Contenu du sommaire : Journal de la Société des océanistes, n°62, tome 35, 1979.

L'environnement de l'Atoll de Takapoto-Tuamotu

• Avant-propos. Recherches sur l'atoll de Takapoto (Tuamotu, Polynésie française) - Bernard Salvat p. 5-7
• Géomorphologie de l'atoll de Takapoto - Jean-Pierre Chevalier, Michel Denizot, Michel Ricard, Bernard Salvat, Alain Sournia, Pierre Vasseur p. 9-18
  This study covers the different parts of the atoll: atoll rim (outer reef flat, elevated reef, inner reef flat) and lagoon. The rim is an almost continuous island, made of an old reef conglomerate which emerges of 50 cm above the sea level (low tide) and on which recent and gravel formations are deposited; Beachrocks appear in some areas. The outer reef is constituted by a reef flat, an algal ridge which is crossed by many grooves, and an outer slope. The appearance of the edge of the reef differs according to the reef position with regard to the winds. There are very few functional channels (hoa) between the ocean and the lagoon, all situated near Teavatika, or Takaï. The inner reef flat often shows gravel or sandy accumulations more or less covering the old reef conglomerate. In the center of the lagoon, there are many patch reefs, surface-rising patches or knolls, which were built when the sea-level was higher than it is now. To day, building fauna is scarce.
• La vie animale terrestre à Takapoto - François Riviere p. 19-29
  The author puts together the data and reports from different specialists who studied, in relation to the UNESCO MAB project "Man and Biosphere", theme 7, the terrestrial biota of the atoll of Takapoto (Tuamotu archipelago, French Polynesia). In order to describe the atoll biota, the author makes out five different habitats, and comments upon the food-cycle structures that control the relations between flora and fauna. Although the determination of all the available species is not yet over as far as entomology is concerned, it is most likely that the animals of Takapoto had a "pioneer" action. This fauna settled very recently and has an universal origin. This is in perfect harmony with the theory of the re-emerging of the Tuamotu atolls. The animal and vegetal species are now spreading over Takapoto, mainly from the Australian and Asian regions, most often via Tahiti. However, due to the fast-developing continent to continent means of conveyance in the Pacific, the latest species that have reached the island may originate from farther countries. But for birds perhaps, the speciation does not seem, up to now, to have developed into new species adapted to the archipelago as it is the case for neighbour high islands like Tahiti or the Marquesas. The inhabitants of Takapoto can only live on and hold out by fulfilling their needs mainly with sea and lagoon products rather than very poor terrestrial ressources. Joining in the modern economic systems, the Paumotu — i.e. the inhabitant of the Tuamotu archipelago — will meet a difficult issue as for the ecology of those South Pacific atolls. The ecosystems of the Tuamotu archipelago enjoy a stable equilibrium, but they are far more fragile than any other because of their very poor ecological diversity.
• Les organismes constructeurs de l'atoll de Takapoto - Jean-Pierre Chevalier, Michel Denizot p. 30-34
  On the outer slope and on the margin of the reef, there are few differences between the reef builders of Takapoto and those of the other atolls. The coral fauna is more abundant on the west coast (leeward-side), compared to what is located on the east coast (windward-side). On the other hand, in the lagoon, the coral fauna is poor (14 genus — mainly Porites) and the populations are heterogeneous, the southern coral patches being most prosperous. The vegetable reef builders are abundant and belong to four main groups : the bulky calcareous algae (Porolithon, Chevaliericrusta), the algae sea- lawns (Jania), the consolidating algae and the Cyanophycae of the bottom. Compared to the corals, the calcareous algae enjoy a particular importance in the building process, both on the reef front and in the lagoon. That comes from the small number of functionnal channels between the lagoon and the ocean; as a result, the renewal of water is slow and the salinity is high.
• Caractéristiques générales du milieu liquide lagonaire de l'atoll de Takapoto - Francis Rougerie p. 35-45
  As the lagoon of the Takapoto atoll is not linked with the ocean, its waters have different physicochemical properties from those of the Pacific ocean. The local hydroclimatical measures since 1974, indicate that the important variation of the salinity of the lagoon waters is in close relation to the precipitation-evaporation ratio; the inflow of oceanic water by some shallow passes (hoa) restitutes the level of the lagoon sporadically, but the lagoon — ocean transfers are practically negligible. The atoll being in an area of strong evaporation, the excess of salt is evacuated through the porous coral mass by a liquid junction as explained in the proposed diagram.
