Spatio-temporal comparison of neustonic microplastic density in Hong Kong waters under the influence of the Pearl River Estuary.

Rivers are recognised as an important source of plastic debris in the open sea. The Pearl River in China is estimated to transport 0.1milliontonnes of plastic waste to the open sea annually. However, no empirical study has been conducted to assess the plastic contamination levels in the Pearl River Estuary. Hong Kong is situated in the east of the Pearl River Estuary; its western waters are strongly influenced by river discharge, whereas the eastern waters are unaffected by the freshwater plume. In this study, we quantified the neustonic plastic debris density in the western and eastern waters of Hong Kong. The mean microplastic (0.355-4.749mm) and large plastic debris (≥4.75mm) densities in the western side were 3.627 and 0.758n/m3, respectively. Seasonal comparisons indicated that both size classes of plastic debris were significantly more abundant by number in the rainy season than the dry season (p<0.001). However, the influence of rivers on plastic density at the sea surface may be highly restricted to the estuarine delta, as no significant spatial difference was found between the western and eastern waters.