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NASA Satellites Eye Coastal Water Quality
Armed with data from two NASA satellites, researchers have invented a way to map the fleeting changes in coastal water quality from space - something that has long evaded researchers and coastal managers relying only on ground-based measurements.

High concentrations of microscopic plants called phytoplankton along the Florida coast and in Tampa Bay are an indicator of ocean health and change as seen in this SeaWiFS image from October 2004. Image right: High concentrations of microscopic plants called phytoplankton (red regions) along the Florida coast and in Tampa Bay are an indicator of ocean health and change as seen in this SeaWiFS image from October 2004. Researchers have successfully used data from similar images to monitor almost daily changes in coastal water quality. + High resolution Credit: SeaWiFS Project

Using data from instruments aboard NASA satellites, Zhiqiang Chen and colleagues at the University of South Florida in St. Petersburg, found that they can monitor water quality almost daily, rather than monthly. Such information has direct application for resource managers devising restoration plans for coastal water ecosystems and federal and state regulators in charge of defining water quality standards.

The team's findings will aid in the effort to tease out factors that drive changes in coastal water quality. For example, sediments entering the water as a result of coastal development or pollution can cause changes in water turbidity – a measure of the amount of particles suspended in the water. Sediments suspended from the bottom by strong winds or tides may also cause such changes. Knowing where the sediments come from is critical to managers because turbidity cuts off light to the bottom, thwarting the natural growth of plants.

"If we can track the source of turbidity, we can better understand why turbidity is changing. And if the source is human related, we can try to manage that human activity," says Frank Muller-Karger, a study co-author from the University of South Florida.

Satellites previously have observed turbidity in the open ocean by monitoring how much light is reflected and absorbed by the water. The technique has not been of much success in observing turbidity along the coast, however. That’s because shallow coastal waters and Earth’s atmosphere serve up complicated optical properties that make it difficult for researchers to determine which colors in a satellite image are related to turbidity, which to shallow bottom waters, and which to the atmosphere. Now with advances in satellite sensors combined with developments in how the data are analyzed, Chen and colleagues show it is possible to monitor turbidity of coastal waters via satellite.

The traditional methods of monitoring coastal water quality require scientists to use boats to gather water samples, typically on a monthly basis because of the high costs of these surveys. The method is sufficient to capture episodic events affecting water quality, such as the seasonal freshwater runoff. Chen and colleagues suspected, however, that the monthly measurements were not capturing fast changes in factors that affect water quality, such as winds, tides and human influences including pollution and runoff.

More particles suspended in the water, a measure called turbidity, show up as yellow, orange and red in December than in July.

Image above: NASA satellite imagery has shown that water quality of Florida's Tampa Bay decreases in winter months compared to summer. More particles suspended in the water, a measure called turbidity, show up as yellow, orange and red in December (left image) than in July (right). Images are composites of turbidity data collected in December and July, respectively, over a span of three years using NASA's MODIS instrument. Credit: NASA/USF

+ Left image high resolution
+ Right image high resolution

The team set out to see if satellites could accurately measure two key indicators of water quality - turbidity and water clarity – in Tampa Bay, Fla. An analysis of turbidity takes into account water clarity, a measure of how much light can penetrate into deep water. Satellites, with their wide coverage and multiple passes per week, provided a solution to frequent looks and measuring an entire estuary within seconds.

To determine water clarity in Tampa Bay, the team looked at more than eight years of imagery from GeoEYE’s Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument, whose data is analyzed, processed, and distributed by NASA for research. The images give a measure of how much light is reflected by the water. The data were put through a two-step calculation to arrive at a measure of clarity. Similarly, data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Aqua satellite was compared with measurements of turbidity gathered on the ground and then applied to each whole image to make the maps.

When compared with results from independent field measurements, collected with the help from the U.S. Geological Survey, the researchers found that the satellites offered an accurate measure of water quality in the bay. The method can be applied to coastal waters worldwide with little change in methods, according to Muller-Karger.

Frequent measurements from space could resolve questions about the specific timing and nature of events that lead to decreases in water quality. Seasonal freshwater discharge from nearby rivers and runoff into the bay can carry nutrients. If these nutrients are not controlled, they can give rise to large and harmful phytoplankton blooms, which can kill sea grass. Wind conditions, however, are the driving force for a decline in water quality in the dry season between October and June, when bottom sediments are disturbed.

Chen thinks the new tools will help people understand how coastal systems change over time, which will provide important information for managing the health of coastal waters. "It’s important to look at baseline conditions and see how they change with the seasons and over the years, and whether that change is due to development, coastal erosion, the extraction and dumping of sediments, or digging a channel," Muller-Karger says.

SeaWiFS was launched aboard the OrbView-2 satellite in 1997. The sensor collects ocean color data used to determine factors affecting global change, particularly ocean ecology and chemistry. MODIS was launched aboard the Aqua satellite in 2002. The instrument, together with its counterpart instrument aboard the Terra satellite, collects measurements from the entire Earth surface every one to two days.

The study was published July 30 in two papers in the journal Remote Sensing of Environment.

Related Links:

+ NASA's Moderate Resolution Imaging Spectroradiometer (MODIS)
+ Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

Kathryn Hansen
Goddard Space Flight Center