Does water column stratification influence the vertical distribution of microplastics?

Microplastic pollution has been confirmed in all marine compartments. However, information on the sub-surface
microplastics (MPs) abundance is still limited. The vertical distribution of MPs can be influenced by water
column stratification due to water masses of contrasting density. In this study, we investigated the vertical
distribution of MPs in relation to the water column structure at nine sites in the Kattegat/Skagerrak (Denmark) in
October 2020.A CTD was used to determine the stratification and pycnocline depth before sampling. Plastic-free
pump-filter sampling devices were used to collect MPs from water samples (1–3 m3) at different depths. MPs
concentration (MPs m 3) ranged from 18 to 87 MP m 3 (Median: 40 MP m 3; n = 9) in surface waters. In the mid
waters, concentrations ranged from 16 to 157 MP m 3 (Median: 31 MP m 3; n = 6), while at deeper depths,
concentrations ranged from 13 to 95 MP m 3 (Median: 34 MP m 3; n = 9). There was no significant difference in
the concentration of MPs between depths. Regardless of the depth, polyester (47%), polypropylene (24%),
polyethylene (10%), and polystyrene (9%) were the dominating polymers. Approximately 94% of the MPs fell
within the size range of 11–300 μm across all depths. High-density polymers accounted for 68% of the MPs, while
low-density polymers accounted for 32% at all depths. Overall, our results show that MPs are ubiquitous in the
water column from surface to deep waters; we did not find any impact of water density on the depth distribution
of MPs despite the strong water stratification in the Kattegat/Skagerrak.