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Coral reefs in French Polynesia, just like many others throughout the world, have been subjected to several natural disturbances including 15 cyclones, seven major bleaching events, and several Acanthaster planci outbreaks since the 1980s. In order to document the effects of these perturbations on coral assemblages, we initiated a long-term monitoring program that extended over both local and regional scales. Coral cover was quantified at 20 sites situated on the outer reef slope of 13 islands. The results from the first decade (1992-2002) are analyzed and the adequacy of our approach is discussed in the context of identifying potential indicators of coral reef health. Among 13 islands in French Polynesia, only two were unaffected by natural disturbances. We found important local and regional variation in the impacts of coral bleaching and cyclones, and three major temporal trends were distinguished: 1) 10 sites where coral cover decreased in relation to the occurrence of major disturbances; 2) nine sites where coral cover increased, despite the occurrence of disturbances affecting seven of them; and 3) a site where no significant variation in coral cover was found. The responses to perturbations were different among coral genera: Acropora species were particularly susceptible to bleaching events, whereas physical damages induced by cyclones concerned mainly branching species of Acropora and Pocillopora. Thus, monitoring surveys could be improved by selecting different and complementary indicators (one on the variation in diversity, one estimating changes in the abundance/cover, and one estimating the potential for recovery), by integrating several spatial scales, and by including at least the most informative species. High frequency recordings of environmental parameters (e.g. sea surface temperature) may be also a complementary tools for identifying causal relationships between changes in coral reef community structure and the factors causing the changes.

influence de la localisation et de la profondeur. Les evenements de blanchissement corallien sont parmi les perturbations les plus frequentes et les plus devastatrices que subissent les recifs coralliens. A Moorea (Polynesie francaise), entre mars et juillet 2002, une moyenne de 55 ± 14% des colonies a blanchi. L'intensite du blanchissement variait fortement en fonction des genres consideres, de la localisation et de la profondeur, atteignant 72% en certaines stations. Les coraux ont davantage blanchi au niveau des stations profondes qu'aux faibles profondeurs, et plus sur la cote nord que sur la cote ouest. Cette forte variabilite a petite echelle spatiale est probablement le resultat d'interactions entre des facteurs extrinseques, comme l'hydrodynamisme, et intrinseques, comme des adaptations differentielles des associations algues/corail a l'augmentation de la temperature.

International audience; We investigated spatial patterns of stony corals, molluscs and echinoderms among six habitat types within the lagoons of six atolls in the Tuamotu Archipelago (French Polynesia). Percent cover (stony corals only), abundance and richness were recorded within habitat types delineated by specific geomorphological features. Among six habitat types defined a priori, four encompassed distinct assemblages of corals and molluscs: (1) pinnacles possessed maximum richness, abundance and coverage, (2) passes showed relatively high abundance and coverage, (3) lagoon floors were marked by the commonness of a few coral genera, and minimum mollusc abundance and richness, and (4) inner reef flat stations, independent of their location in front of spillways, motu or rim zones, had low cover, abundance, and richness. While habitat associations were similar for stony corals and molluscs, echinoderms were preferentially found on the inner reef flat facing the rim. Lowest echinoderm abundances and species richness were recorded on pinnacles and in the vicinity of the pass. Such small-scale spatial heterogeneity appears to be one of the distinctive characteristics of coral communities in French Polynesian reefs. These results, along with those of the TYPATOLL program, demonstrate the importance of both local and regional factors in determining the diversity and structure of coral assemblages. Habitat fidelity was weak: many species/genera were poorly represented among habitats and restricted to a few atolls, while others were present across all lagoons and habitats. Therefore, marine protected areas design based solely on representativeness will require the protecting the vast majority of lagoon habitats.

International audience; This article aims to review 1) the major and most frequent human-induced physical disturbances and their consequences on coral reef habitats using a multi-scale approach, and 2) the scale-related indicators and conceptual aspects used to detect and measure the effects of these physical impacts. By physical disturbances, we mean direct perturbations that lead to the destruction/erosion of the carbonate framework. Human-induced direct physical disturbances are numerous from coastal development, tourism, harvesting, accidents and nuclear/weapon testing. Since methods for monitoring and measuring indicators are generally scale-implicit, coral reefs are first presented according to different ecological-spatial scales of organization, from colony to region (colony, reefscape, reef zone, whole reef, island and region). In this way, it is easier to link a couple {habitat, disturbance} to their potential indicators and to the descriptors they target. Three classes of descriptors, related to the response of the living component of coral reef ecosystem, are considered here: stony coral, reef fishes and the human uses. A synthesis of the different options for coral habitat assessments is proposed. We sort them according to their objectives (monitor, initial status or improvement of knowledge), their specificities (identification or not of a specific disturbances) and their scale of investigation (small, meso- or large scales). Usually, the majority of the indicators of human-induced disturbances are non-specific. They reveal that something is happening but not the actual causality and can only detect differences across time or space. A major weakness lies in the difficulty in deconvoluting the signals from a conjunction of stressors occurring at different scales. As such, a hierarchical concept of disturbances in coral reefs would be the next logical step to enhance our capabilities in monitoring and forecasting coral reefs status.

