|T. `Aulani Wilhelm
Public Information Officer
Hawai`i Department of Land and Natural Resources
1151 Punchbowl Street, Room 130 Honolulu, HI 96813
HYPERSPECTRAL IMAGING AIMS TO REVEAL LONGER TERM IMPACTS TO CORAL REEFS FROM SHIPWRECKS AND OTHER DISTURBANCES
Yesterday, October 11, 2000, divers visited the site of an old shipwreck on the eastern side of Pearl and Hermes Atoll to assess possible continued impacts of the wreck on the barrier reef. At the site, coral biologist Jim Maragos found a 100-square patch of an invasive blue-green algae which he now believes is directly associated with iron shipwrecks in the Pacific.
According to Maragos, the blue-green algae, Lyngbya, has now been found in dense quantities at the site of three shipwrecks in recent years. The first sighting was at Helen Reef off Palau where the shadow, or 'footprint,' of the algae is large enough to be seen from aerial photos. The second sighting was at a shipwreck at Rose Atoll, located at the eastern end of the American Samoan island chain. The Rose Atoll shipwreck occurred in 1993 when a 250-ton ship broke up over the reef and spilled large quantities of fuel along the reef. The fuel killed nearly all the red coralline algae around the wreck site, allowing for a nearly immediate infestation by Lyngbya and other blue-green algae. (Unfortunately, no photo of Lyngbya is available from the expedition.)
Lyngbya, is an opportunistic, invasive algae that establishes itself where disturbances like shipwrecks and diebacks occur in the ocean, particularly at along reefs. Although the algae is naturally occuring and not an alien species, its aggressive nature prevents the re-establishment of coralline algae and other algae species once it settles into an area. "Because of the way Lyngbya alters reef habitat, its presence reduces the quality and integrity of an ecosystem," said Maragos.
At Rose Atoll, Maragos, who served as the contracting officer representative for the federal government during removal operations of the shipwreck, completed detailed monitoring studies documenting the relationship between Lyngbya and iron content on the reef. U.S. Fish and Wildlife Service studies show that the growth of this algae is stimulated by iron, which acts as a fertilizer, suggesting that there is a direct relationship between shipwrecks and the growth of Lyngbya.
If Maragos' assumption proves true, aside from the immediate and obvious impacts to the marine environment from shipwrecks, like scarring and death or illness to marine life, the impacts to coral reef ecosystems from shipwrecks may be long-felt, if the wrecks aren't removed.
To help confirm Maragos's theory and measure, over the long-term, the potential spread of the blue-green algae, doctoral student Daria Siciliano from the University of California at Santa Cruz, took spectral 'signatures' of the algae as part of her hyperspectral imaging work on the Northwestern Hawaiian Islands expedition. These signatures are like fingerprints, possibly unique to specific species, which describe for scientists what they are seeing from a distance. The fingerprints are determined by the way light is absorbed at specific wavelengths by a particular species of plant or animal.
hyperspectral imaging researcher
Hyperspectral imaging is an emergent technology and the latest in the field of remote sensing, which in simplest terms means measurements made from afar. It can precisely identify the composition of particular habitats. It has been used primarily in terrestrial environments to assess the presence or absence of geologic features or certain species of plants for management purposes. For example, hyperspectral imaging has been used to monitor and manage forest reserves, search for suitable habitat for animals on the brink of survival, and assess the health of plants.
According to Siciliano, "While this type of imaging has been used extensively on land where interactions of light with the ground are better understood, it hasn't been used much underwater in marine environments. Light behaves differently underwater, and the way light is absorbed by and behaves in the ocean is affected by the water column and other factors like depth, making remote sensing in the ocean more complicated." Because the use of hyperspectral imagery in marine environments is relatively new, it places Siciliano's work at U.C. Santa Cruz on the cutting edge.
Hyperspectral Imaging, in more technical terms, is the acquisition of images in contiguous wavelength channels, or spectral bands. For each visual pixel in an image, there is a spectrum of reflected light that identifies the composition of a surface, in this case the ocean floor, that is being studied. Every species or substrate absorbs light at different wavelengths. From this data, maps are developed which identify the abundance and distribution of sessile (stationary) species at the seafloor.
For Siciliano, utilizing this kind of technology first requires significant work on-the-ground. Before being able to interpret the hyperspectral maps, scientists need to know what they are looking at. To do this, work has to be done, species-by-species, to identify unique spectral signatures of the species that show up on the maps. In the case of marine environment, these species would be things like coral and algae found at the sea floor. That's her job.
Using a device called a hyperspectral radiometer, Siciliano records the spectral signatures of coral and algae up to depths of 40 to 50 feet. Using the wavelength data she records, Siciliano has been developing a library of spectral signatures that will help in the analysis and interpretation of aerial maps. This on-the-ground work is called 'ground truthing' and allows scientists to measure the depth, complexity and species composition of particular habitats. Combined with the aerial images, this information provides detailed layers of information that provide baseline and on-going monitoring information over large areas in a systematic way.
According to Siciliano, "The best part about hyperspectral imaging and remote sensing is that once you've built an adequate library of spectral signatures, this technology allows us to extrapolate lots of information about various habitats quickly over large areas. The technique is also non-invasive, meaning we don't have to interfere with the subjects we are measuring and studying in order to get our work done."
The hyperspectral imaging is done aerially, using sensing equipment mounted on aircraft. So far this remote sensing technology is in the hands of very few agencies in the world. NASA's Jet Propulsion Laboratory in California is one such agency. Soon, maybe even by the end of this year, the sensing devices will be affixed to satellites, making hyperspectral imaging avaiable on a global scale. With the repetitive passing of satellites, monitoring of habitat across the globe can be done on a regular, continuous basis with relative ease. This is particulary good news for remote areas like the Northwestern Hawaiian Islands.
With its remote location, aircraft-based imagery of the northwestern islands would occur infrequently and would be expensive. With the use of satellites, this technology can make it much easier on natural resource managers and scientists to detect changes in these remote ecosystems quickly and pinpoint the changes down to single species.
At that point, looking at algae around shipwrecks will be merely one of a myriad of ecological issues that can be looked at and better understood by science. In the meantime, at Pearl and Hermes atoll, using hyperspectral imaging, scientists can keep an eye on the aggressive blue-green algae. The can also look for its presence at other locations where shipwrecks have occurred around the globe to determine how Lyngbya may or may not be causing a negative impact on the nearby marine environment.
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