Provtagningsmetoder för mikroplast >300 μm i ytvatten: En jämförelsestudie mellan pump och trål

• Sampling and analyzing microplastics (MPs) comes with a unique set of challenges and currently a wide variety of methods are developed and applied. In order to facilitate future environmental monitoring we compared two methods that are often used for sampling MPs >300 μm; a manta trawl and a filtering pump. Six replicates per method were taken during calm weather conditions in the same location on the same day. The volume per replicate was 20 m3 for the pump and approximately 60 m3 for the trawl. Following analysis was done with visual sorting in a stereo microscope. The same person analyzed all samples and the anthropogenic particles were divided into eleven classification categories. In the pump samples zero to eight MPs were found per sample, rendering an arithmetic mean of 0.17 MPs/m3. In the trawl samples the numbers varied between 9 and 33 MPs, which corresponded to a significantly higher concentration per volume than the pump with an arithmetic mean of 0.32 MPs/m3. The results also indicated that in order to reach a statistical power of 60%, ten pump replicates would be needed to measure a difference between the examined area and an uncontaminated area. For the trawl a corresponding number of two replicates would be required. Alternatively a higher sample volume can be applied, which would render a higher certainty as the distribution data would approach a Gaussian distribution. A higher sampling volume would also lower the measurement uncertainty as it would decrease fluctuations in the counting statistics. Variations in measurement uncertainty between the methods was hypothesized to be primarily related to the differences in sample volume and could also be compensated through increasing the volume sampled, which would be somewhat faster than increasing the amount of replicates. The composition of MPs in the study varied between the replicates but mainly consisted of expanded cellular plastics, films, filaments and fragments. Each pump sample had on average 1.3 films and 0.33 expanded cellular plastics whereas each trawl sample had on average 2.5 films and 9.2 expanded cellular plastics. Per unit of volume the majority of the particles in the pump samples (40%) consisted of films, whereas the particles in the trawl predominantly consisted of expanded cellular plastics (46%). Expanded cellular plastics therefore seem to be sampled more efficiently by trawls, which could be because they float on top of the surface, the pump samples a bit lower in the surface water and the results in this study show that the sample compositions were more comparable for particles with more neutral buoyancy. Not enough particles were however obtained to allow for a more in-depth analysis of the compositional differences. The probability of false null-values increase with a lower true value of numbers of particles per sample and this starts to have a significantly negative effect below five particles per sample. Regardless of which method that is used it is therefore crucial to sample a sufficient number of particles (volume times concentration) suitable for comparing spatial, temporal or compositional differences. Background and purpose of the report Even if methods for sampling, extraction and identification of microplastics have developed rapidly during recent years several challenges remain. One of the challenges that remain is how to sample a group of contaminants that is as heterogeneous, both concerning shape and distribution, as microplastics. Additionally it is important to know to what extent results from different types of sampling devices can be compared. Here we compare two methods that are often used to sample microplastics above 300 μm in surface waters; a manta trawl and a pump. During one day (10th of October 2017) six replicates per sampling method was taken in the same spot in Gullmarsfjorden outside Lysekil. Through counting microplastics and other types of microlitter in the samples the aim was to compare differences between replicates and methods. This study was commissioned by the Swedish Environmental Protection Agency and the Swedish Agency for Marine and Water Management, as a step in the work to develop monitoring of microlitter.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Environmental Sciences (hsv//eng)

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