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Impacts of Typhoon Mangkhut in 2018 on the deposition of marine debris and microplastics on beaches in Hong Kong
Science of the Total Environment 716 (2020) 137172
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Impacts of Typhoon Mangkhut in 2018 on the deposition of marine debris and microplastics on beaches in Hong Kong Hoi-Shing Lo a, Yan-Kin Lee a, Beverly Hoi-Ki Po b, Leung-Chun Wong a, Xiaoyu Xu a, Cheuk-Fung Wong a, Chun-Yuen Wong a, Nora Fung-Yee Tam a, Siu-Gin Cheung a,c,⁎ a b c
Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong Special Administrative Region Department of Zoology, University of British Columbia, Vancouver, BC V6T 1Z4, Canada State Key Laboratory of Marine Pollution, City University of Hong Kong, Tat Chee Avenue, Hong Kong Special Administrative Region
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• The pollution level of marine debris was compared before and after cyclone. • Abundance of macro-debris and microplastics increased tremendously after cyclone. • Pollution level was determined by sitespecific factors. • Plastic was the most frequently found material among all the debris. • Chemical compositions of microplastics became more diverse after cyclone.
a r t i c l e
i n f o
Article history: Received 13 December 2019 Received in revised form 3 February 2020 Accepted 6 February 2020 Available online xxxx Editor: Damia Barcelo Keywords: Cyclone Microplastics Marine debris Plastic pollution
a b s t r a c t Storm surge and waves associated with tropical cyclones carry significant amounts of pollutants into the marine environment. This study evaluated the effects of Typhoon Mangkhut (7–18 September 2018) on marine debris pollution including macro-debris (N2.5 cm) and microplastics (5 μm–5 mm) in Hong Kong. Sampling was repeated on four beaches, two each from protected and exposed coastal areas, spanning from the eastern to western waters before and after the cyclone. For macro-debris, an average density of 0.047 items m−2 and 0.54 items m−2 was obtained before and after the cyclone, respectively or an 11.4-fold increase, with plastic being the most dominant type (61.9–93.3% and 80.7–92.4% before and after the cyclone, respectively) among total beached debris in all four beaches. Likewise, higher mean microplastic abundances were found in the post-cyclone period (335 items kg−1 sediment) when compared with the pre-cyclone period (188 items kg−1 sediment). The depositional dynamics for both macro-debris and microplastic were site-specific due to factors such as wind direction and the associated storm surge, topography and orientation of the site, and proximity to urban areas. This study has demonstrated the role cyclone induced overwash plays on introducing plastic pollution to beach environments. Considering an increase in both the intensity and frequency of cyclone in the future due to global warming, and a tremendous increase in marine plastic debris, more research effort should be spent on this understudied problem. © 2020 Elsevier B.V. All rights reserved.
⁎ Corresponding author at: Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong Special Administrative Region. E-mail address: [email protected] (S.-G. Cheung).
https://doi.org/10.1016/j.scitotenv.2020.137172 0048-9697/© 2020 Elsevier B.V. All rights reserved.
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H.-S. Lo et al. / Science of the Total Environment 716 (2020) 137172
1. Introduction Tropical cyclones (TCs) significantly affect the shoreline dynamics, particularly in the low-lying areas (Woodruff et al., 2013). For example, waves generated from a distant tropical cyclone can mobilise offshore sediment and allow the material to be redistributed along the shore (Cooper et al., 2008). TCs and the associated storm surges facilitate the adjustment of beach face and bar migration (Lee et al., 1998), and shoreline erosion (Woodruff et al., 2013), therefore changing the geomorphology of beaches by deposition and removal of sand (Needham et al., 2015). Since sediment particles of all sizes and compositions, together with marine debris, are transported by cyclone induced overwash (Hansom et al., 2015; Soria et al., 2017; Switzer and Jones, 2008), TCs have tremendous effects on the deposition and distribution of sediments and marine debris. For example, N2 million kg of sediments were flushed down the Salt River Submarine Canyon in U.S. Virgin Islands into deep water during Hurricane Hugo (1989) (Shanmugam, 2008). Extreme weather such as flash floods and storms could aggravate the transfer of plastic debris from land to water. Several studies have demonstrated a significant increase in the abundance of beach debris, including a majority of plastic debris (and microplastics) after heavy rainfall and flash floods (Cheung et al., 2016; Gündoğdu et al., 2018; Lattin et al., 2004; Moore et al., 2002; Rech et al., 2014; Van et al., 2012; Veerasingam et al., 2016; Yonkos et al., 2014). Similarly, TCs could create transient effects on debris source loads (Ribic et al., 2011) and transport a large amount of debris into the marine environment (Critchell et al., 2015; Hidalgo-Ruz et al., 2018; Thompson, 2005; Wang et al., 2019). With future global sea-level rise at an accelerating rate (of 3.0 mm per year, Hay et al., 2015) and a projected increase in the intensity of TCs (Elsner et al., 2008) due to climate change, substantial damages and pollution to vulnerable coastal regions are expected to increase. Western North Pacific is the most active TCs basin, in terms of both the overall number of cyclones and peak wind intensities (Maue, 2011). TCs are named as typhoons in south and east Asia. Typhoon
Mangkhut, formed over the western North Pacific as a Category 5equivalent typhoon, is the third-strongest TC worldwide in 2018, with an estimated peak intensity at maximum sustained wind of 250 km h−1 near the centre. The destructive winds and storm surges caused landslides, damages to buildings and homes, and loss of power over Guam, Philippines and South China, resulting in N130 fatalities and NUS$3.7 billion economic loss. As one of the strongest TCs hitting Hong Kong since WWII, Mangkhut brought severe flooding and inundation due to record-breaking storm surges which also occurred elsewhere in the low-lying areas of Pearl River Delta in south China. Extreme wind velocities, storm surges and rainfall caused by TCs could create a large-scale disturbance to both terrestrial and coastal areas. Wang et al. (2019) reported that cyclones may increase the average concentrations of microplastics in the seawater and seabed sediments by approximately 40%. Yet, there is no study on the relationship between cyclones and the deposition patterns of marine debris and microplastics. This study provided insights into the impacts of TC on the composition and patterns of marine debris deposited on the beaches in Hong Kong. The abundance, size distribution and composition of macro-debris (2.5–50 cm) deposited on beaches and microplastics (b5 mm) in the littoral sediments were compared between the pre- and post-cyclone period. As the frequency and intensity of cyclones as well as the abundance of marine plastic debris and microplastics are expected to increase in the future, there is a pressing need to better understand the significance of cyclones in the distribution and accumulation of marine debris and microplastics on beaches. 2. Materials and methods 2.1. Study area To determine the effect of cyclone on the abundance and composition of marine debris and microplastics on beaches with different degrees of wave exposure, four beaches were chosen for the study with
P1 - Protected shore
Shenzhen
TA
IW
AN
MAINLAND CHINA
P2 - Protected shore
New Territories South China Sea
P2
HONG KONG Kowloon
E2
0 75 150 m P1
Lantau Island E1
Hong Kong Island
E1 - Exposed shore 0
7.5
0 75 150 m
E2 - Exposed shore
15 km
LEGEND Path of Typhoon Mangkhut (7 September - 17 September 2018) Protected shore Exposed shore
0 75 150 m
0 75 150 m
Fig. 1. The trajectory of Typhoon Mangkhut and geographic locations of the sampling sites in Hong Kong. Protected shores (green boxes) are Sham Wat (P1, 22°16′14″N, 113°53′11″E) and Starfish Bay (P2, 22°25′55″N, 114°14′43″E); whereas exposed shores (blue boxes) are Cheung Sha Beach (E1, 22°14′00″N, 113°57′02″E) and Pak Lap Wan (E2, 22°21′05″N, 114°21′38″E). Satellite images are retrieved from ESRI (2019) World Imagery. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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two of them being protected and the others more exposed. They were protected shores at Sham Wat (P1) and Starfish Bay (P2), and exposed shores at Cheung Sha Beach (E1) and Pak Lap Wan (E2) (Fig. 1). Both P1 and E1 are located on Lantau Island in the western part of Hong Kong. Situated in the northwest of the island, P1 is an intertidal flat lying in Sham Wat Bay sheltered from currents and waves. E1 is an exposed sandy beach located on the southern coast of Lantau Island facing South China Sea. The protected shore P2 is lying deep in Tolo Harbour in the eastern New Territories. The harbour has a bottle-neck topography and a small tidal range, resulting in poor water circulation and long water residence time (Hodgkiss and Yim, 1995). Also, the Harbour system is receiving input from six rivers/streams entering Tolo Harbour. They are Lam Tsuen River, Shan Liu Stream, Shing Mun River, Tai Po River, Tai Po Kau Stream and Tung Tsz Stream, with a total annual discharge rate of 4.50 × 107 m3 (Environmental Protection Department (EPD), 2012).These rivers and streams are situated near to several residential towns, namely Shatin, Tai Po and Ma On Shan with a total population of 1 million (Hong Kong Census and Statistics Department, 2018), the major pollution source of domestic sewage, surface runoff and historical pollution from industries in the catchment (Kueh and Lam, 2008). E2 is an exposed sandy beach situated in Sai Kung East Country Park in the eastern New Territories with very little human activities. None of the selected beaches had cleanup activities organized by the authorities before the sampling which was conducted during low tides before (13–14 September 2018) and after (18–19 September 2018) Typhoon Mangkhut hitting Hong Kong. 2.2. Macro-debris survey The marine debris assessment was based on the guideline of NOAA (Opfer et al., 2012). At each site, a designated length (100 m, parallel to the water line) was divided into 5 m × 18–47 m (depending on the distance between strandline and waterline) subsections and four of which were selected from the random number table. Sampling was conducted consistently during ebb tide at 1.0 m above CD. Tidal height was obtained from the real time tidal information from Hong Kong Observatory (https://www.hko.gov.hk/en/tide/marine/realtide.htm) and data from the nearest tide gauges were used. The study sites were located by GPS and sampled before and after the cyclone. Within each subsection, identification of macro-debris (N2.5 cm) was started from the back of the shoreline, usually with vegetation cover, to the water's edge. All the macro-debris items within the subsection area were counted and their types and materials (plastic, metal, glass, rubber, processed lumber, cloth/fabric and others) recorded. The density was calculated as the number of debris divided by the subsection area (items m−2). Mesodebris (5 mm–25 mm) were excluded in this study because of two reasons. Firstly, the time for sampling was very tight as we needed to collect samples on four beaches a few days before and after the cyclone. The low tide period in September only lasted for 4–5 h which further limited the time for the work. After the cyclone was gone, we also had to finish the work in 2 days to reduce the tidal effect on the marine debris. The second reason is past records of macrodebris and microplastics on beaches in Hong Kong are available but not of mesoplastics. Therefore, for comparison sake only macro-debris and microplastics were sampled. 2.3. Microplastic sampling and treatment Surface sediment samples (top 2–3 cm) were collected at three shore heights (strandline, and 1.5 m and 1.0 m above chart datum (CD)). At each shore height, a transect line of 20 m was deployed and top 2 cm sediment within an area of 0.25 m × 0.25 m was collected at 2 m intervals with a stainless steel shovel and put in a cotton bag, hence 10 replicates were collected for each shore height. Three more sediment samples were collected at each shore height for the analysis of sediment particle size distribution.
