French Polynesian Atolls Research Articles

Abiotic drivers of the space use and activity of gray reef sharks Carcharhinus amblyrhynchos in a dynamic tidal environment.

Predators display rhythms in behavior and habitat use, often with the goal of maximizing foraging success. The underlying mechanisms behind these rhythms are generally linked to abiotic conditions related to diel, lunar, or seasonal cycles. To understand their effects on the space use, activity, and swimming depth of gray reef sharks (Carcharhinus amblyrhynchos), we tagged 38 individuals with depth and accelerometer sensors in a French Polynesian atoll channel exposed to strong tidal flow, and monitored them over a year. C. amblyrhynchos used a larger space during nighttime and were more active at night and during outgoing currents. Shark activity also peaked during the full and new moons. The swimming depth of sharks was mostly influenced by diel cycles, with sharks swimming deeper during the day compared to nighttime. The dynamic energyscape may promote the emergence of discrete behavioral strategies in reef sharks that use the south channel of Fakarava for resting and foraging purposes. Turbulence imposed by outgoing tides induces additional foraging cost on sharks, shifting their hunting areas to the southern part of the channel, where turbulence is less pronounced. Understanding when and where sharks are active and foraging is important for our understanding of predator-prey dynamics and ecosystem dynamics. This study highlights how abiotic rhythms in a highly dynamic environment likely generate spatiotemporal heterogeneity in the distribution of predation pressure.

Population Connectivity and Genetic Assessment of Exploited and Natural Populations of Pearl Oysters within a French Polynesian Atoll Lagoon.

In French Polynesia, the production and exportation of black pearls through the aquaculture of the black-lip pearl oyster Pinctada margaritifera provide the second largest economic income for the country after tourism. This industry entirely relies on the collection of natural spats from few highly recruiting lagoons. In recent years, pearl oyster producers have experienced variable success rates in spat collection, with significant spatial and temporal variability in spat supply, driving uncertainty in the future of pearl production. This study combines, for the first time in a farmed lagoon, genetic (SNPs), demographic (sex ratio, age), and biophysical data (larval dispersal modelling) to shed new light on population dynamics, connectivity, and spat recruitment in Ahe Atoll, a well-studied pearl farming site. Our results indicate that the geographical structuring of the natural populations and the contribution of both natural and exploited stocks to the production of spats result from the interaction of hydrodynamic features, life history traits and demographic parameters: the northeastern natural populations are older, not well connected to the southwestern natural populations and are not replenished by larvae produced by adjacent exploited populations. Moreover, we observe that the exploited populations did not contribute to larval production during our experiment, despite a sampling period set during the most productive season for spat collection. This is likely the result of a strong male bias in the exploited populations, coupled with a sweepstakes reproductive strategy of the species. Our results warrant further investigations over the future of the northeastern older natural populations and a reflection on the current perliculture techniques.

Binding of Hg by bacterial extracellular polysaccharide: a possible role in Hg tolerance.

Bacteria employ adaptive mechanisms of mercury (Hg) tolerance to survive in environments containing elevated Hg concentrations. The potential of extracellular polysaccharides (EPS) production by bacteria as a mechanism of Hg tolerance has not been previously investigated. The objectives of this study were to determine if bacterial EPS sorb Hg, and if so does sorption provide protection against Hg toxicity. Purified EPS with different chemical compositions produced by bacterial isolates from microbial mats in French Polynesian atolls and deep-sea hydrothermal vents were assessed for Hg sorption. The data showed that EPS sorbed up to 82% of Hg from solution, that this sorption was dependent on EPS composition, and that sorption was a saturable mechanism. Hg uptake capacities ranged from 0.005 to 0.454mmol Hg/g for the different EPS. To determine if EPS production could alter bacterial Hg tolerance, Escherichia coli K-12 strains and their EPS defective mutants were tested by the disc inhibition assay. Mercury inhibited growth in a dose-dependent manner with wild-type strains having smaller (~1mm), but statistically significant, zones of inhibition than various mutants and this difference was related to a 2-fold decline in the amount of EPS produced by the mutants relative to cell biomass. These experiments identified colanic acid and hexosamine as Hg-binding moieties in EPS. Together these data indicate that binding of Hg to EPS affords a low level of resistance to the producing bacteria.

