Coral mortality event in the Flower Garden Banks of the Gulf of Mexico in July 2016: Local hypoxia due to cross-shelf transport of coastal flood waters?

Remotely sensed and in situ data, in tandem with numerical modeling, are used to explore the causes of an episode of localized but severe mortality of corals, sponges, and other invertebrates at the Flower Garden Banks (FGB) National Marine Sanctuary in July 2016. At about 190 km off the Texas coast, at the top the seamount in the East FGB, up to 82% of coral reef organisms were affected in a 1-2 m thick layer on the local seafloor at similar to 23 in depth. Analysis of available data pointed to low levels of dissolved oxygen being the most likely contributing factor in the observed mortality (Johnston et al., 2019). Observations show that upwelling-favorable winds in June and July 2016 carried brackish and turbid coastal waters across the northwestern Gulf of Mexico continental shelf to the FGB. This plume of coastal water was the result of exceptionally high precipitation and local river run-off. Field data provide clear evidence of thin, localized, subsurface near-hypoxic layers immediately below this turbid, low salinity coastal plume. These midwater layers extended over longer distances (30-40 kin), and reached further offshore (similar to 100 km), than previously reported in the region, associated with large quantities of organic matter carried offshore by the brackish plume. The surface brackish layer was observed to cover the East FGB in satellite ocean color imagery and in situ salinity measurements in late June and July 2016. Model results and sparse observations on the shelf suggest that this surface layer was similar to 20 m thick. It is expected that organic matter carried in the surface layer accumulated on the seafloor of the East FGB, which was just below the brackish plume. In the absence of ventilation, this led to the local formation of a bottom hypoxic layer, similar to what is observed on the Gulf of Mexico inner to midshelf every summer. The conditions experienced at FGB in July 2016 are likely to affect other reefs exposed to brackish plumes with high organic matter loads. The processes of physical connectivity by transport of material is critical for reef colonization and survival, but can also be fatal to coral ecosystems. The monitoring of coral reefs should take the threat of hypoxia due to distant sources of organic matter into account.