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All the sediment samples were dried in a greenhouse for at least 4 weeks and subsequently 500 g of sediment was obtained from each sample and suspended in 1 L of ZnCl2 solution (density = 1.6–1.7 g mL−1) and stirred for 10 min using an overhead stirrer at 500 rpm. The mixture was allowed to settle overnight and afterwards decanted through a stack of sieves (mesh size = 5 mm, 1 mm, 125 μm, respectively). The surface of the glass bottle was rinsed thoroughly with ZnCl2 to collect remaining particles adhered to the surface. This separation process was performed twice to optimize the recovery rate of the particles. All the materials retained on 5 mm sieve were discarded. The solution passed through the sieves was collected and filtered through a filter paper (Advantec Grade No.2, pore size = 5 μm), therefore, particles retained on the filter paper were of the size between 5 and 125 μm. They were then treated with 30% H2O2 for 72 h to digest organic matter. The particles retained on 125 μm and 1 mm sieves were rinsed with deionized water and treated with 30% H2O2 separately. The solutions were then filtered and the size fractions of particles collected were 125–1000 μm and 1000–5000 μm, respectively. The efficiency of this extraction method for microplastics was validated twice for each sampling site (i.e., n = 8). After going through the extraction process, a sediment sample from each site was spiked with 50 pieces of microplastics (a mixture of polyethylene, polypropylene, polyethylene terephthalate, expanded polystyrene and nylon 66, size ranged from 50 to 2000 μm) and the extraction procedure was repeated as described above. The percentage recovery of the particles (ranging from 59 to 95%) was used to correct the microplastics abundance in this study. To assess the contamination in the laboratory, a procedural control was conducted according to Fries et al. (2013). Procedural blanks (n = 20) were prepared using ZnCl2 solution (1 L) but without sediment and the extraction process was conducted the same as that for the treatments. No pellet, fragment, foam or film was found in the procedural blanks. However, fibers with a mean value of 1.6 items were found in the blanks. Therefore, a factor of 1.6 items of fiber was used to correct for the amount of fibers obtained in each 500 g sediment sample. 2.4. Microplastic analysis Microplastics in the size range of 1000–5000 μm were counted with the naked eye, while the 5–125 μm and 125–1000 μm fractions were examined and counted under a microscope with a magnification of 7.3×– 120× (M165C, Leica Microsystems). Microplastics were identified by their shape, surface texture and colour according to the criteria established by Free et al. (2014) and McCormick et al. (2014) and categorized into five classes, namely “fiber”, “fragment”, “foam”, “film” and “pellet”. At least 30 microplastics from each of three size fractions, equivalent to 41.7–100% of all the particles in that fraction, and all the particles that could not be distinguished by microscopic examination were characterized by Fourier transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet iS50 FTIR spectrometer). For the 1000–5000 μm fraction, the particles were characterized using the FTIR instrument equipped with an attenuated total reflectance (ATR) unit with a Ge crystal. For 125–1000 μm and 5–125 μm fractions, the particles were characterized using the FTIR microscope (μFTIR, Thermo Scientific Nicolet iN5 FTIR microscope configured with a Ge-tip ATR). A spectral resolution of 4 cm−1 at a wavenumber ranged between 675 and 4000 cm−1 with 40 co-added scans was performed. The spectra obtained were compared with the reference spectra in the Hummel Polymer library and a matching of N70% was accepted. 2.5. Sediment analysis The beach topography was determined following Emery Method (Emery, 1961). The profile of each beach was measured along three transect lines (50 m apart) perpendicular to the shore length. The median particle diameter was determined for sediment (200 g per sample)
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Fig. 2. Mean (±SE) macro-debris densities for all the sites in the pre- and post-cyclone periods. Asterisks (*p b 0.05; **p b 0.01) denote significant differences (paired t-tests) of the macrodebris density between the pre- and post-cyclone period.
collected from three tidal heights as mentioned earlier following the protocol of Blott and Pye (2001). Sediment particle size was determined by wet sieving using running tap water. Six sieves with different pore sizes, i.e., 2.00 mm, 1.00 mm, 500 μm, 250 μm, 125 μm and 63 μm were prepared. Sediments retained on each sieve were collected, dried at 100 °C overnight and weighted.
data were arcsine transformed. Cyclone period (before and after), tidal height (strandline, 1.5 m above CD, 1.0 m above CD), shore type (exposed and protected), and size class (5–125 μm; 125 μm–1 mm; 1–5 mm) were fixed factors, while site (exposed: E1 and E2; protected: P1 and P2) was the random factor nested within shore types. Either one-way ANOVA or t-test was employed when the main effect or interaction was significant.
2.6. Data analysis 3. Results Statistical analyses were performed using R and Minitab 18. Prior to analysis, data were tested for normality and homogeneity of variance using Shapiro-Wilk test and Levene's test, respectively. Data for macro-debris density failing these tests were Johnson transformed. A parametric three-way nested ANOVA was employed to test the differences in debris density between cyclone period (before and after), shore type (exposed and protected) and site (random factor nested within shore types; exposed: E1 and E2; protected: P1 and P2). In case where the main effect or interaction was significant, either t-test or one-way ANOVA was employed to explain for the differences. The changes in the proportion of plastic in marine debris after the cyclone for each site were tested by the Fisher's exact test. For microplastic abundance, data failed to meet the parametric requirements were log-transformed. A four-way nested ANOVA was employed to test the effect of cyclone (before and after), tidal height (strandline, 1.5 m above CD, 1.0 m above CD), shore type (exposed and protected) and site (random factor nested within shore type; exposed: E1 and E2; protected: P1 and P2) on the microplastic abundance. When the interactive terms in the ANOVA analyses were significant, the effect of individual factors was tested by one-way ANOVA or t-test. The changes in proportions of the three size classes of microplastics were tested by five-way nested ANOVA. Prior to analysis the percentage
3.1. Macro-debris density and characteristics in the pre- and post-cyclone periods The average density of macro-debris was 0.047 ± 0.076 items m−2 (±SE) and 0.54 ± 0.48 items m−2 (±SE) for the pre- and postcyclone period, respectively, or an 11.4-fold increase after the cyclone. For all the sites, although the amount of marine debris increased after the cyclone, the increase was only statistically significant for P2 and E2 (Fig. 2), with a 10-fold and 52-fold increase, respectively. This Table 1 Three-way nested ANOVA showing the effect of cyclone and its interactions with shore types and sites on macro-debris density. Macro-debris density (items m−2)
Cyclone Cyclone × shore type Cyclone × site (nested within shore type)
F
d.f.
p
8.54 0.46 4.57
1, 24 1, 24 2, 24
0.100 0.567 b0.05
Other main effects (shore type, and site nested within shore type) were insignificant therefore are not presented here.
