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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]

Abstract: By using a global data set (NOMAD), improved algorithms were developed for deducing from ocean color remote sensing data the concentrations of phytoplankton pigments, total chlorophyll a (TChl_a), zeaxanthin (Zea) and fucoxanthin (Fuco), and the associated primary phytoplankton groups, namely, Prochlorococcus, Synechococcus, haptophytes and diatoms, in the oceans. A modified classification for the waters of the world''s ocean based on the dominant phytoplankton group was then devised. By using a local data set from the northern South China Sea (NSCS), algorithms were also developed for assessing the cell abundances of picophytoplankton, Prochlorococcus, Synechococcus and pico-eukaryotes, in this region. These algorithms were then applied to the remotely sensed data from the MODerate-resolution Imaging Spectrometer at Aqua sensor (MODIS-Aqua) to study the abundances and the distributional patterns of picophytoplankton in the entire NSCS. The results agreed well with field observations, which were available only from selected locations in the study area. On a basin scale, the results were consistent with expected variations in phytoplankton pigments, phytoplankton community composition, and picophytoplankton biomass, in response to the major known environmental phenomena in the NSCS, such as the progressive increase in biological productivity towards the coast, the runoff from the Pearl River, higher biological productivity in the wintertime, upwelling around the Taiwan Bank, winter upwelling northwest of the Luzon Island, and the transformation and dissipation of internal waves off the Dongsha Atoll. This work represents the first effort in estimating the cell abundances of the principal picophytoplankton groups in the oceans from space. The general application of this approach to the global ocean needs to be further validated and calibrated with a more extensive data base from field observations in other parts of the oceans. [Copyright &y& Elsevier]

Abstract: The abundance, size, and fluorescence of picophytoplankton cells were investigated during the summer (July–August of 2009) and winter (January of 2010) extending from near-shore coastal waters to oligotrophic open waters in northern South China Sea, under the influence of contrasting seasonal monsoons. We found that the median abundance of Prochlorococcus averaged over top 150m decreased nearly 10 times in the winter compared to the summer in the whole survey area, while median abundance of Synechococcus and picoeukaryotes increased 2.6 and 2.4 folds, respectively. Vertical abundance profiles of picoeukaryotes usually formed a subsurface maximum during the summer with the depth of maximal abundances tracking the depth of nutricline, whereas their vertical distributions were more uniform during the winter. Size and cellular fluorescence of Prochlorococcus and Synechococcus usually increased with depth in the summer, while the size of picoeukaryotes was smallest at the depth of maximal abundances. Size, cellular fluorescence, and chlorophyll-to-carbon ratio of Prochlorococcus and Synechococcus in surface waters were generally higher in the winter than in the summer and onshore than offshore, probably resulting from different temperature, nutrient, and light environments as well as different ecotype compositions. Prochlorococcus cells were most abundant in warm and oligotrophic environments, while the abundance of Synechococcus and picoeukaryotes was the highest in waters with intermediate chlorophyll and nutrient concentrations. The distributional patterns of picophytoplankton groups are consistent with their specific physiology documented in previous studies and can be possibly predicted by environmental physical and chemical variables. [Copyright &y& Elsevier]