Localized Hypoxia May Have Caused Coral Reef Mortality at the Flower Garden Banks

On July 25, 2016, hazy water and dead corals, sponges, and other invertebrates were discovered at the East Bank (EB) coral reef within the Flower Garden Banks (FGB) National Marine Sanctuary, with up to 80% coral mortality reported in some areas. Five days later, we conducted a rapid response cruise to uncover the mechanisms leading to this localized mortality event. Hydrographic surveys, mooring data, and a regional hydrodynamic model were used to characterize water chemistry, hydrography, and microbial communities within the FGB. Low salinity (~31-33), total alkalinity (~2284-2330 μmol kg^-1), and dissolved inorganic carbon (DIC, ~1968-2011 μmol kg^-1) were detected in surface waters over the EB, revealing the presence of river-derived water at the surface, but not at the depth of the coral reef ecosystems. The Mississippi/Atchafalaya rivers were the primary sources of freshwater during the event, although Texas rivers, all of which had unusually high discharge during 2016, contributed approximately one fifth to the total freshwater mass. At 75 m depth, high density, salinity, DIC, ammonium, and abundance of microbial taxa associated with deep waters were coincident with low temperature and aragonite saturation state at the northern and eastern stations, indicating a deeper source water at these stations. Cross-slope density gradients were also consistent with an upwelling circulation pattern. Using these observations and data, we hypothesize that the mortality event was most likely caused by the combination of freshwater runoff and upwelling leading to localized hypoxia on EB. The hazy, freshwater layer inhibited photosynthesis, leading to net respiration of coral reef organic matter. Additionally, deep, dense waters upwelled onto the bank and formed a stratified bottom layer, which prevented re-oxygenation from the overlying water column and led to localized areas of hypoxia within pockets on the reef. Hypoxia likely formed rapidly, within two days. Moving forward, high-frequency measurements of water chemistry are needed to assess future effects of freshwater discharge and upwelling on coral reef health.