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Fig. 3. The composition of macro-debris before and after the cyclone.
indicated that the effect of cyclone was site-specific, regardless of the degree of exposure of the sites (Table 1). Plastic dominated the macro-debris at all the sites (Fig. 3) with its proportion increased significantly for E1 and E2 (27.5% and 17.7%, respectively) and decreased for P1 (−12.6%) after the cyclone (Table 2). The most common plastic items were food wrappers (54 out of 167, or 32.3%) and plastic bags (46 out of 167, or 27.5%) before the cyclone but replaced by plastic fragments (635 out of 1701, or 36.7%) and bottle/container caps (424 out of 1701, or 24.4%) after the cyclone (Table S1 of Supplementary information). 3.2. Microplastics in the pre- and post-cyclone periods 3.2.1. Abundances The average microplastic abundance for all the sites increased significantly from 188 ± 223 items kg−1 (±SE) before the cyclone to 335 ± 390 items kg−1 (±SE) after the cyclone (Table 3, p b 0.001). The protected shores also have more microplastics (382 ± 389 items kg−1) than exposed shores (141 ± 166 items kg−1, Table 3, p b 0.05), regardless of the cyclone periods. Since the “tidal height × shore type” interaction was significant (Table 3, p b 0.001), the distribution of microplastics at different tidal heights varied with the degree of exposure of the shores. Further analysis revealed that more microplastics were deposited at 1.0 m and 1.5 m above CD than along the strandline on protected shores but they tended to deposit along the strandline on exposed shores (Table S2 of Supplementary information). The “cyclone × shore type” and “cyclone × tidal heights” interactions were statistically indistinguishable (p N 0.05), indicating that differences in microplastic abundance between shore types and among three tidal heights did not change with the cyclone. The inter-site differences, however, were significant (Table 3, p b 0.001). To better explain the spatial variability, the microplastic abundances of each site before and after
Table 2 Fisher's exact tests showing the change in the proportion of plastics in marine debris collected from the protected and exposed shores before and after the cyclone.
the cyclone were investigated. An increase of microplastic abundance for 3.9 fold (from 180 ± 108 to 701 ± 518 items kg−1) at P2 (Fig. 4), followed by an increase of 1 fold (from 127 ± 113 to 275 ± 262 items kg−1) at E2 were observed. In contrast, there was no significant change of microplastic abundance for P1 and E1 after the cyclone (Fig. 4). These results were in line with the macro-debris density. 3.2.2. Size distributions Microplastics were grouped into three size classes (5–125 μm; 125 μm–1 mm; 1–5 mm) in this study. The size fraction 125 μm– 1 mm was the most frequently found both before and after the cyclone, accounting for 86.0 ± 15.9% and 77.0 ± 24.0% of all the size classes, respectively. The size distribution of microplastics varied neither with any of the main factors, i.e., cyclone, exposure, site, tidal height and size of microplastics nor their interactions except for the “cyclone × size of microplastics × tidal height × site” interaction. Therefore, the effect of cyclone was investigated separately for all the other main factors and the results are shown in Table S3 of Supplementary information. Although the microplastic size distribution was different in a number of comparisons, no specific pattern was observed. 3.2.3. Shape and compositions of microplastic The relative abundance of different shapes and chemical compositions of microplastics is shown in Fig. 5. Fibers were the most common shape of microplastics for all the sites before the cyclone and most of the sites after the cyclone except E2 where foam became dominant. Polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), nylon, cellophane and other polymers (polyester, polycarbonate, acrylate polymer) were found in this study. Higher density polymers (e.g., PET and PVC) were absent before the cyclone but a small amount of them (PET: 0.8%; Table 3 Four-way nested ANOVA showing the effects of cyclone, shore type, site and tidal height and their interactions on microplastic abundances. Microplastic abundance (items kg−1)
Plastic proportions (%)
P1 P2 E1 E2
Pre-cyclone
Post-cyclone
%change
p
93.3 84.2 61.9 74.7
80.7 87.5 89.5 92.4
−12.6 +3.30 +27.5 +17.7
b0.05 0.532 b0.01 b0.001
Cyclone Shore type Tidal height Site Tidal height × shore type
F
d.f.
p
22.7 8.92 1.04 5.50 15.1
1, 217 1, 217 2, 217 3, 217 2, 217
b0.001 b0.05 0.354 b0.005 b0.001
Other interactions were insignificant therefore are not presented here.
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Fig. 4. Mean (±SE) microplastic abundance for all the sites in the pre- and post-cyclone periods. Asterisks (*p b 0.05; ***p b 0.001) denote significant differences (paired t-tests) of the macro-debris density between the two periods.
PVC: 1.9%) were collected after the cyclone. Most of the microplastics identified were nylon, which represented 46.3% and 47.4% of the samples in the pre- and post-cyclone period, respectively. The second most abundant polymer was PE, representing 25.0% and 16.8% before and after the cyclone, respectively. PP ranked the third before the cyclone (11.8%) but was replaced by PS after the cyclone, which increased from 8.0% to 15.0%, although the change was mostly attributed to E2. 4. Discussion 4.1. Effects of Typhoon Mangkhut on marine macro-debris The differential impact of the cyclone on the accumulation of marine debris on individual sites was a combined effect of trajectory of the cyclone, topography and orientation of the sites, and proximity to densely populated areas. P2 received the greatest impact because of three reasons. Firstly, although it is a protected beach in Tolo Harbour, the wind speed inside Tolo Harbour during the cyclone was comparable to that at other sites (Fig. 6a–b). Its northeast facing location rendered it a direct impact from the storms as Typhoon Mangkhut skirted past to the south-southwest of Hong Kong (Fig. 1), generating east to northeasterly storms associated with hurricane force winds (Fig. 6). Secondly, the storm surge, which is one of the major contributors on transportation of marine debris between ocean and beach (Taffs and Cullen, 2005), recorded at the Tai Po Kau tide-gauge station near to P2 was record breaking in Hong Kong with a tidal height of 4.71 m above CD (Fig. 6c). Thirdly, P2 is located in Tolo Harbour along which are several major residential areas (e.g., Shatin, Tai Po, Ma On Shan) with a total population of 1 million. Debris generated on land in these areas would be carried to the sea by wind and major rivers, such as Shing Mun
River in Shatin, and Lam Tsuen River in Tai Po (Fig. 6). In 2018, using video recording Greenpeace estimated that at least 48,000 plastic items were carried by Shing Mun River into Tolo Channel on a no-rain day, and the number doubled after rainfall (Yeung et al., 2018). The problem would be aggravated due to rainstorms associated with the cyclone. E2 received a smaller impact than P2 because it is inside a country park without human settlement and is relatively remote, so human disturbance is minimal. A smaller storm surge and its southeast facing position further reduced the impact. Similar to E2, E1 is facing southeast and located in a country park, but it received less impact from the cyclone than E2 because of the protection by Chi Ma Wan Peninsula in the east (Fig. 6). The impact of the cyclone on P1 was negligible in terms of accumulation of marine debris and microplastics, and was largely due to its northwest facing position and the smallest storm surge. Therefore, wind speed, storm surge, orientation and topography of the site, and proximity to urban areas are major determining factors in introducing plastic pollution to beach environment through cyclone induced overwash. In contrast, the effect of degree of wave exposure of the site was relatively insignificant. Unsurprisingly plastic was the most dominant macro-debris at all the sites (Fig. 3) and its proportion increased further at sites much affected by the cyclone. As compared with other types of marine debris such as glass, rubber and metals, a much lower density rendered plastic debris floating in the sea and eventually washed ashore (Cunningham and Wilson, 2003). Although it is difficult to trace the source of marine debris deposited on the shores, words and logos printed on plastic products have provided some hints on the sources. Being located inside Sai Kung East Country Park, E2 is an exposed shore facing South China Sea, debris collected, therefore, should come from the sea as no urban
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Fig. 5. Composition of microplastics before and after the cyclone in terms of (a) shape and (b) chemical composition. Types of microplastics found included polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), nylon, cellophane and others (polyester, polycarbonate, acrylate polymer). For detailed chemical compositions of microplastic, please see Table S5 of Supplementary information.
