Michel Allenbach

Temporal variability of CO2 fluxes at the sediment-air interface in mangroves (New Caledonia)

Carbon budgets in mangrove forests are uncertain mainly due to the lack of data concerning carbon export in dissolved and gaseous forms. Temporal variability of in situ CO2 fluxes was investigated at the sediment-air interface in different seasons in different mangrove stands in a semi-arid climate. Fluxes were measured using dynamic closed incubation chambers (transparent and opaque) connected to an infra-red gas analyzer. Microclimatic conditions and chl-a contents of surface sediments were determined. Over all mangrove stands, CO2 fluxes on intact sediments were relatively low, ranging from -3.93 to 8.85 mmolCO₂·m(-2)·h(-1) in the light and in the dark, respectively. Changes in the fluxes over time appeared to depend to a great extent on the development of the biofilm at the sediment surface. We suggest that in intact sediments and in the dark, CO2 fluxes measured at the sediment-air interface rather reflect the metabolism of benthic organisms than sediment respiration (heterotrophic and autotrophic). However, without the biofilm, sediment water content and air temperature were main drivers of seasonal differences in CO2 fluxes, and their influence differed depending on the intertidal location of the stand. After removal of the biofilm, Q10 values in the Avicennia and the Rhizophora stands were 1.84 and 2.1, respectively, revealing the sensitivity of mangrove sediments to an increase in temperature. This study provides evidence that, if the influence of the biofilm is not taken into account, the in situ CO2 emission data currently used to calculate the budget will lead to underestimation of CO2 production linked to heterotrophic respiration fueled by organic matter detritus from the mangrove.

Tidal variability of CO 2 and CH 4 emissions from the water column within a Rhizophora mangrove forest (New Caledonia)

We performed a preliminary study to quantify CO2 and CH4 emissions from the water column within a Rhizophora spp. mangrove forest. Mean CO2 and CH4 emissions during the studied period were 3.35±3.62mmolCm-2h-1 and 18.30±27.72μmolCm-2h-1, respectively. CO2 and CH4 emissions were highly variable and mainly driven by tides (flow/ebb, water column thickness, neap/spring). Indeed, an inverse relationship between the magnitude of the emissions and the thickness of the water column above the mangrove soil was observed. δ13CO2 values ranged from -26.88‰ to -8.6‰, suggesting a mixing between CO2-enriched pore waters and lagoon incoming waters. In addition, CO2 and CH4 emissions were significantly higher during ebb tides, mainly due to the progressive enrichment of the water column by diffusive fluxes as its residence time over the forest floor increased. Eventually, we observed higher CO2 and CH4 emissions during spring tides than during neap tides, combined to depleted δ13CO2 values, suggesting a higher contribution of soil-produced gases to the emissions. These higher emissions may result from higher renewable of the electron acceptor and enhanced exchange surface between the soil and the water column. This study shows that CO2 and CH4 emissions from the water column were not negligible and must be considered in future carbon budgets in mangroves.

Semi-automatic detection and labelling of erosion areas using domain knowledge: an appraisal from southeast New Caledonia (SW Pacific)

The management of small Pacific coastal territories has become a crucial issue; these are insular units that often display a high level of biodiversity in a context of changing climate and sea-level rise. In order to preserve and protect populations, infrastructure and living resources, there is a need to understand inland processes that may influence the behaviour of coastal systems and, more particularly, active erosion zones. Cartography of stripped surfaces by remote sensing has become routine and we propose here the first step of a method that aims to monitor erosion features using an automated process. Managing catastrophic erosion and/or landslides needs high frequency image acquisition so as to optimize hazard prevention. On the basis of a single remote sensing map, we propose a generic method for automatically assigning expert-designed labels to erosion areas. Our automated process follows three steps: first, we use classical algorithms to detect stripped zones; second, we assign a label to each extracted zone using domain knowledge. Finally, as a post-processing phase, detected and labelled erosion areas are checked by experts. This method has been validated in an erosion-sensitive area of south-eastern New Caledonia.

