Spatial and temporal variations of picoplankton in three contrasting periods in the Pearl River Estuary, South China

The distribution characteristics and biomass composition of three picophytoplankton (PP) groups (Synechococcus, picoeukaryotes and Prochlorococcus) were identified using flow cytometry in three contrasting periods (August 2010, January 2011 and August 2011) in the Pearl River Estuary (PRE), South China. To eliminate the overestimation of heterotrophic bacteria (HBA), HBA were identified by epifluorescence microscopy and flow cytometry. The average biomass in the three observations was as follows: 22.80, 21.04 and 18.72 μg C/L of HBA, and 12.92, 0.62 and 15.42 μg C/L of PP. The biomass ratio between the PP and HBA measurements increased along the estuarine axis, which suggested the dominance of PP in the outer estuary and HBA in the nearshore waters. The HBA biomass was not related to chlorophyll a or even exhibited a negative correlation with chlorophyll a in the two summer observations; this implied that dissolved organic carbon from other sources or some environmental factors, like suspended solids concentration (SSC), also affected bacterial growth and obscured the relationship between HBA and phytoplankton by shaping phytoplankton distribution. The HBA linked tightly with suspended particles and was mainly shaped by the SSC. We considered that most of the HBA were attached to riverine-originated particulates and consequently exhibited a decreasing trend from the upper estuary to the open shelf waters in the PRE. The low Synechococcus and picoeukaryotes biomass and the undetectability of Prochlorococcus in the winter were probably attributed to high turbidity and low water temperature. The sharp decrease in river flow in the summer of 2011 may have exerted less pressure on Prochlorococcus and resulted in biomass elevation and a further upward distribution scale. Furthermore, Synechococcus and Prochlorococcus exhibited similar distribution patterns and were relevant to the river input. Meanwhile, picoeukaryotes were the least abundant groups among the PP community in our investigations and showed a distinct distribution pattern from that of Synechococcus and Prochlorococcus.

► We examined biomass levels and distribution patterns of four picoplankton groups.
► HBA was dominant in the nearshore waters and most of them were particle-attached.
► Syn and Pro were abundant in the outer estuary.
► Pro biomass and distribution range were controlled by the amount of river input.
► Peuk showed a quite distinct distribution pattern from that of Syn and Pro.