Environmental determinants of coral reef fish diversity across several French Polynesian atolls

The present study aimed at exploring the diversity of coral reef fishes in 10 French Polynesian atolls and sought to determine which environmental variables best explain diversity. A total of 136,614 fish belonging to 302 species were recorded in 1995 and 1996. The stepwise multiple regression analysis showed that the best model of variation in species richness (55% of total variation) incorporated three geomorphologic descriptors (atoll perimeter, submerged rim and abundance of pinnacles) and two habitat descriptors (percentage cover of dead coral and sand). The best model of variation in Shannon-Wiener's species diversity index (43% of total variation) included two geomorphologic descriptors (mean depth and level of water exchange) and three habitat descriptors (percentage cover of mud, dead coral and gravel). Overall, our survey recognises the importance of both geomorphologic and habitat descriptors as leading contenders in explaining biodiversity in relation to energy input and habitat area hypothesis.

Influence of plankton concentration on gametogenesis and spawning of the black lip pearl oyster Pinctada margaritifera in Ahe atoll lagoon (Tuamotu archipelago, French polynesia)

Pearl culture industry represents one of the dominant business sector of French Polynesia. However, it still entirely relies on unpredictable spat collection success. Our aim was to assess the influence of natural plankton concentration fluctuations on maturation and spawning of the black lip pearl oyster Pinctada margaritifera, during a 4months survey conducted in Ahe atoll lagoon. Plankton concentration was assessed by chlorophyll a extraction and by microscope counts while gonadic index, gonado-visceral dry weights and histology were used to measure pearl oysters reproduction activity. We found that (i) plankton concentration fluctuations were mainly related to wind regime, (ii) gametogenesis rate was mainly related to plankton concentration, (iii) spawning occurred when maximal gonad storage was reached, (iv) plankton concentration was the main spawning synchronizing factor. These results contribute explaining P. margaritifera spat collection variability in French Polynesian atoll lagoon.

Development of in situ hybridisation using 16S rRNA gene to monitor black-lip pearl oyster,Pinctada margaritifera, larvae in plankton samples

In French Polynesia, the black-lip pearl oyster Pinctada margaritifera has been farmed to produce pearls since the 1980s, forming the basis of a major industry. The sustainability of this activity relies on spat collection in the lagoons. However, pearl oyster spat can be difficult to identify for the evaluation of stock variations. It is especially hard to distinguish Pinctada spp. larvae at a very early stage of development. In the present study, a whole-mount in situ hybridisation (ISH) technique was developed to allow the discrimination of larvae of closely-related pearl oyster species found in the French Polynesian atolls. Using specific ribosomal 16S-DNA sequence data, we were able to successfully differentiate between Pinctada margaritifera and Pinctada maculata larvae from 5 to 13 days old. This is the first description of a non-destructive method allowing bivalve larvae discrimination between species within this genus. The method allowed us to successfully identify P. margaritifera larvae in natural plankton samples. This result is a key step needed to develop monitoring of P. margaritifera larval distribution in French Polynesian lagoons, a procedure which will increase spat collection efficiency and ensure sustainable development of pearl oyster farming.