development was allowed in country parks of Hong Kong. It is interesting to note that 40% of the plastic debris at E2 were bottle caps with labels in Simplified Chinese (used in China, 45.3%), English (38.8%), Traditional Chinese (used in Hong Kong, Macau and Taiwan, 11.6%) and others (0.04%, including Japanese, Korean, Thai, Vietnamese, Russian). This indicated a possibility that macro-debris at E2 travelled from offshore in South China Sea to nearshore at a short window of time during the cyclone period. 4.2. Effects of Typhoon Mangkhut on microplastics Our previous study (Lo et al., 2018) showed that shores with lower energy are more inductive to microplastic deposition. This is consistent with the present study that the two protected shores were more polluted than the exposed shores before the cyclone. Same as the macrodebris, the abundance of microplastics increased significantly at E2 and P2 after the cyclone and the greatest increase was found at P2. This indicated that the same factors, i.e., wind direction and the associated storm surge, topography and orientation of the sites, and proximity to urban areas were responsible for the increase in both microplastics and macro-debris on the shores. For example, extreme events such as floods generated by heavy rainfall have been shown to transport
terrestrial plastics into the marine environment. This also applies to microplastics as Veerasingam et al. (2016) demonstrated that huge quantities of microplastics were washed through rivers from land to sea during 2015 South India floods. These fresh microplastics were subsequently driven and transported by the winds and surface currents and ultimately deposited, resulted in a 3-fold increase in the microplastic abundance on the Indian beaches. The wave energy generated from TCs and the associated highenergy storms are responsible for erosion and redistribution of sediment, even on protected shores (Hopley, 1974; Galli, 1989). The changes in the shore profile (Fig. S1 of Supplementary information) of all the study sites in this study have indicated redistribution of sediment and removal of prestorm beach crests as reported upon by McIntire and Walker (1964). Sedimentological studies (Table S4 of Supplementary information) have demonstrated that the sediment on the stormdeposited layer often became coarser after severe storm events (Soria et al., 2017) due to onshore sediment overwashing and intense offshore wave activities (Oliva et al., 2017). This phenomenon was observed on the two most affected beaches, E2 and P2 where the grain size increased after the cyclone. The sediment overwashing and redistribution probably led to random deposition of microplastics at different tidal heights
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a
b 2:00 pm @ 16 Sep 2018
11:00 am @ 16 Sep 2018 Tung Tze Stream Lam Tseun River Tai Po River
Tung Tze Stream Lam Tseun River
Shan Liu Stream H Tolo
abou
r
Tai Po River
P2
Shan Liu Stream H Tolo
abou
r
P2
Tai Po Kau Shing Mun Stream River
Tai Po Kau Shing Mun Stream River
E2
E2
P1
P1
E1
E1
an Ma W Chi la insu Pen
an Ma W Chi la insu Pen
c Highest water level recorded
Easterly wind of 90 km h-1 Easterly wind of 18 km h-1 Anemometer
4.71m H Tolo
3.33m
3.09m
r abou
Tide gauge
P2
E2
P1
4.19m E1
3.73m
Fig. 6. Wind speed and direction when Typhoon Mangkhut skirted passed closest to Hong Kong at (a) 11:00 am and (b) 2:00 pm on 16 September 2018. Highest water levels (meter above chart datum) recorded by tide gauges are presented in (c). Data are retrieved from Hong Kong Observatory (2018).
in this study and resulted in unpredictable changes in the distribution of different size groups of microplastics at the three tidal heights. The cyclone increased the diversity of polymer types on the shores with heavier polymers such as PVC and PET found only after the cyclone (Fig. 5b; Table S5 of Supplementary information). This was probably a result of the strong vertical mixing during the passage of cyclone, bringing heavier polymers from the seabed to the shores. This is supported by the fact that PET was one of the major polymer types in seabed sediment collected in Hong Kong (Cheang et al., 2018). A similar increase in the polymer diversity following cyclone was reported upon by Wang et al. (2019). Most of the microplastics were fibers both before and after the cyclone except E2 where foam replaced fibers after the cyclone. As confirmed by the FTIR analysis, most of these fibers were nylon, probably generated by laundry and abandoned fishing net, in contrast to a previous study in which most of the polymers on sandy beaches were PE and PP (Lo et al., 2018). The discrepancy could be explained by methodological differences that Lo et al. (2018) only characterized microplastics down to 500 μm in comparison with 50 μm in this study, resulting in smaller fibers being excluded in the former. More foam items were found at E2 after the cyclone. Since foam has extremely low specific
density and is easily transported in the sea for a very long distance, it is difficult to trace its sources. Nevertheless, based on the location of E2 and a large number of plastic caps found which indicated multiple sources of the debris, the foam may be originated from various sources including foam buoys from fish farms and foam boxes in the logistic industry commonly found in Hong Kong and southern China.
5. Conclusions This study has demonstrated tremendous impacts of Typhoon Mangkhut on the macro-debris and microplastic pollutions on 4 shores in Hong Kong with much higher abundance of both macro-debris and microplastics obtained after the cyclone. The diversity of microplastics also increased. The deposition of macro-debris and microplastics were site-specific and was related to factors such as wind direction and the associated storm surge, orientation and topography of the sites, and proximity to urban areas. In foreseeable future, both the frequency and intensity of cyclone as well as the amount of marine plastic debris are expected to increase, therefore, more research effort should be spent on this relatively understudied problem.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The work described in this paper was supported by the Research Grants Council of Hong Kong SAR (CityU 11302815, CityU 9042495). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2020.137172. References Blott, S.J., Pye, K., 2001. Gradistat: a grain size distribution and statistics package for the analysis of unconsolidated sediments. Earth Surf. Process. Landf. 26, 1237–1248. https://doi.org/10.1002/esp.261. Cheang, C.C., Ma, Y., Fok, L., 2018. Occurrence and composition of microplastics in the seabed sediments of the coral communities in proximity of a metropolitan area. 15, 2270. Int. J. Environ. Res. Public Health https://doi.org/10.3390/ijerph15102270. Cheung, P.K., Cheung, L.T.O., Fok, L., 2016. Seasonal variation in the abundance of marine plastic debris in the estuary of a subtropical macro-scale drainage basin in South China. Sci. Total Environ. 562, 658–665. https://doi.org/10.1016/j. scitotenv.2016.04.048. Cooper, M.J.P., Beevers, M.D., Oppenheimer, M., 2008. The potential impacts of sea level rise on the coastal region of New Jersey, USA. Clim. Chang. 90, 475–492. https://doi. org/10.1007/s10584-008-9422-0. Critchell, K., Grech, A., Schlaefer, J., Andutta, F.P., Lambrechts, J., Wolanski, E., Hamann, M., 2015. Modelling the fate of marine debris along a complex shoreline: lessons from the Great Barrier Reef. Estuar. Coast. Shelf Sci. 167, 414–426. https://doi.org/ 10.1016/j.ecss.2015.10.018. Cunningham, D.J., Wilson, S.P., 2003. Marine debris on beaches of the Greater Sydney Region. J. Coast. Res. 19, 421–430. Elsner, J.B., Kossin, J.P., Jagger, T.H., 2008. The increasing intensity of the strongest tropical cyclones. Nature 455, 92–95. https://doi.org/10.1038/nature07234. Emery, K.O., 1961. A simple method of measuring beach profiles. Limnol. Oceanogr. 6, 90–93. https://doi.org/10.4319/lo.1961.6.1.0090. ESRI, 2019. World imagery. Retrieved from. http://www.esri.com, Accessed date: 12 December 2019. Free, C.M., Jensen, O.P., Mason, S.A., Eriksen, M., Williamson, N.J., Boldgiv, B., 2014. Highlevels of microplastic pollution in a large, remote, mountain lake. Mar. Pollut. Bull. 85 (1), 156–163. https://doi.org/10.1016/j.marpolbul.2014.06.001. Fries, E., Dekiff, J.H., Willmeyer, J., Nuelle, M.T., Ebert, M., Remy, D., 2013. Identification of polymer types and additives in marine microplastic particles using pyrolysis-GC/MS and scanning electron microscopy. Environ. Sci. Processes Impacts 15, 1949–1956. https://doi.org/10.1039/C3EM00214D. Galli, G., 1989. Depositional mechanisms of storm sedimentation in the Triassic Dürrenstein Formation, Dolomites, Italy. Sediment. Geol. 61, 81–93. https://doi.org/ 10.1016/0037-0738(89)90042-0. Gündoğdu, S., Çevik, C., Ayat, B., Aydoğan, B., Karaca, S., 2018. How microplastics quantities increase with flood events? An example from Mersin Bay NE Levantine coast of Turkey. Environ. Pollut. 239, 342–350. https://doi.org/10.1016/j.envpol.2018.04.042. Hansom, J.D., Switzer, A.D., Pile, J., 2015. Extreme waves: causes, characteristics, and impact on coastal environments and society. Coastal and Marine Hazards, Risks, and Disasters. Elsevier, pp. 307–334. Hay, C.C., Morrow, E., Kopp, R.E., Mitrovica, J.X., 2015. Probabilistic reanalysis of twentieth-century sea-level rise. Nature 517, 481–484. https://doi.org/10.1038/ nature14093. Hidalgo-Ruz, V., Honorato-Zimmer, D., Gatta-Rosemary, M., Nuñez, P., Hinojosa, I.A., Thiel, M., 2018. Spatio-temporal variation of anthropogenic marine debris on Chilean beaches. Mar. Pollut. Bull. 126, 516–524. https://doi.org/10.1016/j. marpolbul.2017.11.014. Hodgkiss, I.J., Yim, W.W.-S., 1995. A case study of Tolo Harbour, Hong Kong. In: McComb, A.J. (Ed.), Eutrophic Shallow Estuaries and Lagoons. CRC Press, Boca Raton, pp. 41–57. Hong Kong Census and Statistics Department, 2018. The profile of Hong Kong population analysed by District Council District, 2017. Retrieved from. https://www.statistics. gov.hk/pub/B71807FB2018XXXXB0100.pdf, Accessed date: 11 December 2019. Hong Kong Environmental Protection Department, 2012. River water quality data. Retrieve from. http://epic.epd.gov.hk/ca/uid/riverhistorical/p/1, Accessed date: 17 January 2020. Hong Kong Observatory, 2018. Super typhoon Mangkhut (1822) 7 to 17 September 2018. Retrieved from. https://www.hko.gov.hk/en/informtc/mangkhut18/mangkhut.htm, Accessed date: 11 December 2019.
