Microplastic variability: consecutive surface water microplastic replicate samples collection

Description

Microplastic (MP) abundance in surface water is influenced by both environmental factors and MP properties such as density, size, hydrophobicity, and interactions with aquatic biota. As a result, even samples collected simultaneously using the same method can show considerable variation. To obtain a surface water MP sample that accurately reflects pollution levels with minimal variability, increasing the sample size is recommended. This can be achieved either by collecting larger volumes of water or by taking multiple replicate samples. However, collecting larger volumes is often limited by net clogging when suspended material is present in the water. A practical solution to this challenge is the use of replicate samples. In this study, we collected 15 consecutive replicate samples from four different water bodies to determine the optimal number of replicates needed for reliable assessment of surface water microplastic pollution. The dataset contains information on each sample volume, as well as number and characteristics of identified microplastic particles.

Steps to reproduce

Sampling was conducted in four waterbodies – Lakes Medalfellsvatn, Velnezers and Stameriena, and Fossvogur fjord. Two sets of Manta nets (length 2 m, 300 μm mesh size, 0.15 × 0.30 m frame opening, HydroBios) were deployed from both sides of an inflatable rubber boat equipped outside the wake zone. The submerged frame opening area was 0.021 m2. The nets were towed for 20 minutes at an approximate speed of 1.2 knots to collect 15 replicate samples from each waterbody. The start and end coordinates of all trawls were at roughly the same locations. The filtered surface water volume (V, m3) was determined using a HYDRO-BIOS mechanical flow metre installed in the net frame opening. Before the start and at the end of each sampling, the flow metre rotation measurment was recorded. The filtered water volume was then calculated by applying the following formula: V = r × a × 0.3 where r is the number of flow metre revolutions calculated from registered start and end measurement values, a is the submerged net opening area and 0.3 is the pitch of the flow metre blade impeller (m). After trawling, the sample was concentrated at the cod end of the net, transferred to a metal bowl, and larger non-plastic objects were rinsed over the sample and discarded. The sample was further concentrated through a 50 µm sieve and transferred to labeled glass jars. The samples were kept at room temperature for three months to decompose organic material. For removal of organic material, the samples were treated with 10% sodium hydroxide (sample:solution ratio 1:3) and 30% hydrogen peroxide (sample:solution ratio 1:1) in a shaking water bath at 100 rpm and at a temperature of 50°C for 48 hours. In necessary, Fenton’s wet oxidation was performed. After each treatment step, the previous solution was removed by filtering the sample through a stainless steel sieve (200 µm mesh size, ⌀10 cm, Retsch). Afterward, the samples were filtered through glass fibre filters for visual analysis. The samples were analysed visually using a stereomicroscope, registering colour, size and shape of each particle. Particles were counted as fibres if the relation between the minor and major dimensions was ≤0.11. Particles suitable for manual transfer were subjected to chemical composition analysis using attenuated total reflection-Fourier transform infrared spectroscopy (Thermo Fisher Scientific Nicolet iS20 spectrometer, OMNIC 9 software; 32 scans, spectral resolution 4 cm-1 and energy range 4000-400 cm-1). If the particle spectrum matched a database entry with a percentage higher than 70%, the data were considered reliable. The “hot needle” test was applied for particles not suitable for spectroscopic analysis to differentiate between plastic and non-plastic particles.

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