• Le plancton du lagon de Takapoto - Michel Ricard, Jean-Alfred Gueredrat, Yves Magnier, Jean-Pierre Renon, Jean-Pierre Rochette, Francis Rougerie, Alain Sournia, Bruno Wauthy p. 47-57
  The plankton of the Takapoto lagoon has been investigated and the collected data made it possible to point out some peculiarities which underline the specificity of this ecosystem. PHYTOPLANKTON : in spite of the stability of the nutritive and physical conditions of the lagoon, the distribution of the phytoplankton biomass is heterogeneous. The chlorophyll a concentrations range from 0.07 to 0.84 mg/m3 and are characterized by a very distinct vertical stratification and a very variable horizontal repartition. The biomass is more important during the southern summer than during the winter. Qualitatively, diatoms predominate with the number of taxa but dinoflagellates are the most numerous. ZOOPLANKTON : during the day the zooplankton of the lagoon predominates in the deep layers but, at sunset, there is a quick twilight superficial maximum which results from the rising of the deep plankton. As for the phytoplankton, the distribution of the zooplankton is heterogeneous and it seems to be, partly, determined by winds. Only a few species constitute the essential of the biomass: the main constituants are chaetognaths and copepods for holoplankton, crustacean larvae, mollusc larvae and fish-eggs for meroplankton. The zooplankton of the lagoon is most abundant at the end of spring and during the southern summer. The Takapoto lagoon appears to be a virtually closed ecosystem where great autonomy is possible owing to particular mechanisms : acceleration of mineralisation processes, quick turnover, shortenings of the food-chain.
• Mollusques et faune benthique du lagon de Takapoto - Georges Richard, Bernard Salvat, Olivier Millous p. 59-68
  At least 93 species of Molluscs (58 Gastropods, 34 Bivalves, 1 Cephalopod) exist within the closed lagoon of the Takapoto atoll. The distribution by biotop and by bathymetric level is studied for the most abundant species ; they are Area ventricosa, Chama imbricata and Tridacna maxima for the endofauna, Rhinoclavis fasciatus, Vexillum cadaverosum, Fragum fragum and Lioconcha philippinarum for the epifauna. The Tridacna (14 million individuals — 530 tons of living matter) and the Area (38,5 millions — 340 tons) constitute most of the exemples of the malacological fauna, but the Mother-of-pearl, Pinctada margaritifera, intensively exploited in French Polynesia, is also the subject of quantitative estimates and breeding tests. Along with Molluscs, Corals (mainly Porites, Millepora, Montipora), Echinoderms (Halodeima atra) and Crustaceas (Brachiurs) represent a significant amount of the marine Invertebrates, while the sciaphil fauna (a few species of Sponges) is scarce compared to what is located on the outer slope. In this case we have a fauna typical of a closed atoll, where the uniformity and proliferation of 2 or 3 species in each zoological phylum is opposed to heterogeneousness and wealth of species found in open lagoons like Rangiroa and Mururoa.
• Poissons de Takapoto - Raymond Bagnis, René Galzin, Jack Bennett p. 69-74
  In spite of a geomorphological facies of closed atoll, Takapoto has an ichthyological fauna relatively rich, when compared to some other French Polynesia atolls. Approximatively 170 fish species belonging to 39 families have been counted in the lagoon. This fauna is generally rather poor in individuals, except for some species. Amongst them, three parrotfishes (Scarus harid, S. ghobban, S. sordidus), three mullets (Crenimugil crenilabis, Chelon vaigiensis, Mugil cephalus), three butterflyfishes (Chaetodon auriga, C. ulietensis, C. lunula), two damselfishes (Pomacentrus pavo, abudefduf coelestinus) and a remora (Echeneis naucrates) were very common. More as in the Scilly's lagoon, it has been noted the presence of a relatively large population of Lethrinus miniatus, original feature for a closed atoll. It seems that seasonal variations in the fish density and distribution can occur, changing the above pattern. There was apparently no ciguatera in the lagoon at the survey moment. On the other hand, some species fished outside the barrier reef, including primarily parrotfish, snappers, groupers and jacks, can be toxic for man, as confirmed by biochemical studies.