Monday, December 5, 2016

Another Environmental Assault On The Caribbean: Sargassum

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Sargassum natans (left) and Sargassum fluitans. Ihe beaching of this seaweed is likely tied to increased ocean warming. (Each square shown is one centimeter on a side)

In May of 2014 as we made our way to our favorite south coast beach in Barbados, our nostrils were already being assaulted by the odor of rotten eggs barely a football field distance away.  As we approached to see the source of the odor, we beheld dozens of tourists - not sunbathing or even swimming - but gazing in awe at  an endless carpet of reddish seaweed with dead fish trapped within it.

A scene from a horror film? No, another beaching of mountains of Sargassum, a noxious seaweed that now represents an annually occurring and region-wide phenomenon. Welcome to another likely effect of enhanced global warming, especially of the oceans.  While Barbados at the time was more concerned with the devastating effect on tourism to some of its finest beaches, the fact is the stuff can cause environmental and health problem as well.  The rotten egg odor itself is a result of the hydrogen sulfide gas emitted as Sargassum decomposes onshore. The decomposition also causes other environmental problems including smothering sea turtle nesting sites, killing these and  other species as well as fish.

Such Sargassum beaching events as we beheld in Barbados in May, 2014 led to the largest quantity and frequency increase of beachings ever in 2015. See, e.g.

And no surprise that Sargassum beaching events in the Caribbean, West Africa and other regions have also received wide attention, e.g.

These incidents reached such a pitch in terms of threatening the coastal economy of Mexico that they spawned a Cabinet -level crisis and the Mexican navy was called in to take action. Alas, Barbados and other tiny Caribbean nations lack such resources and so are left to wait until the stuff recedes. That can translate to devastating tourism losses.

Two primary culprits, Sargassum natans and Sargassum fruitans (shown in graphic) are mainly found in the Gulf of Mexico, Caribbean and the Atlantic. John Phillips, a Barbadian biologist who has studied them, noted they reproduce solely by vegetative fragmentation. Generally, the species are not differentiated into true leaves,  stems and roots but rather possess tough, dense branched structures (thalli) composed of axes, blades and air bladders. They weeds do serve as important habitats for a diverse group of marine animals, such as shrimp, crabs and turtles - but when they over reproduce the results are inimical to all these species.

Moreover, excessive amounts of Sargassum on beaches in populated areas represent a nuisance that must be physically removed. Often, as in the May, 2014 beaching in Barbados, it can take weeks. Meanwhile, many people in proximity may suffer respiratory and gastrointestinal symptoms, including running nose, sore throat, and nausea.

Thus, forecasting Sargassum beachings has become a matter of pressing importance.  Enter now the University of South Florida's Optical Oceanography Laboratory  (OOL) which has been working on a virtual antenna system (VAS). This system is capable of downloading and processing raw satellite images into customized images with excellent resolution.  They are then distributed through a web portal within 4-6 hours of the satellite overpass.

In specific terms, the USF OOL system has used NASA's "Terra" and "Aqua" satellites to map the extent of Sargassum blooms.  Each satellite carries a device called "MODIS" or Moderate Resolution  Imaging Spectro-radiometer, which generates the key data.  The latter enables detection of enhanced reflectance in the near infrared region of the spectrum.  The extent of floating vegetation can then be determined using a "floating algal index". More on the Sargassum watch system can be read here:

Another method entails remote sensing via the Landsat satellite. Unlike the MODIS daily cycle, Landsat passes a given region every 16 days.   The advantage is that it has a much higher resolutions (~ 30m) which can reveal much smaller aggregations than the typically 1 km resolution of MODIS.  There is also the capacity to use mixed system, both MODIS and Landsat. Below is shown the reach of the University of South Florida's Sargassum watch system using MODIS and Landsat 8:
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USF real time Sargassum detection with MODIS and Landsat 8showing regions covered.

What is the takeaway from the USF and other studies? Well, data synthesis is still underway but preliminary results show distinct patterns of distribution for each Sargassum species as well as pathological proliferation of one previously rare form (Sargassum natans VIII) accompanied by decreased abundance of marine animal diversity during the 2014-15 Caribbean event.

We are hoping the next time we return to Bim, which may be next year, our favorite beaches are not contaminated with this unwanted, malodorous species. 
Crane Beach at the height of the Sargassum weed invasionÂș.
The Crane Resort Beach mucked up by Sargassum in 2014. The Crane has since installed a boom 1,100 feet long and 1.5m deep to prevent further Sargassum beachings.

No, admittedly the sight of pelagic Sargassum isn't new, hell even Columbus reported seeing it. But according to biologist John Phillips the beaching events are reaching much more malignant displays now, with more impacts on local environments.  Like the Crane Beach Resort (see above) other hotels in Barbados and around the Caribbean may have to resort to extreme measures - including installing booms - to prevent further invasions.

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