Fate of plastics in Hong Kong mangroves : from macro to micro

Abstract

Plastic pollution has been one of the major environmental issues to address since the rise of plastic production in the last century. Due to the mismanagement of waste disposal, plastic waste has entered the ocean and dispersed ubiquitously in our living environment. Marine environments, especially coastal habitats, are under the threat of plastic pollution. Mangroves are important coastal habitats that provide a wide spectrum of ecosystem services to maintain the world’s functioning and human well-being. With their proximity to rivers and urban centres, as well as the complex aerial root systems, mangroves are vulnerable to plastic pollution. Thus, understanding the sources and impacts of mangrove plastic pollution is crucial to conserve this valuable ecosystem. In this thesis, I presented a comprehensive assessment of microplastic pollution across Hong Kong mangroves respect the abundance, distribution and composition of microplastics in the three compartments – water, sediment and macrofauna within mangrove ecosystems. Through quantifying the abundance, monitoring the distribution, and analysing the composition, in terms of shapes and polymer types, of microplastics in water and sediment, I identified the potential sources of microplastic accumulated in the habitat. I also confirmed the ingestion of microplastics by mangrove crabs and gastropods. Through analysing their ingestion level, I found that microplastic ingestion is linked to the foraging habits of organisms. This linkage is helpful in identifying which species are more vulnerable to plastic pollution regardless of habitats. By consolidating the existing knowledge of interactions between plastics and macrofauna, I discovered that when macrofauna ingest plastics, the feeding behaviours including biting and chewing could result in plastic fragments. Plastics ingested could be further shattered into smaller plastics by stomach contractions and enzymatic degradation occurred within the digestive system. Next, I conducted a laboratory exposure experiment to examine whether mangrove crabs could perform this fragmentation mechanism and form microplastics. Results obtained show that mangrove crabs are potential contributors to mangrove microplastics. In summary, my findings provide new insights into plastic pollution in mangroves and and shed light on plastic fragmentation mechanisms. These insights can help obtain more accurate estimations of the abundance and distribution of plastic debris in the environment, thereby a better evaluation of the extent of impacts caused by plastic pollution.