Microplastics are a problem for the oceans and their ecosystems, especially coastal regions. Waves create currents that move the plastic particles in the direction of the surf zone, where they accumulate among plants such as seagrass. This makes it difficult to effectively remove the tiny particles from these zones. However, if the conditions under which the microplastics tend to accumulate were understood, it would be possible to determine where it makes sense to collect the plastic and thereby protect important coastal areas.
For this reason, researchers working with Dr.-Ing. Nils Kerpen, operations manager at the Ludwig Franzius Institute of Hydraulic, Estuarine and Coastal Engineering at Leibniz University Hannover are conducting model experiments to investigate more closely how currents move the plastic particles as part of a study currently underway. Previous studies have examined the behaviour of the particles with a consistent current only. In the present study, the researchers have used a wave flume, or wave channel, to simulate currents in the surf zone, which change over time.
“To do this we have simulated realistic swell from the North and Baltic seas with over 40,000 individual waves per experiment,” explains Nils Kerpen. “This lengthy experiment time is necessary to capture fluctuations in the transportation of the particles in a statistically representative way.” This means that the tiny plastic particles do not move in exactly the same direction with each wave, but over the course of many waves, a tendency can be observed.
In order to hold the plants’ characteristics constant over the long experiment period, the researchers used sections of astroturf in the wave flume. From experiment to experiment, they removed individual stems in order to reduce the density of the vegetation. This systematic approach makes it possible to reproduce the experiments and verify the results.
The researchers thus identified the factors that have the largest impact on the movement of microplastics: the plant density, the length of the area covered by vegetation and the settling velocity of the particles. “The denser the vegetation and the lower the resulting wave strength, the more likely it is that particles can accumulate on the plants,” says Nils Kerpen. “We have discovered a new service provided to the ecosystem by coastal plants: By retaining the microparticles, the plants help us collect the microplastics.” Particularly at the edges of the vegetated areas, an especially large amount of microplastics accumulated.
The laboratory tests were carried out at a scale of 1 to 20 in the wave flume, which means they were 20 times smaller than they would be in nature. “However, the results have been processed without dimensions, which means that all the observations and statements regarding the transport and accumulation of particles also apply to particles in nature,” says Kerpen.
The studies were carried out as part of the research project MPCOAST – MicroPlastic transport processes in the COASTal environment, in cooperation with researchers at the Technical University of Denmark. The project is funded by the Independent Research Fund Denmark (grant no. 0136-00227B). The researchers have now published their results in the journal Science of the Total Environment (https://doi.org/10.1016/j.scitotenv.2023.169280).
Note to editors:
For further information, please contact Dr.-Ing. Nils Kerpen, operations manager at the Ludwig Franzius Institute of Hydraulic, Estuarine and Coastal Engineering (tel. +49 511 762 3740, email: kerpen@lufi.uni-hannover.de).
