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Microplastic abundance and composition in western Lake Superior as determined via microscopy, Pyr-GC/MS, and FTIR. Erik Hendrickson, Elizabeth C. Minor, and Kathryn Schreiner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05829 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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Environmental Science & Technology
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Microplastic abundance and composition in western Lake Superior as determined via microscopy, Pyr-GC/MS, and FTIR.
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Erik Hendrickson1, Elizabeth C. Minor2*, and Kathryn Schreiner3 1
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Water Resources Science Program, University of Minnesota, 2205 East 5th St, Duluth, MN 55812. E-mail;
[email protected] 2
Large Lakes Observatory and Department of Chemistry and Biochemistry, University of Minnesota Duluth, 2205 East 5th St. Duluth, MN 55812. Email:
[email protected] 3
Large Lakes Observatory and Department of Chemistry and Biochemistry, University of Minnesota Duluth, 2205 East 5th St. Duluth, MN 55812. Email:
[email protected] *corresponding author For submission to Environmental Science and Technology
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Abstract: While plastic pollution in marine and freshwater systems is an active area of
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research, there is not yet an in-depth understanding of the distributions, chemical
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compositions, and fates of plastics in aquatic environments. In this study, the magnitude,
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distribution, and common polymers of microplastic pollution in surface waters in
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western Lake Superior are determined. Analytical methodology, including estimates of
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ambient contamination during sample collection and processing, are described and
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employed. Microscopy, Pyrolysis-Gas Chromatography/Mass spectrometry (Pyr-
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GC/MS), and Fourier Transform Infrared spectroscopy (FTIR) were used to quantify and
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identify microplastic particles. In surface waters, fibers were the most frequently
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observed morphology, and, based upon PyGC/MS analysis, PVC was the most frequently
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observed polymer, followed by polypropylene and polyethylene. The most common
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polymer identified by FTIR was polyethylene. Despite the low human population in Lake
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Superior’s watershed, microplastic particles (particularly fibers, fragments, and films)
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were identified in western-lake surface waters at levels comparable to average values
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reported in studies within Lake Michigan, the North Atlantic Ocean, and the South
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Pacific Ocean. This study provides insight to the magnitude of microplastic pollution in
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western Lake Superior, and describes in detail methodology to improve future
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microplastics studies in aquatic systems.
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Introduction: Since the advent of mass-produced plastic in the middle of the 20th century, plastic pollution has become a significant environmental concern and a focus of recent 2
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study and remediation efforts in aquatic systems. Input vectors of plastic pollution to
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aquatic systems include wastewater treatment plants, shoreline debris, river outflows,
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landfills, industrial pollution, illegal dumping into aquatic systems, and atmospheric
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deposition.1–4
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Most plastic particles in oceanic gyres are less than 10 mm in length,5,6 which
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encompasses part of the mesoplastic size range as well as microplastic size range.4
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Historically, “microplastic” particles have been inconsistently defined in terms of size.7
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The definition established for the this study, defined by the National Oceanographic and
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Atmospheric Administration (NOAA), is a particle greater than 333 µm (as set by the
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plankton net used for collection) with no dimension longer than 5.0 mm.8,9 Microplastic
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particles have several recognized morphologies, including beads, foams, films,
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fragments, fibers, and other shapes resulting from the degradation of larger plastics.1,4,10,11
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Negative or potentially negative effects of microplastic pollution can include
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physical damage to organisms upon ingestion, false satiation, toxicological effects from
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adsorbed substances, speculated toxicity from polymers and additives, and providing a
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vector for invasive species.11–13 Much of the focus on potential toxicological damage
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from microplastic pollution regards hydrophobic substances adsorbing to microplastic
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particles, including PCBs (polychlorinated biphenyls), DDT
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(dichlorodiphenyltrichloroethane) related compounds, and PAHs (polyaromatic
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hydrocarbons),12 although the potential amount of uptake of these adsorbed substances
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from hydrophobic microplastic surfaces into organism biomass is not well known. Recent
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research has revealed the presence of inorganic contaminants in microplastic paint
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pigments as well, including copper (Cu), lead (Pb), and cadmium (Cd),14 and the uptake
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of these pigment-related inorganic contaminants into aquatic organisms is unknown.
