ODU RESEARCHER TURNS SATELLITE IMAGES INTO MEASURES OF WATER QUALITY
A little past noon each day the NASA MODIS satellite swings over the Chesapeake Bay and sends back to Earth a digital, color record of what it sees. It is the job of scientists such as Old Dominion University oceanographer Richard Zimmerman and his research team to turn these "snapshots" of the 190-mile-long Bay into reliable measures of water quality.
For Zimmerman, who is chair of the ODU Department of Ocean, Earth and Atmospheric Sciences, the work has all the urgency of a spy thriller in which the good guys work fervently to solve a puzzle and prevent a catastrophe.
In the summer of 2005, about 5 percent of the Chesapeake Bay became anoxic, or oxygen depleted. This comprised the bay's largest ever "dead zone," and urged along the work of Zimmerman and his ODU bio-optical research group.
"Our mission is to develop algorithms specifically for the Chesapeake Bay," he explained. These mathematical instructions will wring more and better information from the satellite imagery, and could provide the incontrovertible evidence necessary to "mobilize the body politic to take the bold actions needed" to improve water quality in the Bay, he said.
At the base of Zimmerman's work is this one simple fact: Healthy expanses of water have what might be called a healthy complexion. A favored complexion comes from a favored productivity level, which means that organisms are growing in the water and tend to be the organisms that scientists want to be growing.
Any change from healthy colors can mean trouble. A common culprit is a harmful algae bloom-and often a chlorophyll-rich green color-that is caused by excessive flows of nutrients from agricultural and urban sources, especially in warm summer waters. The blooms can block sunlight from reaching sea grass meadows growing on the sea floor, killing the grasses and robbing the water of the oxygen that plants produce. When the bloom-forming algae die, they sink and decompose, a process that further consumes the dissolved oxygen in the water.
In addition to the anoxic zones that appeared in the Chesapeake Bay during the unusually warm summer of 2005, there were even larger hypoxic zones-those with significantly reduced oxygen levels. Some experts estimated that about one-third of the Bay suffered from a lack of oxygen during 2005's warmest months. Mobile marine creatures can flee an oxygen-depleted zone; stationary creatures, such as oysters, die.
By the spring of 2006, some areas of the bay had not recovered from last summer's die-off of bottom-dwelling seagrass, making it imperative that marine scientists keep close tabs on the bay's water quality this summer, Zimmerman said.
Up until about a quarter century ago, marine researchers did not have access to regular color imagery of a body of water. They had to rely upon very coarse-grained observations collected from boats and from fixed instrument perches on lighthouses and buoys, and the occasional photographic image collected from aircraft.
Today's satellite observing platforms provide a tremendous flow of earth observing data, including ocean color that has vastly improved what we know about oceans and coastal or inland waters. The acronym MODIS for the satellite that makes the daily pass over the Chesapeake Bay stands for "moderate resolution imaging spectroradiometer. (More about it can be found at http://modis.gsfc.nasa.gov/.)
It is the goal of ocean researchers such as Zimmerman to improve our ability to interpret and extract useful knowledge from the satellite data.
He and the bio-optical research group at ODU are working with the National Oceanographic and Atmospheric Administration (NOAA), as well as NASA, to improve the algorithms that enable computers to interpret the satellite imagery. Group members include Victoria Hill, a postdoctoral research associate; David Rule and Ilaria Nardello, research associates; Jasmine Cousins and Xiaoju Pan, doctoral students; and Mandy Stoughton, master's student.
In addition to the Chesapeake Bay, the ODU research group has bio-optical projects under way in the coastal waters of Florida, California and the Arctic. These coastal waters need study because the mouths of rivers, shallow water, and bottom characteristics such as sand bars and oyster or seagrass beds can give patchwork variety to what the satellite sees. This makes it difficult to pinpoint those dots of color that actually are telltale signs of water quality slippage.
Although deep and blue oceans may submit to one-algorithm-fits-all analysis, coastal waters must be treated as a collection of "highly parochial" zones requiring customized analysis for each zone, Zimmerman explained.
At present, the lower Chesapeake Bay project of the bio-optical research group involves regular transects in the ODU research vessel, Fay Slover, from just off Little Creek to the Chesapeake Light tower. At each of several research stations, the oceanographers make measurements of ocean color and take samples to analyze the constituents responsible for that color. They measure concentrations of phytoplankton chlorophyll, dissolved organic matter and suspended sediment in the water. They take core samples of the floor of the bay. They also use a special spectrometer to measure the light leaving the water, the light that will be converted into color imagery by the satellite cameras.
The data the researchers collect can be used to validate and calibrate computational processes that glean water-quality measurements from satellite images. Among the processes are those that make allowances for the atmospheric disturbances-clouds, haze, etc.-that can prevent a satellite from seeing the true colors of light leaving the water's surface.
Over time, the researchers expect to develop algorithmic instructions that will automatically convert digital image data from satellites into detailed and precise water quality information for the lower bay.
"We want to take the skilled interpreter out of the raw data analysis, to eliminate the judgment calls and to deliver image products in near real-time for use in decision-making by resource managers," Zimmerman said. Too often in the past, he added, experts have had to "allow for this, and allow for that" in their interpretations of satellite imagery because algorithms weren't sophisticated enough to provide reliable analysis. Reliance on human experts also slows the delivery of image products into the hands of water-quality managers, reducing their utility.
The goal is rapid, pinpoint accuracy, such as early detection of algae blooms, and early differentiation between benign and harmful blooms. It is also important to be able to tell from satellite imagery whether a green patch is caused by algae or a kelp forest. In a project in California, the ODU research group is developing ways not only to tell kelp from algae, but also to zero in on the age and health of the kelp.
Zimmerman believes the day is coming when satellite imagery analysis will allow scientists to stay on top of marine environmental issues, even to the point of being prescient. "It is helpful to be right with your predictions," he explained. "For example, to be able to say when there will be die-offs of grass beds. The prediction would demonstrate to everyone the impact of nutrient loading from agricultural and urban runoff."
Authoritative analysis and predictions could point precisely to places along the Chesapeake Bay where preventative measures and environmental cleanups are likely to have the largest impacts, he added. "Some problems need more attention than others. The problem with generic cleanups is that a region can spend a lot of money but not measurably improve the quality of the environment. And the lack of clear results can undermine public support for cleaning up the bay."
This article was posted on: June 8, 2006
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