The distribution and influence of heavy metals in mangrove forests of the Tamshui Estuary in Taiwan

Abstract

The Tamshui estuary area is one of the few locations in Taiwan where mangroves are still growing. Heavy metals are generally carried by the water of the Tamshui river and accumulate in the estuary soil. Their accumulation is closely related to the frequency and duration of tidal flood. The concentration of heavy metals in Kandelia candel showed a sequential decline from roots to stems, leaves, and seedlings. There was a correlation between the amount of extractable heavy metals in soils and that in the tissues in Phragmites communis, while in Kandelia candel the correlation varied largely with the habitats except for the roots. The availability of heavy metals in mangrove soil may decrease due to the flooding which subsequently reduces their uptake in plant tissues. Sequential extractions of mangrove soil Indicated that the labile fraction contained a relatively small proportion of the total amount under the mangrove estuary environment. The tolerance of Kandelia candel seedlings to copper and zinc was much higher than that of rice plants (Oryza sativa). The value of 50% inhibition concentration (IC₅₀) of root development in Kandelia candel was estimated at 3.8 mg liter^-1 for Cu and 46.33 mg liter^-1 for Zn. For comparison, the correspecting values of the IC₅₀ in rice was 0.39 mg liter^-1 for Cu and 9.63 mg liter^-1 for Zn. Field studies and greenhouse experiments have revealed that Kandelia candel can adapt to areas polluted with heavy metals partly because of its tolerance and partly because of the low availability of heavy metals in mangrove soil.