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Biogeographic variations of picophytoplankton in three contrasting seas: the Bay of Bengal, South China Sea and Western Pacific Ocean

Yuqiu Wei, Danyue Huang, Guicheng Zhang, Yuying Zhao, Jun Sun*

*Corresponding author:

ABSTRACT: Marine picophytoplankton are abundant in many oligotrophic oceans, but the known geographical patterns of picophytoplankton are primarily based on small-scale cruises or time-series observations. Here we conducted a wider survey (5 cruises) in the Bay of Bengal (BOB), South China Sea (SCS) and Western Pacific Ocean (WPO) to better understand the biogeographic variations of picophytoplankton. Prochlorococcus (Pro) were the most abundant picophytoplankton (averaging 1.9-3.6×104 cells ml-1) across the three seas, while average abundances of Synechococcus (Syn) and picoeukaryotes (PEuks) were generally 1-2 orders of magnitude lower than Pro. Average abundances of total picophytoplankton were similar between the BOB and SCS (4.7×104 cells ml-1), but were 2-3 folds less abundant in the WPO (2.5×104 cells ml-1). Pro and Syn accounted for a substantial fraction of total picophytoplankton biomass (70-83%) in the three contrasting seas, indicating the ecological importance of Pro and Syn as primary producers. Pro were generally abundant in the oligotrophic open waters, however, the exceptional presence of Pro near the SCS coast was potentially associated with the Kuroshio intrusion. Syn and PEuks abundances were higher near the freshwater-dominated areas, which was likely due to the dilution waters. Water temperature and cold eddy were also major drivers responsible for the biogeographic distributions of picophytoplankton. Although Pro, Syn and PEuks showed negative correlations with nutrient concentrations, the maximal abundances of them in vertical distribution showed positive correlations with the nutricline depth, indicating the nutrient availability has two-faceted role in regulating the biogeographic variations of picophytoplankton.