Physical and chemical control of the phytoplankton of Ahe lagoon, French Polynesia

► Ahe lagoon was oligotrophic. ► Phytoplankton growth is mainly constrained by nitrogen availability. ► Phytoplankton is dominated by picoplankton. ► Spatial distribution is heterogeneous while vertical distribution is homogeneous. ► Impact of pearl oyster aquaculture does not appear onto phytoplankton biomass. The environmental characteristics of Ahe deep lagoon (Tuamotu Archipelago, French Polynesia) were studied over 3years with the aim of explaining the spatial and temporal variability of the natural food available for pearl oysters with a special focus on phytoplankton biomass and global photosynthesis/respiration ratio of the lagoon. Chlorophyll averaged 0.34±0.01μgL−1 and our findings did not confirm increased phytoplankton biomass in deep lagoonal waters. Phytoplankton production appears to be limited firstly by nitrogen and respiratory processes overpass photosynthetic processes at least in the north-eastern edge of the atoll. Grazing by pearl oysters in culture seems to decrease the POC concentration but not the phytoplankton biomass. Oysters graze mainly on non chlorophyllian particles.