Picophytoplankton community dynamics in a tropical river estuary and adjacent semi-enclosed water body

Abstract

The dynamics of picophytoplankton, a vital component of the aquatic microbial food web, are least explored in the river-dominated estuaries, especially those subjected to human modifications that can alter the hydrodynamics and the biota. Here, picophytoplankton community dynamics was assessed seasonally and spatially in the tropical monsoon-influenced Hooghly River estuary and an adjacent semi-enclosed dock. We hypothesized that the seasonal and spatial picophytoplankton community structure and abundance would be controlled by synergistic effects of the environmental and hydrographic factors with different responses in the semi-enclosed water body compared to the flowing estuary. The relatively lower turbidity and higher water transparency suggest that the dock's water residence time was relatively longer than the estuary due to its semi-enclosed nature. A phycocyanin-rich Synechococcus group dominated the community seasonally and spatially throughout the study region, with the highest population during the pre-monsoon. The higher water temperature, salinity, transparency, and residence time were the supporting factors. During the warm monsoon season, the relatively lower abundance of the phycocyanin-rich Synechococcus group despite the high nutrient concentrations implied impairment of growth due to high turbidity, low light availability, and residence time. However, the relatively higher salinity, water transparency, lower turbidity due to a longer residence time failed to stimulate the picophytoplankton during the cooler post-monsoon, suggesting temperature as the main controlling factor. Seasonal introduction of phycoerythrin-rich Synechococcus and two other picocyanobacteria (freshwater Microcystis and Cyanobium like) groups ensued from the open sea and river, respectively, due to varying hydrodynamics. The presence of the latter picocyanobacteria group during non-monsoon indicates higher salinity tolerance. Among the two zones, the higher abundance of picophytoplankton in the dock implies the stimulatory effect of the relatively higher water transparency, longer residence time, and enhanced resource utilization capabilities. A synergistic role of environmental parameters and hydrodynamics becomes apparent from these observations, thereby supporting the hypothesis.