Water column

Litter might submerge below the surface

Wind distributes plastics deeper

CORNER: Vertical distribution of microplastics in the Baltic Sea

References

Litter might submerge below the surface

Plastics are rarely sampled from the water column beneath the sea surface. This is mainly because plastics were long assumed being submerged in the water column only temporarily and were thought to either be floating on the sea surface or sinking towards the bottom. Plastics can, however, be also neutrally buoyant and float in a certain depth in the water column. Knowledge about macro-sized debris in the water column is extremely scarce, but lately few studies have demonstrated the presence of microplastics in sub-surface waters and deeper layers of water column.

For example seawater samples collected at 4.5 m below the surface in the north-east Pacific Ocean were reported to contain microplastics in all 34 sampling stations. The mean concentration was 2080 particles m^-3, but concentrations varied from 8 to 9180 particles m^-3 depending on the location. Since the concentrations were higher near the coast compared to the offshore locations, these results indicate that land-based activities may be the major source of microplastics in the sampled area. Majority of particles were fibres which accounted for three quarters of all the found particles. The polymer types were, however, not identified, so the origin of fibres remain unknown.

Similarly, microplastics were also found in the sub-surface waters of the Arctic waters near Svalbard, Norway, in the depth of 6 meters. 93 % of the samples contained microplastics, and their concentration ranged between 0 and 11.5 particles m^-3 and averaged 2.68 particles m^-3. Fibres accounted for 95 % of all found particles followed by fragments and films. From the fibres analysed, the identified polymers included polyester (15 %), polyamide (15 %), acrylic (10 %), polyethylene (5 %), polyvinylchloride (5 %) and cellulose (30 %), possibly rayon. In another study conducted in the northeastern Atlantic Ocean, fibres also made up 95.9 % of the microplastic litter sampled from the depth of 3 meters, where the overall concentration of microplastics was on average 2.46 particles per m^-3.

Wind distributes plastics deeper

Microplastics can be distributed even deeper than couple of meters below the surface. Sub-surface net tows in North Atlantic subtropical gyre have revealed that microplastics are found also from the depth of 20 meters. Based on the observations and subsequent modelling, the vertical distribution of microplastics in the upper part of water column seems to be affected by wind speed. Wind-induced mixing of water can distribute microplastics from the surface deeper in the water column. The model predicts that the largest decrease in plastic concentration occurs over the first meters of water column. Vertical mixing may therefore reduce the amount of plastic caught by surface tows and result in underestimations of plastic concentrations. These predictions have been confirmed in an observational study conducted in the North Atlantic subtropical gyre, where the study detected highest concentrations of microplastics on the surface (0–0.5 m) below which layer the microplastic concentrations gradually decreased.

Wind-induced vertical mixing has been also shown to affect the distribution of bigger plastic particles (5–207 mm) in the water column in the depth of 0.5–5 meters. However, the mixing effect is stronger for smaller particles due to their lower rising velocities. It has been proposed that the vertical distribution of different-sized plastics may affect the animal-plastic interactions in the ocean by influencing the likelihood to encounter plastic in different depths.