[Coral-List] Mortality event at Flower Garden Banks
Vladimir.Kosmynin at dep.state.fl.us
Sun Jul 31 16:10:30 EDT 2016
Thank you for sharing with us Dr. Steve DiMarco's commentaries on the event. I think details of current situation with water masses at FGB provided by Steve give us by far the most probable scenario of what happened. Stable stratification of water masses lead to depletion of oxygen in the near-bottom layer, die-off some organisms, and then the decay of dead organisms lead to even lower oxygen content, causing farther mass-die-off of the benthos. It is important to keep in mind, that such die-off happens in the environment that is not anoxic yet (Steve mentioned it), but with low and continuing to drop level of oxygen at the bottom layer. It looks like records of the oxygen content in the water and the stratification of water masses continues.
I guess now somebody will follow with a question if water circulation/oxygenation could be artificially enhanced at such large area as FGB.
Vladimir N. Kosmynin, Ph.D.
Division of Water Resource Management
Florida Department of Environmental Protection
2600 Blair Stone Road,
Tallahassee, Florida 32399-2400
e-mail: vladimir.kosmynin at dep.state.fl.us
From: Emma Hickerson [mailto:emma.hickerson at noaa.gov]
Sent: Sunday, July 31, 2016 12:10 PM
To: Kosmynin, Vladimir <Vladimir.Kosmynin at dep.state.fl.us>
Cc: coral-list at coral.aoml.noaa.gov
Subject: Re: [Coral-List] Mortality event at Flower Garden Banks
From Dr. Steve DiMarco, Texas A&M University:
It is highly unlikely that the movement of the area known as the Deadzone is responsible here. The benthic water mass that makes up that near shore (20-30 m depth) region cannot simply move that far without ventilating. The advection of that water also involves mixing with high oxygen water from the open boundary (the waters at the shelf edge). At small spatial scales (a few km), a hypoxic water mass might remain hypoxic, but when moving tens of km, particularly offshore, the hypoxia is ventilated and levels go up. The time scales of this are also not favorable with this mechanism as it would take several months not days or weeks for the benthic water mass to advect completely across the shelf. The surface plume can move that fast, but the benthic waters do not.
This process of advection and ventilation and respiration is directly addressed in Hetland and DiMarco (2008; Journal of Marine Systems). This is my most cited paper.
The physical processes at work here, in my view, are the stratification and the temperature. The stratification has been persistent for nearly six weeks. This has inhibited the oxygen ventilation of the lower layers. The high temperature lead to high Q (biological activity).
When we were at FGB on June 30, we saw a very stratified system. Oxygen levels were depleted but not at all close to being hypoxic. According to the TABS buoys, the stratification has persisted since then. Because the sub-pycnocline water is cut off form the atmospheric oxygen source (because stratification inhibits ventilation) oxygen levels will continue to drop because respiration continues in the lower layer.
My guess is a tipping point than was reached: when oxygen levels get low enough then other biological processes can become more prevalent and may accelerate the depletion. For example once sponges and other organisms begin to die, than the biological process associated with the decay of these organisms will increase oxygen demand. As long as the system is stratified, oxygen levels will continue to be low.
This process above does not exclude the presence of hydrocarbons, fracking waste products, surface advection of the chl plume from the coast, transport of harmful contaminants related to the elevated rainfall events in Texas or Louisiana, eutrophication from Miss River nutrients (however, we saw no surface nutrients in out June trip), attack of an invasive species/organism, undocumented human contamination, upwelling processes, downwelling processes,…..and many others I have seen in the emails that have circulated.
Hopefully, on our rapid response on the Manta (out right now), we are collecting a large sweet of parameters that help to exclude or include some of these processes listed.
This should than point us to more targeted process work that will unambiguously focus on a cause for why this has happened.
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