Response of a Phytoplankton Community in a Subtropical Estuary to a Pulse Disturbance Driven by a Combo Hurricane and Extreme Rainfall Event

Major precipitation events from destructive hurricanes can change the hydrodynamics of estuaries. Indeed, water rather than wind was the major destructive force behind Hurricane Harvey, the category 4 storm that stalled over Texas in August 2017. Herein, we present a unique examination of the phytoplankton community as it responded to the changing hydrodynamic conditions in Galveston Bay. We used traditional (primary productivity, pigments, Fluorescence Induction and Relaxation System, pulse-amplitude modulation fluorometer), and modern (Imagining FlowCytobot [IFCB], 16S rRNA, and 18S rRNA gene sequencing) approaches to characterize the phytoplankton community and their physiological response. Phytoplankton communities transitioned from pre-storm estuarine to freshwater species and then began to transition back to an estuarine community with similar but not identical structure to that present before the storm. Further, phytoplankton communities were initially displaced by the flood waters, which lowered biomass and productivity despite the high nutrient concentrations and dissolved inorganic nitrogen to phosphate ratios of 16. We found for cyanobacteria, factors such as time (i.e., sampling event after the storm), temperature (°C), salinity, NH4+ (µM), NO3− (µM), and turbidity (Secchi disk depths and total suspended solids) were the most critical environmental factors that explained the variation in community composition relative to the major eukaryotic groups. Only factors such as time, followed by salinity, and NO3− played a crucial role for eukaryotes, with time having the most impact on diatoms. The results herein provide a greater understanding of the succession and natural variability of phytoplankton species following large flood events in subtropical estuaries, which are predicted to occur more frequently with increasing effects of climate change.