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Nutrient concentration and phytoplankton biomass were investigated in Huangyan Atoll in May 2015. The concentrations of nutrients were very low, and dissolved inorganic nitrogen was composed mainly of ammonia. Nitrogen likely was the primary limiting factor for phytoplankton growth. The spatial variation of phytoplankton biomass was significant among the lagoon, reef flats, and outer reef slopes. Extremely high chlorophyll a concentration and micro-phytoplankton abundance were found in the region near the lagoon mouth. This high phytoplankton biomass might be due to nutrient input from fishing vessels and phytoplankton aggregation driven by the southwestern wind. Our results indicate that phytoplankton biomass could be a reliable indicator of habitat differences in this coral reef ecosystem, and micro-phytoplankton seems to be more sensitive to nutrient input than pico-phytoplankton., (Copyright © 2016. Published by Elsevier Ltd.)

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. [ABSTRACT FROM AUTHOR]

We investigated the distribution of several picophytoplankton groups (Prochlorococcus, Synechococcus, and picoeukaryotes) in relation to a warm eddy in the northern South China Sea in summer 2012. An anticydonic eddy centered on 117°E longitude was identified during the sampling period using satellite data and hydrologic mapping. The layer of maximum Prochlorococcus and Synechococcus abundance within the eddy core dropped from 50 to 75 m, which was consistent with the subsurface chlorophyll a maximum. The water-column integrated abundance of Prochlorococcus, Synechococcus, and picoeukaryotes in the eddy core (9.67±0.23,1.56±0.04, and 0.28±0.01 x 103 cells ml-1, respectively) was significantly lower (P<0.05) than that of the reference stations (25.10±2.32,2.71±0.63, and 0.92±0.15 x 103 cells ml-1, respectively), and the abundance of Prochlorococcus in the core was also significantly lower than that at eddy edges (15.75±1.78 x 103 cells ml-1). However, there were no differences in the water-column integrated Chi a between the eddy core and edge. Our findings show that the warm eddy led to the reduced picophytoplankton abundance, especially of Prochlorococcus and picoeukaryotes, causing the layer of maximum picophytoplankton abundance to differ from that of the subsurface Chl a maximum. [ABSTRACT FROM AUTHOR]