Dataset on microplastic concentrations, characteristics, and chemical composition in the marine surface waters of Latvia – the Eastern Gotland basin and the Gulf of Riga

The dataset provides information on spectroscopically verified microplastics, both particles and fibers, from 44 marine surface water samples of two Baltic Sea sub-basins – the semi-enclosed Gulf of Riga and the Eastern Gotland Basin. Sampling was performed by using Manta trawl with a mesh size of 300 µm. Thereafter, the organic material was digested with sodium hydroxide, hydrogen peroxide and enzymes. Samples were filtered on glass fiber filters and analyzed visually, registering the shape, size, and color of each item. Where feasible, the polymer type was determined using Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy method. The number of plastic particles per m3 of filtered water was determined. The data presented in this article may be useful for further research on microplastic pollution, meta-analysis and calculation of microplastic flow. Interpretation and analysis of total acquired data on micro debris and microplastics are reported in the article “Occurrence and spatial distribution of microplastics in the surface waters of the Baltic Sea and the Gulf of Riga”.

Environmental Science, Pollution Specific subject area Occurrence and characterisation of microplastic contamination in marine surface water Type of data Table  Figure How data were acquired Visual analysis was performed using stereomicroscope Leica DM400 B LED fitted with camera DFC 295 and software Leica Application Suite V4.1. For chemical composition analysis Thermo Fisher Scientific Nicolet iS20 spectrometer and OMNIC software was used. Data format Raw Description of data collection Collection of 44 samples from May to September, 2018. Recording of start and end coordinates of the transect and meteorological parameters (wind speed and direction, wave height, air temperature). Characterization of microplastic particles by shape, size, colour, and detection of polymer. Quantification per one cubic meter of filtered water. Acquisition of microplastic particle images, spectra and reference spectra identified by ATR-FTIR spectroscopy. Quality assurance for sample loss and contamination prevention. Positive controls for determining recovery rates. Data

Value of the Data
• The comprehensive original dataset from Baltic Sea 44 coastal and offshore stations in marine surface waters under the jurisdiction of Latvia, what has not been studied before for microplastic pollution, substantially contributes to already existing knowledge base on composition and spatial distribution of spectroscopically verified microplastics encountered in the marine environment, • The data would be useful for researchers attempting to model spatial distribution of spectroscopically verified microplastics in marine environment and for calibration of existing models, • Obtained data can also be used as baseline information for further research to identify the differences of microplastic contamination between regions, sampling sites, as well as a reference value to facilitate establishment of environmental quality standards, • This data set can serve as a base for further research activities attempting to pinpoint sources and transport pathways of microplastics encountered in marine environment.

Objective
An extensive study investigating the composition and spatial distribution of micro debris and microplastic pollution in the coastal and open sea surface waters of the Gulf of Riga and the Eastern Gotland Basin, Baltic Sea [1] suggested the necessity of follow up studies by arguing that surface water microplastics debris pollution levels might be highly variable depending on different environmental factors, e.g., meteorological conditions and hydrodynamical characteristics distinctive for certain region. However, the available data on this is relatively scarce. With the data made accessible in this article, we aim to contribute to filling the existing knowledge gap and provide detailed information on spectroscopically verified microplastics characteristics and environmental conditions during sampling events for further studies and research activities. We also encourage to use this data for data-driven decision making and as a baseline for assessment of changes in the level of pollution.

Data Description
The dataset contains information on micro debris and microplastic particle presence and characteristics (shape, size, color and polymer) in 44 samples of marine surface waters under the jurisdiction of Latvia -Eastern Gotland basin and Gulf of Riga. The map showing study area and sampling sites is shown in the Fig. 1 . The Table 1 presents the geographical location of each sampling site and related background information, such as transect length, filtered water volume and meteorological parameters.
The Table 2 presents total number of micro debris and microplastic particles visually identified in each sample for extended comparability as well as visually identified microplastic particles verified by chemical composition analysis in each sample, and the background airborne contamination registered during samples visual analysis. The concentrations are expressed as the total number of particles and particles per one cubic meter of filtered water.
The characteristics (shape, size, color) of visually identified microplastic particles verified by chemical composition analysis are expressed as the total number of items found in the specific sample and provided below in the Tables 3-5 . In the cases of classification by size and color, fiber and non-fiber shaped particles are presented separately.
The Table 6 presents information on microplastic particle polymer composition as the total number of items found in the samples. Polymers of fiber and non-fiber shaped particles are given separately.  ( continued on next page )   Table 3 Classification by shape of the microplastic particles found in the samples. The negative controls in three-replicate proved background contamination by fibers during samples treatment (blank samples) to be 7, 12 and 23 fibers per blank sample. The positive controls of particles in the three-replicate spike-and-recovery test showed recovery rates of 90%, 95% and 95%. Table 4 Classification by size of the microplastic particles (non-fiber and fiber) found in the samples.

