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Occurrence and characteristics of microplastic pollution in Xiangxi Bay of Three Gorges Reservoir, China Kai Zhang, Xiong Xiong, Hongjuan Hu, Chenxi Wu, Yonghong Bi, Yonghong Wu, Bingsheng Zhou, Paul Kwan-Sing Lam, and Jiantong Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00369 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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Environmental Science & Technology

TOC Art 270x151mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Occurrence and characteristics of microplastic pollution in Xiangxi Bay of Three

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Gorges Reservoir, China

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Kai Zhang1,2,4, Xiong Xiong1, Hongjuan Hu1, Chenxi Wu1*, Yonghong Bi1, Yonghong

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Wu3, Bingsheng Zhou1, Paul K. S. Lam2, Jiantong Liu1

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Hydrobiology, Chinese Academy of Sciences, Wuhan 430072, China

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Kong SAR, China

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State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of

State Key Laboratory in Marine Pollution, City University of Hong Kong, Hong

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Sciences,

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Chinese Academy of Sciences, Nanjing 21008, China

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University of Chinese Academy of Sciences, Beijing 100039, China

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* Corresponding author

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Chenxi Wu

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Address: Donghu South Road No.7, Wuhan, P.R.China, 430072

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Tel: +86-27-68780712

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Fax: +86-27-68780123

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E-mail: [email protected]

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ABSTRACT: Microplastic pollution in inland waters is receiving growing attentions.

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Reservoirs are suspected to be particularly vulnerable to microplastic pollution.

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However, very limited information is currently available on pollution characteristics

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of microplastics in reservoir ecosystems. This work studied the distribution and

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characteristics of microplastics in the backwater area of Xiangxi River, a typical

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tributary of the Three Gorges Reservoir. Microplastics were detected in both surface

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water and sediment with concentrations ranging from 0.55×105 to 342×105 items km-2

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and 80 to 864 items m-2, respectively. Polyethylene, polypropylene, and polystyrene

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were identified in surface water, while polyethylene, polypropylene, and polyethylene

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terephthalate, and pigments were observed in sediment. In addition, microplastics

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were also detected in the digestion tracts of 25.7% of fish samples, and polyethylene

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and nylon were identified. Redundancy analysis indicates a weak correlation between

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microplastics and water quality variables but a negative correlation with water level of

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the reservoir and Secchi depth. Results from this study confirm the presence of high

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abundance microplastics in reservoir impacted tributaries, and suggest that water level

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regulated hydrodynamic condition and input of nonpoint sources are important

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regulators for microplastic accumulation and distribution in the backwater area of

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reservoir tributaries.

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INTRODUCTION

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Plastic products are extensively used in the modern world. The annual global

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production of plastics increased 40% in the past decade and reached 322 million

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tonnes in 2015.1 However, improper use and disposal of plastics are causing severe

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environmental problems.2, 3 Most plastics are resistant to degradation and are able to

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persist in the environment for tens to hundreds of years.4 Worse still, smaller plastic

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debris can be generated during the breakdown of large plastic products and plastics

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measuring less than 5 mm are commonly referred to as microplastics.2, 5 In the past

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decade, detection of microplastics in marine and terrestrial environments has been

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increasingly reported, and microplastics are considered as contaminants of emerging

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concern.6-8

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Microplastics in aquatic ecosystems can be mistakenly ingested by aquatic

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animals and water birds,9-11 and food chain transfer has also been found to be

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possible.12, 13 Therefore, microplastic pollution in aquatic environments has raised

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growing concerns regarding their potential threats to aquatic animals and even

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seafood safety.14-16 Recently, microplastics were found to be able to inhibit hatching,

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decrease growth rate, and alter the feeding preferences and innate behavior of

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European perch (Perca fluviatilis) larvae at environmentally relevant concentrations.17

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Leachates from plastics are reported to affect larval development in brown mussels

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and inhibit barnacle settlement.18, 19

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Information on microplastic pollution from freshwater environments is limited

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compared to that from marine environments.7, 20 However, the majority of plastic 3


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debris in oceans is derived from the use of plastics on land although discharges from

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ocean vessels and general shipping activities can also contribute a minor portion.21 It

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was estimated that 275 million tonnes of plastic wastes were generated in 2010 in 192

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coastal countries, and about 2.5 to 6.6% of the total plastic wastes ended up in the

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ocean.21 The remainder of plastic wastes could be either recycled, disposed of in

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landfills, incinerated (with or without energy recovery), or stay in terrestrial

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environment.3 Recent works show the ubiquitous presence of microplastics in many

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inland waters, and their abundances in some inland waters are found to be much

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higher than those reported in marine environments.22-25 Therefore, microplastic

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pollution from inland waters deserves more attention.