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Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Impacts of Typhoon Mangkhut in 2018 on the deposition of marine debris and microplastics on beaches in Hong Kong Hoi-Shing Lo a, Yan-Kin Lee a, Beverly Hoi-Ki Po b, Leung-Chun Wong a, Xiaoyu Xu a, Cheuk-Fung Wong a, Chun-Yuen Wong a, Nora Fung-Yee Tam a, Siu-Gin Cheung a,c,⁎ a b c
Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong Special Administrative Region Department of Zoology, University of British Columbia, Vancouver, BC V6T 1Z4, Canada State Key Laboratory of Marine Pollution, City University of Hong Kong, Tat Chee Avenue, Hong Kong Special Administrative Region
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• The pollution level of marine debris was compared before and after cyclone. • Abundance of macro-debris and microplastics increased tremendously after cyclone. • Pollution level was determined by sitespecific factors. • Plastic was the most frequently found material among all the debris. • Chemical compositions of microplastics became more diverse after cyclone.
a r t i c l e
i n f o
Article history: Received 13 December 2019 Received in revised form 3 February 2020 Accepted 6 February 2020 Available online xxxx Editor: Damia Barcelo Keywords: Cyclone Microplastics Marine debris Plastic pollution
a b s t r a c t Storm surge and waves associated with tropical cyclones carry significant amounts of pollutants into the marine environment. This study evaluated the effects of Typhoon Mangkhut (7–18 September 2018) on marine debris pollution including macro-debris (N2.5 cm) and microplastics (5 μm–5 mm) in Hong Kong. Sampling was repeated on four beaches, two each from protected and exposed coastal areas, spanning from the eastern to western waters before and after the cyclone. For macro-debris, an average density of 0.047 items m−2 and 0.54 items m−2 was obtained before and after the cyclone, respectively or an 11.4-fold increase, with plastic being the most dominant type (61.9–93.3% and 80.7–92.4% before and after the cyclone, respectively) among total beached debris in all four beaches. Likewise, higher mean microplastic abundances were found in the post-cyclone period (335 items kg−1 sediment) when compared with the pre-cyclone period (188 items kg−1 sediment). The depositional dynamics for both macro-debris and microplastic were site-specific due to factors such as wind direction and the associated storm surge, topography and orientation of the site, and proximity to urban areas. This study has demonstrated the role cyclone induced overwash plays on introducing plastic pollution to beach environments. Considering an increase in both the intensity and frequency of cyclone in the future due to global warming, and a tremendous increase in marine plastic debris, more research effort should be spent on this understudied problem. © 2020 Elsevier B.V. All rights reserved.
⁎ Corresponding author at: Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong Special Administrative Region. E-mail address: [email protected] (S.-G. Cheung).
https://doi.org/10.1016/j.scitotenv.2020.137172 0048-9697/© 2020 Elsevier B.V. All rights reserved.
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1. Introduction Tropical cyclones (TCs) significantly affect the shoreline dynamics, particularly in the low-lying areas (Woodruff et al., 2013). For example, waves generated from a distant tropical cyclone can mobilise offshore sediment and allow the material to be redistributed along the shore (Cooper et al., 2008). TCs and the associated storm surges facilitate the adjustment of beach face and bar migration (Lee et al., 1998), and shoreline erosion (Woodruff et al., 2013), therefore changing the geomorphology of beaches by deposition and removal of sand (Needham et al., 2015). Since sediment particles of all sizes and compositions, together with marine debris, are transported by cyclone induced overwash (Hansom et al., 2015; Soria et al., 2017; Switzer and Jones, 2008), TCs have tremendous effects on the deposition and distribution of sediments and marine debris. For example, N2 million kg of sediments were flushed down the Salt River Submarine Canyon in U.S. Virgin Islands into deep water during Hurricane Hugo (1989) (Shanmugam, 2008). Extreme weather such as flash floods and storms could aggravate the transfer of plastic debris from land to water. Several studies have demonstrated a significant increase in the abundance of beach debris, including a majority of plastic debris (and microplastics) after heavy rainfall and flash floods (Cheung et al., 2016; Gündoğdu et al., 2018; Lattin et al., 2004; Moore et al., 2002; Rech et al., 2014; Van et al., 2012; Veerasingam et al., 2016; Yonkos et al., 2014). Similarly, TCs could create transient effects on debris source loads (Ribic et al., 2011) and transport a large amount of debris into the marine environment (Critchell et al., 2015; Hidalgo-Ruz et al., 2018; Thompson, 2005; Wang et al., 2019). With future global sea-level rise at an accelerating rate (of 3.0 mm per year, Hay et al., 2015) and a projected increase in the intensity of TCs (Elsner et al., 2008) due to climate change, substantial damages and pollution to vulnerable coastal regions are expected to increase. Western North Pacific is the most active TCs basin, in terms of both the overall number of cyclones and peak wind intensities (Maue, 2011). TCs are named as typhoons in south and east Asia. Typhoon
Mangkhut, formed over the western North Pacific as a Category 5equivalent typhoon, is the third-strongest TC worldwide in 2018, with an estimated peak intensity at maximum sustained wind of 250 km h−1 near the centre. The destructive winds and storm surges caused landslides, damages to buildings and homes, and loss of power over Guam, Philippines and South China, resulting in N130 fatalities and NUS$3.7 billion economic loss. As one of the strongest TCs hitting Hong Kong since WWII, Mangkhut brought severe flooding and inundation due to record-breaking storm surges which also occurred elsewhere in the low-lying areas of Pearl River Delta in south China. Extreme wind velocities, storm surges and rainfall caused by TCs could create a large-scale disturbance to both terrestrial and coastal areas. Wang et al. (2019) reported that cyclones may increase the average concentrations of microplastics in the seawater and seabed sediments by approximately 40%. Yet, there is no study on the relationship between cyclones and the deposition patterns of marine debris and microplastics. This study provided insights into the impacts of TC on the composition and patterns of marine debris deposited on the beaches in Hong Kong. The abundance, size distribution and composition of macro-debris (2.5–50 cm) deposited on beaches and microplastics (b5 mm) in the littoral sediments were compared between the pre- and post-cyclone period. As the frequency and intensity of cyclones as well as the abundance of marine plastic debris and microplastics are expected to increase in the future, there is a pressing need to better understand the significance of cyclones in the distribution and accumulation of marine debris and microplastics on beaches. 2. Materials and methods 2.1. Study area To determine the effect of cyclone on the abundance and composition of marine debris and microplastics on beaches with different degrees of wave exposure, four beaches were chosen for the study with
P1 - Protected shore
Shenzhen
TA
IW
AN
MAINLAND CHINA
P2 - Protected shore
New Territories South China Sea
P2
HONG KONG Kowloon
E2
0 75 150 m P1
Lantau Island E1
Hong Kong Island
E1 - Exposed shore 0
7.5
0 75 150 m
E2 - Exposed shore
15 km
LEGEND Path of Typhoon Mangkhut (7 September - 17 September 2018) Protected shore Exposed shore
0 75 150 m
0 75 150 m
Fig. 1. The trajectory of Typhoon Mangkhut and geographic locations of the sampling sites in Hong Kong. Protected shores (green boxes) are Sham Wat (P1, 22°16′14″N, 113°53′11″E) and Starfish Bay (P2, 22°25′55″N, 114°14′43″E); whereas exposed shores (blue boxes) are Cheung Sha Beach (E1, 22°14′00″N, 113°57′02″E) and Pak Lap Wan (E2, 22°21′05″N, 114°21′38″E). Satellite images are retrieved from ESRI (2019) World Imagery. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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two of them being protected and the others more exposed. They were protected shores at Sham Wat (P1) and Starfish Bay (P2), and exposed shores at Cheung Sha Beach (E1) and Pak Lap Wan (E2) (Fig. 1). Both P1 and E1 are located on Lantau Island in the western part of Hong Kong. Situated in the northwest of the island, P1 is an intertidal flat lying in Sham Wat Bay sheltered from currents and waves. E1 is an exposed sandy beach located on the southern coast of Lantau Island facing South China Sea. The protected shore P2 is lying deep in Tolo Harbour in the eastern New Territories. The harbour has a bottle-neck topography and a small tidal range, resulting in poor water circulation and long water residence time (Hodgkiss and Yim, 1995). Also, the Harbour system is receiving input from six rivers/streams entering Tolo Harbour. They are Lam Tsuen River, Shan Liu Stream, Shing Mun River, Tai Po River, Tai Po Kau Stream and Tung Tsz Stream, with a total annual discharge rate of 4.50 × 107 m3 (Environmental Protection Department (EPD), 2012).These rivers and streams are situated near to several residential towns, namely Shatin, Tai Po and Ma On Shan with a total population of 1 million (Hong Kong Census and Statistics Department, 2018), the major pollution source of domestic sewage, surface runoff and historical pollution from industries in the catchment (Kueh and Lam, 2008). E2 is an exposed sandy beach situated in Sai Kung East Country Park in the eastern New Territories with very little human activities. None of the selected beaches had cleanup activities organized by the authorities before the sampling which was conducted during low tides before (13–14 September 2018) and after (18–19 September 2018) Typhoon Mangkhut hitting Hong Kong. 2.2. Macro-debris survey The marine debris assessment was based on the guideline of NOAA (Opfer et al., 2012). At each site, a designated length (100 m, parallel to the water line) was divided into 5 m × 18–47 m (depending on the distance between strandline and waterline) subsections and four of which were selected from the random number table. Sampling was conducted consistently during ebb tide at 1.0 m above CD. Tidal height was obtained from the real time tidal information from Hong Kong Observatory (https://www.hko.gov.hk/en/tide/marine/realtide.htm) and data from the nearest tide gauges were used. The study sites were located by GPS and sampled before and after the cyclone. Within each subsection, identification of macro-debris (N2.5 cm) was started from the back of the shoreline, usually with vegetation cover, to the water's edge. All the macro-debris items within the subsection area were counted and their types and materials (plastic, metal, glass, rubber, processed lumber, cloth/fabric and others) recorded. The density was calculated as the number of debris divided by the subsection area (items m−2). Mesodebris (5 mm–25 mm) were excluded in this study because of two reasons. Firstly, the time for sampling was very tight as we needed to collect samples on four beaches a few days before and after the cyclone. The low tide period in September only lasted for 4–5 h which further limited the time for the work. After the cyclone was gone, we also had to finish the work in 2 days to reduce the tidal effect on the marine debris. The second reason is past records of macrodebris and microplastics on beaches in Hong Kong are available but not of mesoplastics. Therefore, for comparison sake only macro-debris and microplastics were sampled. 2.3. Microplastic sampling and treatment Surface sediment samples (top 2–3 cm) were collected at three shore heights (strandline, and 1.5 m and 1.0 m above chart datum (CD)). At each shore height, a transect line of 20 m was deployed and top 2 cm sediment within an area of 0.25 m × 0.25 m was collected at 2 m intervals with a stainless steel shovel and put in a cotton bag, hence 10 replicates were collected for each shore height. Three more sediment samples were collected at each shore height for the analysis of sediment particle size distribution.
