[Coral-List] Mortality event at Flower Garden Banks

Melanie McField mcfield at healthyreefs.org
Mon Aug 1 11:24:11 EDT 2016

A weak hurricane or TS should suffice for mixing that stratified water
mass.   There may be one on the way - if it veers North a bit more.

Sunday afternoon video discussion on Invest 97L:

On Sun, Jul 31, 2016 at 4:10 PM, Kosmynin, Vladimir <
Vladimir.Kosmynin at dep.state.fl.us> wrote:

> Emma,
> 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.
> Thank you,
> Vladimir
> Vladimir N. Kosmynin, Ph.D.
> Environmental Consultant
> Division of Water Resource Management
> Florida Department of Environmental Protection
> 2600 Blair Stone Road,
> MS 3590
> Tallahassee, Florida 32399-2400
> e-mail: vladimir.kosmynin at dep.state.fl.us
> Tel: (850)245-7622
> -----Original Message-----
> 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.
> —steve
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Melanie McField, PhD
Director, Healthy Reefs for Healthy People Initiative, Smithsonian
1648 NE 47th St, Ft Lauderdale FL 33334
Cell: 754 610 9311   Tel: 954 990 8842
email: mcfield at healthyreefs.org

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