Supporting Information

Spatio-temporal comparison of neustonic microplastic density in Hong Kong waters under the influence of the Pearl River Estuary

Pui Kwan Cheunga, Lincoln Fokb,*, Pui Lam Hungc, Lewis T.O. Cheungc

a Department of Geography, University of Hong Kong, Pokfulam Road, Hong Kong SAR, Chinab Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong SAR, Chinac Department of Social Sciences, The Education University of Hong Kong, Hong Kong SAR, China

About this SI: Six tables, five figures and unit conversion method.

S1

Table S1Names and coordinates of the sampling sites.

Region Site Name Sampling date Dry season Rainy season Latitude (°E) Longitude (°N)

Western waters (Pearl River Estuary)

W1 Lung Kwu Tan West 28th February 2015 17th July 2015 22.399728 113.902485W2 Lung Kwu Chau West 28th February 2015 17th July 2015 22.366557 113.865703W3 Airport Southwest 28th February 2015 17th July 2015 22.288598 113.855987W4 Tai O West 28th February 2015 17th July 2015 22.246125 113.831765W5 Fan Lau South 28th February 2015 17th July 2015 22.189939 113.849680W6 Cheung Sha South 26th February 2015 17th July 2015 22.207281 113.942255W7 Cheung Chau Northeast 26th February 2015 18th July 2015 22.223415 114.069382W8 Stanley Southwest 26th February 2015 18th July 2015 22.206843 114.192885W9 Shek O Wan West 26th February 2015 18th July 2015 22.233287 114.262884

Eastern waters E1 North Ninepin Island East 26th February 2015 18th July 2015 22.271453 114.365632E2 High Island Reservoir South 1st March 2015 29th July 2015 22.352056 114.377896E3 Tai Long Wan South 1st March 2015 20th July 2015 22.389243 114.393968E4 Grass Island Southeast 1st March 2015 20th July 2015 22.449889 114.385996E5 Port Island Northeast 1st March 2015 20th July 2015 22.507844 114.365451E6 Crooked Island North 1st March 2015 20th July 2015 22.557787 114.305867

S2

Table S2Summary statistics of the densities of microplastics (0.355 – 4.75 mm) and large plastic debris (≥ 4.75 mm) in number and weight (per km2).

Microplastics (0.355 – 4.75 mm) Large plastic debris (≥4.75 mm)

OverallSeason

OverallSeason

Rainy Dry Rainy DryN 30 15 15 30 15 15

Number (n/km2)

Mean 334,780 588,169 28,708 104,485 197,570 11,400SEM 81,097 126,278 11,036 36,743 65,847 4403Median 60,321 393,077 21,673 19,634 78,563 5987IQR 351,929 647,217 17,118 68,930 249,400 10,196Minimum 1858 42,849 1858 0 7093 0Maximum 1,675,982 1,675,982 174,723 803,243 803,243 65,087

Weight* (g/km2)

Mean 35 49 38 12,497 24,187 806SEM 9 12 14 5669 10,653 378Median 32 36 20 234 2119 135IQR 48 36 40 3079 34,774 807Minimum 0 0 1 0 9 0Maximum 202 165 202 130,926 130,926 3340

*For microplastics, the weight data only included those in the range of 0.71 to 4.75 mm.

S3

Table S3aSummary statistics of microplastic densities in the western and eastern waters of Hong Kong by number and by weight (per km2).

Overall Rainy season Dry seasonWest East West East West East

N 18 12 9 6 9 6

Number (n/km2)

Mean 304,816 313,871 575,522 607,138 34,110 20,604SEM 109,610 124,460 180,041 183,594 18,060 6917Median 56,036 101,342 393,077 574,092 21,673 16,720IQR 34,6157 425,439 575,502 668,158 16,947 15,167Minimum 1858 5815 42,849 151,267 1858 5815Maximum 1,675,982 1,242,221 1,675,982 1,242,221 174,723 51,418

Weight* (g/km2)

Mean 55 27 59 34 50 21SEM 14 7 18 10 22 11Median 32 23 52 35 22 5IQR 39 39 32 32 34 27Minimum 1 0 4 0 1 2Maximum 202 68 165 63 202 68

*The weight data only included microplastics in the range of 0.71 to 4.75 mm.

S4

Table S3bSummary statistics of large plastic debris densities in the western and eastern waters of Hong Kong by number and by weight (per km2).