3
All the sediment samples were dried in a greenhouse for at least 4 weeks and subsequently 500 g of sediment was obtained from each sample and suspended in 1 L of ZnCl2 solution (density = 1.6–1.7 g mL−1) and stirred for 10 min using an overhead stirrer at 500 rpm. The mixture was allowed to settle overnight and afterwards decanted through a stack of sieves (mesh size = 5 mm, 1 mm, 125 μm, respectively). The surface of the glass bottle was rinsed thoroughly with ZnCl2 to collect remaining particles adhered to the surface. This separation process was performed twice to optimize the recovery rate of the particles. All the materials retained on 5 mm sieve were discarded. The solution passed through the sieves was collected and filtered through a filter paper (Advantec Grade No.2, pore size = 5 μm), therefore, particles retained on the filter paper were of the size between 5 and 125 μm. They were then treated with 30% H2O2 for 72 h to digest organic matter. The particles retained on 125 μm and 1 mm sieves were rinsed with deionized water and treated with 30% H2O2 separately. The solutions were then filtered and the size fractions of particles collected were 125–1000 μm and 1000–5000 μm, respectively. The efficiency of this extraction method for microplastics was validated twice for each sampling site (i.e., n = 8). After going through the extraction process, a sediment sample from each site was spiked with 50 pieces of microplastics (a mixture of polyethylene, polypropylene, polyethylene terephthalate, expanded polystyrene and nylon 66, size ranged from 50 to 2000 μm) and the extraction procedure was repeated as described above. The percentage recovery of the particles (ranging from 59 to 95%) was used to correct the microplastics abundance in this study. To assess the contamination in the laboratory, a procedural control was conducted according to Fries et al. (2013). Procedural blanks (n = 20) were prepared using ZnCl2 solution (1 L) but without sediment and the extraction process was conducted the same as that for the treatments. No pellet, fragment, foam or film was found in the procedural blanks. However, fibers with a mean value of 1.6 items were found in the blanks. Therefore, a factor of 1.6 items of fiber was used to correct for the amount of fibers obtained in each 500 g sediment sample. 2.4. Microplastic analysis Microplastics in the size range of 1000–5000 μm were counted with the naked eye, while the 5–125 μm and 125–1000 μm fractions were examined and counted under a microscope with a magnification of 7.3×– 120× (M165C, Leica Microsystems). Microplastics were identified by their shape, surface texture and colour according to the criteria established by Free et al. (2014) and McCormick et al. (2014) and categorized into five classes, namely “fiber”, “fragment”, “foam”, “film” and “pellet”. At least 30 microplastics from each of three size fractions, equivalent to 41.7–100% of all the particles in that fraction, and all the particles that could not be distinguished by microscopic examination were characterized by Fourier transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet iS50 FTIR spectrometer). For the 1000–5000 μm fraction, the particles were characterized using the FTIR instrument equipped with an attenuated total reflectance (ATR) unit with a Ge crystal. For 125–1000 μm and 5–125 μm fractions, the particles were characterized using the FTIR microscope (μFTIR, Thermo Scientific Nicolet iN5 FTIR microscope configured with a Ge-tip ATR). A spectral resolution of 4 cm−1 at a wavenumber ranged between 675 and 4000 cm−1 with 40 co-added scans was performed. The spectra obtained were compared with the reference spectra in the Hummel Polymer library and a matching of N70% was accepted. 2.5. Sediment analysis The beach topography was determined following Emery Method (Emery, 1961). The profile of each beach was measured along three transect lines (50 m apart) perpendicular to the shore length. The median particle diameter was determined for sediment (200 g per sample)
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Fig. 2. Mean (±SE) macro-debris densities for all the sites in the pre- and post-cyclone periods. Asterisks (*p b 0.05; **p b 0.01) denote significant differences (paired t-tests) of the macrodebris density between the pre- and post-cyclone period.
collected from three tidal heights as mentioned earlier following the protocol of Blott and Pye (2001). Sediment particle size was determined by wet sieving using running tap water. Six sieves with different pore sizes, i.e., 2.00 mm, 1.00 mm, 500 μm, 250 μm, 125 μm and 63 μm were prepared. Sediments retained on each sieve were collected, dried at 100 °C overnight and weighted.
data were arcsine transformed. Cyclone period (before and after), tidal height (strandline, 1.5 m above CD, 1.0 m above CD), shore type (exposed and protected), and size class (5–125 μm; 125 μm–1 mm; 1–5 mm) were fixed factors, while site (exposed: E1 and E2; protected: P1 and P2) was the random factor nested within shore types. Either one-way ANOVA or t-test was employed when the main effect or interaction was significant.
2.6. Data analysis 3. Results Statistical analyses were performed using R and Minitab 18. Prior to analysis, data were tested for normality and homogeneity of variance using Shapiro-Wilk test and Levene's test, respectively. Data for macro-debris density failing these tests were Johnson transformed. A parametric three-way nested ANOVA was employed to test the differences in debris density between cyclone period (before and after), shore type (exposed and protected) and site (random factor nested within shore types; exposed: E1 and E2; protected: P1 and P2). In case where the main effect or interaction was significant, either t-test or one-way ANOVA was employed to explain for the differences. The changes in the proportion of plastic in marine debris after the cyclone for each site were tested by the Fisher's exact test. For microplastic abundance, data failed to meet the parametric requirements were log-transformed. A four-way nested ANOVA was employed to test the effect of cyclone (before and after), tidal height (strandline, 1.5 m above CD, 1.0 m above CD), shore type (exposed and protected) and site (random factor nested within shore type; exposed: E1 and E2; protected: P1 and P2) on the microplastic abundance. When the interactive terms in the ANOVA analyses were significant, the effect of individual factors was tested by one-way ANOVA or t-test. The changes in proportions of the three size classes of microplastics were tested by five-way nested ANOVA. Prior to analysis the percentage
3.1. Macro-debris density and characteristics in the pre- and post-cyclone periods The average density of macro-debris was 0.047 ± 0.076 items m−2 (±SE) and 0.54 ± 0.48 items m−2 (±SE) for the pre- and postcyclone period, respectively, or an 11.4-fold increase after the cyclone. For all the sites, although the amount of marine debris increased after the cyclone, the increase was only statistically significant for P2 and E2 (Fig. 2), with a 10-fold and 52-fold increase, respectively. This Table 1 Three-way nested ANOVA showing the effect of cyclone and its interactions with shore types and sites on macro-debris density. Macro-debris density (items m−2)
Cyclone Cyclone × shore type Cyclone × site (nested within shore type)
F
d.f.
p
8.54 0.46 4.57
1, 24 1, 24 2, 24
0.100 0.567 b0.05
Other main effects (shore type, and site nested within shore type) were insignificant therefore are not presented here.
