Anthropogenic marine debris accumulation and its ecological impact on the highly urbanized mangroves of Hong Kong

Mangroves are recognized as a sink for plastic and other anthropogenic marine debris (AMD). The accumulation rates of AMD within these ecosystems, however, have not yet been assessed anywhere in the world. Here, we investigated the standing stock and accumulation rate of AMD at four of the most polluted mangroves in Hong Kong over one year, focussing on its ecological impact on the diversity and abundance of vegetation and benthic macrofauna. Plastics accounted for ~80 % of the recorded AMD. The AMD accumulated at an average rate of 0.025 ± 0.141 items m^-2. d^-1 across all study sites. Mangroves on the east coast presented the highest rate (0.071 ± 0.246 items m^-2. d^-1), while those on the west had the lowest (0.001 ± 0.005 items m^-2. d^-1). The accumulation rate of debris varied significantly across zones (landward vs seaward), months, and sites, with landward zones generally serving as sinks for debris items. Mangrove sapling density also varied significantly among sites, species, and zones. A significant positive relationship between the surface area covered by debris and mangrove sapling density was observed. No significant effect on the diversity of macrobenthic crabs and gastropods was observed, which may reflect the resilience of the ecosystem's faunal components.