Microplastics in Four Estuarine Rivers in the Chesapeake Bay, U.S.A.

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Microplastics in Four Estuarine Rivers in the Chesapeake Bay, USA Lance T. Yonkos, Elizabeth A Friedel, Ana C. Perez-Reyes, Sutapa Ghosal, and Courtney D Arthur Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5036317 • Publication Date (Web): 12 Nov 2014 Downloaded from http://pubs.acs.org on November 20, 2014

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Environmental Science & Technology

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MANUSCRIPT

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Microplastics in Four Estuarine Rivers in the Chesapeake Bay, USA

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Lance T. Yonkos*†‡, Elizabeth A. Friedel‡, Ana C. Perez-Reyes†, Sutapa Ghosal§, and Courtney

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D. Arthur∥

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†

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College Park, MD, USA

University of Maryland, Department of Environmental Science and Technology

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University of Maryland, Wye Research and Education Center, Queenstown, MD, USA

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§

California Environmental Health Laboratory, California Department of Public Health,

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Richmond, CA, USA

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∥

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MD, USA; and I.M. Systems Group, Rockville, MD, USA

Marine Debris Program, National Oceanic and Atmospheric Administration, Silver Spring,

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*To whom correspondence may be addressed ([email protected])

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ACS Paragon Plus Environment


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ABSTRACT―Once believed to degrade into simple compounds, increasing evidence suggests

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plastics entering the environment are mechanically, photochemically and/or biologically

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degraded to the extent that they become imperceptible to the naked eye yet are not significantly

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reduced in total mass. Thus, more and smaller plastics particles, termed microplastics, reside in

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the environment and are now a contaminant category of concern. The current study tested the

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hypotheses that microplastics concentration would be higher in proximity to urban sources, and

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vary temporally in response to weather phenomena such as storm events. Triplicate surface

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water samples were collected approximately monthly between July and December 2011 from

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four estuarine tributaries within the Chesapeake Bay, USA using a manta net to capture

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appropriately sized microplastics (operationally defined as 0.3‒5.0 mm). Selected sites have

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watersheds with broadly divergent land use characteristics (e.g., proportion urban/suburban,

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agricultural and/or forested) and wide ranging population densities. Microplastics were found in

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all but one of 60 samples with concentrations ranging over three orders of magnitude (

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560 g/km2). Concentrations demonstrated statistically significant positive correlations with

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population density and proportion of urban/suburban development within watersheds. Greatest

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microplastics concentrations also occurred at three of four sites shortly after major rain events.

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GRAPHICAL ABSTRACT:

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INTRODUCTION

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Marine microplastics, a type of marine debris generally accepted as all plastic particles

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measuring ≤ 5.0 mm in size, have become a growing concern in North American waters since

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they were first reported in southern New England coastal waters in the 1970s.1 Across numerous

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studies, plastic particles were found to vary in size, shape, chemical composition, and spatial and

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temporal distribution.2 Plastics have been detected in surface waters around the globe and are

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likely the most abundant form of marine debris.3 Low density plastics tend to remain buoyant

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and float on the water’s surface, allowing them to travel long distances.4 It is generally accepted

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that most microplastics in aquatic systems derive from secondary sources (e.g., progressively

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reductive fragmentation of larger material) rather than from primary sources (e.g., manufactured

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particles and pelletized pre-production materials).5,6 Patterns in variety and concentration

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suggest proximity to human population centers, river mouths7,8, prevailing ocean currents, and

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weather events all influence introduction and dispersion of plastics into marine systems, and

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determine regions of eventual accumulation.3,9-12

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Once in aquatic systems, plastics provide substratum for sorption of various contaminants

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including persistent bioaccumulative organics (e.g., PCBs, PBDEs, etc) and metals (e.g., Cu,

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Pb).1,6,13-16 While at sea, plastic fragments can accumulate various attachment organisms (e.g.,

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barnacles, polychaete worms, vegetative cysts) potentially functioning to transport invasive17 or

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pathogenic species.18 As plastics degrade, their small sizes, varied shapes and conspicuous

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colors can lead to ingestion by a wide range of marine organisms, including planktonic fish and

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invertebrates, seabirds and cetaceans.6,19,20 Such ingestion is non-nutritive and raises obstruction

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concerns.21-23 Moreover, the potential for toxicity arises from leaching of plastic constituents or


