Mangroves: A filter for heavy metals

New Caledonia is the3rd largest nickel producer worldwide, with over 30% of the planet's resources, and it has been the location of intense mining activity since the end of the 19th century. Around 300 million m³ of spoils rich in heavy metals have been created until now. A significant amount of this mining waste has been transported to coastal areas by the dramatic climate events, (thunderstorms and tropical tempests, cyclones) that often occur in this region. Accentuated by mining activity, this erosion is the most important cause of deterioration of the coastline, the mangroves, the fringing reef and the lagoon.

Polluted mangroves

An IRD team and their partners(1) have recently demonstrated that concentrations of mineral metals such as iron, nickel and chrome are 10 to 100 times higher in mangroves situated downstream from mining sites. Recently, two studies have been published, one concerning the mangrove downstream from a nickel mine that operated in the 20th century at the mouth of the Dumbéa river in the south west of the island, the other being unaffected by mining activity in its catchment area (used as a control) in Conception Bay, near Nouméa.

Core samples of sediment 70cm in length were taken at low tide from the different zones in the mangroves, in order to account for differing coverage in vegetation. The mangrove ecosystem is clearly divided into different zones, each dominated by a separate type of mangrove tree, according to the topography of the land and the duration of tidal immersion. Rhizophora trees are found where the mangrove meets the sea, large in size with their root systems above ground. In the central zone where high tides are intermittent, the medium-sized Avicennia mangrove shrubs are found. Located at the rear is the 'tanne', the area least often submerged by water, consisting salt-saturated soil, bare or sparsely populated with vegetation.

The core samples extracted were examined using a variety of chemical treatments designed to dissolve minerals containing metallic elements. Such analysis has enabled a comparison between the concentrations of metals in the sediment from the two mangroves under study, in addition to their potential toxicity, and revealed the biogeochemical processes that are specific to the various species of mangrove trees.

The mangrove -- a highly adapted forest

Mangrove trees use a real arsenal of survival techniques to deal with the extreme conditions of their natural habitat. To counteract the absence of oxygen in the mud, they have developed remarkable root systems, enabling air to penetrate the soil. The Rhizophora located on the waterfront have developed roots that form stilts, emerging from their branches, in order to combat the swell and currents. As a result, there is a major accumulation of litter within the sediment, where anoxic(2) processes take place, leading to the precipitation of 'sulphide' type minerals. In this type of forest, metals can thus merge with decomposing organic matter, or co-precipitate with the sulphides, and are thus trapped by the mangrove.

The Avicennia are characterised by a star-shaped root system that develops a sub-surface level, with vertical growths emerging skywards. These formations are known as 'pneumatophores', and allow the mangrove tree to extract oxygen from the atmosphere. However, these breathing organs are not watertight, and lose a portion of their oxygen to the sediment. As such, beneath the vegetation, metallic elements linked to iron oxides are dissolved and transferred to the mangrove trees.

This research has led to a better global understanding of the processes that control the mangrove ecosystem. It has confirmed that mangroves act as a well for contaminants over the long term. However, their surface coverage is decreasing by 1 to 2% each year. The cause: demographic growth along the tropical coastlines and urbanisation, as well as prospecting for and exploitation of natural resources, such as nickel in New Caledonia. Without the dense network of vegetation provided by the mangrove trees, sediment that is loaded with pollutants could be returned to the lagoon, a haven for biodiversity and a major source of revenue for the local population through fishing and aquaculture.

1. This research has been carried out by the University of New Caledonia, Koniambo Nickel SAS and the AEL/LEA laboratory in Nouméa, the University of Orléans and Paris-Sud University.
2. produced in the absence of oxygen