Méthodes d’estimation des surfaces développées de coraux scléractiniaires à l’échelle d’une colonie ou d’un peuplement

Abstract

Methods to estimate actual surface areas of scleractinian coral at the colony- and community-scale. This study presents an original method for the determination of the actual surface area of Scleractinian corals at colony-scale and at the scale of a whole reef. Firstly, at colony-scale, the technique consists of dipping coral colonies in a liquid paraffin bath and measuring the weight of the coral before and after the dipping. It is based on the relationship between the weight and the surface area of paraffin deposited and allows one to obtain the actual surface area of coral colonies in the laboratory. The main advantages of this method compared to others is that it uses easily obtainable materials and it is adaptable to all colony shapes. It was used on coral colonies representing the six most frequently seen colony shapes on the reefs of the island of Moorea (French Polynesia). It is shown that, within a wide range of sizes of any given shape, the relationship between vertical projection surface area and actual surface area is linear for most of the colony shapes. The shapes tested were: branching ( Synaraea rus), encrusting ( Montipora aequituberculata), massive ( Porites lobata), sub-massive ( Pocillopora verrucosa), mushroom ( Fungia scutaria) and tabular ( Acropora hyacinthus). Conversion indices for each shape (vertical projection surface area versus actual surface area) were determined from this preliminary study. Secondly, to estimate the actual surface area of coral colonies at the scale of the reef for nine different coral communities, the conversion indices determined above were used to convert vertical projection percent cover to actual surface area. Because of differences in shape composition among the studied coral communities, differences between communities in classical planar percentage cover are not proportional to differences observed in actual coral surface area. These results show that actual surface area may provide a more appropriate measure than planar percentage cover for future quantitative studies of coral communities. © 2000 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS actual surface area / coral colony / coral reef