Pico- and nanoplankton biomass and production in the two largest atoll lagoons of French Polynesia

Pico- and nanoplankton concentrations were measured in the lagoonal waters of the 2 largest atolls of French Polynesia (Fakarava and Rangiroa). Growth, production and grazing rates were estimated using diffusion chambers near the reef flat spillways and near the main channels of the atolls. The overall microbial biomass was dominated by picoplankton, with very high abundances of bacteria and cyanobacteria, these 2 groups representing each 20 to 50% of the total carbon. Nanoplankton (auto- and heterotrophic flagellates) constituted only 10 to 15% of the total biomass. Microbial concentrations were 1.5 to 3-fold lower near the reef flat spillways than near the channels. This suggests an important biomass production inside the lagoon. At Rangiroa, growth rates varied from 0.02 to 0.06 h(-1) for bacteria and from 0.01 to 0.04 h(-1) for the other groups (cyanobacteria, auto- and heterotrophic flagellates). At Fakarava, growth rates were in the same range except for the heterotrophic flagellates (0.05 to 0.17 h(-1)). Growth rates were significantly higher near the reef flat spillways than near the channels. More than 50% of both bacterial and cyanobacterial production was grazed by the higher trophic levels in both atolls. Bacterial production was enhanced by a nitrogen enrichment whereas production of cyanobacteria and flagellates was enhanced by both a nitrogen and a phosphorus enrichment.