Assessment of sediment quality in Avicennia marina-dominated embayments of Sydney Estuary: the potential use of pneumatophores (aerial roots) as a bio-indicator of trace metal contamination.

Currently, coastal intertidal environments are under stress from increased contaminant loads due to urbanization and other anthropogenic disturbances. Mangrove habitats are abundant in tropical and sub-topical intertidal zones and frequently act as a metal bio-filter in estuarine systems. Mangrove reforestation is often considered as one of the management options to protect estuarine-marine habitats. The main objective of the present investigation was to assess the bio-indicator potential of Avicennia marina by determining heavy metal concentrations in pneumatophore (aerial root) tissues and ambient sediments from Sydney Estuary (Australia). We collected mangrove sediments and pneumatophores in fifteen locations covering five major embayments of the estuary for a detailed biogeochemical investigation. Metal concentrations in sediment were mostly above Australian interim sediment quality guidelines (ISQG)-Low and in few instances above ISQG-High values. Enrichment factors (EFs >6, especially of Cu, Pb and Zn) suggest "very severe" modification of sediment in Sydney Estuary in all but one embayment which was mainly due to rapid changes in land use in connection with urbanization. High bio-concentration factors (BCFs) were observed for Cu and Ni in comparison with other metals (i.e., As, Cd, Co, Cr, Pb and Zn). A strong, positive relationship between metals in sediments and pneumatophores suggests potential use of these tissues as a bio-indicator of estuarine contamination and that metals are entering the biotic environment. The study further highlights a positive role of mangroves in sequestering metals from sediments and the water column and thus protecting estuarine environments from pollution.