Trace metals dynamics in a tropical mangrove-dominated estuary (Can Gio, Viet Nam)

Mangroves are considered as efficient barriers between land and sea, being sinks for trace metals and protecting coastal water from pollutions. However this ability may depend on sediment characteristics and hydrology. The fate of trace metal is complex due to various biogeochemical processes across the intertidal zone. Because of their toxicity to mangrove biodiversity and also to human health, the trace metal cycling is a serious question addressed by many scientists during the last few decades and nowadays. In contrast to the rest of the world, trace metal distribution in Viet Nam’s mangrove forest has received little attention. The aim of this research was to characterize some trace metals dynamics in the Can Gio mangrove, which is located in the Southern Viet Nam, being situated at the edge of a densely populated megacity – Ho Chi Minh City (i.e. almost 10 million inhabitants) and the South China Sea. Along the Can Gio estuary, trace metals, which mainly originated from upstream lateritic soils, were transported mostly associated with suspended solids, mainly oxihydroxides forms. After being deposited in mangrove sediments, the oxihydroxides were dissolved by bacteria during organic matter decay processes under suboxic condition, releasing trace metals in pore-waters. The enrichment of mangrove derived-OM from the mudflat to the Rhizophora stand played a key role in controlling diagenetic processes. Dissolved metals were then reprecipitated with new bearing phases such as organometallic complexes, suphides, carbonate etc. depending on the redox condition and on metal characteristics. Furthermore, part of these dissolved metals were available for biota uptake or export to tidal creek by pore-water seepage. Consequently, this PhD thesis also demonstrated that mangrove ecosystems can be sources of trace metals for adjacent ecosystems.