Distribution and retention efficiency of micro- and mesoplastics and heavy metals in mangrove, saltmarsh and cordgrass habitats along a subtropical coast

Understanding how coastal ecosystems mitigate pollution is essential due to their critical role in safeguarding environmental health, and supporting restoration efforts. This study, for the first time, evaluated the contamination levels and retention capacities of micro- and mesoplastics, and heavy metals across coastal habitats—specifically mangrove (MH), invasive Kikuyu grass (KH), and salt marsh cord grass (SH)—along a subtropical intertidal beach. Of the 120 sediment samples collected, 60 were analyzed for micro- and mesoplastics using wet peroxide oxidation and FTIR spectroscopy, while the remaining 60 were examined for heavy metal concentrations via ICP-MS. Results showed that KH habitats retained the highest plastics (153 ± 10.9 items/kg), followed by MH (112 ± 4.58 items/kg), SH (73.17 ± 6.81 items/kg), and NV (50.83 ± 10.87 items/kg) areas with significantly different retention in MH and KH habitats. Heavy metals followed a decreasing retention order of Mn > Zn > Cu > Cr > Pb > Ni > As > Cd > Hg. Significant difference was observed in Pb, Cr retention by an invasive Kikuyu grass (KH1) station, and Cu retention in two invasive Kikuyu grass stations (KH1 and KH3). However, in general no habitats were significantly different in retaining the metals. Principal Component Analysis and Canonical Correspondence Analysis revealed that micro- and mesoplastics were strongly associated with Zn, Cu, and Pb. KH habitats showed the highest retention efficiency, however, the associated toxicity risk increased with retention levels, indicating a higher risk in KH habitats compared to NV areas. The study highlighted Kikuyu grass habitats as both efficient pollutant sinks and potential ecological risk zones, emphasizing the need for targeted remediation to optimize retention while safeguarding ecosystem health. • Microplastics, mostly fibrous, varied from 153 ± 10.9–50.83 ± 10.87 items/kg across habitats. • Heavy metals followed the order: Mn > Zn > Cu > Cr > Pb > Ni > As > Cd > Hg. • Grass habitat showed the highest plastic retention than mangroves and saltmarsh. • Multivariate analyses showed strong associations between micro-/mesoplastics and Zn, Cu, Pb. • Risk indices revealed a higher associated ecological threat in grass habitat.