Trapping of Microplastics in Halocline and Turbidity Layers of the Semi-enclosed Baltic Sea

Abstract

Microplastic pollution in semi-enclosed seas is gaining attention since microplastics are more likely to accumulate there. However, research on the vertical distribution of microplastics and impact factors is still limited. In this study, we focus on the Baltic Sea, which has distinguished salinity stratification, and we assume that the resulting strong density stratification (halocline) can influence the vertical distribution of microplastics in the water column. Therefore, we analyzed the vertical abundance distribution, the composition, and the sizes of microplastics (27.3–5,000.0 μm) in the Baltic Sea. The results showed that microplastics comprising fibers, fragments, and films occurred throughout the water column at an abundance of 1.1–27.7 items L−1. The abundance of microplastics (3.2–27.7 items L−1) at haloclines was significantly higher than those at other water depths except the near surfaces (p < 0.05), contributing 24.1–53.2% of the microplastics in the whole water column. Small microplastics (<100 μm) were more likely to accumulate in the water layers above halocline. Moreover, the current with high turbidity might be another carrier of microplastics in the near-bottom water layer due to its strong correlation with microplastics abundance. This study provides valuable evidence for the accumulation trend of microplastics in water columns and its influencing factors in the semi-enclosed marginal sea. Further research on the vertical distribution of microplastics under the control of multiple factors should be conducted in the future.

Microfiber Hotspots’ Association with Ships in a Remote Port before and during Covid-19

“…This is similar to the results before Covid-19 in that the size distribution of MP at deeper layers was different from the surface, which can be attributed to the formation of halocline that entraps MP. 21,22 The mean and median size also increased as depth increased, but the larger size during Covid-19 compared to that before Covid-19 can be attributed to the difference in the resolutions of stereomicroscopes. Eight polymer types were identified for MP samples during Covid-19 (n = 599), including cellulosic fibers (86.8%), PET (7%), PAN (3.17%), PP (1.5%), PS (0.5%), PA (0.2%), and one alkyd paint particle.…”

Does Water Column Stratification Influence the Vertical Distribution of Microplastics?

“…Water stratification is one of the significant physical factors that influence the distribution and sinking rates of MPs within the water column (Liu et al, 2020;Uurasjärvi et al, 2021).In the case of the Kattegat region, the water column is strongly stratified (Pedersen, 1993;Nielsen, 2005;Bendtsen et al, 2009;Lehmann et al, 2022;Ni et al, 2023).The pycnocline can act as a density barrier and hinder the sinking of MPs from surface water to deep waters (Reisser et al, 2015;Isobe et al, 2017).There are studies which have found that MPs are trapped at the beginning of the pycnocline in the Baltic Sea, showing that the change in water density from low to high salinity affects the sinking of MPs within the water column (Zobkov et al, 2019;Uurasjärvi et al, 2021;Zhou et al, 2021).In our study area, we observed no significant difference in the concentration of MPs among different water masses despite the strong pycnocline present in the Kattegat (Figs. S2 and S4).…”

“…Large-scale oceanographic factors like ocean currents influence the horizontal and vertical distribution of MPs in the ocean (Law et al, 2010;Zobkov et al, 2019) and mesoscale processes such as eddies, convective flows, and upwelling (Brach et al, 2018;Díez-Minguito et al, 2020;Vega-Moreno et al, 2021).Vertical water mixing also affects the distribution of both positively and negatively buoyant MPs (Liu et al, 2020;Uurasjärvi et al, 2021).Several studies found that water column stratification influences the distribution of MPs (Wu et al, 2019;Zhou et al, 2021), even causing the accumulation of MPs in thin sub-surface water layers (Uurasjärvi et al, 2021).For instance, Uurasjärvi et al, 2021 found 1000 times more MPs at the beginning of the thermocline than in the other water layers in Baltic waters. However, other studies have found a less clear/no influence of water stratification on MP vertical distribution (Eo et al, 2021;Zhou et al, 2021).…”

Settling Velocities of Small Microplastic Fragments and Fibers

“…According to particle numbers, the predominant MPs found in marine and freshwater ecosystems are irregularly shaped fragments smaller than 500 μm and fibers with lengths below 1 mm, − although cellulose and semisynthetic fibers are sometimes probably misidentified as MPs as well. , While many studies reported empirical settling velocities of larger MPs, − only few investigated the settling of these smaller particles yet, despite their prevalence and a supposedly higher ecotoxicological relevance . Certain flow conditions might reduce the significance of small MPs’ settling velocities, yet it was concluded by Hoellein et al that nominal settling velocities of small MPs could provide good estimations of their actual deposition velocity in natural streams.…”