L'hydrodynamique du lagon sud-ouest

La circulation des eaux dans le lagon sud-ouest Nouvelle-Calédonie est due à trois agents principaux : la marée, le vent local et le déferlement la houle par-dessus le récif-barrière

Biogeochemical typology and temporal variability of lagoon waters in a coral reef ecosystem subject to terrigeneous and anthropogenic inputs (New Caledonia)

International audienceConsidering the growing concern about the impact of anthropogenic inputs on coral reefs and coral reef lagoons, surprisingly little attention has been given to the relationship between those inputs and the trophic status of lagoon waters. The present paper describes the distribution of biogeochemical parameters in the coral reef lagoon of New Caledonia where environmental conditions allegedly range from pristine oligotrophic to anthropogenically influenced. The study objectives were to: (i) identify terrigeneous and anthropogenic inputs and propose a typology of lagoon waters, (ii) determine temporal variability of water biogeochemical parameters at time-scales ranging from hours to seasons. Combined ACP-cluster analyses revealed that over the 2000 km2 lagoon area around the city of Nouméa, "natural" terrigeneous versus oceanic influences affecting all stations only accounted for less than 20% of the spatial variability whereas 60% of that spatial variability could be attributed to significant eutrophication of a limited number of inshore stations. ACP analysis allowed to unambiguously discriminating between the natural trophic enrichment along the offshore-inshore gradient and anthropogenically induced eutrophication. High temporal variability in dissolved inorganic nutrients concentrations strongly hindered their use as indicators of environmental status. Due to longer turn over time, particulate organic material and more specifically chlorophyll a appeared as more reliable nonconservative tracer of trophic status. Results further provided evidence that ENSO occurrences might temporarily lower the trophic status of the New Caledonia lagoon. It is concluded that, due to such high frequency temporal variability, the use of biogeochemical parameters in environmental surveys require adapted sampling strategies, data management and environmental alert methods

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

International audienceThe 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