Comparisons of picophytoplankton abundance, size, and fluorescence between summer and winter in northern South China Sea

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 150 m 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. ► Population dynamics of picophytoplankton in South China Sea in summer and winter were studied. ► Prochlorococcus abundance decreased, while Synechococcus and picoeukaryote increased in the winter. ► Picophytoplankton size, fluorescence, and chlorophyll-to-carbon ratio all increased in the winter. ► The distribution of picophytoplankton was consistent with their physiology.