Fiber
Non-fiber Eastern Gotland Basin coastal area    Table 5 Classification by color of the microplastic particles (non-fiber and fiber) found in the samples.

Sampling
Altogether 44 micro debris samples (28 in Gulf of Riga and the 16 Eastern Gotland Basin) were collected. Sampling occurred over the time period from May to September 2018 during 7 cruises to cover coastal and open waters of the study area representatively. The samples were taken from the State Environmental Service (SES) fishing control and marine monitoring vessel "MARE" and Tallinn University of Technology research vessel "SALME".
Sampling was performed by deploying Manta net (HydroBios, mesh size 300 μm, frame opening area 0.28 m 2 , length 3 m) at 7 m distance from the side of the vessel and trawled for 1 hour at the speed of approximately 2 knots, following the recommendations of Gago et al., 2018 [2] , Hydro-bios Apparatebau GmbH, 2017 [3] and Viršek et al. 2016 [5] . The meteorological parameters were recorded immediately before sampling, using devices available on the sampling vessel -anemometer and wind vane for detection of wind speed and direction, thermometer for determining air temperature, and online weather data portal "Windy" [4] to find out current wave height. The start coordinates and time was recorded at the moment when the net was placed in the water, and end coordinates and time -when the net was raised from the water. Trawling duration and distance were altered (reduced) in cases when surface water algal blooms caused clogging of the net. The volume of water filtered through the net was calculated by multiplying After sampling, the net was rinsed from the outside with sea water to concentrate the sample at the cod end of the net and exclude the possibility of contamination. Then, the cod end was removed, and its content was rinsed into previously decontaminated stainless-steel sieve (mesh size 200 μm, Ø 100 mm, RETSCH production) using filtered distilled water. To facilitate sample treatment process better, largest non-synthetic origin particles that were easily visually recognizable, such as leaves, branches, feathers, marine organisms, insects etc. were discarded by first picking them up with tweezers and thereafter rinsing them over the steel sieve with filtered distilled water in order to retain any micro debris particles that could have been attached to them. Finally, the sample from the sieve was transferred to previously decontaminated glass jar by rinsing with filtered distilled water. The jar opening was then covered with aluminum foil and a metal lid, and frozen at the temperature of -4 ±2 °C until sample treatment.

Quality assurance
In order to minimize external sample contamination, several precautionary measures were taken while working with the samples. During sampling, the Manta net was towed from the side of the vessel to avoid airborne or waterborne vessel-origin contamination. Only glass or metal equipment was used for samples handling, storage and treatment process. All equipment that came in contact with samples was thoroughly rinsed before use with filtered distilled water. All reagents used for samples treatment were filtered through glass fiber (GF) filters (equivalent to pore size 1.2 μm, Ø 47 mm, Whatman®) before use. Cotton or linen laboratory coats with specific color (green or purple) were used to maximize capacity to determine airborne contamination from clothing during samples handling and treatment. Nitrile gloves were worn throughout the treatment process. The polymer spectrum of nitrile gloves and Manta net was taken, and corresponding polymer types were excluded from data set after samples polymer analysis. Sample treatment was done in fume hood and samples were covered with aluminum foil at all times when not worked with or when placed in the shaking water bath. To minimize possible particle loss, the same thoroughly rinsed beaker was used for each treatment step of the respective sample. Three procedural blank samples were run to assess the level of background contamination degree in the laboratory by applying the same processing steps as for the samples. For estimation of airborne contamination during visual analysis, for each sample separate Whatman glass fiber filter in Petri dish was exposed next to the microscope in the proximity of sample.
Spike-and-recovery approach in triplicate was used to assess the potential non-fiber particle loss during sample treatment with standardized polystyrene beads (Ø 100 μm density 1.05 g/cm −3 100 μm size) of red color (Sigma-Aldrich product no. 56969-10ML-F). Each of the positive control samples contained 100 beads in distilled water and was treated in the same manner as field samples.

