MARINE ECOLOGY PROGRESS SERIES Vol. 191: 91-100.1999 Published December 30 Mar Ecol Prog Ser l Spatial and temporal patterns of settlement of the brown macroalgae Turbinaria ornata and Sargassum mangarevense in a coral reef on Tahiti V. Stiger*,C. E. Payri Laboratoire d'Ecologie Marine, Universite Franqaise du Pacifique, BP 6570 FaaaIAeroport, Tahiti, French Polynesia 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 abihty and lower dispersal distances than S. mangarevense. A model of dispersal is suggested for the 2 spe- cies 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 explana- tion of the metapopulation distribution of the 2 species. The 2 species did not release all oogonia pre- sent 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 tendgncy for fertlle thalli of T ornata to float over long &stances, combined with its oogonia stock and hgher settlement efficiencies, could account for its greater capacity to colonize new areas, as con~paredto S. mangarevense. KEY WORDS: Settlement efficiencies . Dispersal &stances . Oogonia stock . Spatial and seasonal variations . Turbinana ornata . Sargassum n~angarevense. Coral reefs . French Polynesia INTRODUCTION short-distance dispersal of germlings released by at- tached thalli, and long-distance dispersal of germlings Coral reefs provide a highly specialized habitat in released by whole or partial thalli drifting over many which benthic algae are abundant, and in many places kilometers (Morrissey 1985, Van den Hoek 1987, Kilar dominant. In French Polynesia, Turbinaria ornata and & Norris 1988, Norton 1992). The dispersal distance is Sargassum mangarevense, 2 brown macroalgae, have known to be low for fucalean algae such as species of thrived on the reefs of high islands over the past 2 Sargassum (Fletcher & Fletcher 1975, Deysher & Nor- decades (Payri & Naim 1982, Done et al. 1991). Since ton 1982, Critchley 1983, De Ruyter van Steveninck & 1985, only T. ornata has settled in the lagoons of atolls Breeman 1987, Kendrick & Walker 1991) and Fucus (Montaggioni et al. 1985, Payri & N'Yeurt 1997), de- (Menge et al. 1993). monstrating that it has the ability to disperse over long In the present study, the short-distance dispersal was distances. Some species appear to have 2 modes of dis- investigated for Turbinaria ornata and Sargassum man- persal for habitat colonization (Santelices 1990), i.e. garevense. These 2 species are able to produce and re- lease sperm and eggs throughout the year ulth minor 'Present address: Division of Biological Sciences, Graduate spatial and temporal variations (Stiger & Payri 1999). School of Science, Hokkaido University, Sapporo 060-0810, Hence they are in direct competition for space with Japan. E-mail: [email protected] other benthic organisms such as corals, which broad- O Inter-Research 1999 Resale of full article not permitted 92 Mar Ecol Prog Ser 191.91-100,1999 cast their gametes or larvae seasonally (Jardin 1994). Experimental design. Experiments were carried out The colonization of new substrata is the most funda- between December 1995 and April 1996 (hot season), mental process in the life history of attached benthic or- and in July 1996 (cold season), to test for seasonal vari- ganisms. Hence a better understanding of colonization ation in dispersal. Two experimental setups were fixed will lead to a better understanding of patch size, distrib- on the flat upper part of 2 dead coral colonies of more ution and invasive tendencies of benthic marine algae. than 4 m*, located on the internal reef flat: about 10 m Previous measurements of dispersal have not consid- (Cl) and 50 m (C2) from the margin of a channel ered the fertility of the parent plants and the losses (Fig. 1B).The setups consisted of ceramic tiles laid out incurred during dispersal, as it is difficult to recognize approximately parallel (W to E) and perpendicular the source of the germlings. Here we identify the (N to S) to the water current direction. Generally the germlings by staining them at their source and we esti- water circulation at the study site occurs from the mate dispersal directly from parent thalli using settle- ocean towards the shore (Fig. 1B);however, depending ment plates in the field. Spatial and seasonal variations on the wind regime