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Much of the research regarding microplastic pollution has focused on marine
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systems with less research in freshwater systems.11,12 Recently, however, there has been
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an increasing interest in studying microplastics in freshwater systems, including
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investigations of lakes in Italy, Switzerland, Mongolia, East Africa, Canada, and the
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United States.15–24 Previous studies in the Laurentian Great Lakes have shown that
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microplastic pollution has a propensity to be elevated in surface waters in nearshore
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areas, especially near areas of high human population density and industrial activity,
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where extensive clean-up projects have not been performed.1,18,25 These studies have
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found that average open-water plastic densities resembled those in severely polluted
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ocean gyres1,18 and plastic was often found to be greater than 80% of anthropogenic
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shoreline debris items.1 As with surface water studies, investigations of microplastics in
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benthic settings have typically been in marine environments,23 although a study on Lake
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Ontario sediments, as well as sediments from affiliated tributaries and beaches, was
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conducted in 2016, revealing elevated concentrations in sediments near or in Humber
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Bay and Toronto Harbor.24
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Despite Lake Superior’s status as the largest freshwater lake on earth by area,
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there is a lack of data, quantitatively and qualitatively, regarding microplastic pollution in
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this Laurentian Great Lake. Existing published data is from single measurements at five
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stations in the relatively unpopulated eastern basin of Superior and two tributaries
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flowing into the lake.18,26
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Quantification of microplastic particles often relies on visual sorting from a
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processed sample and recording the number of particles observed.4 For reporting
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microplastic quantities in the environment, typical units are simply a count of plastic
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items per unit area or volume, i.e. “items per km2” or “items per m3”, although, some
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previous studies have used mass per mass units as well.10,27–29 Pyrolysis-Gas
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Chromatography/Mass Spectrometry (Pyr-GC/MS), Fourier Transform Infrared
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Spectroscopy (FTIR), Focal-Plane Array (FPA) FTIR, Raman spectroscopy, and
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Scanning Electron Microscopy (SEM) are qualitative techniques used for identifying and
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characterizing microplastic polymers.4,10,30,31 These techniques are typically used after
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isolating microplastic particles via a density separation procedure. Density separations
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are conducted by subjecting environmental samples to concentrated or saturated salt
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solutions, followed by filtration or other separation techniques, and then optical
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identification/selection of microplastic particles.4,10,12,30,32
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The goals for the present study are to quantify the distribution of microplastic
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pollution in surface waters of western Lake Superior, to identify polymers present in
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microplastic debris, and to improve methods for the quantification and identification of
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microplastics in aquatic environments. Little quantitative data exists for the extent of
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microplastic pollution in Lake Superior, and no published data exists for far western Lake
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Superior. Western Lake Superior is a high stress region of the lake33,34 impacted by the
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most populated (110,000 people) and most densely populated urban area (1100 people
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per square mile) of Lake Superior: Duluth, Minnesota and Superior, Wisconsin (as of the
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2016 censuses for the United States and Canada).35,36
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In addition, the polymer composition of Lake Superior microplastics has not yet
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been investigated. In this study, we apply Pyr-GC/MS and ATR-FTIR for qualitative
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compositional characterization. These two techniques provide different but
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complementary interrogations of sample composition. Pyr-GC/MS uses pyrolysis to
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break polymers into smaller moieties, of which the volatile pieces are then separated and
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identified using GC-MS. In ATR-FTIR the molecular vibrations of transiently polar
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covalent bonds are measured, through the change they impart to an evanescent wave
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penetrating the sample.
