Data on microplastic contamination of the Baltic Sea bottom sediment samples in 2015–2016

The contamination by microplastics particles (MPs, 0.2–5 mm) in bottom sediments of the Baltic Sea is quantified. In total, 53 sediment samples were obtained in 8 cruises of research vessels in July–October 2015 and March–December 2016. The depths from 3 to 215 m in the Gotland, Gdansk, and Bornholm Basins are covered. Primary data is provided, along with exhaustive information on sampling dates and coordinates, depths, sampling methods, extracting procedures, control measures, detection techniques, and verification by μ-Raman spectroscopy. Number of pieces per kg dry weight is determined separately for fibres, films, and fragments. Distributions by size, plastic colour, and plastic type are presented. Modified NOAA method and μ-Raman spectroscopy were applied to obtain the data, thus they can be used for comparative analyses.


Sample preparation
Microplastics were extracted from the sediment samples using the method employed by Ref. [1] with modifications [2,3]. To maximize extraction rates, sediments with high clay content were washed through a sieve cascade (0.333 mm, 174 mm, 174 mm) before the extraction to remove clayey mud fractions, which hampers the extraction process [3]. The sediment retained by the sieves was subjected to flotation (Fig. 2).
In brief, the modified NOAA method consists of the following main steps [2,3]: (1) Multiple MPs extraction from a sediment sample by means of density separation with the ZnCl 2 solution (specific density 1.6 g mL À1 ), (2) Filtering of supernatant solution above the sediment with the filter funnel, (3) Wet peroxide oxidation on the water bath, (4) Calcite fraction digestion with HCl solution, (5) Filtering with filter funnel, (6) Density separation to detach oxidized organic matter, (7) Filtering with filter funnel, (8) MPs detection with a stereomicroscope, and additionally (9) MPs identification with a Raman spectrometer (Fig. 2).

Analytical techniques
The MP particles were optically analysed and photographed using a stereomicroscope (Micromed MC2 Zoom Digital) with magnification from Â10 to Â40 directly on the filter surface according to recommendations for microscopic determination [6].  Table 3 Polymer type and types of synthetic dyes identified using m-Raman spectroscopy.  All the analysis and detection procedures were performed by the single operator to exclude interoperator variability. Since plastics particles cannot be fully exactly identified only by visual observation [7e11], m-Raman spectroscopy was used to verify the result and attain the composition of plastic-like particles [12]. Raman Centaur U (LTD «NanoScanTechnology», Russia) spectrometer was used to obtain plastic spectra [13].

Contamination control and quality analysis
Metal laboratory equipment and glass tableware were used where possible to minimize external contamination. All instruments used during the extraction process were washed with distilled water and dried before the analysis. Cotton lab coats and clothing from non-synthetic materials were used to minimize airborne contamination during samples handling and extraction.
Twelve blank samples were run to assess the level of background contamination according to Ref. [3].
As an additional measure to control the extraction efficiency, artificial reference particles (ARPs) were added to each sample prior to the extraction procedure. Rectangular ARPs with the side dimension of 0.88 ± 0.41 mm (p ¼ 0.05; n ¼ 40) were prepared from a sheet of fluorescent PET 0.46 mm ± 0.02 mm thick (p ¼ 0.05; n ¼ 40). These ARPs, with their artificial shape and characteristic fluorescence, are easily distinguishable from MPs of natural sediments, and provide a clear indication of the quality of the extraction procedure [3].

Classification methods
A visual assessment was performed to identify the shape, size, and colour of MPs according to the physical characteristics of the particles. The extracted MPs were classified into three groups: fragments, films, and fibres according to Ref. [14].
Particle colour was divided into the following categories: transparent, white, green, blue, yellow, red, brown, and black, which is close to categories according to Refs. [8,15]. The blue category included deep blue, light blue, and violet particles. The yellow category also included orange particles. The transparent category included colourless and muddy particles. The red category also included pink and purple particles. The black category included transparent black and grey particles.
The extracted particles were divided into 24 categories using similarity of their visual appearance (shapes, colours), mechanical quality (rigid, soft, elastic, foamed, etc.), and behaviour during a hotneedle test.

m-Raman spectroscopy verification
The analysis procedure followed [13]. Out of the identified MPs, the core polymer type of some specimens was impossible to identify because of the strong signal induced by synthetic dyes (SD) or strong background fluorescence. Still, the fact of presence of SD was considered as confirmation of synthetic origin of a particle. So, all such specimens were accounted as MPs (for example, Fig. 3).
Polymer type and types of synthetic dyes identified using m-Raman spectroscopy are presented in Table  3. In other cases, the identification by m-Raman spectroscopy was not possible due to too small particle size or chemical compounds remaining on the surface of a particle. Raman spectra of top 8 typical MPs are presented in Fig. 4.