Circulation and suspended sediment transport in a coral reef lagoon: The south-west lagoon of New Caledonia

Abstract

The south-west lagoon of New Caledonia is a wide semi-open coral reef lagoon bounded by an intertidal barrier reef and bisected by numerous deep inlets. This paper synthesizes findings from the 2000–2008 French National Program EC2CO-PNEC relative to the circulation and the transport of suspended particles in this lagoon. Numerical model development (hydrodynamic, fine suspended sediment transport, wind-wave, small-scale atmospheric circulation) allowed the determination of circulation patterns in the lagoon and the charting of residence time, the later of which has been recently used in a series of ecological studies. Topical studies based on field measurements permitted the parameterisation of wave set-up induced by the swell breaking on the reef barrier and the validation of a wind-wave model in a fetch-limited environment. The analysis of spatial and temporal variability of suspended matter concentration over short and long time-scales, the measurement of grain size distribution and the density of suspended matter (1.27 kg l⁻¹), and the estimation of erodibility of heterogeneous (sand/mud, terrigenous/biogenic) soft bottoms was also conducted. Aggregates were shown to be more abundant near or around reefs and a possible biological influence on this aggregation is discussed. Optical measurements enabled the quantification of suspended matter either in situ (monochromatic measurements) or remotely (surface spectral measurements and satellite observations) and provided indirect calibration and validation of a suspended sediment transport model. The processes that warrant further investigation in order to improve our knowledge of circulation and suspended sediment transport in the New Caledonia lagoon as well as in other coral reef areas are discussed, as are the relevance and reliability of the numerical models for this endeavour.

Introduction

A main interest (and also difficulty) of scientific research in coastal waters is due to the overlap of large variety of biotic and abiotic processes, the biological richness of these systems, the socio-economic importance of these areas and the interrelationship between all these factors. Particle dynamics is a topic which illustrates this complexity. The south-west lagoon of New Caledonia is of particular interest because it is typical example of a South Pacific barrier reef lagoon. In addition, sediment transport is intrinsically linked with metal contamination resulting from industrial and urban waste, coastal management, changes in land use and, above all, mining activities, all of which have an impact on the lagoon ecosystem.

New Caledonia is currently the third largest producer of nickel in the world. The saprolitic layers and enriched laterites have been exploited for more than a century using the opencast mining technique. Moreover, the prospecting and open cast mining on the hilltops has resulted in major alterations to landscape and habitat strongly enhancing natural soil erosion and subsequent river transport of terrigenous material and associated metals toward the lagoon (Bird et al., 1984). Although all pelagic and benthic communities are affected to some extent by the increases in sediment deposition and decreases in light penetration, fringing reefs are especially sensitive to these changes.

This paper synthesizes findings from the 2000–2008 French “Programme National Environnement Côtier” (PNEC) relative to the circulation and the transport of suspended particles in the south-west lagoon of New Caledonia. The questions asked in this project are detailed in Section 1.2. Section 2 presents the study area, its geomorphology, climate, temperature and salinity climatology, tide and wave regimes. Hydrodynamic results (circulation, residence time, exchanges with the ocean, wind waves, atmospheric forcing) are presented and discussed in Section 3. After a detailed presentation of the lagoon sedimentology, Section 4 deals with suspended sediment transport. This section analyzes the spatial and temporal variability of turbidity, the grain size distribution of suspended particles, the optical measurements, modelling of sediment transport, erodibility of sediment beds, and ends with a sedimentary budget.

The Institute of Research for Development (IRD, formerly called ORSTOM) initiated a pluridisciplinary scientific program (“ECOTROPE”) in 1996 to study the impact of anthropogenic activities on the functioning of the south-west lagoon of New Caledonia (e.g. Bujan et al., 2000). Based on ECOTROPE, the PNEC funded a large interdisciplinary study where scientists from the IRD and other French scientific institutions such as the universities and CNRS worked together to improve our knowledge of its physical, chemical, biological functioning in relation to its use.