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Fig. 3. The composition of macro-debris before and after the cyclone.
indicated that the effect of cyclone was site-specific, regardless of the degree of exposure of the sites (Table 1). Plastic dominated the macro-debris at all the sites (Fig. 3) with its proportion increased significantly for E1 and E2 (27.5% and 17.7%, respectively) and decreased for P1 (−12.6%) after the cyclone (Table 2). The most common plastic items were food wrappers (54 out of 167, or 32.3%) and plastic bags (46 out of 167, or 27.5%) before the cyclone but replaced by plastic fragments (635 out of 1701, or 36.7%) and bottle/container caps (424 out of 1701, or 24.4%) after the cyclone (Table S1 of Supplementary information). 3.2. Microplastics in the pre- and post-cyclone periods 3.2.1. Abundances The average microplastic abundance for all the sites increased significantly from 188 ± 223 items kg−1 (±SE) before the cyclone to 335 ± 390 items kg−1 (±SE) after the cyclone (Table 3, p b 0.001). The protected shores also have more microplastics (382 ± 389 items kg−1) than exposed shores (141 ± 166 items kg−1, Table 3, p b 0.05), regardless of the cyclone periods. Since the “tidal height × shore type” interaction was significant (Table 3, p b 0.001), the distribution of microplastics at different tidal heights varied with the degree of exposure of the shores. Further analysis revealed that more microplastics were deposited at 1.0 m and 1.5 m above CD than along the strandline on protected shores but they tended to deposit along the strandline on exposed shores (Table S2 of Supplementary information). The “cyclone × shore type” and “cyclone × tidal heights” interactions were statistically indistinguishable (p N 0.05), indicating that differences in microplastic abundance between shore types and among three tidal heights did not change with the cyclone. The inter-site differences, however, were significant (Table 3, p b 0.001). To better explain the spatial variability, the microplastic abundances of each site before and after
Table 2 Fisher's exact tests showing the change in the proportion of plastics in marine debris collected from the protected and exposed shores before and after the cyclone.
the cyclone were investigated. An increase of microplastic abundance for 3.9 fold (from 180 ± 108 to 701 ± 518 items kg−1) at P2 (Fig. 4), followed by an increase of 1 fold (from 127 ± 113 to 275 ± 262 items kg−1) at E2 were observed. In contrast, there was no significant change of microplastic abundance for P1 and E1 after the cyclone (Fig. 4). These results were in line with the macro-debris density. 3.2.2. Size distributions Microplastics were grouped into three size classes (5–125 μm; 125 μm–1 mm; 1–5 mm) in this study. The size fraction 125 μm– 1 mm was the most frequently found both before and after the cyclone, accounting for 86.0 ± 15.9% and 77.0 ± 24.0% of all the size classes, respectively. The size distribution of microplastics varied neither with any of the main factors, i.e., cyclone, exposure, site, tidal height and size of microplastics nor their interactions except for the “cyclone × size of microplastics × tidal height × site” interaction. Therefore, the effect of cyclone was investigated separately for all the other main factors and the results are shown in Table S3 of Supplementary information. Although the microplastic size distribution was different in a number of comparisons, no specific pattern was observed. 3.2.3. Shape and compositions of microplastic The relative abundance of different shapes and chemical compositions of microplastics is shown in Fig. 5. Fibers were the most common shape of microplastics for all the sites before the cyclone and most of the sites after the cyclone except E2 where foam became dominant. Polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), nylon, cellophane and other polymers (polyester, polycarbonate, acrylate polymer) were found in this study. Higher density polymers (e.g., PET and PVC) were absent before the cyclone but a small amount of them (PET: 0.8%; Table 3 Four-way nested ANOVA showing the effects of cyclone, shore type, site and tidal height and their interactions on microplastic abundances. Microplastic abundance (items kg−1)
Plastic proportions (%)
P1 P2 E1 E2
Pre-cyclone
Post-cyclone
%change
p
93.3 84.2 61.9 74.7
80.7 87.5 89.5 92.4
−12.6 +3.30 +27.5 +17.7
b0.05 0.532 b0.01 b0.001
Cyclone Shore type Tidal height Site Tidal height × shore type
F
d.f.
p
22.7 8.92 1.04 5.50 15.1
1, 217 1, 217 2, 217 3, 217 2, 217
b0.001 b0.05 0.354 b0.005 b0.001
Other interactions were insignificant therefore are not presented here.
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Fig. 4. Mean (±SE) microplastic abundance for all the sites in the pre- and post-cyclone periods. Asterisks (*p b 0.05; ***p b 0.001) denote significant differences (paired t-tests) of the macro-debris density between the two periods.
PVC: 1.9%) were collected after the cyclone. Most of the microplastics identified were nylon, which represented 46.3% and 47.4% of the samples in the pre- and post-cyclone period, respectively. The second most abundant polymer was PE, representing 25.0% and 16.8% before and after the cyclone, respectively. PP ranked the third before the cyclone (11.8%) but was replaced by PS after the cyclone, which increased from 8.0% to 15.0%, although the change was mostly attributed to E2. 4. Discussion 4.1. Effects of Typhoon Mangkhut on marine macro-debris The differential impact of the cyclone on the accumulation of marine debris on individual sites was a combined effect of trajectory of the cyclone, topography and orientation of the sites, and proximity to densely populated areas. P2 received the greatest impact because of three reasons. Firstly, although it is a protected beach in Tolo Harbour, the wind speed inside Tolo Harbour during the cyclone was comparable to that at other sites (Fig. 6a–b). Its northeast facing location rendered it a direct impact from the storms as Typhoon Mangkhut skirted past to the south-southwest of Hong Kong (Fig. 1), generating east to northeasterly storms associated with hurricane force winds (Fig. 6). Secondly, the storm surge, which is one of the major contributors on transportation of marine debris between ocean and beach (Taffs and Cullen, 2005), recorded at the Tai Po Kau tide-gauge station near to P2 was record breaking in Hong Kong with a tidal height of 4.71 m above CD (Fig. 6c). Thirdly, P2 is located in Tolo Harbour along which are several major residential areas (e.g., Shatin, Tai Po, Ma On Shan) with a total population of 1 million. Debris generated on land in these areas would be carried to the sea by wind and major rivers, such as Shing Mun
River in Shatin, and Lam Tsuen River in Tai Po (Fig. 6). In 2018, using video recording Greenpeace estimated that at least 48,000 plastic items were carried by Shing Mun River into Tolo Channel on a no-rain day, and the number doubled after rainfall (Yeung et al., 2018). The problem would be aggravated due to rainstorms associated with the cyclone. E2 received a smaller impact than P2 because it is inside a country park without human settlement and is relatively remote, so human disturbance is minimal. A smaller storm surge and its southeast facing position further reduced the impact. Similar to E2, E1 is facing southeast and located in a country park, but it received less impact from the cyclone than E2 because of the protection by Chi Ma Wan Peninsula in the east (Fig. 6). The impact of the cyclone on P1 was negligible in terms of accumulation of marine debris and microplastics, and was largely due to its northwest facing position and the smallest storm surge. Therefore, wind speed, storm surge, orientation and topography of the site, and proximity to urban areas are major determining factors in introducing plastic pollution to beach environment through cyclone induced overwash. In contrast, the effect of degree of wave exposure of the site was relatively insignificant. Unsurprisingly plastic was the most dominant macro-debris at all the sites (Fig. 3) and its proportion increased further at sites much affected by the cyclone. As compared with other types of marine debris such as glass, rubber and metals, a much lower density rendered plastic debris floating in the sea and eventually washed ashore (Cunningham and Wilson, 2003). Although it is difficult to trace the source of marine debris deposited on the shores, words and logos printed on plastic products have provided some hints on the sources. Being located inside Sai Kung East Country Park, E2 is an exposed shore facing South China Sea, debris collected, therefore, should come from the sea as no urban
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Fig. 5. Composition of microplastics before and after the cyclone in terms of (a) shape and (b) chemical composition. Types of microplastics found included polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), nylon, cellophane and others (polyester, polycarbonate, acrylate polymer). For detailed chemical compositions of microplastic, please see Table S5 of Supplementary information.
