The Study of Marine Debris and Microplastic Pollution on Beaches in Hong Kong


Student thesis: Doctoral Thesis

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Award date5 Mar 2020


The number of marine debris keeps increasing globally. Most of them that ends up in the ocean has the potential to be washed ashore. In fact, marine debris is found on many shores around the world, even in the most remote area. An increase in the number of beach debris constitutes a threat to ecological, economic, recreational and aesthetic values. The occurrence of marine debris, and more recently microplastics (< 5 mm), in the marine ecosystem has become a major global environmental issue because the microplastics are widespread in the oceans, bioavailable and able to carry toxic chemicals. The uptake of microplastics by marine organisms shows adverse health impacts including reduction in food uptake and altering physiological functions. The bioavailability of microplastics towards lower trophic levels such as zooplankton and invertebrates holds great potential for bioaccumulation and biomagnification. The present study assessed the status of beach debris and microplastic pollution in Hong Kong and provided an innovative approach for their monitoring.

Plastic waste enters the coastal zones and oceans through rivers, water drainage, sewerages, and transport by wind or tides. Its pattern of transport and deposition is determined by climatic and oceanic conditions, specifically wind, nearshore currents, wave motion, tidal dynamics, as well as shoreline geomorphology. Routine monitoring of marine debris is vital in identifying hotspots of beach debris and providing valuable information for the establishment of an efficient cleanup strategy. The first study of this thesis investigated the transient effects of typhoon-induced marine debris on shores. The amount of macro-debris (>2.5 cm) and microplastics (<5 mm) were compared before and after a typhoon. For macro-debris, the average density increased from 0.047 items m–2 before the typhoon to 0.54 items m–2 several days later, an 11.4-fold increase with plastic being the most dominant type of debris (61.9–93.3% and 80.7–92.4% before and after the typhoon, respectively). Likewise, a significantly higher microplastic abundance was found in the post-typhoon period (mean = 335 items kg-1 sediment) than the pre-typhoon period (188 items kg-1 sediment). These data have demonstrated that the typhoon can dramatically increase both the marine debris and microplastics on the shore.

The visual survey is the most common method to quantify and characterize beach debris. However, it is very labour intensive and difficult to carry out on beaches which are remote or difficult to access. The second study of this thesis tested an alternative approach to monitor macro-debris on two beaches using an unmanned aerial vehicle (UAV), or aerial drone, with automated image requisition and automatic image processing. Debris of size between 2.5–50 cm on the beaches was identified by untrained personnel from the photos with the highest accuracy of >75%. The quantification of beach debris using the drone was three times faster than the visual census. How different operating conditions affected the performance of the drone in the beach debris assessment were investigated for optimizing the protocol for the assessment. This study has revealed the potential of using an automated approach for beach debris assessment at different spatial and temporal scales.

Locally, recent studies have shown that Hong Kong is one of the hotspots of microplastic pollution. The third study of the thesis compared microplastics in sediment samples collected from 10 beaches in Hong Kong spanning from the eastern to western waters. Abundance of microplastics ranged between 4.50 and 188 items kg–1 sediment. Polyethylene (46.9%) was the most abundant and followed by polypropylene (13.8%) and polyethylene terephthalate (13.5%). Both input from the Pearl River estuary and local activities were major sources of microplastics on Hong Kong shores. The amount of microplastics on the beaches was at the lower range as compared with similar shore types in Asia and other continents. This study has also shown that microplastic abundance and composition varied with tidal height in an unpredictable way. This supports recommendations from authorities in USA and EU on the sampling of microplastics at multiple tidal heights.

Microplastics are harmful to marine organisms not only because they block the digestive system of the organisms after being ingested, they also adsorb pollutants in water due to their high surface area to volume ratio and hydrophobicity. The fourth study determined the spatial distribution, composition and source of HOCs including polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organchlorinated pesticides (OCPs) in sedimentary microplastics in Hong Kong. The concentration of ΣPAHs ranged between 70.8 and 1509 ng g–1 with inter-site differences although the regional difference was insignificant, indicating localised pyrolytic and petrogenic input of PAHs. The concentration of ΣPCBs (13–1083 ng g–1) varied with both study site and region with higher concentrations obtained in the western waters, possibly due to the input from Pearl River. Significantly higher concentrations of OCPs on eastern shores highlighted fishing activities in South China Sea a potential major source of OCPs. DDT and its metabolites (DDX, ranged from 1.96–626 ng g–1) were the dominant forms of OCPs (45%–80%). Since most of the DDX existed as DDT, this suggested that there was a fresh input of DDT into the microplastics.

The final chapter concludes the current situation of marine debris and microplastic pollution in Hong Kong. The challenges may remain for the next few decades due to the persistency of microplastics in the environment, therefore monitoring its abundance and potential hazards are particularly important. Some recommendations are provided for tackling the problem locally and for the scientific community in future research. The very first step for local authorities is to establish a regular monitoring program to understand the extent of the problem and hopefully identify major sources of microplastic pollution in Hong Kong. The legislation enforcement is in need, not only in Hong Kong but also in the East Asia, for the removal of HOCs. In-depth studies on the mechanisms and dynamics of transportation, degradation and ingestion of microplastics are urgently needed before any realistic ecological risk assessment can be established.