Spatial and temporal patterns of settlement of the brown macroalgae Turbinaria ornata and Sargassum mangarevense in a coral reef on Tahiti

Abstract

Turbinaria ornata and Sargassum mangarevense are the most conspicuous macroalgal species that grow on the reefs of Tahiti and other high islands of French Polynesia. This study reports on a quantitative investigation of spatial and seasonal settlement efficiencies and dispersal distances for these 2 species in a coral reef habitat on Tahiti. Settlement patterns of germlings were observed in situ on settlement plates placed around parental thalli during 2 seasons (hot and cold). For both species, the dispersal of germlings was limited to within 90 cm of the parental thalli, and the greatest number of settled germlings was observed during the cold season. T. ornata showed a higher attachment ability and lower dispersal distances than S. mangarevense. A model of dispersal is suggested for the 2 species showing a decrease in germling number with distance from parental thalli. Dispersal of germlings appears to be influenced by the dominant current during their release. This short-distance dispersal allows rapid establishment and maintenance of local populations, and is consistent with the explanation of the metapopulation distribution of the 2 species. The 2 species did not release all oogonia present in their reproductive structures. T. ornata had a stock of oogonia that varied seasonally (with low amounts during the hot season), whereas there was no seasonal variation in the oogonia stock of S. mangarevense. Recently, isolated atolls have been settled by T ornata but not by S. mangarevense. It is proposed that the tendency for fertile thalli of T. ornata to float over long distances, combined with its oogonia stock and higher settlement efficiencies, could account for its greater capacity to colonize new areas, as compared to S. mangarevense.