Overall Rainy season Dry seasonWest East West East West East

N 18 12 9 6 9 6

Number (n/km2)

Mean 121,343 79,199 232,072 145,817 10,613 12,580SEM 55,994 39,150 101,056 70,344 6915 4522Median 12,557 25,657 37,484 86,557 5579 10,864IQR 52506 71,492 368,376 93,769 6585 16,521Minimum 0 0 7093 13,065 0 0Maximum 803,243 481,850 803,243 481,850 65,087 27,117

Weight* (g/km2)

Mean 16,462 6549 31,803 12,763 1120 336SEM 8845 5010 16,532 9743 605 215Median 539 224 1484 3218 49 171IQR 3080 2558 61,098 7172 1611 171Minimum 0 0 36 9 0 0Maximum 130,926 61,005 130,926 61,005 4882 1395

*The weight data only included microplastics in the range of 0.71 to 4.75 mm.

S5

Table S4 Mean numeric densities (n/m3) of plastic debris by category and their changes (%) between seasons.

Microplastics Large plastic debrisSeason PF FB FL FM PL PF FB FL FM PLDry 0.104 0.058 0.011 0.078 0.006 0.013 0.014 0.028 0.016 0.001Rainy 3.690 0.182 0.207 1.918 0.126 0.522 0.183 0.196 0.326 0.007Change (%) 3438 216 1802 2356 2166 3929 1196 610 1995 658

S6

Table S5Polymer composition of the confirmed non-PF plastics.

Size range (mm) Total PP/EPR LDPE PE PP Others

0.355 - 0.709N 279 86 38 32 24 99% 100.0% 30.8% 13.6% 11.5% 8.6% 35.5%

>0.71N 549 163 46 213 28 99% 100.0% 29.7% 8.4% 38.8% 5.1% 18.0%

All N 828 249 84 245 52 198% 100.0% 30.1% 10.1% 29.6% 6.3% 23.9%

PP/EPR: Polypropylene + poly(ethylene:propylene).LDPE: Low-density polyethylene.PE: Polyethylene.PP: Polypropylene.

S7

0

50

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450

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Aver

age

tota

l rai

nfal

l (m

m)

Month

Fig. S1. Monthly average total rainfall in Guangdong Province in 2015 (Source: Guangdong Meteorological Service, http://www.grmc.gov.cn/qxgk/tjsj/index.html)

S8

0102030405060708090

100

0.355 -0.499

0.500 -0.709

0.710 -2.799

2.800 -4.749

≥4.750Prop

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(a) PF FB FL FM PL

0102030405060708090

100

0.355 -0.499

0.500 -0.709

0.710 -2.799

2.800 -4.749

≥4.750Prop

ortio

n by

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egor

y (%

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Size class (mm)

(b) PF FB FL FM PL

Fig. S2. Categorical compositions of plastic debris of different size classes of the (a) rainy season and (b) dry season by number.

S9

0510152025303540

05

1015202530354045

W1 W2 W3 W4 W5 W6 W7 W8 W9 E1 E2 E3 E4 E5 E6

Salin

ity (

ppt)

Tota

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deb

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(n/m

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Site

Plastic density Salinity(a)

0510152025303540

00.250.5

0.751

1.251.5

1.752

W1 W2 W3 W4 W5 W6 W7 W8 W9 E1 E2 E3 E4 E5 E6

Salin

ity (

ppt)

Tota

l pla

stic

deb

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(n/m

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Site

Plastic density Salinity(b)

Fig. S3. Total plastic debris density and salinity at each sampling site in the (a) rainy season and the (b) dry season. Please refer to Figure 1 for the locations of the sites.

S10

Fig. S4. Convergence front of the Pearl River Estuary (site W6), which is characterised by a strip of flotsam and foam.

S11

Plastic density unit conversion method

In this paper, plastic and microplastic densities are reported in items/m3 and items/km2. The sampled area is given by the following equation:

Area (km2) = Width of mental frame (km) × Tow distance (km)

= 0.00087 × (Flow meter counts) × C

where C = flow meter constant = 26863/999999 (model: 2030R, General Oceanics Inc.).

It should be noted that the flow meter constant is unique to a specific model of flow meter.

The volume of sea water filtered by the net is given by the following equation:

Volume (m3) = Area (m2) × Height of mental frame (m)

= Area (m2) × 0.18

Plastic density in items/m3 and items/km2 is therefore the number of plastic items divided by volume of filtered sea water and by sampled area respectively.

S12