Comparison between manta trawl and in situ pump filtration methods, and guidance for visual identification of microplastics in surface waters

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.

Microplastic Characterization by Infrared Spectroscopy

“…The classifications are also often selected based on the analyst's perception rather than objective particle descriptors. Therefore, a visualphysical-chemical classification scheme with guidelines "MyVISPEC" was recently presented (Karlsson et al 2020).…”

Solving the Nonalignment of Methods and Approaches Used in Microplastic Research to Consistently Characterize Risk

“… 17 At the other end of the scale, sampling volume determines the detection limit for large particles, where larger particles with a low frequency of occurrence are simply missed when the volume is too small. 15 , 18 Therefore, the aforementioned differences in methods have resulted in data incomparability. Third, conversion from number to mass concentrations (or vice versa) is usually done assuming microplastic particles to have a certain fixed shape (e.g., spherical) with the density of a discrete polymer.…”

Non-breaking Wave Effects on Buoyant Particle Distributions

“…In addition to hydrodynamic conditions, observations can be affected by the microplastic sampling method, e.g., net tows, grab samples, or filtering pumps (Hidalgo-Ruz et al, 2012;Prata et al, 2019). Some work has been done to address variability in measurements across methods (e.g., Setälä et al, 2016;Barrows et al, 2017;Karlsson et al, 2019); these studies have demonstrated that net tows can both under-sample and over-sample relative to other measurement techniques. While net tows are ideally conducted under calm conditions (e.g., sea states with Beaufort scales of 0-2) (Viršek et al, 2016), microplastics observations have been reported from conditions with Beaufort scales up to 5 (Eriksen et al, 2014;Kooi et al, 2016;Poulain et al, 2019).…”