Hurricane Harvey delivered over 124 trillion liters of freshwater to the Texas-Louisiana coast and the northwestern Gulf of Mexico (GOM) in late August-early September 2017. Environmental conditions, size-fractionated phytoplankton biomass, and pico- and nanoplankton abundances (picocyanobacteria, picoeukaryotes, autotrophic, and heterotrophic nanoplankton) were characterized along nearshore-offshore transects prior to Hurricane Harvey (late July 2017) and in the 3 weeks to 6 months following the storm (September 2017 to March 2018). To understand the extent to which observed changes in the aquatic environment and plankton communities could be attributed to Hurricane Harvey (vs. seasonal or interannual variability), salinity, temperature, and phytoplankton biomass from historical data (2006-2018) were also analyzed. Nearshore stations from September and October 2017 showed significantly lower salinities and overall phytoplankton biomass compared to historical data. Inorganic nitrogen concentrations were minimal in October. Pico- and nanoplankton abundances were lower in September and October than prior to the storm, with the exception of picocyanobacteria. In contrast, post-storm biomass at mid-shelf stations was within the historical average, while pico- and nanoplankton abundances were higher. Offshore stations showed little change in biomass or abundances following the storm. Pre-storm assemblages of pico- and nanoplankton in July 2017 were distinct from those in post-stormmonths, and variance in these assemblages and specific group abundances was tied to inorganic nutrients, salinity, and temperature. These results point to significant changes in important members of the plankton that occurred in GOM continental shelf waters following a major hurricane, with important implications for oceanic food webs and biogeochemical cycles.
