Latitudinal distribution of the picoplankton community in the eastern equatorial Indian Ocean during the boreal fall intermonsoon period

Abstract: A flow cytometric analysis of picoplankton (Prochlorococcus (Pro), Synechococcus (Syn), picoeukaryotes (Peuk) and heterotrophic bacteria (Hbac) was performed along a meridional transect (88°E, 10°S – 4°N) in the eastern equatorial Indian Ocean (EEIO) during the boreal fall intermonsoon period. The transect spanned contrasting physical and chemical conditions, allowing investigation of the effects of environmental gradients on picoplankton distributions. Hbac accounted for more than 80% of the total picoplankton abundance. Among picophytoplankton, Pro was numerically dominant, with abundances at least one order of magnitude greater than those of Syn and Peuk. Vertically, Pro was mainly distributed in the upper mixed layer while the subsurface Peuk abundance maxima usually coincided with the subsurface chlorophyll a maxima. Physical processes controlled the picoplankton distribution patterns. The maximum prokaryotic picoplankton (Pro, Syn and Hbac) abundances occurred at the center of a cyclonic eddy at 2°N. The depth-integrated picoplankton abundances increased northwards along the transect and were inversely related to the stratification strength. This gradient probably resulted from a combination of physical processes, such as the intrusions of the Bay of Bengal runoff and Indonesia throughflow, advection by the Wyrtki jets and strong stratification south of the equator. This study highlighted the dynamic response of picoplankton to complex physico-chemical conditions in the EEIO.