Seasonal variability of picoplankton in the Northern South China Sea at the SEATS station

Flow cytometric analysis of picoplankton from the Southeast Asia Time-series Station (SEATS) in the South China Sea was performed for samples collected every 1–4 months during October 2001 to September 2002 (six cruises) and between November 2004 and December 2005 (four cruises). Prochlorococcus was the most abundant autotrophic picoplankton with the maximum abundance occurring in summer. Synechococcus and picoeukaryotes were 1 or 2 orders of magnitude less abundant during most of the year, but showed a much larger seasonal variation, with maximum abundance occurring in winter to early spring. The winter peak of Synechococcus and picoeukaryotes coincided with the deepening of the mixed-layer depth caused by surface cooling and the strong NE monsoon winds. Maximum bacterial biomass was observed in spring, and generally followed the winter peak of Synechococcus and picoeukaryotes with a time lag. Prochlorococcus contributed up to 80% of the total autotrophic biomass during summer, but Synechococcus and picoeukaryotes accounted for 60–80% of the total autotrophic biomass in winter. Bacterial biomass was less than the autotrophic biomass and the two were only weakly correlated. The interannual variability in the structure and composition of the picoplankton community may be related to the El Niño-Southern Oscillation (ENSO) cycle. During an El Niño year (2001–2002), with higher sea-surface temperature, chlorophyll a was lower and Prochlorococcus and bacterial biomass were higher. In contrast, the higher picoeukaryote biomass during winter was probably responsible for the higher chlorophyll a during the La Niña year of 2004–2005.