Aménagements du bas delta de la Néra et du cordon littoral de la Roche percée (Bourail, Nouvelle-Calédonie)

ABSTRACT Managements concern the lower Néra delta and The Roche percée offshore bar (Bourail, New Caledonia). The Roche percée offshore bar at Bourail (New Caledonia), an urbanized place since the seventies submitted to a significant erosion for more ten years has shown important and significant wakening tracks after hurricane Béti spell in march 1996. The coastal sedimentary dynamic study ordered by the territorial collectivities has allowed to draw up a precise balance sheet of the area evolution dynamic. An integrated management of the Néra Delta has been decided. The management associates classic enrockment cordon in the medium part of the beach with an intervention for restoring the natural operating scheme of northern mouth of the Néra River. The topographic survey of the area, after hurricane Frank spell in 1999 shows a good functioning of the engineering works since their building.

Biofilm and temperature controls on greenhouse gas (CO2 and CH4) emissions from a Rhizophora mangrove soil (New Caledonia)

Seasonal variations of CO2 and CH4 fluxes were investigated in a Rhizophora mangrove forest that develops under a semi-arid climate, in New Caledonia. Fluxes were measured using closed incubation chambers connected to a CRDS analyzer. They were performed during low tide at light, in the dark, and in the dark after having removed the top 1-2 mm of soil, which may contain biofilm. CO2 and CH4 fluxes ranged from 31.34 to 187.48 mmol m-2 day-1 and from 39.36 to 428.09 μmol m-2 day-1, respectively. Both CO2 and CH4 emissions showed a strong seasonal variability with higher fluxes measured during the warm season, due to an enhanced production of these two gases within the soil. Furthermore, CO2 fluxes were higher in the dark than at light, evidencing photosynthetic processes at the soil surface and thus the role of biofilm in the regulation of greenhouse gas emissions from mangrove soils. The mean δ13C-CO2 value of the CO2 fluxes measured was -19.76 ± 1.19‰, which was depleted compared to the one emitted by root respiration (-22.32 ± 1.06‰), leaf litter decomposition (-21.43 ± 1.89‰) and organic matter degradation (-22.33 ± 1.82‰). This result confirmed the use of the CO2 produced within the soil by the biofilm developing at its surface. After removing the top 1-2 mm of soil, both CO2 and CH4 fluxes increased. Enhancement of CH4 fluxes suggests that biofilm may act as a physical barrier to the transfer of GHG from the soil to the atmosphere. However, the δ13C-CO2 became more enriched, evidencing that the biofilm was not integrally removed, and that its partial removal resulted in physical disturbance that stimulated CO2 production. Therefore, this study provides useful information to understand the global implication of mangroves in climate change mitigation.

Variability of CO 2 emissions during the rearing cycle of a semi-intensive shrimp farm in a mangrove coastal zone (New Caledonia)

In New Caledonia, shrimp ponds are built not on cleared mangroves but on salt flats behind the mangroves. The objectives of this study were to determine the variability of CO2 fluxes from a semi-intensive shrimp pond during active and non-active periods of the farm and to determine the carbon dynamics from the upstream tidal creek to the downstream creek, which receives the farms effluents. CO2 emissions from the active pond were estimated at 11.1 ± 5.26 mmol CO2 m-2 d-1. By modifying the hydrodynamics of the creeks, farm practices also influenced CO2 emissions from both the upstream and downstream creeks. After tillage, all the organic carbon deposited at the pond bottom during the active period was mineralized, resulting in CO2 emissions to the atmosphere estimated at 7.9 TCO2 ha-1. Therefore, shrimp farming is an anthropogenic source of CO2 to the atmosphere, but suitable and optimized rearing practices limit these emissions.

