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)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(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-storm months, 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.Tropical cyclones, such as hurricanes, tropical depressions and storms, and typhoons have significant ecological effects on coastal systems, changing the population density and evenness of organisms ranging from vegetation to fauna with wind velocity, storm duration, and rainfall (Ackerman et al. 1991). Aquatic ecosystems are subject to direct impacts of tropical cyclones from wind, rainfall, and changes in water circulation, which trigger physical effects including water mixing, changes in stratification, and upwelling (Jullien et al. 2012). Most of this study has focused on impacts in estuaries (e.g., Valiela et al. 1998;Peierls et al. 2003;Hu and Muller-Karger 2007;, where tropical cyclones winds and river input can induce vertical mixing and result in the increase of suspended materials in shallow water columns (Hu & Muller-Karger 2007). The increased rainfall also accelerates river input which leads to hyposaline, high nitrogen and phosphorus conditions in the system (Peierls et al. 2003). Research on storm effects further offshore have largely focused on direct impacts of wind, circulation, sea surface temperature (S...