New Caledonia’s lagoon: Better understanding for better protection

IRD scientists and their research partners* have delivered their report on the research. One of their studies describes in detail the lagoon hydrodynamics which controls the transport and fate of incoming terrigenous deposits. As a result of this intense scientific survey on Noumea lagoon, decision-makers and industry leaders are now aware of the need to reduce the impact of nickel mining. Various measures are now being applied to limit soil erosion and marine pollution, including revegetation and installation of effluent retention basins.

New Caledonia 's lagoon harbours one of the largest coral-reef ecosystems on Earth, classified by UNESCO since 2008 as a World Heritage Site on the strength of its high biodiversity level. However, the archipelago also holds the world's third largest nickel deposits. The mining operations to extract this resource lead to erosion of mountainsides and transport of substantial mineral sediment inputs to coastal areas and waters.

The processes involved in the ejection, transport and deposit of metal particles in the lagoon are still not well known, even though they are the result of over a Century of mining activity. From 2000 to 2008, IRD researchers and their scientific partners¹ ran an ambitious multidisciplinary programme. The teams delivered their report on about ten years of work, covering such aspects as the physical characteristics and chemical composition of marine waters and sediments, the currents behind the suspended particle transport and circulation patterns of terrigenous sediments, marine habitats and communities. The objective is to explain the processes at work in the lagoon ecosystem and assess the impact of nickel mining.

Mining triples the sediment input

A series of measurements on the particles which accumulate on the lagoon floor enabled the researchers to track the lagoon's sedimentary history. Input of these materials from the land tripled over the course of the 20^th Century in some bays affected by nickel extraction. As the scientists recall, soil erosion can be increased five-fold when a mine is brought into operation.

On a several-day scale, trends in terrigenous inputs are closely linked also to climatic events, like strong episodes of tropical rainfall and cyclones. The researchers noted that such events can increase enormously the amounts of materials carried towards the lagoon, metals and various other pollutants included with them.

The investigations on sediments in the lagoon showed these to be highly variable, ranging from almost pure types of mud originating from terrigenous inputs lying in the coastal areas where they were deposited, to white sands at the backreef resulting from destruction of the corals by breakers pushed by the ocean swell. This strongly marked sediment pattern, which limits the distribution of habitats and species, is governed by the tidal currents but also by the winds, especially where the depth is less than 20 m.

The water can undergo quick renewal…

The hydrodynamic system, that is all the mechanisms involved in the movement of water masses (currents, swell, tides, turbulence), governs the transport and fate of metal particles in the lagoon. The researchers described the principal mechanisms. Numerical models of the lagoon circulation enabled them to draw up an atlas of currents and map residence times in the lagoon. These parameters play a major role in the lagoon biology and geochemistry by significantly regulating the distribution of dissolved metals and biogenic matter. This system results in renewal of the backreef waters in less than two days. And on average it takes only 12.5 days for 65% of the water in the lagoon to be changed.

… or stagnate in bayheads

Yet these overall figures hide a strong spatial irregularity. Water can stay for up to 25 days in the coastal current band running along the shoreline, or can even stagnate for up to two months in the bayheads near Noumea. This explains the accumulation of organic matter and high residual turbidity observed along the coast.

Almost ten-years' concentration of research effort on this complex site has revealed some of its secrets and helped to define avenues for future work, particularly in the framework of GOPS. Beyond the scientific prospects, these investigations have kindled an awareness among political decision-makers and mine operators. The industry's leaders are now setting up a number of measures to limit soil erosion and pollution of the lagoon. These include mining methods that are less destructive for the ground, installation of retention basins to hold effluents, replanting of areas stripped of their vegetation. The whole issue at stake for local actors is to reconcile objectives of development derived from what is the island's primary economic activity with protection of this unique place, a jewel of marine biodiversity.

* These research investigations were conducted mainly by teams from the IRD, CNRS, IFREMER, the Universities of Toulouse and Bordeaux as part of the Programme National Environnement Côtier (PNEC) (French Coastal Environment Research Programme) and with the support of the Programme Néo-calédonien ZoNéCo.