[Coral-List] new paper on Flower Garden Banks coral reef mortality event

[Katie Shamberger]

Hi all,
We want to share some exciting new work our lab just published in Coral
Reefs, “Localized hypoxia may have caused coral reef mortality at the
Flower Garden Banks”, led by Dr Andrea Kealoha. The full citation, press
release, and abstract are below.

Katie Shamberger

Kealoha A.K., S.M. Doyle, K.E.F. Shamberger, J.B. Sylvan, R.D. Hetland,
S.F. DiMarco. 2019. Localized hypoxia drives coral reef mortality at the
Flower Garden Banks. Coral Reefs. https://doi.org/10.1007/s00338-019-01883-9

Press Release:
https://today.tamu.edu/2020/01/03/answer-found-for-die-off-of-coral-reefs-in-gulf-of-mexico/

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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Kathryn E. F. Shamberger
Assistant Professor
Department of Oceanography
Texas A&M University
College Station, TX 77843
1-979-845-5752
katie.shamberger at tamu.edu
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