Dominant benthic mollusks in closed atolls, French Polynesia

We investigated the mollusk fauna of 53 closed lagoons (atolls without passes) among the 83 French Polynesian atolls. These confined lagoons of closed atolls reveal a limited diversity of mollusks but with wide distribution of some species. A few species occur in these lagoons but each lagoon is unique with a particular qualitative mollusk composition related to only 5 species: Tridacna maxima, Pinctada margaritifera, Pinctada maculata, Chama iostoma and Arca ventricosa. Due to the absence or poor data on 14 closed lagoons, our detailed study was limited to 39. We characterised the mollusk fauna of each closed lagoon and attempted to identify group of lagoons with the same dominant mollusk composition. T. maxima, the giant clam, appeared to be the most characteristic species of these confined lagoons with large densities and populations in 32 of the 39 lagoons. P. margaritifera, the mother-of-pearl oyster, also occurs as large populations in 18 lagoons; these two species are of economic importance now as they were in the past. The three other species mentioned above occurred respectively in 25, 28 and 20 lagoons. There are 11 lagoons with original and unique characteristics (their assemblages of dominant species do not occur in any other lagoon), 28 are grouped in 7 categories of assemblages. The most numerous groups contain all 5 species or at least 4 of them. Research topics are suggested in order to understand the ability of each mollusk species to colonize and maintain its dominance in a confined lagoon.

Evaluation of insecticide impregnated baits for control of mosquito larvae in land crab burrows on French Polynesian atolls.

Land crab burrows are larval mosquito habitats of major significance in the Pacific region. They are constituted by a sinuous tunnel leading to a chamber in contact with the water table, where mosquito larvae proliferate. Controlling larvae in these sites is difficult, because the configuration of burrows prevents the use of standard techniques. An experiment was carried out in French Polynesia to control Aedes polynesiensis Marks and Culex spp. breeding in burrows of the land crab Cardisoma carnifex (Herbst). The technique was based on the crab's behavior, which involves the crab carrying food into its burrow. It was shown that appetizing baits impregnated with an insecticide were carried by crabs into the flooded chamber of their burrows. A field treatment of burrows was carried out by sowing insecticide impregnated baits on the ground. The treatment coverage was almost perfect and the easy implementation of the technique enabled large areas to be treated in a short time. The bait was developed by compacting various flours, which easily incorporate a large variety of insecticide formulations. Although the baits can be easily stocked, a reliable insecticide is still to be found. The results indicate that our technique could be a method of choice for treating crab burrows.

Patterns of Local Distribution of Labroides Dimidiatus in French Polynesian Atolls

Cleaning behaviour by fishes has been described to be either facultative or specialised. If being specialised in cleaning is more advantageous than to be a facultative cleaner, cleaner fishes would prefer to settle on reefs were ecological conditions promote specialisation. To test this hypothesis, we looked at the influence of physical and ecological factors on cleaner wrasse abundance, studying variations in L. dimidiatus density between 10 atolls of French Polynesia. We used a multiple regression method based on permutations of distance matrices. Our study reveals that L. dimidiatus are significantly more numerous at sites where ecological factors, such as the species richness of the fish community, may promote their cleaning activity. Moreover, they were significantly more abundant in atolls presenting a large number of sedentary fish species, few predators and few fishes living in large groups. Finally, physical factors, such as the distance between atolls, the lagoon surface, the mean percentage of hard substrate, the mean current index, and the mean depth, play a minor role in the occurrence of L. dimidiatus.

Physical, chemical, and microbiological characteristics of microbial mats (KOPARA) in the South Pacific atolls of French Polynesia

Microbial mats that develop in shallow brackish and hyposaline ponds in the rims of two French polynesian atolls (Rangiroa and Tetiaroa) were intensively investigated during the past three years. Comparative assessment of these mats (called kopara in polynesian language) showed remarkable similarities in their composition and structure. Due to the lack of iron, the color of the cyanobacterial pigments produced remained visible through the entire depth of the mats (20–40 cm depth), with alternate green, purple, and pink layers. Profiles of oxygen, sulfide, pH, and redox showed the anoxia of all mats from a depth of 2–3 mm. Analyses of bacterial pigments and bacterial lipids showed that all mats consisted of stratified layers of cyanobacteria (mainly Phormidium, Schizothrix, Scytonema) and purple and green phototrophic bacteria. The purple and green phototrophic bacteria cohabit with sulfate reducers (Desulfovibrio and Desulfobacter) and other heterotrophic bacteria. The microscopic bacterial determination emphasized the influence of salinity on the bacterial diversity, with higher diversity at low salinity, mainly for purple nonsulfur bacteria. Analyses of organic material and of exopolymers were also undertaken. Difference and similarities between mats from geomorphological, microbiological, and chemical points of view are discussed to provide multicriteria of classification of mats.Key words: microbial mats, cyanobacteria, bacterial pigments, French Polynesia atolls, exopolymers.