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Furthermore, previously established methods have been criticized for having a
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lack of “clean techniques”.37 While a number of investigations of aquatic microplastic
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pollution have taken place, various publications provide detailed critiques of the
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commonly used and published methods. These methods have been plagued by variable
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sample contamination during sampling and processing, difficulty distinguishing among
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natural materials and microplastics, bias associated with not selecting small and
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uncolored particles, and inefficient isolation of denser plastics unless heavier and
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expensive salts like NaI, ZnCl2, and polytungstates are used in density
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separations.4,25,31,32, 37, 38 Here, we attempt to address some of these analytical issues.
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“Clean” analytical techniques are employed, including washing sampling gear
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methodically between samples, collecting ambient (field) controls, collecting sample
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processing method blanks, and using lab control samples (LCS) of analytical plastic
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standards to assess the efficiency of our techniques in isolating microplastics.37,39
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Methods: 6
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Methodology Motivation and Influence:
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Methods used here for sampling, sample processing, and analysis were strongly
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influenced by previous works, in particular NOAA’s publication regarding standardized
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microplastic sampling and sample processing by Masura and colleagues.8 Microplastic
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polymers identified included polyethylene (PE), polypropylene (PP), polyvinylchloride
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(PVC), polystyrene (PS), and polyethylene terephthalate (PET). These polymers were
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chosen because they are the most commonly produced and encountered in the
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environment.7,25 These polymers correspond with designator codes 1-6 created by the
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Society of The Plastics Industry (SPI) and currently used by the American Society for
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Testing and Materials (ASTM).4 While these polymers were the primary references for
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analysis (see Table S-1 for further details), other polymers isolated, analyzed, and
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identified were reported as well.
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Microplastic particles were defined as belonging to one of 6 morphological
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categories (shown in Figure 1, see also SI, Table S-2), similar to those defined by
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Crawford and Quinn.4 Note that while the effective upper-limit based upon sieving is that
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at least 2 dimensions were less than 4.0 mm, any non-fibrous particles with a dimension
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of 4.0 to 5.0 mm that passed through the 4.0 mm sieve, and were captured on the 250 µm
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sieve, were isolated during microscopy and included in subsequent analyses. Non-fibrous
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particles determined by microscopy to exceed 5.0 mm in any dimension were not counted
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as microplastic particles. Despite the use of a 250 µm sieve, the effective minimum sizes
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of microplastic particles collected was 333 µm as established by the mesh size of the
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manta net (see below).
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Water Sampling:
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Twelve sample sites in western Lake Superior were chosen to include key
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environments speculated to differ in their microplastic distributions based upon proximity
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to believed sources (local wastewater treatment plant, urban shorelines, and river
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outflows). These sites were categorized into generalized, regional environments:
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nearshore, estuary/harbor, and offshore/open water (Figure S-1 and Table S-3).
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An NQS-45-60 manta tow net (333 µm mesh size, 3 m length, frame opening 14
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cm deep by 85 cm wide, Ocean Instruments, San Diego, CA, USA) was used for surface
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water sampling. At each sample site, a target tow length of 500 to 2000 m was
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established with length based upon the amount of floating debris. The distance sampled
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was measured using a Model 2030R flowmeter (General Oceanics Inc., Miami, FL,
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USA). To avoid sample contamination, the manta net and collection vessel were rinsed
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thoroughly before each tow. The net was rinsed in surface water without the collection
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vessel in place, and then was thoroughly rinsed from the outside using an on-vessel lake
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water hose to prevent sample contamination. Finally, the collection vessel was tripled-
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rinsed using Millipore water before attaching to the net and beginning a tow.
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After a tow was completed, the net was rinsed thoroughly from the outside using
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the on-vessel hose to concentrate collected materials into the collection vessel. Contents
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captured in the collection vessel were quantitatively transferred to 4.0 mm and 250 µm
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metal sieves in series. Materials were hand-shaken on the sieves and washed with
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Millipore water until it was visually apparent that only collected materials larger than 4.0
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mm were present. Contents isolated on each sieve were transferred via forceps and/or
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water rinses (either Millipore water or
