Impacts of Typhoon Mangkhut in 2018 on the deposition of marine debris and microplastics on beaches in Hong Kong

Storm surge and waves associated with tropical cyclones carry significant amounts of pollutants into the marine environment. This study evaluated the effects of Typhoon Mangkhut (7–18 September 2018) on marine debris pollution including macro-debris (>2.5 cm) and microplastics (5 μm–5 mm) in Hong Kong. Sampling was repeated on four beaches, two each from protected and exposed coastal areas, spanning from the eastern to western waters before and after the cyclone. For macro-debris, an average density of 0.047 items m⁻² and 0.54 items m⁻² was obtained before and after the cyclone, respectively or an 11.4-fold increase, with plastic being the most dominant type (61.9–93.3% and 80.7–92.4% before and after the cyclone, respectively) among total beached debris in all four beaches. Likewise, higher mean microplastic abundances were found in the post-cyclone period (335 items kg⁻¹ sediment) when compared with the pre-cyclone period (188 items kg⁻¹ sediment). The depositional dynamics for both macro-debris and microplastic were site-specific due to factors such as wind direction and the associated storm surge, topography and orientation of the site, and proximity to urban areas. This study has demonstrated the role cyclone induced overwash plays on introducing plastic pollution to beach environments. Considering an increase in both the intensity and frequency of cyclone in the future due to global warming, and a tremendous increase in marine plastic debris, more research effort should be spent on this understudied problem.