3.1: The View From Space
When looking at the landscape level changes on the scale encountered in Louisiana's brown marsh dieback, scientists have what amounts to four levels of resolution to choose from: satellite imagery, high-altitude aerial photography, lower-altitude aerial surveys, and "get-your-boots-muddy" ground assessments. All four approaches have their places in creating the composite view needed for good science to move forward, but getting the "big picture" first-be it from space or air-usually makes the most sense.
Distressed smooth cordgrass could be found from the state's western chenier plains to the deltaic plains in the east, but the greatest concentration of distressed plants, by far, was in the Barataria-Terrebonne region.
Beginning with the "biggest picture" possible, the project used the NASA platform Landsat 7, the seventh in the Landsat series and the first to employ the Enhanced Thematic Mapper sensing apparatus. In a cooperative effort with PixSell, Inc., Andrew Beall, the remote sensing manager at the University of New Orleans (UNO) Coastal Research Laboratory, used the Landsat 7 to image the entire Louisiana coastal zone once every 16 days across the summers of 2000 and 2001. Almost as important, given the lack of a vegetative baseline, was his and his group's peek into archived images of the Barataria-Terrebonne basin taken in 1999, images taken prior to the dieback.
The light bands the Enhanced Thematic Mapper can read are three visible spectra (red, green, and blue) and five nonvisible, including one near-infrared band. By simultaneously using two of the eight bands (red and near infrared), Beall and his fellow researchers were able to employ the Normalized Difference Vegetation Index (NDVI). Because the near infrared is read as reflected energy and the red is absorbed by plants, a precise coefficient of the two that represents the amount of chlorophyll present can be produced. The amount of chlorophyll, in turn, can be used to assess the health of species such as smooth cordgrass if their seasonally adjusted healthy coefficients have been previously established.
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| Taken in August of 2000, the above image depicts the eastern half of Louisiana's coastline, with the areas most severely affected by the dieback appearing as brown. The Mississippi River, with the healthy marsh areas surrounding it appearing in green, can be seen winding its way southwest in the right hand portion of the image |
Employing six of the eight spectra at their disposal, Beall's analysis of the images taken in 1999 showed nothing out of the ordinary in the Barataria-Terrebonne region, a notable finding given that this would become the area most severely affected by the dieback. The story, of course, was far different in 2000. Imaging the entire Louisiana coast in seven scenes rather than the single scene that had been required to examine the Barataria-Terrebonne region, the investigators delineated their brown marsh findings into four distinct categories: severe, afflicted, impacted, and healthy.
While the three classifications showing distressed smooth cordgrass could be found from the state's western chenier plains to the deltaic plains in the east, by far the greatest concentration of distressed plants-along with the greatest severity-was in the Barataria-Terrebonne region. The story in the summer of 2001, however, was different yet again. For the most part, the afflicted and impacted areas-the healthier of the three "distressed plant" categories-had shown overall improvement, with some areas improving markedly over the previous year. However, those areas that had fallen into the "severe" category the previous year remained largely unchanged.