Physical, chemical, and microbiological characteristics of microbial mats (KOPARA) in the South Pacific atolls of French Polynesia

Microbial mats that develop in shallow brackish and hyposaline ponds in the rims of two French polynesian atolls (Rangiroa and Tetiaroa) were intensively investigated during the past three years. Comparative assessment of these mats (called kopara in polynesian language) showed remarkable similarities in their composition and structure. Due to the lack of iron, the color of the cyanobacterial pigments produced remained visible through the entire depth of the mats (20-40 cm depth), with alternate green, purple, and pink layers. Profiles of oxygen, sulfide, pH, and redox showed the anoxia of all mats from a depth of 2-3 mm. Analyses of bacterial pigments and bacterial lipids showed that all mats consisted of stratified layers of cyanobacteria (mainly Phormidium, Schizothrix, Scytonema) and purple and green phototrophic bacteria. The purple and green phototrophic bacteria cohabit with sulfate reducers (Desulfovibrio and Desulfobacter) and other heterotrophic bacteria. The microscopic bacterial determination emphasized the influence of salinity on the bacterial diversity, with higher diversity at low salinity, mainly for purple nonsulfur bacteria. Analyses of organic material and of exopolymers were also undertaken. Difference and similarities between mats from geomorphological, microbiological, and chemical points of view are discussed to provide multicriteria of classification of mats.

Jurassic lagoonal environments and quasi‐abiotic platy limestone accumulation: microbial interventions

Within the regressive sequence (eustatic sea‐level fall) that characterizes the French Southern Jura carbonate platform during the Kimmeridgian, a succession of three facies (called here units 1–3) may be observed in a palaeolagoon near Orbagnoux. Unit 1 is a shallow‐water, intensely burrowed, carbonate deposit; unit 2 (consisting of laminated, organic matter‐rich carbonate) represents a shallow lagoonal environment, prone to stromatolite development and organic matter accumulation; unit 3 consists of platy limestones almost devoid of identifiable biota. Trace element distributions indicate that depositional and early diagenetic conditions must have been reducing during deposition of unit 1 and, especially, unit 2, whereas they were much less reducing during deposition of unit 3. However, the latter does not contain body or trace fossils, with the exception of structures interpreted as calcified bacteria. The reasons why the apparently normal marine environment of unit 3 did not allow a normal biota to develop are explored and lead us to propose that: (i) unit 2 may be a Jurassic counterpart of modern kopara deposits (stromatolites of French Polynesian atolls); and (ii) unit 3 may be the result of palaeokopara reworking. This model invokes the extensive intervention of microbes in the formation of these carbonate deposits.

The elasto-plastic response of underground excavations in rock masses that satisfy the Hoek–Brown failure criterion

Abstract This paper is intended to illustrate the relationship between the Hoek–Brown parameters describing the strength of rock masses and the mechanical response of underground openings. A formulation of the elasto-plastic behavior of rock in terms of the Hoek–Brown criterion is presented. The analysis assumes that the joint system present in the rock mass has no preferred orientation so that the medium can be considered to behave as an isotropic continuum. It is shown that appropriate scaling of the Hoek–Brown parameters leads to considerable simplification in defining the elasto-plastic response of the rock mass. The classical case in which the excavation process is treated as a uniform reduction of internal pressure in symmetrically loaded cylindrical and spherical cavities is considered. Closed-form expressions are given for the extent of plastic behavior and the related stress and displacement fields. A dimensionless graphical representation of these solutions is provided that allows accurate estimates of the response of excavations in Hoek–Brown materials to be made quickly and easily. Examples are given to illustrate the use of the graphs. Illustrative applications of the derived closed-form solutions are also described. The construction of ground reaction curves for the design of cylindrical tunnels according to the convergence–confinement method and a case study of stability analysis of spherical cavities produced by underground nuclear explosions in French Polynesian atolls are discussed.

