Fournier et al. (2012a) investigated in Ahe Atoll the influence of natural plankton concentration on maturation and spawning of P. margaritifera, during a 4 months survey. Plankton concentration (chlorophyll a) and microscope counts were compared with oysters reproduction activity, measured with gonadic index, gonado-visceral dry weights and histology. Fournier find more et al. (2012a) concluded that gametogenesis rate was mainly related to plankton concentration and that spawning occurred when maximal gonad storage was reached. The main spawning synchronizing factor was plankton concentration. Understanding
at least the chlorophyll spatio-temporal variations are thus a priority for predicting the timing of spawning. In their sampling stations, Fournier Epacadostat cost et al. (2012a) reported that plankton concentration fluctuations were mainly related to the wind regime, and to the overturning circulation and upwelling effects described by Dumas et al. (2012). The hydrology of the lagoon was characterized during the larval experiments (Thomas et al., 2010), during the hydrodynamic surveys (Dumas et al., 2012) and during the plankton surveys (Charpy et al., 2012). Because different depth limits and stations were considered, and because of the fairly high wind regime experienced during each field period, conclusions were not always in agreement between studies in terms of
stratification. Neither Charpy et al. (2012) and Thomas
et al. (2010) reported stratification for any of their campaigns. However, according to Dumas et al. (2012), slight thermal and salinity stratifications can occur. The general overturning circulation evidenced by Dumas et al. is likely to be responsible for the mixing of the lagoon water body. In light to medium wind conditions, the overturning circulation weakens, allowing the development of a slight vertical stratification. In more intense wind, the circulation HSP90 is strong enough to prevent stratification, by upwelling to windward of the bottom cold water and downwelling to leeward of the surface warm water. Charpy et al. (2012) reported on the general hydrologic characteristics of the lagoon, and compared them to previously studied atolls. The vertical and spatial distribution observed on phytoplankton biomass (extracted chlorophyll) in Ahe was fairly homogeneous, with a significant increase in the southwest of the lagoon under windy conditions. Phytoplankton biomass was also in the same range as other atoll lagoons, as well as nutrient concentrations. Nitrogen is probably the first limiting factor for phytoplankton production (DIN: P ratio <3) but N-enrichment by benthic N2-fixing cyanobacteria needs to be precisely investigated. The benthic interface was assumed to deliver only up to 28% of the nitrogen phytoplankton demanded. Lefebvre et al.