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sorbed contaminants after ingestion and subsequent biomagnification within aquatic food

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webs.14,24-27

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While microplastic marine debris has been extensively documented in Atlantic and Pacific

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Ocean gyres,27-29 similar studies in estuarine and near-shore waters are comparatively

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scarce.8,10,31,32 The purpose of this study was to collect and quantify microplastics in surface

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waters of the Chesapeake Bay to compare with historical concentrations and to establish modern

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baselines for future monitoring. Collection was directed toward presumed sources of

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microplastics (i.e., estuarine regions) rather than eventual regions of accumulation (e.g., mid-

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ocean gyres). The study was also designed to investigate spatial and temporal trends within and

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between sample locations. Collections occurred across multiple seasons with sample sites

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chosen according to land use characteristics and urban intensity within watersheds. It was

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hypothesized that microplastics concentrations would: (1) be greater near urban sources; and (2)

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vary temporally in response to climate/weather phenomena. As the largest estuary in the US, the

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Chesapeake Bay and associated watershed currently accommodate a human population of over

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17 million, provide habitat for numerous and diverse fish, waterfowl, and migratory shorebird

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populations, and sustain culturally and economically important commercial and recreational fin

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and shellfish fisheries. Understanding sources, transport, fate, and potential impacts of

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microplastics on the Chesapeake Bay and its resources is an important step to further the state of

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the science about microplastics as an emerging pollution issue.

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MATERIALS AND METHODS

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Sample Sites



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Surface waters from four estuarine tributaries within the Chesapeake Bay, USA were sampled

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for enumeration of microplastics (Figure 1). The four sample sites (Patapsco, Magothy, Rhode,

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and Corsica Rivers) were located within navigable tidal regions of estuarine rivers that varied

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substantially in watershed size and land use characteristics (Table 1). Watershed margins were

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delineated using digital US Geological Survey quarter quad topographic maps with catchment

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areas calculated in ArcMap 10.0.3 (ESRI, Redlands, California, USA). Land cover (percent

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agricultural, urban/suburban, and forest) was calculated for catchments using the 2006 National

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Land Cover Database.33 Population within watersheds was extrapolated from 2010 US census

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data.34 The Patapsco River was by far the largest and most urban of estuarine sites, with a

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population density of 550 persons per sq. km within a 1,637 sq. km watershed (total population

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899,000; Table 1). While the Upper Patapsco watershed is largely rural and forested, the lower

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Patapsco and Patapsco River Basin contain the entirety of Baltimore, MD, a densely urban city

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and historically industrial harbor. The three other estuarine systems had significantly smaller

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watersheds (range 67 – 97 sq. km) with varying population densities and proportions of

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suburban, agricultural and forested land use: the Magothy, predominantly residential (59%); the

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Corsica, predominantly agricultural (60%); and the Rhode, predominantly forested (78%).

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Sample Collection, Preparation, Processing, and Quantification

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Samples were collected by surface trawl (1.0–2.0 km) using a 70 cm wide manta net with a mesh

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size of 0.33 mm, designed to capture floating material to a depth of 15 cm (Figure 2A).35 Sites

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were surveyed by collecting contents from triplicate surface trawls on five discrete occasions

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between July and December 2011. Therefore, each estuarine site had a total of 15 samples

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examined for microplastics. Materials captured by trawl were passed through nested 5.0 mm and


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0.3 mm stainless steel sieves. Debris retained on the 0.3 mm screen was rinsed using site water

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into pre-labeled jars for future processing and analysis. Trawl distance, water volume sampled,

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weather (e.g., air temperature, wind speed and direction, recent precipitation), and water quality

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parameters (e.g., surface water temperature, salinity) were recorded during each sampling event.

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Materials collected from all trawls were manipulated to isolate and identify microplastics using a

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combination of chemical, thermal, physical, and mechanical processes (Figure 2B-C) modified

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from the methods of Baker et al.36 Briefly, samples were rinsed onto 0.3 mm stainless steel

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sieves using deionized (DI) water and then transferred to pre-weighed 600 mL glass beakers

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where they were oven-dried at 75°C for approximately 24 h. After recording dry weights,

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contents were treated with 20 mL aliquots each of a 0.05 M Fe(II) solution and 30% hydrogen

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peroxide (H2O2) to facilitate chemical digestion of labile organic material (Figure 2B). Mixtures

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were placed on heated stir-plates maintained at 75°C and gently stirred. At 30 min intervals,

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samples were re-examined and additional H2O2 added and heating repeated, as necessary, until

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all organic material was digested.