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The Three Gorges Dam (TGD) is the largest hydropower project in the world. It

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is of great importance to electric power generation, river transportation, and water

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conservancy and control. But controversy has never stopped since the project was first

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proposed in the early 20th century due to its unprecedented magnitude and broad

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impacts especially on environmental and ecological aspects.26 Many environmental

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problems have emerged in the Three Gorges Reservoir (TGR) area since the

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impoundment of the TGD, such as algal blooms, water quality deterioration, and

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reduction of domestic fish species.27 Floating garbage, including many plastic wastes,

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is another issue in the TGR especially during rainy season.28 In our previous work,

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surface water from the TGR was found to be seriously polluted by microplastics, and

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tributaries are believed to be contributors of microplastic pollutants in the Yangtze

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River mainstream in the TGR.23 4



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Since the impoundment of the TGD, intense algal blooms have been observed in

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many tributary backwater areas due to a drastic decrease in flow rate.29 Many villages

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and towns are located near the banks of tributaries. But anthropogenic wastes in these

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underdeveloped areas are poorly managed and can be discharged into the tributaries

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intentionally, or following surface runoff, causing pollution of the tributaries. In this

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work, microplastic pollution was studied in Xiangxi River, a typical tributary within

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the TGR area which has attracted great research interest previously.29-31 Planktonic

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samples were collected seasonally from upstream to downstream sites in the

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backwater area of Xiangxi River to reveal the spatial and temporal distribution

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patterns of microplastics in surface water and to investigate their relation with

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phytoplankton, zooplankton, and environmental variables. Sediment and fish samples

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were also collected and analyzed for microplastics to illustrate their fate. This

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comprehensive study contributes to a better understanding of the occurrence,

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distribution, and potential impacts of microplastics in reservoir ecosystems.

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MATERIALS AND METHODS

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Study Area. Xiangxi River is the largest Yangtze River tributary in the TGR area

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in Hubei Province. It originates from Shennongjia mountain area and flows through

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Xingshan and Zigui Counties with a watershed area of 3099 km2.30 The main human

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activities in this area include mining, chemical industry, agriculture, and tourism. The

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water level of Xiangxi River has risen about 40 m since 2003, and varies seasonally as

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regulated by the TGD. The TGR is usually operated at a high water level in winter for 5


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electric power generation and at a low water level in summer for flood control. Seven

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sampling sites were selected from upstream to downstream of the Xiangxi River

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backwater area (Fig. 1). The most upstream site ZJ was located near Zhaojun Town

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within the uppermost area that backwater can reach in summer. The most downstream

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site XX was located near Xiangxi Town within the estuary area entering the Yangtze

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River mainstream. Site GL was at the mouth of Gaolan River - a major tributary of

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Xiangxi River. Another four sites were roughly evenly distributed along the Xiangxi

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River main channel.

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Sample Collection. Microplastics in surface water were collected by surface

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trawling in April, July, and October 2015 and in January 2016. Surface trawling was

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performed following a previous method.23 Briefly, a trawl net (50 × 100 × 150 cm,

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with a mesh size of 112 µm) was towed vertically to the flow direction by the side of a

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boat at a speed of ~5 km h-1 from one side to the other side of the river and back. The

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towing distance varied from 190 to 480 m depending on the river width at the

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sampling site. After each towing, the net was rinsed with river water from outside and

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floating debris retained in the net was transferred into a 1-L glass bottle and preserved

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with 5% methyl aldehyde before analysis. The net was turned over and rinsed three

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times with distilled water before next sampling to reduce cross contamination. At the

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same site, phytoplankton, zooplankton, and water samples were collected according to

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the standard methods for plankton survey,32 and analyzed after being transported back

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to the laboratory in a cooler. Dissolved oxygen (DO), pH, water temperature (T), and

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conductivity (EC) were measured in situ using a HQ40d meter (Hach, Loveland, CO, 6



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USA). Secchi depth (SD) was measured using a Secchi disk. Water level (WL) data

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were obtained from China Three Gorges Corporation. Sediment samples were

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collected from the midstream channel in January 2016 with a Petersen grab sampler

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with an opening area of 0.0625 m2. Collected sediment samples were passed through

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a 0.3-mm mesh sieve, and debris retained on the sieve was rinsed into a 500-mL glass

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bottle with distilled water. Sediment samples were preserved with 5% methyl

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aldehyde before analysis. Fish samples were acquired from local fishermen in June

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2016. Thirty five fish belonging to 13 species were obtained and were dissected after

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weight and length measurements. Digestive tracts were stored in glass vials and

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transported back to the laboratory in a cooler for further treatment.

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Sample Analysis. Surface water and sediment samples were processed following

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previous methods.23, 33 Detailed procedure is provided in Supplementary Information.

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The digestion tract of each fish was weighed and digested using a 10% KOH solution

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following Foekema et al.34 The digested solution was filtrated onto Whatman GF/C

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glass fiber filters (1.2-µm pore size). Samples on filters were examined under a

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stereomicroscope up to 40× magnification, suspected microplastics were identified

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primarily based on their shape, surface texture, color and luster, and picked out with

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tweezers for further confirmation. During sample analysis, nitrile gloves and cotton

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laboratory coat were worn, all containers were washed with distilled water and

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covered with aluminum foil if not in use, and sample pretreatment were performed in

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hood with laminar flow to reduce contamination.

