HETEROGENEITY OF ZOOXANTHELLAE DENSITY IN THE CORAL ACROPORA GLOBICEPS AROUND MOOREA (FRENCH POLYNESIA)

Understanding the natural heterogeneity of coral zooxanthellae density appears as very important to understand variations in coral survival due to bleaching (the loss of these micro-algal symbionts). Heterogeneity of coral bleaching at different observation scales (within a colony, among neighbouring colonies of the same species or on a wider scale) remains largely misunderstood. The present work explores intracolonial, spatial and temporal variations of zooxanthellae density in the coral Acropora globiceps Dana 1846, over a period of three months on the forereef of Moorea, French Polynesia. In our study, intracolonial zooxanthellae densities did not vary significantly. However, zooxanthellae densities differed slightly between inner and outer branches but this trend was not significant at 6, 12 and 18 meters depth. On a wider scale, zooxanthellae densities also vary spatially : a positive correlation was observed between depth and symbiont density (density increases when light intensity decreases, so according to depth). Moreover, the location of colonies exposed to different hydrodynamical conditions was not a parameter controling the concentration of zooxanthellae. Finally, the temporal variation of zooxanthellae densities did not show significant variations even if it decreased slightly over the study period. The results of this study highlight the importance of accounting for variations within and among colonies to determine zooxanthellae densities and to assess the evolution of zooxanthellae populations

High spatial variability in coral bleaching around Moorea (French Polynesia): patterns across locations and water depths

Mass coral bleaching events are one of the main threats to coral reefs. A severe bleaching event impacted Moorea, French Polynesia, between March and July 2002, causing 55+/-14% of colonies to suffer bleaching around the island. However, bleaching varied significantly across coral genera, locations, and as a function of water depth, with a bleaching level as high as 72% at some stations. Corals in deeper water bleached at a higher rate than those in shallow water, and the north coast was more impacted than the west coast. The relatively small scale of variability in bleaching responses probably resulted from the interaction between extrinsic factors, including hydrodynamic condition, and intrinsic factors, such as differential adaptation of the coral/algal association

Maintien des populations de coraux Scléractiniaires en milieu insulaire fragmenté (archipel de la Société, Polynésie française) : influence du recrutement et de la mortalité post-fixation

To improve our understanding of maintenance processes of Scleractinian coral assemblages in an insular fragmented system, the influence of recruitment fluctuations and post-settlement mortality on spatial patterns of juvenile and adult assemblages was investigated. Spatio-temporal variability of recruitment was characterized at an insular scale around Moorea over five years and at a regional scale in the Society archipelago (French Polynesia) over one year. This technique enabled a muti-scale spatio-temporal analysis of the data. Benthic mortality of recruits and juveniles was also quantified around Moorea and the major factors causing mortality were identified. Our results emphasize the importance of post-recruitment events, particularly those occurring during the first weeks after settlement, in structuring and maintaining adult assemblages, regardless of the spatio-temporal scales considered. However, for some taxa, spatial patterns of adult populations around Moorea are mainly driven by spatial variability of recruitment during previous years. These results illustrate the importance of life history traits in mechanisms of population maintenance. Around Moorea, recruit mortality is particularly high (50 % in 7 days), and is notably linked with predation by Scaridae and Balistidae fishes (especially Scarus psittacus, Chlorurus sordidus, and Melichthys vidua), and competition by encrusting organisms. Moreover, at two of the three sites studied, spatial variability of early post-settlement mortality convincingly explains differences observed between recruit and juvenile spatial patterns. Juvenile mortality is less intense (40 % in 14 months), but also shows strong spatial variability, linked with the abundance spatial variability of Chaetodontid fishes (especially Chaetodon pelewensis) and with living coral cover. This research highlights the preponderance of post-recruitment events in structuring adult populations and assemblages, and the importance of events occurring during the first weeks of benthic life (recruit stage), particularly mortality due to biotic interactions (predation and competition).De façon à améliorer la compréhension des mécanismes de maintien des populations de Scléractiniaires en milieu insulaire fragmenté, l'influence des variations du recrutement, d'une part, et de la mortalité post-recrutement, d'autre part, sur la structure des peuplements juvéniles et adultes a été explorée. Pour cela, dans un premier temps, la variabilité spatio-temporelle du recrutement a été caractérisée à l'échelle insulaire sur 5 ans autour de Moorea et à l'échelle régionale sur 1 an dans l'archipel de la Société (Polynésie française) de façon à mener des analyses spatio-temporelles à multi-échelles. Dans un deuxième temps, la mortalité benthique des stades recrue et juvénile a été quantifiée autour de Moorea, et les principaux facteurs de mortalité identifiés. Nos résultats montrent l'importance des événements post-recrutement dans la structuration et le maintien des peuplements de Scléractiniaires, quelles que soient les échelles spatio-temporelles considérées. Cependant, pour certains taxons, la structure spatiale des populations adultes à Moorea semble majoritairement gouvernée par la variabilité spatiale du recrutement sur plusieurs années, illustrant l'implication des différences de traits d'histoire de vie dans les mécanismes de maintien des populations. Autour de Moorea, la mortalité des recrues est particulièrement élevée (50 % en 7 jours), et notamment liée à la prédation par les poissons des familles Scaridae et Balistidae (en particulier les espèces Scarus psittacus, Chlorurus sordidus et Melichthys vidua), et à la compétition avec les autres organismes encroûtants. En outre, la variabilité spatiale de cette mortalité post-fixation précoce explique de façon convaincante les différences observées entre la structure spatiale des recrues et celle des juvéniles, pour deux des trois sites étudiés. La mortalité des juvéniles est de moindre intensité (40 % en 14 mois), mais présente également une variabilité spatiale marquée, notamment en lien avec les variations de l'abondance des poissons de la famille Chaetodontidae (Chaetodon pelewensis) et avec le recouvrement en coraux vivants. Cette étude souligne ainsi la prépondérance des événements post-recrutement dans la structuration des populations et des peuplements adultes, et l'importance des événements ayant lieu au cours des premières semaines de la vie benthique (stade recrue), parmi lesquels la mortalité liée aux interactions biotiques (prédation et compétition) semble particulièrement importante

