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WHAT FACTORS FAVOUR THE ECOLOGICAL DIVERSITY OBSERVED IN ATOLL LAGOONS ?

April 14, 1999

The first step in the investigation was the mapping by remote sensing techniques of more than two-thirds of the 76 atolls of the Tuamotu archipelago. High resolution maps, which did not exist before, were then established. Using the cartographic data thus gleaned alongside morphometric parameters picked out (size and shape of the lagoons, degree of communication with the open ocean, the direction of communicating passages in relation to the prevailing winds), 14 representative atolls were selected. They included the celebrated pearl-oyster atoll Takapotot and Tikehau, known for its abundance in fish. Water samples taken underwent chemical analysis and biological examination, and the researchers made biomass determinations and an inventory of pelagic species (fish) and benthic ones (organisms inhabiting the lagoon floor, such as corals, algae, molluscs and crustaceans).
Five years of research and six prospection campaigns have revealed that there can be very wide differences between lagoons. For example the richest biologically produce 40 times as much phytoplankton per unit volume as the poorest. This diversity is partly linked to the amount of communication existing between lagoon and ocean. The more the atoll is shut off from the ocean, the greater is its biomass in phytoplankton or bivalve molluscs (especially oysters). One reason, among others, is that oceanic waters, which are biologically poor, enter a lagoon they dilute and wash through the water found there. Conversely, among corals diversity and abundance are greater in lagoons well exposed to the ocean. Corals need light and naturally prefer transparent water with little suspended matter.
The study has moreover indicated that both the number of species and the biomass of fish increase with the size of the lagoon. Nearly 300 000 fish have been counted during research diving trips in 14 lagoons; but among the 300 species represented, 28 make up threequarters of the biomass. Furthermore, inside a given lagoon some habitats appear richer than others. Channels and reef knolls (coral 'pies' in Polynesian) thus harbour the most fish. The littoral zones on the West (unprotected from prevailing winds) are also some of the richest habitats.
The results of this research point to the main parameters which in their different proportions create the great diversity of ecological conditions found among atolls. Such findings should help in the management of the different sites, particularly fragile and threatened today by uncontrolled development. The data should assist those responsible in choosing the most suitable usage for each type of atoll: such as pearl culture, fishing, tourism or as a nature reserve. The forthcoming creation of a reference data bank and a CD-Rom which gives describes the research will make the results available for specialist scientists and a wide public who might be concerned: teachers and students, managers in this domain and decision makers.

Typatoll forms part of the National Programme of Research on COral Ecosystems) (PNCRO). Coordinated by IRD, it has brought into association 32 scientists belonging to the Marine Resources Service of Polynesia, the Ecole Pratique des Hautes Etudes (EPHE), UFP (French Univesity of the Pacific), IFREMER, the CNRS and scientists from the Universities of San Francisco and Laval (Canada).

FOR FURTHER INFORMATION

CONTACT : Philippe Dufour, IRD, Station marine d'Endoume, rue de la Batterie-des-Lions, 13007 Marseille, tél. (33) 4 50 75 58 71 ou 04 91 04 16 14, fax : (33) 4 50 75 34 39, e-mail : dufour@orstom.rio.net

Bibliographie :
- L. Charpy and J. Blanchot "Photosynthetic picoplankton in French Polynesia atoll lagoons: estimation of taxa contribution to biomass and production by flow cytometry" Marine Ecology Progress Series, 162 : 57-70, 1998
- P. Dufour and B. Berland, "Nutrient control of phytoplanktonic biomass in atoll lagoons and Pacific ocean waters: studies with factorial enrichment bioassays" Journal of Experimental Marine Biology & Ecology 234(2):147-166, 1999.
- M. Kulbicki "How acquired behaviour of commercial reef fish may influence results obtained from visual censuses" Journal of Experimental. Marine Biology & Ecology, 222: 11-30, 1998.
- C.E Payri and A.R. DeN'Yeurt " A revised checklist of polynesian benthic marine algae" Australian Systematic Botany, 10 (6) : 867-910, 1998.

Pour obtenir des illustrations de ces recherches, contacter : Indigo Base, Banque d'images de l'IRD, Claire Lissalde, tél. 33 1 48 03 78 99, e-mail : lissalde@paris.ird.fr


Institut de Recherche pour le Développement, Paris (IRD)