What You Net Depends on if You Grab: A Meta-analysis of Sampling Method’s Impact on Measured Aquatic Microplastic Concentration

Microplastic pollution is measured
with a variety of sampling methods.
Field experiments indicate that commonly used sampling methods, including
net, pump, and grab samples, do not always result in equivalent measured
concentration. We investigate the comparability of these methods through
a meta-analysis of 121 surface water microplastic studies. We find
systematic relationships between measured concentration and sampled
volume, method of collection, mesh size used for filtration, and waterbody
sampled. Most significantly, a strong log–linear relationship
exists between sample volume and measured concentration, with small-volume
grab samples measuring up to 104 particles/L higher concentrations
than larger volume net samples, even when sampled concurrently. Potential
biasing factors explored included filtration size (±102 particles/L), net volume overestimation (±101 particles/L),
fiber loss through net mesh (unknown magnitude), intersample variability
(±101 particles/L), and contamination, the potential
factor with an effect large enough (±103 particles/L)
to explain the observed differences. On the basis of these results,
we caution against comparing concentrations across multiple studies
or combining multiple study results to identify regional patterns.
Additionally, we emphasize the importance of contamination reduction
and quantification strategies, namely that blank samples from all
stages of field sampling be collected and reported as a matter of
course for all studies.