From samples taken in the Mediterranean Sea, researchers reveal that the quantity of microplastics accumulating in the seabed is increasing at the same rate as global plastic production. They also found that once sequestered in sediment, the particles do not degrade.
The presence of microplastics in the oceans raises concerns regarding the harm they represent for the functioning of these ecosystems. Their dispersion in water is a phenomenon that is still poorly understood today, partly because of constraints linked to the analytical methods available. Nevertheless, some studies point to the fact that the seabed represents an important sink for these pollutants. Scientists from the Institute of Environmental Science and Technology at the Autonomous University of Barcelona (ICTA-UAB) and the University of Aalborg in Denmark have carried out a study on the presence of microplastics in marine sediments and their accumulation over time. It was published in the journal Environmental Science and Technology.
To carry out this research work, the researchers used samples taken in November 2019 in the Ebro delta, in the south of Catalonia. This area of the Mediterranean Sea was selected because the rivers are known to be hotspots for several pollutants, including microplastics. In addition, the influx of sediments from the Ebro provides higher sedimentation rates than in the open sea.
A total of five sediment cores were taken, then only one, located regarding a hundred meters deep, was the subject of an in-depth analysis. It was cut into slices in order to study the microplastic composition of each layer, as well as to observe their degradation. The researchers managed to date the samples with lead-210 to establish the chronology of each portion of sediment, with the aim of understanding what happens to these microplastics over time. Through the use of state-of-the-art imaging techniques, the quantities of particles ranging in size from 11 to 1,000 micrometers might be studied.
The first result put forward by this research reveals that the amount of microplastics accumulating in sediments has increased exponentially over the period from 1965 to 2016, at the same rate as global plastic production. “Specifically, the results show that, since 2000, the amount of plastic particles deposited on the seabed has tripled and that, far from decreasing, the accumulation has not stopped growing, mimicking the production and global use of these materials, says Laura Simon-Sánchez, researcher at ICTA-UAB.
Three types of particles strongly present in the seabed
The second lesson of this study concerns the state of degradation of buried particles. Thanks to chemical analyses, scientists have found that once trapped in the seabed, microplastics no longer degrade and are preserved as they are, due to a lack of erosion, oxygen or light. “The process of fragmentation occurs mainly in beach sediments, on the sea surface or in the water column, explains Patrizia Ziveri, professor at ICTA-UAB. Once deposited, degradation is minimal, so plastics from the 1960s remain on the seabed, leaving the signature of human pollution there. »
For Michael Grelaud, a researcher at the ICTA-UAB, this research work has made it possible to “see how, since the 1980s, but especially in the last two decades, the accumulation of particles of polyethylene and polypropylene from packaging, bottles and food films has increased, as well as polyester from synthetic fibers in clothing fabrics. » The amount of these three types of particles reaches 1.5 milligrams per kilogram of sediment collected, with polypropylene being the most abundant, followed by polyethylene and polyester.
This study provides a better understanding of the transfer of microparticles from the ocean surface to the seabed. It paves the way for other lines of research, in particular on better understanding the impact of these microplastics on the organisms that develop in the seabed.