Sediment composition influences microplastic trapping in seagrass meadows

Seagrass meadows are highly-valued coastal ecosystems that have the potential to act as sinks for microplastic pollution through their particle-trapping capabilities. Using wave tank simulations, this study tested how microplastic capture can be affected by particle size, sediment composition, and the presence of seagrass (cores collected from two Zostera muelleri meadows). Two synthetic crumb rubber polymers of differing densities were chosen as model contaminants due to their widespread use in road and sporting infrastructure. Sediment composition and seagrass presence influenced microplastic capture, with site-specific differences between cores from the two meadows. The meadow with greater silt and clay content captured more microplastics in the upper 0-2 cm layer. In contrast, the sand-dominated site captured fewer microplastics in the upper layers, but the lower packing fraction of the sediment allowed for penetration to 6 cm depth. Microplastic size also influenced trapping efficiency, with smaller microplastic particles (250-499 μm) being captured at a rate 177 % higher than larger particles (1.18-2 mm). Polymer density (high vs. low) had no significant effect on trapping capability. These findings demonstrate the importance of sediment composition in the ability of seagrass meadows to act as microplastic sinks. Fine-grained sediments trap microplastics in the upper sediment layers (0-2 cm) where interactions with benthic organisms are more likely to occur, while sandy sediments allow for greater penetration, potentially creating deeper reservoirs. This knowledge highlights the need for policymakers and environmental managers to consider sediment composition and site-specific strategies when addressing microplastic pollution in coastal ecosystems.