Sensors in Space Allow Daily Coastal Water Quality Monitoring

ST. PETERSBURG, Florida, August 31, 2007 (ENS) - Using data from instruments aboard two NASA satellites, Florida researchers have created 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.

This information has direct application for resource managers working on restoration plans for coastal water ecosystems as well as federal and state regulators in charge of defining water quality standards.

Scientists at the University of South Florida in St. Petersburg, found that they can monitor water quality almost daily, rather than monthly as most scientists monitoring coastal water quality do.

The team's findings will help pinpoint 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.

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

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 are the driving force for a decline in water quality in the dry season between October and June, when bottom sediments are disturbed.

"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.

Satellites previously have observed turbidity in the open ocean by monitoring how much light is reflected and absorbed by the water. But this technique has not been of much success in observing turbidity along the coast.

The researchers say 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 advances in satellite sensors combined with developments in how the data are analyzed, have made it possible for the scientists to monitor turbidity of coastal waters via satellite.

The two sensors generating the data used by the Florida scientists are the Sea-viewing Wide Field-of-view Sensor, SeaWiFS, and the Moderate Resolution Imaging Spectroradiometer, MODIS.

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 surface of the Earth every one to two days.

Usually, scientists interested in monitoring coastal water quality use boats to gather water samples. High costs of these surveys mean they take place on a monthly basis.

The method works well for capturing data episodic events affecting water quality, such as the seasonal freshwater runoff.

But the Florida team suspected 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 the satellite-mounted sensors could accurately measure two key indicators of water quality, turbidity and water clarity, in Tampa Bay.

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.

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.

Muller-Karger says the method his group used can be applied to coastal waters worldwide.

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