Spatial distribution patterns of phytoplankton biomass and primary productivity in six coral atolls in the central South China Sea

Nutrients’ concentration, phytoplankton biomass, and primary productivity were investigated in six coral atolls in the central South China Sea during the southwest monsoon in the early summer. Nutrients’ concentrations were generally very low in these atolls, and showed no obvious variation among outer reef slope, reef flat, and lagoon. The spatial variation of phytoplankton biomass and primary productivity were significant among the different regions of the atolls, and generally followed the order of outer reef slope < reef flat < lagoon. Phytoplankton biomass might be a more sensitive indicator than nutrients for the habitat differences in coral atoll ecosystem. The lagoon of Huangyan Atoll showed the highest phytoplankton biomass and primary productivity. The phytoplankton biomass and primary productivity were mainly contributed by pico-phytoplankton. Moreover, our results suggested that there might have a threshold value of Chl a for pico-phytoplankton abundance. When Chl a was higher than this threshold value (about 0.5 µg L−1), the pico-phytoplankton abundance should not linearly increase with the Chl a concentration. Due to the significantly high-phytoplankton biomass and primary productivity, the anthropogenic eutrophication should be paid attention in the lagoon of Huangyan Atoll.