development was allowed in country parks of Hong Kong. It is interesting to note that 40% of the plastic debris at E2 were bottle caps with labels in Simplified Chinese (used in China, 45.3%), English (38.8%), Traditional Chinese (used in Hong Kong, Macau and Taiwan, 11.6%) and others (0.04%, including Japanese, Korean, Thai, Vietnamese, Russian). This indicated a possibility that macro-debris at E2 travelled from offshore in South China Sea to nearshore at a short window of time during the cyclone period. 4.2. Effects of Typhoon Mangkhut on microplastics Our previous study (Lo et al., 2018) showed that shores with lower energy are more inductive to microplastic deposition. This is consistent with the present study that the two protected shores were more polluted than the exposed shores before the cyclone. Same as the macrodebris, the abundance of microplastics increased significantly at E2 and P2 after the cyclone and the greatest increase was found at P2. This indicated that the same factors, i.e., wind direction and the associated storm surge, topography and orientation of the sites, and proximity to urban areas were responsible for the increase in both microplastics and macro-debris on the shores. For example, extreme events such as floods generated by heavy rainfall have been shown to transport
terrestrial plastics into the marine environment. This also applies to microplastics as Veerasingam et al. (2016) demonstrated that huge quantities of microplastics were washed through rivers from land to sea during 2015 South India floods. These fresh microplastics were subsequently driven and transported by the winds and surface currents and ultimately deposited, resulted in a 3-fold increase in the microplastic abundance on the Indian beaches. The wave energy generated from TCs and the associated highenergy storms are responsible for erosion and redistribution of sediment, even on protected shores (Hopley, 1974; Galli, 1989). The changes in the shore profile (Fig. S1 of Supplementary information) of all the study sites in this study have indicated redistribution of sediment and removal of prestorm beach crests as reported upon by McIntire and Walker (1964). Sedimentological studies (Table S4 of Supplementary information) have demonstrated that the sediment on the stormdeposited layer often became coarser after severe storm events (Soria et al., 2017) due to onshore sediment overwashing and intense offshore wave activities (Oliva et al., 2017). This phenomenon was observed on the two most affected beaches, E2 and P2 where the grain size increased after the cyclone. The sediment overwashing and redistribution probably led to random deposition of microplastics at different tidal heights
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a
b 2:00 pm @ 16 Sep 2018
11:00 am @ 16 Sep 2018 Tung Tze Stream Lam Tseun River Tai Po River
Tung Tze Stream Lam Tseun River
Shan Liu Stream H Tolo
abou
r
Tai Po River
P2
Shan Liu Stream H Tolo
abou
r
P2
Tai Po Kau Shing Mun Stream River
Tai Po Kau Shing Mun Stream River
E2
E2
P1
P1
E1
E1
an Ma W Chi la insu Pen
an Ma W Chi la insu Pen
c Highest water level recorded
Easterly wind of 90 km h-1 Easterly wind of 18 km h-1 Anemometer
4.71m H Tolo
3.33m
3.09m
r abou
Tide gauge
P2
E2
P1
4.19m E1
3.73m
Fig. 6. Wind speed and direction when Typhoon Mangkhut skirted passed closest to Hong Kong at (a) 11:00 am and (b) 2:00 pm on 16 September 2018. Highest water levels (meter above chart datum) recorded by tide gauges are presented in (c). Data are retrieved from Hong Kong Observatory (2018).
in this study and resulted in unpredictable changes in the distribution of different size groups of microplastics at the three tidal heights. The cyclone increased the diversity of polymer types on the shores with heavier polymers such as PVC and PET found only after the cyclone (Fig. 5b; Table S5 of Supplementary information). This was probably a result of the strong vertical mixing during the passage of cyclone, bringing heavier polymers from the seabed to the shores. This is supported by the fact that PET was one of the major polymer types in seabed sediment collected in Hong Kong (Cheang et al., 2018). A similar increase in the polymer diversity following cyclone was reported upon by Wang et al. (2019). Most of the microplastics were fibers both before and after the cyclone except E2 where foam replaced fibers after the cyclone. As confirmed by the FTIR analysis, most of these fibers were nylon, probably generated by laundry and abandoned fishing net, in contrast to a previous study in which most of the polymers on sandy beaches were PE and PP (Lo et al., 2018). The discrepancy could be explained by methodological differences that Lo et al. (2018) only characterized microplastics down to 500 μm in comparison with 50 μm in this study, resulting in smaller fibers being excluded in the former. More foam items were found at E2 after the cyclone. Since foam has extremely low specific
density and is easily transported in the sea for a very long distance, it is difficult to trace its sources. Nevertheless, based on the location of E2 and a large number of plastic caps found which indicated multiple sources of the debris, the foam may be originated from various sources including foam buoys from fish farms and foam boxes in the logistic industry commonly found in Hong Kong and southern China.
5. Conclusions This study has demonstrated tremendous impacts of Typhoon Mangkhut on the macro-debris and microplastic pollutions on 4 shores in Hong Kong with much higher abundance of both macro-debris and microplastics obtained after the cyclone. The diversity of microplastics also increased. The deposition of macro-debris and microplastics were site-specific and was related to factors such as wind direction and the associated storm surge, orientation and topography of the sites, and proximity to urban areas. In foreseeable future, both the frequency and intensity of cyclone as well as the amount of marine plastic debris are expected to increase, therefore, more research effort should be spent on this relatively understudied problem.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The work described in this paper was supported by the Research Grants Council of Hong Kong SAR (CityU 11302815, CityU 9042495). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2020.137172. References Blott, S.J., Pye, K., 2001. Gradistat: a grain size distribution and statistics package for the analysis of unconsolidated sediments. Earth Surf. Process. Landf. 26, 1237–1248. https://doi.org/10.1002/esp.261. Cheang, C.C., Ma, Y., Fok, L., 2018. Occurrence and composition of microplastics in the seabed sediments of the coral communities in proximity of a metropolitan area. 15, 2270. Int. J. Environ. Res. Public Health https://doi.org/10.3390/ijerph15102270. Cheung, P.K., Cheung, L.T.O., Fok, L., 2016. Seasonal variation in the abundance of marine plastic debris in the estuary of a subtropical macro-scale drainage basin in South China. Sci. Total Environ. 562, 658–665. https://doi.org/10.1016/j. scitotenv.2016.04.048. Cooper, M.J.P., Beevers, M.D., Oppenheimer, M., 2008. The potential impacts of sea level rise on the coastal region of New Jersey, USA. Clim. Chang. 90, 475–492. https://doi. org/10.1007/s10584-008-9422-0. Critchell, K., Grech, A., Schlaefer, J., Andutta, F.P., Lambrechts, J., Wolanski, E., Hamann, M., 2015. Modelling the fate of marine debris along a complex shoreline: lessons from the Great Barrier Reef. Estuar. Coast. Shelf Sci. 167, 414–426. https://doi.org/ 10.1016/j.ecss.2015.10.018. Cunningham, D.J., Wilson, S.P., 2003. Marine debris on beaches of the Greater Sydney Region. J. Coast. Res. 19, 421–430. Elsner, J.B., Kossin, J.P., Jagger, T.H., 2008. The increasing intensity of the strongest tropical cyclones. Nature 455, 92–95. https://doi.org/10.1038/nature07234. Emery, K.O., 1961. A simple method of measuring beach profiles. Limnol. Oceanogr. 6, 90–93. https://doi.org/10.4319/lo.1961.6.1.0090. ESRI, 2019. World imagery. Retrieved from. http://www.esri.com, Accessed date: 12 December 2019. Free, C.M., Jensen, O.P., Mason, S.A., Eriksen, M., Williamson, N.J., Boldgiv, B., 2014. Highlevels of microplastic pollution in a large, remote, mountain lake. Mar. Pollut. Bull. 85 (1), 156–163. https://doi.org/10.1016/j.marpolbul.2014.06.001. Fries, E., Dekiff, J.H., Willmeyer, J., Nuelle, M.T., Ebert, M., Remy, D., 2013. Identification of polymer types and additives in marine microplastic particles using pyrolysis-GC/MS and scanning electron microscopy. Environ. Sci. Processes Impacts 15, 1949–1956. https://doi.org/10.1039/C3EM00214D. Galli, G., 1989. Depositional mechanisms of storm sedimentation in the Triassic Dürrenstein Formation, Dolomites, Italy. Sediment. Geol. 61, 81–93. https://doi.org/ 10.1016/0037-0738(89)90042-0. Gündoğdu, S., Çevik, C., Ayat, B., Aydoğan, B., Karaca, S., 2018. How microplastics quantities increase with flood events? An example from Mersin Bay NE Levantine coast of Turkey. Environ. Pollut. 239, 342–350. https://doi.org/10.1016/j.envpol.2018.04.042. Hansom, J.D., Switzer, A.D., Pile, J., 2015. Extreme waves: causes, characteristics, and impact on coastal environments and society. Coastal and Marine Hazards, Risks, and Disasters. Elsevier, pp. 307–334. Hay, C.C., Morrow, E., Kopp, R.E., Mitrovica, J.X., 2015. Probabilistic reanalysis of twentieth-century sea-level rise. Nature 517, 481–484. https://doi.org/10.1038/ nature14093. Hidalgo-Ruz, V., Honorato-Zimmer, D., Gatta-Rosemary, M., Nuñez, P., Hinojosa, I.A., Thiel, M., 2018. Spatio-temporal variation of anthropogenic marine debris on Chilean beaches. Mar. Pollut. Bull. 126, 516–524. https://doi.org/10.1016/j. marpolbul.2017.11.014. Hodgkiss, I.J., Yim, W.W.-S., 1995. A case study of Tolo Harbour, Hong Kong. In: McComb, A.J. (Ed.), Eutrophic Shallow Estuaries and Lagoons. CRC Press, Boca Raton, pp. 41–57. Hong Kong Census and Statistics Department, 2018. The profile of Hong Kong population analysed by District Council District, 2017. Retrieved from. https://www.statistics. gov.hk/pub/B71807FB2018XXXXB0100.pdf, Accessed date: 11 December 2019. Hong Kong Environmental Protection Department, 2012. River water quality data. Retrieve from. http://epic.epd.gov.hk/ca/uid/riverhistorical/p/1, Accessed date: 17 January 2020. Hong Kong Observatory, 2018. Super typhoon Mangkhut (1822) 7 to 17 September 2018. Retrieved from. https://www.hko.gov.hk/en/informtc/mangkhut18/mangkhut.htm, Accessed date: 11 December 2019.
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