Pico‐ and nanoplankton communities on a near to offshore transect along the continental shelf of the northwestern Gulf of Mexico in the aftermath of Hurricane Harvey

Abstract

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...

Multi-storm analysis reveals distinct zooplankton communities following freshening of the Gulf of Mexico shelf by Hurricane Harvey

“…Conversely, high salinities indirectly reduced zooplankton evenness via greater water column stratification. Lower fluorescence at higher salinities supports the spatial patterns in NWGOM phytoplankton identified by Kurtay et al 20 following Hurricane Harvey. Those authors observed declines in overall phytoplankton abundance and communities increasingly dominated by cells < 20 µm in size waters along a coastal to oceanic gradient.…”

“…The PSEM “multigroup” analysis showed that relationships between salinity and primary production (i.e., fluorescence) switched signs and was reduced by an order of magnitude after Harvey. This reflects the post-Harvey shift in underlying phytoplankton community, towards smaller heterotrophic organisms reported by Kurtay et al 20 . Finally, we see that after Harvey, higher temperatures led to lower zooplankton community evenness whereas the reverse was true for non-storm years.…”

“…In the same area, Topor et al 22 reported higher abundances 1 month after the storm in September as well as subsequent shifts in mesozooplankton abundance and diversity relative to the 6 months following the storm. A caveat to these findings is that NWGOM zooplankton abundance is highly variable year-to-year 23 , 24 , and these results must be placed within historic context to determine if Harvey was indeed anomalous 20 , 22 .…”

“…This freshening subsequently altered phytoplankton community composition 17 , 18 and homogenized zooplankton communities in both space and time in the bay 2 . A large freshwater plume extended to the coastal margin of the NWGOM 19 , decreasing salinity, temperature, and phytoplankton biomass on the shelf 20 . The coastal margin showed prolonged impacts of Harvey.…”

“…However, it is unknown if shelf mesozooplankton communities were altered as well, as residence times vary considerably between environments (13 days in Galveston Bay versus < 1 day on the shelf 15 , 21 ). Kurtay et al 20 reported decreases in overall phytoplankton biomass relative to historic levels and numerical increases of pico- and nanoplankton on the northwestern GOM shelf 2 months following Harvey. In the same area, Topor et al 22 reported higher abundances 1 month after the storm in September as well as subsequent shifts in mesozooplankton abundance and diversity relative to the 6 months following the storm.…”