Photosynthetic picoplankton in French Polynesian atoll lagoons:estimation of taxa contribution to biomass and production by flow cytometry

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 162:57-70 (1998) - doi:10.3354/meps162057 Photosynthetic picoplankton in French Polynesian atoll lagoons: estimation of taxa contribution to biomass and production by flow cytometry L. Charpy1,*, J. Blanchot2 1ORSTOM, COM, Traverse de la Batterie des Lions, F-13007 Marseille, France 2ORSTOM, CNRS, Station Biologique, BP 74, F-29682 Roscoff, France *E-mail: lcharpy@com.univ-mrs.fr Picophytoplankton was studied by flow cytometry in 11 Tuamotu (French Polynesia) atoll lagoons and in the surrounding ocean. The respective contribution of Prochlorococcus, Synechococcus and picoeukaryotes on biomass and primary production was evaluated. Red fluorescence was demonstrated to be a proxy for chlorophyll. The relative sizes of the 3 picoplankton groups were estimated using forward light scattering measured in Takapoto lagoon on living cells. The average diameters for Prochlorococcus, Synechococcus and picoeukaryotes were estimated as 0.62 ± 0.08 (SD), 0.89 ± 0.09 and 3.11 ± 0.22 µm, respectively. The lowest values occurred before sunrise and the highest in the afternoon. Cellular carbon content was estimated using C/biovolume ratios from the literature. The average biomass for Prochlorococcus,Synechococcus and picoeukaryotes was calculated as 60 ± 20, 178 ± 52 and 4695 ± 834 fg C cell-1 respectively. Synechococcus formed the predominant group in terms of abundance and carbon biomass and had the highest planktonic primary production in most lagoons. As it is generally scarce in deep water with limited light availability, its biomass contribution was reduced in deep lagoons. Average lagoonal picoplankton abundance varied by a factor of 200 for the different populations and was affected by the geomorphology of the atolls. In very shallow lagoons, no general trend could be observed, as the dominant group appeared to depend on the water renewal rate within the lagoon. In the surface layer of the surrounding ocean the community structure was dominated by Prochlorococcus. However, the observed percentage of Synechococcus (>10%) is usual for the coastal zone. In the upper 120 m of ocean waters surrounding Takapoto, the integrated picoplankton biomass (1242 mg C m-2) consisted of 65% Prochlorococcus, 1% Synechococcus and 34% picoeukaryotes. Flow cytometry · Cyanobacteria · Prochlorococcus · Chlorophyll · Primary production · Atoll lagoons · Diurnal cell size variations Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 162. Publication date: February 12, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Texture of Microbial Sediments Revealed by Cryo-Scanning Electron Microscopy

ABSTRACT Textures of modern lacustrine stromatolites on Kiritimati (Line Islands, Central Pacific Ocean), and of buried layers in the stromatolitic carbonate sediments from French Polynesian atoll lakes (kopara), have been studied using cryo-scanning electron microscopy (SEM equipped with a freeze-drying sample preparation device). This study confirms that microscopic three-dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria, and from other extracellular polymer secretions, are common components of microbial sediments, of which they may form the framework, In addition to this role in sediment cohesion and formation of microstructure, the organic framework appears to be involved in carbonate precipitation with n the stromatolites, through chemical (nucleating), steric, and hydrodynamical controls. The role of the dead organic constituents of the stromatolites is not only confirmed in micrometer-size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments, and to the formation of peloids that evolve into spherulites. Bacterial, including nannobacterial, carbonate bodies, and carbonate-impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied. All these carbonate precipitation processes may cooperate in lithification of a given sediment.