Physical processes in coral reef lagoons have already been individually studied in detail and the variability of some physical parameters have been synthesized. Hydrodynamics has been widely studied (Wolanski and Pickard, 1985, Hearn and Parker, 1988, Atkinson et al., 1992), occasionally on the basis of numerical models (Smith, 1985, Frith and Mason, 1986, Prager, 1991, Tartinville et al., 1997, Kraines et al., 1998, Kraines et al., 1999, Douillet et al., 2001). Wave-associated processes in lagoons have been investigated by, e.g. Hardy, 1993, Symonds et al., 1995, Hearn, 1999, Callaghan et al., 2006. The impact of hydrodynamics on lagoon functioning was analyzed by Bradbury and Young, 1981, Hamner et al., 1988, Roberts et al., 1992, Wolanski, 2001. Studies have also dedicated to variability in temperature (Liston et al., 1992, Nadaoka et al., 2001a, Ouillon et al., 2005) and in salinity (Ouillon et al., 2005) in lagoons. Among coastal systems, coral reefs are thought to be particularly vulnerable to the amount of sediment provided by anthropogenic activities (Smith and Buddenmeier, 1992). As sedimentation is a potential factor driving reef degradation (Rogers, 1990), sediment transport and fluxes have been monitored and analyzed over coral reef lagoons (Hamilton, 1994, Larcombe et al., 1995, Larcombe et al., 2001, Orpin et al., 1999, Kraines et al., 2001), with some studies describing and quantifying the impact of storms and hurricanes on sediments (Bonem, 1988, Gagan et al., 1990). Although the variability of suspended matter concentrations at different time scales has been analyzed in continental shelves and bays (Castaing et al., 1999, Schoellhamer, 2002), few authors have conducted such studies in coral reef lagoons (Thomas et al., 2003).

Even if physical processes in reef lagoons have been investigated, few studies have yet presented integrated physical functioning of coral reefs over areas of great extent, with the exception of the Great Barrier Reef (Wolanski, 1994). Macroscale processes influence large portions of a continental shelf, mesoscale processes determine the degree to which individual reef systems are connected to their oceanic environment, and microscale processes affect zones within reefs (Andrews and Pickard, 1990). Physical analysis integrating temperature, salinity, currents and waves have been presented by Wing and Leichter, 1997, Nadaoka et al., 2001b. However, it is clear that additional integrated physical studies of reefs at different scales and over extended periods are needed. This paper is an attempt to summarize and synthesize the physical studies (circulation and suspended sediment transport) which were performed in the south-west lagoon of New Caledonia.

This program follows on from research funded by the IRD and from the French National Program on Coral Reefs, led by Jacques Clavier and Renaud Fichez. During these two programs, a large number of hydrological and current measurements were made (Douillet et al., 1989, Douillet et al., 1990), enabling the set up a 2D model of tide propagation (Douillet, 1998) followed by the development of a 3D model of hydrodynamics in collaboration with Ifremer. At the same time, the leading paper by Hearn (2001) of the special issue of Coral Reefs devoted to “Coral reef hydrodynamics” highlighted the context and importance of such studies. Hearn explained that the 9th International Coral Reef Symposium in Bali in 2000 saw for the first time a session devoted entirely to hydrodynamics that “in a sense, marked the coming of age of ‘coral reef hydrodynamics’ ”. Obviously, prior to this conference, scientists had not been ignoring the study of lagoon hydrodynamics, particularly given the nuclear testing that was carried out by the United States and France in some Pacific atolls (e.g. von Arx, 1948). Andrews and Pickard (1990) wrote a brilliant synthesis of this pioneer work on lagoon hydrodynamics. However, the massive coral bleaching episodes in the 1980’s and that of the El Niño event of 1997–98 brought renewed interest in the physics, biology, chemistry and ecology of coral reef environments.

The physics-focused paper published in the special issue of Coral Reefs, Douillet et al. (2001), presented the first application of a 3D model of fine suspended sediment transport in the south-west lagoon of New Caledonia and posed the central questions relevant to this review:

• What kind of easy-to-use parameters can we propose for characterization of hydrodynamics for chemical or biological applications?

• Is it possible to estimate the fluxes of water and momentum above the barrier reef induced by the swell surge?

• Is it possible to estimate the wind wave action on resuspension within the lagoon?

• How can we take into account a realistic wind stress field rather than a uniform stress in the circulation model (in relation with orographic effects)?

• What is the size distribution of in-water particles and their density?

• What is the sediment erodibility and how can it be connected to sedimentological parameters?

These questions were approached progressively during the PNEC program from 2000 to 2008. The answers are synthesized in the present paper. As always in research, the initial questions have evolved and some new directions that may help structure of future research are given at the end of the paper.