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After rinsing with DI water to remove residual H2O2, sieve contents were rinsed using a hyper-

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saline solution (300 g/L table salt in DI water) into glass funnels with attached and clamped

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flexible tubing (Figure 2C). Funnels were filled with the hyper-saline solution, covered, and left

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undisturbed for approximately one hour to allow density-separation of remaining solids. After

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completion of material separation, settled debris was dried in 50 mL beakers, transferred to 110

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mm glass petri dishes, and visually examined using a dissecting scope at 7.5× magnification

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(Stereomaster SKZ-5) to determine presence/absence of microplastics. Remaining floating



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materials were collected separately onto 0.3 mm stainless steel sieves, rinsed with DI water,

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covered loosely with aluminum foil and dried at ambient temperature for 24 to 48 h. Once dry,

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sieve contents were examined for microplastics by dissecting scope. Putative plastics were

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carefully separated from residual woody/organic debris, removed using microforceps, placed into

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pre-labeled aluminum weigh-boats (pre-weighed to the nearest 0.01 mg), and dried at 75°C for

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12 to 24 h. Total foil weights were determined on a microbalance and resulting microplastics

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weights calculated to the nearest 0.01 mg. The number of microplastic particles collected was

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also determined for each trawl by counting total particles collected. Final environmental

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concentrations of microplastics were calculated both as mass per unit surface area (g/km2) and

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particles per unit surface area (pieces/km2).

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Detected microplastics were sorted by category into groups that included: synthetic fibers, thin

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flexible sheets, hard multi-colored fragments of various sizes, pre-production pellets, and

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extruded polystyrene (e.g., Styrofoam®). Validation of visually-based micrplastics identification

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was achieved using Raman micro-spectroscopy (RMS) analysis, a vibrational spectroscopy

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technique providing compositional information with micrometer-scale spatial resolution,

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following the methods of Ghosal and Wagner.37 Ten randomly selected small fragments (≤ 2

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mm; the particle category deemed most likely to be misidentified) were analyzed by RMS using

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a Renishaw inVia Raman microscope (Renishaw Plc., Old Town, Wotton-under-Edge,

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Gloucestershire, U.K.) equipped with a near-infrared 785 nm diode laser.

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Statistical Analysis


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Spatial and temporal collection of microplastics from Chesapeake Bay sites provided data

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amendable to comparison by two-way analysis of variance (ANOVA) using site and sample date

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as variables. Concentrations calculated both by microplastic weight (g/km2) and number of

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particles (pieces/km2) were analyzed by two-way ANOVA followed by an all-pairwise multiple

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comparison procedure (Holm-Sidak method) to investigate statistical differences between sites

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and/or sample dates. Pearson Product Moment Correlation analysis was employed to investigate

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the association between microplastics concentration and various watershed characteristics (e.g.,

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population density, land use prevalence). All analyses were performed using SigmaStat version

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3.5 (Systat Software Inc.) with statistical significance reported at p = 0.05.

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RESULTS

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Microplastics were found in all but one of 60 samples (Corsica, December 1, 2011) (Figure 3).

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Where detected, concentrations of microplastics were generally low and variable between

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replicates and across sample locations and time periods (Table 2). Measured concentrations

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ranged over three orders of magnitude (< 1.0 g/km2 to 563 g/km2). Plots of microplastics

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concentration (g/km2) (Figure 4) show generally greater abundances in Patapsco River samples

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and comparatively lower abundances in Rhode and Corsica River samples. With the exception

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of the Corsica River, all sites had peak mean microplastics concentrations during September

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sampling (Figure 4). Two-way ANOVA did not indicate a statistically significant interaction

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between sample location and sample time on microplastics concentration (p = 0.175) (two-way

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ANOVA results provided in Supplementary information: Table S1). The power of the test,

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however, was only 0.219 (at α = 0.05) suggesting that the negative result be interpreted with

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caution. Individually, spatial and temporal variables were both confirmed to have highly



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statistically significant effects on resulting microplastics concentrations (sample location p =

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Microplastics in Four Estuarine Rivers in the Chesapeake Bay, U.S.A.

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