Variation in diversity of coral reef fish between French Polynesian atolls

The diversity of coral reef fish in seven atolls in French Polynesia is analyzed with respect to geomorphological characteristics of the atolls. The results show that size of the lagoon is more important than confinement in affecting overall fish diversity. This result suggests that island biogeographic theory, as developed by MacArthur and Wilson for terrestrial animals, also applies to reef fish in that more area gives more habitat complexity which, in turn, supports higher fish diversity. However, species diversity within a given family appears to be affected more by ecological parameters, such as living coral cover, food diversity, and reproductive behavior, than geomorphological features.

Kopara in Polynesian atolls: early stages of formation of calcareous stromatolites

Widespread modern calcareous stromatolites known as kopara in the vernacular language on the Tuamotu Archipelago have been found accreting on the bottom of shallow (generally < 1 m depth) lakes located on the rims of these French Polynesian atolls. They are flat, gelatinous sediments, several tens of centimetres thick, with lamination comprised of vertically alternating red organic-rich, and white carbonate-rich, millimetre-thick laminae. They originate from benthic microbial communities mainly composed of cyanobacteria, generally dominated by the genus Phormidium. The living cyanobacteria occupy only the top 1–3 cm of the deposit, the red organic matter below being made of remnants of the dead microbes, essentially of polysaccharide microfibrils inherited from the sheaths and arranged into a three-dimensional network enclosing pores ranging from a few tens of ångstroms to a few micrometres wide. The carbonates are predominantly high-Mg calcite (9–19 mole% MgCO 3) which precipitated as micron-size bunches, within the pores, on walls of the organic network. It is sometimes accompanied by aragonite (less than 22% of the total mineral fraction when present), and lesser high-Mg calcite, allochthonous bioclasts, which occasionally form up to few centimetres-thick detrital intercalations. The pore waters are variable mixtures of freshwater (rain or ground water) and sea water from the ocean or lagoon which are feeding waters of the lakes. Their chemistry deviate from a straight mixture gradient with respect to Ca 2+, Mg 2+, and alkalinity under the influence of local calcification and bacterial processes (e.g. ammonia production, sulphate reduction) within the sediments. It is hypothesized that the calcification is initiated at carboxylic sites on the walls of the polysaccharide network, the pores acting as confining organic compartments with increased internal supersaturation. Lamination is suggested to be due to the combined effects of the stratification of the microbial activity of the deposits, and alternations of fresh and saline periods within the lakes. Lacustrine environments on oceanic atolls are found to be important habitats of modern microbialite formation.

Fine-grained carbonates in nearly closed reef environments: Mataiva and Takapoto atolls, Central Pacific Ocean

The nature, origin and mode of accumulation of fine-grained carbonate sediments have been investigated in the nearly enclosed lagoons of two French Polynesian atolls (Mataiva and Takapoto). The fine-sized fraction (< 63 μm) averages 50% of the total sediment on the lagoon floors, whereas it is replaced by sand-grained material at the vicinity of the lagoonal buildups, and along the peripheral inner talus. The silt fraction ( > 20 μm ) is substantially higher in amount (70% of the total fine sediments on average) than the mud portion and may increase volumetrically (90–99%) in some localities. Aragonite is the predominant mineral present (90% of the total calcium carbonate), the fines appear to be mainly skeletally-derived. The silt fraction includes coral and molluscan detritus, sponge chips, juvenile benthic foraminiferans and other biogenic debris. The mud is chiefly composed of aggregated nanograins, ascidian spicules, skeletal nanograins (coral-generated, mainly) and coccoliths. A minor part of the mud is certainly of non-skeletal origin. Some portion of freely deposited fines fill cavities or are attached upon sand-grain surfaces. They are associated with scattered inorganic precipitates, and contribute to the formation of micrite envelopes and aggradation of host grains. Such a process is highly significant in the total carbonate budget of the reef. The fine carbonate sediments result directly from the biological disintegration of nearby buildups, mainly through the boring activity of sponges and lithophagid molluscs. We suggest that the authigenic mud sized crystallites are controlled in their precipitation both by carbonate supersaturation and anoxic conditions seasonally affecting the interstitial sediment waters. Enclosed atoll lagoons are relatively self-governing environments of deposition which are influenced by long-term, quiescent water situations rather than by rare, catastrophic events.