Sample treatment
To remove biological material from the sample, several step procedure was conducted ( Fig. 2 ) based on the protocol described by Gago et al. 2018 [2] . Before the treatment, samples were thawed and transferred to a glass beaker using filtered distilled water. For the first treatment step alkaline digestion method was used, adding 10% sodium hydroxide (NaOH, Firma Chempu) to the sample in proportion 1:3 (1/4 of final volume sample and 3/4 NaOH). Then the samples were covered with foil and incubated in a shaking water bath for 24h at the temperature of + 50 °C and shaking frequency 100 rpm. After that, the NaOH solution was removed by filtering the samples through stainless steel sieve (mesh size 200 μm) and thoroughly rinsing the beaker with filtered distilled water. Then samples from the sieve were transferred to the previously used beaker by rinsing with filtered distilled water. For the second treatment step oxidative digestion method was used by adding 30% hydrogen peroxide (H 2 O 2 , Carl Roth) to the sample in proportion 1:2 (1/3 of the final volume sample, 2/3 H 2 O 2 ), covering the sample with foil. If the oxidation reaction was active, the sample was kept in fume hood for 24h at room temperature. If the oxidation reaction was not active or if 24h at room temperature had passed, the samples were incubated in a shaking water bath for 24h at the temperature of + 50 °C and shaking frequency 100 rpm. After that, the H 2 O 2 solution was removed by filtering the samples through stainless steel sieve (mesh size 200 μm) and thoroughly rinsing the beaker with filtered distilled water. Then samples from the sieve were transferred to the previously used beaker by rinsing with 100 mL of acetate buffer (pH 4.8). For the last treatment step, enzymatic digestion method was used by adding acetate buffer until 300 mL mark, and 0.5 mL cellulase (Cellulase enzyme blend, activity > 10 0 0 U/mL, Sigma Aldrich) and 0.5 mL viscozyme (Viscozyme L. cellulolytic enzyme mixture, activity > 100 FBGU/g, Sigma-Aldrich) to the sample. Then the samples were covered with foil and incubated in a shaking water bath for 48h at the temperature of + 50 °C and shaking frequency 100 rpm. After that, the acetate buffer solution was removed by filtering the samples through stainless steel sieve (mesh size 200 μm) and thoroughly rinsing the beaker with filtered distilled water. If after this step a considerable amount of microcrustaceans was present in the samples, the samples were transferred from the sieve to the previously used beaker by rinsing with 100 mL of acetate buffer (pH 4.8), later fixing the buffer volume until 300 mL mark and adding 0.1-0.7 mL chitinase (activity > 100 U/mL, ASA Spezialenzyme GmbH). Then the samples were covered with foil and incubated in a shaking water bath for 48h at the temperature of + 37 °C and shaking frequency 100 rpm. After enzymatic treatment, acetate buffer solution was removed by filtering the samples through stainless steel sieve (mesh size 200 μm) for the last time, thoroughly rinsing the beaker with filtered distilled water. Then samples were filtered on GF filters. In cases when the described sample treatment procedure was not sufficient to remove all organic material from the samples, a series of several GF filters were used.

Sample visual analysis
Particle visual identification was performed by using the automated upright microscope system Leica DM400 B LED equipped with digital microscope color camera DFC 295 and microscope software platform Leica Application Suite V4.1. At the same time particle morphology was registered -all potential microplastic particles on the GF filters were counted and characterized by their visual features such as shape, size and color, and photographed. Particles were categorized into five types according to their shape ( Fig. 3 ) similarly as done by Gago et al. 2018 [2] and Viršek et al. 2016 [5] . Category A is fragments that have irregular form, thickness, sharp or crooked edges as well as variety of different colors. Category B is fibers that are supple, can have different colors and lengths but width in relation to length is considerably smaller. Category C is beads that have regular round shape, diverse colors and are usually smaller than 1 mm in diameter. Category D is films that also have irregular form and different colors, but in comparison to fragments, they are thin, flexible and usually transparent. Category E is foams that are soft, with irregular form and usually white or yellow color.
In regard to the color of particles, they were classified as transparent/translucent, black, white, blue, red, pink, purple, green, grey, orange, yellow, brown or multi-colored.

Polymer composition analysis
During visual analysis particles in sizes suitable for manual handling were individually transferred with tweezers from GF filters into vial plates for further polymer type identification. Analysis was done applying Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy method using Thermo Fisher Scientific Nicolet iS20 FTIR spectrometer combined with Thermo Fisher Scientific OMNIC computer software. The particles were manually transferred from vial plates to ATR-FTIR where they were one by one fixed to a cleaned diamond crystal and, for the acquisition of polymer spectra, scanned with a resolution of 4 cm −1 and an IR range of 40 0 0-40 0 cm −1 . The acquired spectra were automatically compared to reference spectra in 30 spectral libraries that contained more than 15 0 0 0 spectra of both synthetic and natural materials and compounds. If the compatibility was higher than 80%, the match was considered to be credible, but if it was lower than 80% and higher than 60%, the compatibility was critically evaluated by manually comparing the distinctive spectral peaks of particles spectra to the best matches offered from spectral libraries. Compatibility below 60% was not considered and was recognized as invalid.

Ethics Statements
This study did not involve human or animal subjects, and no data from social media platforms were used.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data Availability
Dataset on microplastic concentrations, characteristics, and chemical composition in the marine surface waters of Latvia -the Eastern Gotland basin and the Gulf of Riga (Original data) (Mendeley Data).