LAUSR

Marine picophytoplankton are abundant in many oligotrophic oceans, but the known geographical patterns of picophytoplankton are primarily based on small-scale cruises or timeseries 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 3 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 close to 2-fold 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 3 contrasting seas, indicating the ecological importance of Pro and Syn as primary producers. Pro were generally abundant in oligotrophic open waters; however, the exceptional presence of Pro near the SCS coast was potentially associated with the Kuroshio intrusion. Syn and PEuk abundances were higher near freshwater-dominated areas, which was likely due to dilution waters. Water temperature and cold eddies were also major drivers responsible for the biogeographic distributions of picophytoplankton. Although Pro, Syn and PEuks showed negative correlations with nutrient concentrations, their maximal abundances in vertical distribution showed positive correlations with the nutricline depth, indicating that nutrient availability plays a 2-faceted role in regulating the biogeographic variation in picophytoplankton.