Heavy metals in mangroves: methodology, monitoring and management

An extensive study of heavy metals in Australian mangroves has revealed strong regional differences in the concentrations of several metals. The results also indicate that (i) fine textured mangrove sediments are highly efficient and effective sinks for heavy metals; (ii) metal concentrations in mangrove tissues, particularly in young leaves, correlate with concentrations in the sediments; (iii) bioaccumulation of metals occurs in mangroves but differs between species and mangrove tissue types; lead is selectively concentrated in bark and wood of mangroves whereas zinc and copper reach their highest concentrations in young leaves; (iv) mangrove leaf litter can return heavy metals to the environment in a bioavailable form; (v) young leaves are reliable and convenient bioindicators of heavy metal contamination, possibly because their salt-glands are not active and there is no tendency to excrete soluble metalchloride complexes; and (vi) there are several ways in which the presence of mangroves influences the distribution and dispersion of metals and all have management implications. From a management point of view, any modification to the mangrove forest is highly likely to alter the chemical and physical conditions in the forest in such a way that the metal uptake is likely to be reversed and a high proportion of the metals that were trapped while the forest existed may be released to the environment. The question is really not whether metals will be released, but how fast and in what form? Hence, removal of a mangrove forest would not only remove a potential pollutant trap that can be very effective in protecting adjacent environments, but it is also very likely to release some of the pollutants that were trapped while the forest existed.