this situation can sometimes be in settlement efficiencies and dispersal distances of reversed (Lenhardt 1991). The 2 experimental setups gerrnlings were investigated in order to propose a dis- were approximately 50 m apart (Fig. 1C) and perma- persal model for both species. The efficiency of repro- nently submerged, even during low tides, and they duction was estimated by determining: (1) the ratio of were carried out at the same times. Each experimental germlings produced to germlings released by parent setup had 4 sets of 5 settlement tiles (0.3 X 0.3 m) thalli, and (2) the losses during dispersal in relation to arranged in a cross (Fig. 2A).Each arm of the cross was the fertility of parent thalli before and after dispersal. A composed of 3 contiguous tiles located towards the better understanding of the settlement of these 2 algae middle of the setup, covering a distance of 0.9 m, and will lead to possible future management of popu- 2 tiles placed 2 m from the middle of the setup. Each lation~. tile was divided into 169 small squares to facllitate the counting of settled germlings (Fig. 2B). The roughness of the ceramic tiles is similar to coral surfaces and pro- MATERIALS AND METHODS vides an inert texture with presumably no negative impact on germling settlement. The ceramlc tiles were The study was conducted on the Taapuna barrier reef brushed and cleared of epiphytes before each experi- on the northwest coast of Tahiti, French Polynesia ment and fixed with nails on the coral colonies after all (Fig. 1A).The reef site spreads over 500 m from the reef the fucalean algae were removed around the setup. margin and consists of a relatively deep area, 2.5 m Staining of germlings. Germlings were stained with average depth, where coral colonies are well devel- Toluidine Blue while they were still attached to parent oped and scattered amongst coralline sand accumula- thalli, as described in Kendrick & Walker (1991).Stiger tions, as described previously by Stiger & Payri (1999). (1997) has shown that staining has no effect on release and dispersal of germlings. In order to recover enough germlings following staining, approximately 70 to 80 fe- male thalli in Turbinaria ornata and 80 to 120 mature laterals in Sargassum mangarevense were stained (Table 1). All thalli were of similar mean size (19 to 20 cm for the 2 species), and over Nonh 50°& of the thalli had visible oogonia attached at the surface of the concep- tacles. Staining was done in th.e field in a 1.arge bin of 0.05 % Toluidine Blue Shore in seawater for 1 to 2 h. Stained thalli were then tied to stakes with soft wires and fixed in the center part of each South experimental setup. Four to 5 d later, the settlement plates were collected, I) Watcr current and stained germlings were counted Fig. 1. The study site. (A) Map of Tahiti island. (B) Localization of the study area within each grid of the (Fig. 2B) on, the inner barrier reef. (C) Disposition of the 2 setups on the study area. and mapped ln a matrix of coordinates RC.reef crest. BR: barrier reef. C1 and C2 are the 2 setups in the field. Stiger & Payri: Settlement patterns of brown macroalgae 93 Table 1 Number of individuals collected at each sampllng Table 2. Distances and sampling areas used for the measure- date in Turbinaria ornata and Sargassum mangarevense ment of dispersal distance in Turblnana ornata and Sargas- sum mangarevense Species Date of Number of sampling individuals Turbinaria ornata Sargassum mangarevense Distance Sampling area Distance Sampling area Turbjnafia ornata Dec 1995 7 0 (m) (mZ) (m) (mZ) Feb 1996 80 Jul 1996 80 0.046 0.0139 0.092 0.0139 Sargassurn rnangarevense Mar 1996 80 0.162 0.0208 Apr 1996 100 0.254 0.0277 Ju1 1996 120 0.485 0.0692 0.900 0.1246 2.000 0.1800 Settlement and dispersal patterns. Settlement effi- ciency and dispersal distances were investigated in Turbinaria ornata and Sargassum mangarevense in Within each stratum, the mean number of germlings separate experiments. Settlement efficiency was mea- was determined, and for the 4 directions the same sured as the number of stained germlings attached to strata were considered throughout the study for a the ceramic tiles per unit area (m2)and divided by the given species to carry out all statistical comparisons.