Heavy metals in mangroves: methodology, monitoring and management

Mangroves occur widely throughout Australia and, at some sites, are exposed to elevated heavy metal loads. The value of mangrove communities, and particularly of mangrove forest sediments, as a buffer between potential sources of metalliferous pollutants and marine ecosystems has been noted previously (Harbison, 1981, 1986; Saenger et al., 1991). How mangroves respond to the metal load in their environment remains largely unknown. Some work on the response of mangrove seedlings to heavy metal exposure (Montgomery and Price, 1979; Peterson et al., 1979; Walsh et al., 1979; Thomas and Ong, 1984; Chiu et al., 1995; Chen et al., 1995; Wong et al., 1997) has been published, but detailed studies of heavy metals in mangrove ecosystems are rare (Yudman et al., 1988; Mackey et al., 1992; Saenger et al., 1991; Lacerda et al., 1993; Clark et al., 1997, 1998). Mangrove areas are also nursery or breeding grounds for several commercially important species of marine fauna (Saenger, 2002), and because many mariculture operations are sited in or near mangroves, it is important to determine how mangroves respond to metallic pollutants, and to what extent they can shield commercially important marine species from these pollutants. To understand the relationship between mangroves and heavy metals, several questions need to be answered, including: How do mangroves respond to high concentrations of metals in their environment? Are heavy metals accumulated in mangroves and, if so, where? Do mangroves return their metal load to the environment in a more or a less bioavailable form? Can mangroves be used to monitor heavy metal concentrations in their environments? How effective are mangroves at shielding the marine environment from metallic pollutants? What are the limits to their buffering capacity? Do mangroves act as a step in a metal biomagnification pathway? How can mangroves be protected from the build-up of heavy metal loads? As a first step in answering these questions, heavy metal concentrations were compared in this study between different mangrove communities from areas (Table 1) where elevated levels are likely and where they are highly improbable. Subsequently, the distribution of copper, lead, zinc and cadmium were determined within the principal parts of common mangrove species and any tendencies towards selective metal accumulation in, or exclusion from, particular parts of the plants were assessed. Currently, the principal pathways for the transfer of metals, the significance of these pathways, and the potential of mangroves as bioindicators of heavy metal contamination in aquatic ecosystems are being examined.