Actualité scientifique
Scientific news
January 2011
New Caledonia possesses
the second largest coral
reef lagoon on Earth and
harbours an exceptional
biodiversity. The island is
also the world’s third most
important nickel producer.
Ore extraction over the
20th Century has in places
tripled the input of
sediments and accompanying pollutants, such as
metals, in the marine
environment. This observation was the impulse for a
wide-ranging multidisciplinary research project,
run from 2000 to 2008,
aiming to determine the
impact of this process on
the lagoon ecosystem. IRD
scientists and their research
partners1 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: Better
understanding for better protection
© IRD / Jean-Michel Boré
N° 365
Actualidad cientifica
A wide-ranging research programme was conducted for nearly 10 years in Noumea lagoon system to explain in detail the physical mechanisms involved.
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 20th 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
For further information
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
Contact
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 GOPS2. 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.
References
Ouillon S., Douillet P., Lefebvre J.P.,
Le Gendre R., Jouon A., Bonneton P.,
Fernandez J.M., Chevillon C., Magand O.,
Lefèvre J., Le Hir P., Laganier R., Dumas F.,
Marchesiello P., Bel Madani A., Andrefouët S., Panché J.Y., Fichez R., 2010.
Circulation and suspended sediment
transport in a coral reef lagoon: the
southwest lagoon of New Caledonia,
Marine Pollution Bulletin, 61 (7-12),
269-296.
doi: 10.1016/j.marpolbul.2010.06.023
Copy editor – Gaëlle Courcoux - DIC, IRD
Key words
New Caledonia, lagoon, mines, nickel
Sylvain OUILLON,
director of research IRD
Tel: +33 (0)5 61 33 30 55
sylvain.ouillon@ird.fr
Laboratoire d’études en géophysique et
océanographie spatiales – LEGOS (UMR
IRD/CNES/CNRS/Université Paul Sabatier
- Toulouse 3)
Address
14 avenue Edouard BELIN
31400 Toulouse
France
Translation – Nicholas FLAY
1. T
hese 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.
© IRD / Jean-Michel Boré
2. See Le Grand Observatoire de l’environnement du Pacifique Sud sur les rails (web link: http://www.ird.fr/toute-l-actualite/
actualites/communiques-et-dossiers-de-presse/le-grand-observatoire-de-l-environnement-du-pacifique-sud-sur-les-rails)
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Soil erosion, accelerated by nickel mining on the island, in places tripled the sediment input to the lagoon during the 20th Century.
© IRD / Jean-Michel Boré
© IRD / Jean-Michel Boré
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