Trace metal accumulation and environmental risk assessments of adjacent mangroves and salt marshes in Queensland, Australia

Coastal wetlands are impacted by both terrestrial and marine pollution, which can threaten their ecological functions and the services they provide. Monitoring sediment contaminants is essential to identify potential risks and prevent catastrophic or irreversible harm. However, monitoring is expensive, and there are few long-term monitoring programs for coastal wetlands; thus, studies that can prioritise ecosystems at risk and identify environmental parameters associated with contaminants are necessary. Here, we evaluate and compare the enrichment and ecological risk of trace metals in three pairs of adjacent mangroves and salt marshes in southeast Queensland, Australia. We also investigate the relationship between trace metals (concentrations and indices) and physicochemical soil properties. Concentrations of nine metals were significantly higher in mangroves compared to salt marshes, associated with distinct soil physicochemical characteristics of each wetland type. The concentration of Ni was above the lower limit of the national guidelines, and As and Se had a potential risk to the wetland biota. Trace metal concentrations were significantly correlated with soil grain size composition, organic carbon and acid volatile sulphide soils. Overall, trace metal contamination risk was higher at mangroves than at salt marshes, especially in sites with soils rich in organic carbon dominated by clay or silt. In southeast Queensland, trace metal contamination is generally low, although some metals are close to reaching points of ecological concern. Future monitoring programs could target these trace metals in mangroves to ensure their future ecological integrity.