[Coral-List] Mortality event at Flower Garden Banks
emma.hickerson at noaa.gov
Sun Jul 31 12:10:04 EDT 2016
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.
Sent from iPad
> On Jul 29, 2016, at 10:12 PM, Kosmynin, Vladimir <Vladimir.Kosmynin at dep.state.fl.us> wrote:
> Apparently water was 26 ppt when the die-off was discovered. Was it the only property of water masses that could cause mass-mortality of several taxa or there were other properties of water masses that caused it, remains to be discovered. Some corals can tolerate considerable drop in salinity, even below 26 ppt, but not for long time. For example, Siderastrea siderea can withstand such low salinity. It seems that water could be also anoxic. The origin of this water mass of such low salinity, and located at the depth of 20-30 m also needs to be traced. The distance from Mississippi River delta is about 300 km; eddies that can carry water from the delta westward and WSW are quite usual. However, nothing says that such event like what happened at FGB could happen in recent past; coral community in FGB consists of very large coral colonies, some of which are hundreds of years old. It seems that cause of reef mortality is not just the drop in salinity. Can the water masse
> s from Texas and Mississippi delta, which, according to Frank, has moved over the FGBNMS area be anoxic?
> Certainly would be beneficial to take water samples for the analyses not just at FGB, but also at adjacent banks and over the shelf north and east.
> Vladimir N. Kosmynin
> Message: 5
> Date: Fri, 29 Jul 2016 14:23:51 -0400
> From: Frank Muller-Karger <carib at usf.edu>
> Subject: Re: [Coral-List] NEED ADVICE!! - ACTIVE MORTALITY EVENT AT
> FLOWER GARDENS!
> To: <coral-list at coral.aoml.noaa.gov>, Villy Kourafalou
> <vkourafalou at rsmas.miami.edu>, Matthieu Le Henaff
> <mlehenaff at rsmas.miami.edu>
> Message-ID: <579B9F37.3020704 at usf.edu>
> Content-Type: text/plain; charset="UTF-8"; format=flowed
> Dear colleagues -
> I had sent this to the coral list yesterday (THURS 7/28/16) morning but
> it seems it was not posted. In the meantime we are working with the
> FGBNMS and the ONMS and the satellite data images to show how it
> impacted different parts of the Sanctuary.
> The FGBNMSanctuary is working on sampling waters and the benthos.
> Also working with Villy Kourafalou and Matt Lehenaff to understand the
> causes for the offshore dispersal of the plume in this area at this time.
> My message to Steve Gittings and the coral-list yesterday ---
> Hello Steve -
> physically I am not sure what it is we can do but my sense is that water
> samples be collected and examined for nutrient, oxygen, salinity, and
> preserve it for analysis of toxic materials especially metals like
> copper and heavier.
> I don't think this is temperature or just temperature.
> There has been a massive amount of rain over Texas and Louisiana over
> the last few months.
> The satellite images for this area over the past few days are pretty
> poor due to cloud cover, but the images for several days ago show
> massive amounts of coastal water from the LA-TEX area, likely also much
> of Mississippi water, has moved over the FGBNMS area.
> See these images:
> JUL 12
> JUL 13
> JUL 18
> JUL 20
> We'd need to see if we can get better images, and build a time series
> with anomalies to see how often this really happens.
> The FGBNMS area normally does not get the direct input from the
> Mississippi or coastal low salinity water - this water stays closer to
> the coast. This year this plume offshore does seem anomalous.
> __________________ FMK __________________
> Frank Muller-Karger
> Institute for Marine Remote Sensing/IMaRS
> College of Marine Science
> University of South Florida
> 140 7th Ave. South
> St Petersburg, FL 33701
> (727) 553-3335 Office
> (727) 553-1186 Lab.
> (727) 553-1103 FAX
> << carib at usf.edu>>
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