AbstractMany coral reef ecosystems have undergone profound ecological changes over the past decades leading sometimes to a shift from coral to macroalgal-dominated areas. In Martinique (Caribbean region), the proliferation of macroalgae is an important phenomenon. Coral reef resilience, involving reef building species recruitment, might be modified by macroalgal presence. This work aimed at understanding reef recruitment processes in areas dominated either by macroalgae, coral or intermediate, based on scuba diving observations, manipulative experiments and laboratory studies. Particular attention was given to the physical and chemical effects of Sargassum (one of the most represented species: 100-200 g.m-2 (wet weight) in algal beds) on benthic invertebrates’ larvae recruitment. Further experiments focused on the effects of surface molecules and of the waterborne cues produced by Sargassum polyceratium on the development of marine invertebrates’ embryos.
This study demonstrated that juvenile coral diversity and density vary between the considered habitat types (i.e: dominated by algae, coral or intermediate with numerous sea urchins). It was low in algal areas (0.9-1.4 recruit.m-2) as compared to coral ones (7-8 recruit.m-2) and intermediate in urchin zones (2-3.2 recruit.m-2). Moreover, species recruiting differed according to their reproductive mode. Brooders recruited more in coral areas, which suggested that they settled in the vicinity of their parent colonies. Settlement and recruitment experiments demonstrated the barrier effect of Sargassum species on settlement but no allelochemical impacts could be identified in situ. However, the laboratory based experiments demonstrated that S. polyceratium surface molecules were active against the early stages of development of Arenicola brasiliensis (annelid), Codakia orbicularis (bivalvia) and Diadema antillarum (sea urchin, a reef key stone species) (LC50 between 25 and 51 μg.mL-1).
These results give insight into the coral recruitment capacities in several habitats, which is of major importance for reef managers.
|Date of Award||2012|
|Supervisor||Claire Hellio (Supervisor) & Jean-Philippe Marechal (Supervisor)|