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Localized hypoxia may have caused coral reef mortality at the Flower Garden Banks

Abstract

On July 25, 2016, turbid water and dead corals, sponges and other invertebrates were discovered at the East Bank (EB) of the Flower Garden Banks (FGB) National Marine Sanctuary. Mortality was spread over 0.06 km2, with up to 80% coral mortality reported in some areas. Within days, response efforts were underway to investigate the potential mechanisms leading to the mortality event. Hydrographic surveys, moored buoy 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 and eastern stations, revealing the presence of river-derived water. 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 two processes. The turbid 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 temporal measurements of oxygen and carbonate chemistry are critical for monitoring risks (e.g., hypoxia and acidification) associated with freshwater discharge and upwelling, since these processes may adversely affect coral reef health.

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Acknowledgements

Thanks go to the Captain and crew of the RV Manta, operated by the FGB Foundation, for sea going operations during the rapid response cruise; staff of FGBNMS/ONMS (E. Hickerson, M, Johnston, G.P. Schmahl and S. Gittings) for access to early reports of the event and for multiple discussions pertaining to this manuscript; J. Schiff, E.A Whitaker and B. Buckingham for collecting carbonate chemistry, microbiology, and oxygen samples. This work is dedicated to Matthew K. Howard (TAMU), who served as Chief Scientist for the 2016 rapid response cruise and passed away suddenly in February 2018; he was a valued friend and dedicated scientist. Special thanks to the TAMU Geochemical and Environmental Research Group (GERG) for access to the Texas Automated Buoy System (https://tabs.gerg.tamu.edu) historical and real-time data. Funds for this work were provided by the GoM Coastal Ocean Observing System (GCOOS), the College of Geosciences (TAMU), the Department of Oceanography (TAMU), FGBNMS (NOAA), the Geochemical and Environmental Research Group (TAMU), the Harte Research Institute, the Texas General Land Office, the Texas OneGulf Center of Excellence, the GoM Research Initiative to support the Aggregation and Degradation of Dispersants and Oil by Microbial Exopolymers (ADDOMEx) consortium, and a NOAA Nancy Foster Scholarship to A. Kealoha. Partial funding of this research was provided by the Texas Governor Fund through the Texas Restore Program as well as the G. Unger Vetlesen Foundation to A. H. Knap (TAMU). The author’s thank A.H. Knap for his assistance in facilitating the response cruise aboard the R/V Manta in July/August 2016. We also thank two anonymous reviewers for their constructive and thoughtful comments that substantially improved this manuscript. The funders had no role in study design, data collection, and interpretation, or the decision to submit the work for publication.

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Correspondence to Andrea K. Kealoha.

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Kealoha, A.K., Doyle, S.M., Shamberger, K.E.F. et al. Localized hypoxia may have caused coral reef mortality at the Flower Garden Banks. Coral Reefs 39, 119–132 (2020). https://doi.org/10.1007/s00338-019-01883-9

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Keywords

  • Coral reef
  • Hypoxia
  • Mortality
  • Gulf of Mexico
  • Upwelling