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All suspected microplastics were examined using a Renishaw inVia Raman 7


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microscope (Wotton-under-Edge, Gloucestershire, UK). Raman spectra were recorded

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from 300-3200 cm-1 with a laser excitation wavelength of 785 nm. The spectra of the

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samples were searched in the Renishaw Polymeric Materials Database and types of

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microplastics were identified based on the similarity to the spectra of standards.

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Presence of pigments was observed in sediment samples, which interfered with the

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identification. To distinguish between pure pigments and pigment containing

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microplastics, we followed the protocol of Imhof et al,35 and particles showing C-H

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stretching peaks around 2800-3100 cm-1 were considered as microplastics and were

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identified based on the match of characteristic peaks with references. The Raman

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spectra of typical samples are presented in Fig. S1.

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Phytoplankton and zooplankton identification and quantification were performed

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by reference to Zhang and Huang.36 Total phosphorus (TP), total nitrogen (TN),

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ammonia nitrogen (NH4+-N), and nitrate nitrogen (NO3--N) in surface water samples

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were analyzed following standard methods.37

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Data Analysis. The abundance of microplastics in surface water and sediment

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from each sampling site was calculated by dividing the number of identified

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microplastics by the towing area (found by multiplying the towing distance by the

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width of the trawl net) and the opening area of the Peterson grab sampler expressed as

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items km-2 and items m-2, respectively.

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Microplastics were treated as a taxon of plankton. Together with other taxa of

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planktons, their relationship with environmental variables was determined using

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ordination analysis. Linear ordination method was applied according to the Detrended 8



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Correspondence Analysis (DCA) of the taxa and environmental variables, and

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Redundancy Analysis (RDA) was performed to obtain appropriate ordination. Taxa

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data and environmental variables (except pH) were logarithmically transformed

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before RDA to eliminate the influence of extreme values on ordination scores.

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Environmental variables with high correlation coefficients (r > 0.6) were excluded in

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the final RDA analysis, and 6 out of 10 environmental variables were selected using a

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forward selection of environmental variables. The analysis of RDA was performed

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using Canoco Version 4.5.

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RESULTS AND DISCUSSION

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Occurrence of Microplastics in Surface Water. Microplastics were detected in

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all sampling sites and their abundances varied from 0.55×105 to 342×105 items km-2

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(Table 1). A typical microplastic sample from surface water is presented in Fig. 2a.

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Microplastic abundances observed in this study were similar to our previous

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observation in the Yangtze River mainstream and the tributary estuaries

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(1.92×105-136×105 items km-2) of the TGR.23 Relatively high abundance of

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microplastics has also been reported in Taihu Lake (0.1×105-68×105 items km-2),24

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North Shore Channel in Chicago (7.3×105-67×105 items km-2),38 and Rhine River

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(1.45×105-30.7×105 items km-2).39 Whereas, relatively lower abundance of

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microplastics was detected in the Laurentian Great Lakes (0-4.66×105 items km-2),

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estuarine rivers in the Cheaspeake Bay (0.055×105-2.6×105 items km-2),22 and Swiss

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lakes (0.11×105-2.2×105 items km-2).40 9


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Spatially, relatively higher microplastic abundances were observed at XKD

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downstream from Xiakou Town except in January 2016 (Fig. 3a), which could be

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related to the input of plastic wastes from Xiakou - the largest town along the

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riverside of the backwater area with a population of 24,582 in 2010 according to the

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data from Xingshan County Bureau of Statistics. Sewer systems are not installed in

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this town, and municipal wastewater is treated primarily using onsite septic systems

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or is discharged directly. Wastes are poorly managed, and many untended trash dump

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sites were observed in the town. Therefore, wastewater discharge and surface runoff

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may carry plastic wastes and other pollutants into the river. Additionally, input from

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Gaolan River could also increase microplastic abundance in the main channel. All

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these sources together might have led to the higher microplastic abundances at XKD.

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Seasonally, the highest and lowest average abundances of microplastics were

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observed in July 2015 (65.6×105 item km-2) and in January 2016 (4.5×105 item km-2),

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respectively. Microplastics collected in July 2016 at GL had the highest abundance

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among all samples, which was one or two orders of magnitude higher than those

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observed at other sampling sites. According to the records from a local experimental

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station, the study area experienced heavy precipitation in July 2015 (Fig. S2).

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Therefore, higher microplastic abundances observed in July may be related to an

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increase in surface runoff during heavy rainfall evens, which could carry plastic

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wastes within the watershed into the river. Relatively steep slopes along both sides of

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the river also exacerbate the transport of plastic wastes from the river bank to the

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water. 10



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The distinctive hydrodynamic characteristics of tributaries in the TGR can also

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influence the spatial and seasonal distribution of microplastics. Based on water flow

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velocity and direction, Xiangxi River can be divided into riverine, transitional,

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lacustrine, and mainstream influenced zones.41 The site XKD is within the lacustrine

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zone (Fig. S3), which is characterized by a very low surface flow velocity (

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