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.