Abstract
Owing to the development and adoption of a variety of methods for sampling and identifying microplastics, there is now data showing the presence of microplastics in surface waters from all over the world. The difference between the methods, however, hampers comparisons, and to date, most studies are qualitative rather than quantitative. In order to allow for a quantitative comparison of microplastics abundance, it is crucial to understand the differences between sampling methods. Therefore, a manta trawl and an in situ filtering pump were compared during realistic, but controlled, field tests. Identical microplastic analyses of all replicates allowed the differences between the methods with respect to (1) precision, (2) concentrations, and (3) composition to be assessed. The results show that the pump gave higher accuracy with respect to volume than the trawl. The trawl, however, sampled higher concentrations, which appeared to be due to a more efficient sampling of particles on the sea surface microlayer, such as expanded polystyrene and air-filled microspheres. The trawl also sampled a higher volume, which decreased statistical counting uncertainties. A key finding in this study was that, regardless of sampling method, it is critical that a sufficiently high volume is sampled to provide enough particles for statistical evaluation. Due to the patchiness of this type of contaminant, our data indicate that a minimum of 26 particles per sample should be recorded to allow for concentration comparisons and to avoid false null values. The necessary amount of replicates to detect temporal or spatial differences is also discussed. For compositional differences and size distributions, even higher particle counts would be necessary. Quantitative measurements and comparisons would also require an unbiased approach towards both visual and spectroscopic identification. To facilitate the development of such methods, a visual protocol that can be further developed to fit different needs is introduced and discussed. Some of the challenges encountered while using FTIR microspectroscopic particle identification are also critically discussed in relation to specific compositions found.
Highlights
Responsible editor: Christian GagnonElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Early plastic pollution research built on lessons from planktologists
It was recognized early on that one method would not be adequate to assess microplastic pollution, Environ Sci Pollut Res (2020) 27:5559–5571 since microplastics come in many different shapes, sizes, and densities (Moore et al 2005)
Between 0 and 13 particles identified as microplastics were found, a concentration that corresponds to 0–0.4 microplastics/m3
Summary
A common cutoff size limit in many microplastic studies is 0.3 mm since that is a common mesh size for studying zooplankton, fish eggs, larvae, and other organisms in the nekton It was, recognized early on that one method would not be adequate to assess microplastic pollution, Environ Sci Pollut Res (2020) 27:5559–5571 since microplastics come in many different shapes, sizes, and densities (Moore et al 2005). This was further emphasized in a recent paper by Rochman et al (2019). Several scientists have noted the need to harmonize sampling methods (European Commission Joint Reseach Centre 2013; Lusher 2015; Van Cauwenberghe et al 2015; Setälä et al 2016; Rochman et al 2017)
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