Downward particle fluxes off the Taotoi Pass, a typical Polynesian lagoon pass, have been measured over an annual cycle by means of an array of three mooring lines deployed at 40 m depth on the external sandy plain seawards of the Tiahura reef. Each line was equipped with two hand-operated sediment traps located 1.5 and 10 m above the bottom. During the 1-year survey, the traps were deployed for seven periods of 2-3 weeks. One line equipped with an automated trap was further deployed at 400 m depth on the deep fore-reef slope to collect deep settling particles over two consecutive 6-month periods. For all periods investigated, fluxes on the external plain were highest in front of the pass, and decreased by half on either side (up- and downstream with respect to general circulation), indicating a net export through the pass. At all three locations, the highest downward fluxes recorded (60-100 g m(-2) d(-1)) corresponded to coarse panicles mostly carbonate, whereas the lower fluxes (a few g m(-2) d(-1)) corresponded to smaller and lighter material mostly organic carbon. Fluxes on the deep slope were much lower than on the sandy plain (1.5-7.3 g m(-2) d(-1)), but followed a broadly similar seasonal trend. Comparison of our trap data with published production values for the Tiahura reef system suggests that inorganic and organic carbon export through the pass is limited. The minimum estimates represent around 6% of total calcification but only 1% of the net production of the reef system. The qualitative and quantitative temporal evolution of downward fluxes showed a seasonally-contrasted trend, related to local hydrodynamic conditions. Fluxes were always higher during the southern summer season, and lower during winter. Carbon export appears to be ultimately controlled by climatic factors. i.e. seasonal changes in the orientation and heights of swells and waves that modulate the amount of water flowing into the lagoon, before being flushed out into the open ocean, entraining particles. (C) 2002 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS. All rights reserved., Les flux particulaires ont été mesurés devant la passe lagonaire de Taotoï (côte Nord de Moorea, Polynésie française) sur une période annuelle, au moyen de trois lignes de mouillage de pièges à particules déployées à 40 m de profondeur sur la plaine sableuse située à l’extérieur du récif barrière de Tiahura. Chaque ligne comporte deux pièges à particules manuels situés à 1,5 m et 10 m au-dessus du fond. Ces pièges ont été déployés durant sept périodes de deux à trois semaines. Un piège à particules automatique a, en outre, été déployé à 400 m de profondeur sur le tombant afin de collecter, de façon séquentielle, des échantillons mensuels de particules pendant deux périodes consécutives de six mois. Les flux mesurés sur la plaine sableuse sont toujours plus élevés devant la passe. Ils diminuent de moitié environ pour les stations placées en « amont » et en « aval » de la passe (en référence au courant de dérive générale à l’extérieur du récif barrière), indiquant une exportation nette de matière par cette dernière. Les flux les plus élevés (60 à 100 g m–2 j–1) sont systématiquement composés de particules lourdes et carbonatées. À l’inverse, les flux les plus faibles (quelques g m–2 j–1) se composent dˈun matériel plus fin, plus léger, enrichi en carbone organique. En profondeur, les flux sont beaucoup plus faibles que sur la plaine sableuse (1,5 à 7,3 g m–2 j–1), mais présentent un signal saisonnier similaire. La comparaison de nos résultats avec les valeurs de production publiées pour le système récifal de Tiahura suggère que l’exportation de carbone inorganique et organique par le récif reste un phénomène limité, avec des minima dˈenviron 6% de la calcification totale et de 1% de la production nette. L’évolution temporelle des flux présente une tendance saisonnière marquée, liée à la variation des conditions hydrodynamiques locales: les flux sont toujours plus élevés en période d’été austral et plus faibles en hiver. L’exportation de carbone semble ainsi contrôlée par les facteurs climatiques, c’est-à-dire les changements saisonniers d’orientation des houles et des vagues qui modulent les arrivées d’eau dans le lagon et donc le transport particulaire associé au flux dˈeau sortant.