Microplastic retention in marine vegetation canopies under breaking irregular waves

Nils B Kerpen; Bjarke Eltard Larsen; Torsten Schlurmann; Maike Paul; Hasan Gokhan Guler; Koray Deniz Goral; Stefan Carstensen; Erik Damgaard Christensen; David R Fuhrman

doi:10.1016/j.scitotenv.2023.169280

Microplastic retention in marine vegetation canopies under breaking irregular waves

Sci Total Environ. 2024 Feb 20:912:169280. doi: 10.1016/j.scitotenv.2023.169280. Epub 2023 Dec 19.

Authors

Nils B Kerpen¹, Bjarke Eltard Larsen², Torsten Schlurmann¹, Maike Paul¹, Hasan Gokhan Guler³, Koray Deniz Goral², Stefan Carstensen², Erik Damgaard Christensen², David R Fuhrman⁴

Affiliations

¹ Gottfried Wilhelm Leibniz University Hannover, Ludwig-Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering, Nienburger Str. 4, D-30167 Hannover, Germany.
² Technical University of Denmark, Department of Civil and Mechanical Engineering, DK-2800 Kgs. Lyngby, Denmark.
³ Technical University of Denmark, Department of Civil and Mechanical Engineering, DK-2800 Kgs. Lyngby, Denmark; Middle East Technical University, Department of Civil Engineering, Ocean Engineering Research Center, Cankaya, Ankara, Turkey.
⁴ Technical University of Denmark, Department of Civil and Mechanical Engineering, DK-2800 Kgs. Lyngby, Denmark. Electronic address: drfu@dtu.dk.

PMID: 38128667
DOI: 10.1016/j.scitotenv.2023.169280

Abstract

The present study provides indications and underlying drivers of wave-induced transport and retention potential of microplastic particles (MP) in marine vegetation canopies having different densities. The anthropogenic occurrence of MP in coastal waters is well documented in the recent literature. It is acknowledged that coastal vegetation can serve as a sink for MP due to its energy dissipating features, which can mimic a novel ecosystem service. While the transport behavior of MP in vegetation has previously been investigated to some extent for stationary flow conditions, fundamental investigations for unsteady surf zone flow conditions under irregular waves are still lacking. Herein, we demonstrate by means of hydraulic model tests that a vegetation's retention potential of MP in waves increases with the vegetation shoot density, the MP settling velocity and decreasing wave energy. It is found that particles migrating by traction (predominantly in contact with the bed) are trapped in the wake regions around a canopy, whereas suspended particles are able to pass vegetated areas more easily. Very dense canopies can also promote the passage of MP with diameters larger than the plant spacing, as the canopies then show characteristics of a solid sill and avoid particle penetration. The particle migration ability through a marine vegetation canopy is quantified, and the key drivers are described by an empirical expression based on the particle settling velocity, the canopy length and density. The findings of this study may contribute to improved prediction and assessment of MP accumulation hotspots in vegetated coastal areas and, thus, may help in tracing MP sinks. Such knowledge can be considered a prerequisite to develope methods or new technologies to recover plastic pollutants and rehabilitate valuable coastal environments.

Keywords: Canopy density; Canopy flow; Particle Dean number; Particle retention; Particle transport; Submerged canopy.