Plastic pollution in deep seafloor of the South China Sea

Plastic pollution is at the forefront of environmental problems, and has invaded every sphere on Earth. We describe the abundance, distribution, transport pathways and mechanisms, fate and ecological impacts of plastics in the South China Sea (SCS), emphasizing on the deep seafloor microplastics and plastics. We document presence of plastics, primarily based on more than 100 dives in manned submersibles, backed-up by in-depth analysis of dive track videos and images from all locations, providing the first distribution maps of microplastic and plastics on the bottom of the SCS. Abundance of large plastics have been observed to be highest at typical V-shaped geomorphological units, such as canyons, where hydrodynamic conditions are stronger. However, high concentrations of microplastics occur commonly in the sediments of coastal zones. Sources and transportation mechanism of microplastics and plastics in the SCS are thus distinctly different from each other. While a vast majority of microplastics is possibly transported through riverine inputs, most of plastics have a sea-based origin and are discarded from fishing boats, entertainment vessels, and merchant ships. Apart from surface currents, deep-water, seasonal currents, as well as gravity flow, facilitate the transport of marine plastics to deep seafloor sediments. We present two types of models for the transportation of microplastics and plastics in SCS, respectively. We further elucidate the ecosystem which has emerged as a new hot spot with the plastics in SCS. Both large plastics and microplastics additionally act as vectors of chemical pollutants, resulting in ecotoxicological damages. Interaction of biota with deep-sea plastics has been observed and documented in terms of ingestion, entanglement, or proximity, resulting in potential negative effects. The extent of plastic pollution in the SCS is at an alarming level. Therefore, strengthened mitigation procedures, reuse and recycling structures, and waste water management have to be urgently incorporated in National Action Plans to control the burden of plastics entering the SCS.