Maintien des populations de coraux Scléractiniaires en milieu insulaire fragmenté (archipel de la société, Polynésie française) (influence du recrutement et de la mortalité post-fixation)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Response of coral assemblages to thermal stress: Are spatial and taxonomic patterns in bleaching consistent between events?

International audienceMass bleaching events resulting in coral mortality are among the greatest threats to coral reefs, and are projected to increase in frequency and intensity with global warming. Achieving a better understanding of the consistency of the response of coral assemblages to thermal stress, both spatially and temporally, is essential to determine which reefs are more able to tolerate climate change. We compared variations in spatial and taxonomic patterns between two bleaching events at the scale of an island (Moorea Island, French Polynesia). Despite similar thermal stress and light conditions, bleaching intensity was significantly lower in 2007 (approximately 37 % of colonies showed signs of bleaching) than in 2002, when 55 % of the colonies bleached. Variations in the spatial patterns of bleaching intensity were consistent between the two events. Among nine sampling stations at three locations and three depths, the stations at which the bleaching response was lowest in 2002 were those that showed the lowest levels of bleaching in 2007. The taxonomic patterns of susceptibility to bleaching were also consistent between the two events. These findings have important implications for conservation because they indicate that corals are capable of acclimatization and/or adaptation and that, even at small spatial scales, some areas are consistently more susceptible to bleaching than others

Spatial Patterns and Short-term Changes of Coral Assemblages Along a Cross-shelf Gradient in the Southwestern Lagoon of New Caledonia

Coral reef assemblages generally form gradients of spatial structures which are governed by a variety of interacting physical and biological processes that vary in intensity, frequency, and spatial scale. Assessing the structure of contemporary reef assemblages may help to understand future changes and to identify appropriate conservation actions. The spatial distribution and interannual variability (from 2006 to 2008) of coral assemblages were investigated at 10 stations in the southwestern lagoon of New Caledonia, and the strength of the cross-shelf gradient was evaluated. Coral cover, generic richness, and abundance of adult and juvenile assemblages were highly variable within and among the three major reef habitats (fringing, mid-shelf, and barrier reefs). Abundance increased with distance from shore, whereas generic richness and cover were not correlated with shelf position. Assemblage composition was generally related to habitat, even though some mid-shelf and fringing reef assemblages resembled those observed on other habitats. A significant correlation between juvenile and adult distributions was recorded, suggesting that adult assemblages are partly controlled by the short-term history of recruitment patterns. The interannual variation of coral assemblages was far less pronounced, with significant changes only detected at some mid-shelf and barrier reefs, for a few genera characterised by high turn-over

Spatial distribution of juvenile and adult corals around Moorea (French Polynesia): implications for population regulation

Spatial patterns of juvenile and adult scleractinian corals were analyzed and compared at Moorea, French Polynesia, in order to better understand the processes that influence coral population dynamics and maintenance. Variation in the abundance and composition of juvenile (< 5 cm in diameter) vs adult corals were examined at three depths (6, 12, 18 m) at each of three locations (Vaipahu, Tiahura, Haapiti) on the outer reef slope. A marked spatial heterogeneity for both juvenile and adult assemblages was found with significant variation across depths and locations. Trends in spatial variation were highly similar between juvenile and adult assemblages, and the overall abundance (all genera pooled) of juvenile corals was positively and highly correlated with the abundance of adults. However, within the six dominant genera, which are all broadcast spawners, we found positive and significant correlations between adult and juvenile abundance for only Acropora, Fungia, Montipora, and Pavona, and not for Pocillopora and Porites. These findings suggest that underlying mechanisms for the establishment of the adult patterns may vary among coral taxa, even among broadcast spawners