The restoration of mangrove forests in urban environments aims to improve water quality and provide valuable habitat for wildlife. However, restoration sites in these areas are likely to have higher contaminant concentrations due to their proximity to urban and industrial activities. Despite the known effects of contaminants on biota, sediment-based contaminants are rarely considered in mangrove restoration projects.
Aims: The aim of this study was to evaluate the risk of restoring mangroves in contaminated environments, by assessing the influence of sediment chemistry on contaminant bioaccumulation and the physiological health of shore crabs from restored and natural mangroves.
Methods: Three sites in southeast Queensland were selected; a contaminated restored habitat, a contaminated natural habitat and an undeveloped natural habitat with low contaminant concentrations. From each site, sediment cores were collected to analyse total organic carbon, reactive iron speciation, acid volatile sulfides, particle size, trace elements, perfluoroalkyl substances (PFAS) and polycyclic aromatic hydrocarbons (PAHs). From each site, the tissue of shore crabs was also analysed for trace elements and polar metabolites (via NMR spectroscopy) to investigate the bioaccumulation of contaminants and their sub-lethal effects on shore crab physiology.
Results: Several trace elements (As, Cd, Co, and Pb), PAHs, and PFAS compounds were present at significantly higher concentrations in the sediment and crab tissues collected from the restored contaminated site compared to those from the natural control site. Metabolite profiles in the three crab populations indicated potential effects on physiological status associated with sediment-based contaminants, which may negatively impact overall metabolic health.
Conclusions: Our findings show that the restoration of contaminated environments could create ecological sinks, where animals take residence in a poor-quality habitat, their health is impacted, and populations decline. This study highlights the importance of considering, and at times mitigating, contaminant burden on urban ecosystems when restoring mangrove wetlands.