Effects of stratified water column on settling dynamics of microplastics and implications for interactions between microplastics and marine ecosystem

, Mrokowska Magdalena.

Recent studies have indicated that microplastics are likely to accumulate at pycnoclines, that is the regions of sharp density gradients induced by the vertical variation of salinity and/or temperature. Moreover, pycnoclines present in the ocean, coastal regions, and estuaries cause a pronounced decrease in settling velocity of natural marine particles such as plankton, faecal pellets, and detritus causing their accumulation at density transitions. In such conditions, intensified interactions between microplastics and natural marine particles may occur. The knowledge on the interactions between microplastics and marine biological components in stratified systems is poor and research on this topic poses a challenge, since interdisciplinary studies are necessary to gain understanding of these processes. Results shown in this presentation may help understand the role of physical aspects of microplastic settling that could be used in further studies on their fate in marine system. I demonstrate the effects of water column stratification on settling dynamics of plastic non-spherical particles. I present laboratory experiments in which disks made of ABS were settling in a column of salt-water solution. Disks were of diameters less than 5 mm, and showed a complex pattern of reorientations and velocity variation during settling through density transition. Visualization (cameras with macro lenses and backlight) combined with image analysis methods were used to measure the pattern of trajectory, variation of particle orientation, and settling velocity. The results indicate that disks decelerate significantly in the pycnocline, and existing methods of particle flux assessment overestimate settling velocities. Consequently, microplastics reside in the transition layer much longer than predicted, which may explain to some extent enhanced interactions with biota such as ingestion by zooplankton, adsorption of heavy metals, biofouling, aggregation, and incorporation of microplastics into marine snow in stratified aquatic systems. Reorientations may additionally induce interactions between microplastics and other types of marine particles.

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