Nasa, USF 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.

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.

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, which is then refined to determine water clarity. Concurrently, 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 Frank Muller-Karger, a study co-author working at the University of South Florida.

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 seagrasses.

“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 Muller-Karger, who has since moved to the University of Massachusetts.

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 because shallow coastal waters and Earth’s atmosphere serve up complicated optical properties. Advances in satellite sensors, combined with developments in how the data are analyzed, now make it 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.

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.

Frequent measurements from space could resolve questions about the specific timing and nature of events that lead to decreases in water quality. 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 said.