Développements récents dans la connaissance de l'océanologie du lagon de Wallis

Résumé : De nouvelles acquisitions de données courantologiques obtenues dans le suivi océanologique du lagon de Wallis entrepris depuis 2006, ont permis de mieux définir le schéma de circulation général du lagon de Wallis. L’importance des entrées d’eau océaniques dues au déferlement des vagues par-dessus la barrière récifale, mesurées dans plusieurs situations météo-océanologiques, aboutissent à la définition du concept de bourrelet de surpression créé par le déferlement des vagues par-dessus la barrière récifale qui se développe du coté au vent de l’île. L’importance de ce bourrelet de surpression conditionne la circulation dans le lagon par le biais des échanges entre bassins lagonaires au travers des chenaux internes et des passes. En fonction des conditions météo-océanologiques, il se développe une circulation circulaire conduisant au déplacement des masses d’eau excédentaires dans les bassins situés du côté opposé de l’île, et à leur évacuation par les passes et par-dessus le récif. Mots-clés : Courantologie doppler, Lagon, Marées, Houles, Suivi du réchauffement climatique, Wallis, Pacifique Sud-Ouest. Abstract: New ADCP current meter acquisitions data recorded in the oceanologic monitoring of the Wallis lagoon, undertaken since 2006, in the context of assessing the impact of global warming, have helped to define a general circulation pattern for the Wallis lagoon. The importance of ocean water inputs due to the waves breaking over the barrier reef, measured in several ocean-weather situations, lead to the definition of a pressure gradient that develops on the windward side of the island. The importance of this pressure gradient affects the residual circulation in the lagoon and the inter basins exchanges through the lagoon channels. This induces a circular movement of the water masses in excess, toward the lagoon basins located on the opposite side of the island, for their evacuation through the passes and over the barrier reef.

Les variantes granulats dans le contexte des îles océaniennes affectées par le changement climatique. Application au cas de l’île de Wallis (Pacifique sud-ouest)

Resume : Les incidences majeures des changements climatiques futurs sur les petites iles oceaniennes annoncees par le GIEC (2007) sont l’intensification des inondations, des tempetes et de l’erosion, l’atteinte aux ressources locales, le blanchissement des coraux et la reduction des ressources en eau. Ces tendances sont amplifiees par les erreurs anthropiques : polderisation du littoral, prelevements anarchiques de sables et graviers sur les plages et dunes. La communication se propose de faire le point, a partir d’une monographie regionale consacree au cas de l’archipel des iles Wallis (Pacifique sudouest) sur les variantes utilisables pour pallier le manque recurrent de granulats sur les espaces insulaires oceaniens. Ces variantes, par ailleurs classiques, doivent etre etudiees en regard, d’une part, de leur impact environnemental sur un ecosysteme fragile, le milieu recifal et d’autre part, des realites socio-economiques et techniques de ces espaces isoles au sein de l’ocean Pacifique. Mots-cles : Granulats – Changement climatique global – Wallis et Futuna – Pacifique sud-ouest – Ecosysteme recifal – Developpement durable – Scories de nickel Abstract: The major impacts of future climate change on small Pacific islands announced by the IPCC (2007) are the increase in floods, storms and erosion, damage to local resources, coral bleaching and reduced resources water. These tendencies are amplified by human error : polders, unregulated gathering sand and gravel on beaches and dunes. The paper aims to take stock, from a regional monograph devoted to the case of the Wallis archipelago (Southwest Pacific) the variants can be used to offset the recurrent lack of aggregates on the Pacific island areas. These variants, also classic, are to be studied with their environmental impact on a fragile ecosystem : the reef on one hand and socioeconomic and technical issues of the isolated areas within the Pacific Ocean on the other. Keys words: Aggregates – Global Climate Change – Wallis and Futuna – Pacific Southwest – Reef ecosystems – Sustainable Development – Nickel Slag

Correlation between SO2 emissions rate and S contained in fuel used in a power plant, Noumea, New Caledonia

SO2 emissions from fossil fuel power plants can have significant impacts on human health and ecosystems. Consequently, numerous techniques are in use to monitor these emissions, in order to comply with environmental legislations. Here we highlight the correlation between SO2 emissions rate and the S contained in fuel used in power plant. We obtained a maximum of 1.3 kg.s-1 of SO2 emissions rate and a minimum of 0.4 kg.s-1 corresponding respectively to 2.9 % and 1.2 % of S contained in fuel. Measurements also indicate that high concentration of SO2 released from the Noumea 121 MW power plant is rapidly diluted in the first 10 minutes, corresponding to 3-4 km distance from the source downwind. Thus inhabitants living within the 3-4 km radius are potentially exposed to power plant emissions.

Enjeux et perspectives de la gestion intégrée du littoral insulaire français du Sud-Ouest Pacifique

ABSTRACT This article presents an analysis of coastal zone management in the French insular spaces of the south-western Pacific Ocean. After describing the regional context and the problems at stakes induced by the climatic changes, it approaches the specificities of the French territories of the zone (geography, settlements, status). It presents the existing points of convergence compared to the development of the coastline, and the oceanian perspective of this space. Finally, it enumerates and comments on the prospects that are offered for a development and an integrated management of the coastal area.