Experimental study of buoyant macro- and mesoplastic trapping by Spartina maritima

A set of laboratory experiments was conducted to assess the trapping efficiency of buoyant plastic debris by the estuarine vegetation Spartina maritima. Different hydrodynamic conditions typical of salt marshes were simulated in a hydraulic flume. These conditions included varying water levels between 40% and 90% of stem height, wind speeds up to 2 m/s, and unidirectional current velocities between 0.1 and 0.3 m/s. Moreover, three vegetation densities (small/medium/high) and nine plastic debris types varying in shape (elongated/two-dimensional/three-dimensional) and size (macro/meso) were tested. The results indicate that Spartina maritima functions as a natural trap. Specifically, the study highlights that lower surface velocities, higher stem densities, greater emergent heights, and larger debris sizes significantly enhance trapping efficiency. It was also inferred that for a Spartina maritima density comparable to that observed in marshes, the dominance of surface velocity or debris size effects on trapping efficiency is primarily dependent on debris shape. Consequently, surface velocity has a greater impact on two-dimensional elements, whereas debris size is more significant for three-dimensional elements. Finally, a preliminary trapping model was developed to integrate all the aforementioned variables. This model has the potential to enhance the accuracy of numerical predictions regarding the transport and fate of plastic debris using Lagrangian modeling, and can be further refined by incorporating additional data.

The authors acknowledge the support of the Programa de Ciencias Marinas (PCM), funded by the Ministry of Science and Innovation of the Spanish Government and the Regional Governments of Andalucía, Baleares, Canarias, Cantabria, Comunidad Valenciana, Galicia, and Murcia (PRTR-C17⋅I1). This program, part of the ThinkInAzul I + D + i Complementary Plan, has provided essential funding for this research, in line with the 2030 Agenda for Sustainable Development and the Decade of the Oceans initiative. Moreover, this research was also partially funded by PID2021-127358NB-I00-MICIU/AEI/10.13039/501100011033 and by FEDER as a way of making Europe. Finally, we express our deep gratitude to Margot Sánchez for her invaluable support and dedication throughout the development of this work.