC) were less abundant, except near sewage outfalls in Victoria Harbour. High concentrations of polystyrene (PS) in Deep Bay, an oyster culture area, suggest that Styrofoam used in local oyster culture rafts is a potential pollution source. Multivariate analyses indicate that microplastic pollution in Hong Kong is influenced by local environmental factors, with four environmental proxies-salinity, chlorophyll a, E. coli, and total inorganic nitrogen-accounting for a significant portion of the spatial pattern in microplastic abundance. Overall, our study disentangles the complex influences of the Pearl River and local human activities on the abundance and distribution of microplastics in Hong Kong waters, highlighting the complexity of tracing the sources of these pollutants.