Trace metal dynamics in soils and plants along intertidal gradients in semi-arid mangroves (New Caledonia)

Trace metal dynamics were investigated in mangroves developing in semi-arid New Caledonia, where Avicennia and Rhizophora stands grow in the upper and lower intertidal zone, respectively. We collected soil samples and mangrove tissues in an undisturbed site, a mining-influenced site and in a mining and aquaculture-influenced site. Differences in duration of immersion and organic matter (OM) cycling resulted in a sharp decrease of metal concentrations in soils and plants from landside to seaside. Both species were tolerant to metals mainly via exclusion, (i.e. metal bioaccumulation restricted to roots and leaf litter). Strong correlations (p < 0.05) were found between Na and Fe, Mn, Cu and Zn in green and senesced leaves of Avicennia marina, indicating a possible role of those metals in mechanisms to cope with hypersaline conditions. Increasing metal pollution, aridity and sea-level rise are likely to result in a decrease in mangrove efficiency in filtering trace metals seaward.