| Processing Steps |
- Parameter or Variable: microplastic concentration (measured); Units: items kg-1 d.w.; Observation Category: in situ; Sampling Instrument: Van Veen grab sampler; Sampling and Analyzing Method: This study was carried out to estimate the amount of microplastics on shorelines and seabed sediments in Narragansett Bay, Rhode Island, USA. Shoreline transects were carried out at seven sites between September and October of 2020, with one site (Bold Point, RI) sampled in March 2021. Sites were chosen to provide a spatial distribution ranging from the Lower Providence River to the mouth of Narragansett Bay. At each site, three transects perpendicular to the shoreline were completed within 2 h of low tide. Along each transect, four sample locations were chosen: (1) the upper beach, near the dune toe; (2) mid-beach; (3) intertidal zone, between the high tide line and edge of the swash zone; and (4) a subaqueous sample taken approximately 50 m from the shoreline. The wrack line was not sampled at any site. Samples were limited to the upper 5 cm of sediment. All sediment was collected in pre-rinsed glass jars using a metal spoon. Cotton clothing was worn during collection. To evaluate seabed plastics, sediment grab samples were collected at five locations throughout Narragansett Bay. Samples were collected using a Van Veen grab sampler deployed from the R/V Cap’n Bert on two occasions, in Fall 2020 and Summer 2021. Sediment was collected in pre-rinsed glass jars using a metal spoon. Shoreline and seabed sediment samples were processed identically. Shoreline and seabed sediment samples were processed identically. One liter of sediment was wet sieved at 1 mm using metal sieves. Debris > 1 mm were dried in an oven at 60 °C for 24 h. Debris was then transferred to a large glass pan and any suspected macroplastics (> 5 mm) or large microplastics (1–5 mm) were removed using metal forceps. Plastics were transferred to a glass petri dish for further analysis. A description of each particle along with the mass, color, and plastic type (e.g., film, fiber, fragment, foam, pellet) was recorded. The plastic research literature analyzes plastics at a wide range of sizes, and there is a methodological limit to the size of particles that can be confidently measured (approximately < 40 microns). Given the sedimentological focus of this study, this study decided to set a lower limit of 62.5 microns, the sand-mud size cutoff for sediments, for plastics to analyzed as this is a standard sieve size used for geological analyses and can confidently be measured. This lower size limit may limit broader comparisons to other studies, which apply a range of size cutoffs including at 38 μm50, 45 μm52, or 100 μm4. Approximately 100 g of wet sediment was weighed and sieved at 62.5 μm to remove fine sediment. The remaining sediment was transferred to a 500 mL glass beaker for microplastic extraction. The sieve was then inspected under the dissecting microscope to ensure no fibers or other particles had been left behind. A dense solution of sodium iodide (NaI, 1.8 g/cm3) was used for extractions. The NaI solution was added until the volume of NaI was twice the volume of sediment. The beaker was covered with aluminum foil and stirred for 5 min using a glass stir bar. The solution was left to settle for a minimum of 6 h and a maximum of 24 h. Once the NaI solution was clear, the supernatant containing any floating particles was carefully decanted through a 62.5 μm sieve, avoiding resuspending the sediment. The sieved NaI was poured back into the sediment sample, stirred for 5 min, and left to settle for a second extraction. The particles caught in the sieve were rinsed into a small glass beaker using pre-filtered (0.2 μm) DI water. Organic digestions were carried out on Bold Point Park samples, which were from a marshy shoreline and thus contained a high presence of organic material in the post-extraction material. An aliquot of 2 mL of 30% H2O2 was added to the beaker containing the extracted material and allowed to digest overnight at 60 °C. All extracted material was filtered through a pre-weighed 47 mm diameter, 1.6 μm pore size GF/F filter using vacuum filtration with a glass filter funnel. The filter was then placed in a small glass jar for future analysis. Once dry, filters containing extracted material were weighed. Filters were examined using light microscopy under a dissecting microscope, with all suspected plastic particles counted and categorized into fragments, fibers, and films. Fluorescence was also used to help locate plastic particles on the filter. NIGHTSEA royal blue (440–460 nm excitation) illuminated the filter, which was viewed using the dissecting microscope fit with a 500 nm long pass emission filter (NIGHTSEA SFA Stereo Microscope Fluorescence Adapter). When possible, plastic particles were removed from the filter using fine forceps and adhered to a glass slide using double-sided tape. Plastic particles were imaged at 40X zoom using an AmScope dissecting microscope fit with an Amscope MU1803 digital camera. The size of particles > 500 μm was measured using ImageJ. To verify efficacy of the extraction methods, fragments of polystyrene and polyethylene terephthalate, low density polyethylene film, as well as polypropylene fibers, were created using a coffee grinder followed by sieving to attain a representative particle size distribution and imaged and sized using ImageJ. Sediment samples were spiked with a known number of particles and extracted as described above. Extraction efficiency following the second extraction ranged from 77.1 to 100%, with a mean efficiency of 92%. Fourier-transform infrared spectroscopy (FTIR) was used to determine the polymer makeup of all particles able to be picked by forceps, typically > 250 μm in size (Shimadzu IRTracer-100). Sample spectra were collected over the range of 500–4000 cm−1, with a data interval of 1 cm−1 and resolution of 4 cm−1. The ATR diamond crystal was cleaned with 70% 2-propanol and a background scan was performed between each sample. All collected spectra were compared to the Shimadzu spectra libraries for identification. A minimum match of 80% was required for spectra to be accepted as plastic particles.; Data Quality Method: Several measures were applied to minimize the potential microplastic contamination during the whole microplastic analysis procedures. 100% cotton clothing was worn during sampling, and a cotton lab coat was worn during all laboratory procedures. All extractions were performed under a laminar flow hood, which was cleaned before each extraction began. Plastic materials were avoided at all possible steps of sampling and extraction, and all glassware was pre-rinsed. Filters were placed in the laminar flow hood to serve as air blanks during each extraction. Filters were also collected every 12 samples to test the DI water and NaI solution for contamination. All particles found on blanks were recorded. If a particle of the same color and shape was found in the blank and sample, corrective action was taken and that particle was subtracted from the total count for that sample. The maximum number of particles on any single blank was 4, and 49% of counted filters had at least one particle removed through blank subtraction..
- Parameter or Variable: microplastic concentration (measured); Units: items kg-1 d.w.; Observation Category: in situ; Sampling Instrument: Metal spoon.
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