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Muddy Waters

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Muddy Waters

Is it safe to swim in your local lake? Are the fish from that lake good to eat? NASA satellites will soon help find out....

NASA

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see captionSept. 5, 2002:  Dozens of shallow-draft bass fishing boats creep along the cypress-lined shore, each guided by a seasoned fisherman.  It's the annual bass tournament everyone's been waiting for ... yet the fish aren't biting. 

Tugging at the rim of his threadbare cotton fishing hat, one veteran angler eyes the clear blue sky above, the ripples on the lake surface, and the pattern of tree trunks and reeds along the water's edge. Sensing a clue, he toes the control of the trolling motor and glides slowly toward a shallow cove where a trophy-winning fish is certainly lurking--just out of view.

Indeed, there's something there. But it's not a bass.

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Above:  A fisherman patrols Lake Everett near Ely, Minnesota. Credit and copyright Kevin Judd.

Buried down in the sand and silt of the lake's bottom lies a rainbow of different noxious chemicals--relics of 100 years of industry in the region. The boat's trolling motors stir them up and so do wind-driven waves.  The fisherman doesn't notice what's happening, but a satellite passing 400 km overhead does. It snaps a picture of the lake and beams the data to Earth, where scientists note areas of water that are less reflective than usual--a result of the stirred-up sediments.

City officials and environmental regulators can't wait to see the data. They hope it will help answer some important questions: Are the lake's legendary bass fit for the dining room table? How much sediment is dumped into the lake by the adjoining river? Do pollutants buried in a patch of lakebed near an abandoned paper mill pose any threat to swimmers at a beach on the far side of the lake? And why is this year's tournament a bust?

see captionIn real-life, they'll have to keep waiting. Satellite views of stirred-up murky water (scientists call it "resuspended sediments") aren't yet available to answer their questions. Currently, monitoring suspended sediments is done by hand, a challenge for bodies of water that cover hundreds or even thousands of acres. Scant data gathered at a few monitoring stations provide only a glimmer of what's going on. 

Above: One way to measure water clarity by hand: a "Secchi disk." The photographs show increasing murkiness from left to right. Credit: Minnesota Pollution Control Agency.

Around the country, there are dozens of reasons to monitor stirred-up sediments. Shellfish harvests in Northeastern bays, for example, are affected by sediment levels. So is the rich biodiversity of Atlantic, Pacific and Gulf coastal estuaries. Further inland, nutrients released by stirred-up sediments can nourish microscopic phytoplankton in freshwater lakes and trigger algal blooms that choke-off the lakes' plant and animal life.

This need for wide-area monitoring is what has motivated scientists at NASA's Stennis Space Center in Mississippi to explore how satellites might help. And after 6 months studying Lake Pontchartrain, just north of New Orleans, Louisiana, they think they have a system that works.

see caption

Above: This high-resolution satellite image (Terra/MODIS) shows sediments from the Mississippi River spilling into the Gulf of Mexico. Lake Pontchartrain lies near the top center of the photo. [more]

"We've talked to city planners, [environmental regulators, and other] decision makers--and they've said they would like this," says Richard Miller, chief scientist for NASA's Earth Science Applications Directorate and the manager of the project.

Miller's team monitored Lake Pontchartrain using two instruments in space: NASA's SeaWiFS satellite and NOAA's Advanced Very High Resolution Radiometer (AVHRR). Both measure the reflectance of the water--an indicator of turbidity and stirring.

A certain amount of stirring will occur just because of the action of wind-driven waves. This is called "natural resuspension." To account for it, Miller's group uses a computer model to calculate the expected amount of stirring based on wind speed, wind direction, and the depth and shape of the body of water.

see captionThe computer runs its simulation and "spits out" a number the scientists call the "index of resuspension intensity." Plotted over the area of the body of water (in the form of false colors or contours) this number maps out the expected resuspension due to wind and waves.

Left:  In this false-color map of Lake Pontchartrain red squares denote cloudy water; blue squares denote clear water. [more]

"At least for the environments in Lake Pontchartrain, our index of resuspension intensity correlates really well with our satellite imagery," Miller says. Sometimes, though, they spot suspended particles in a place not predicted by the computer model. Such anomalies might be evidence of human activity--such as fishing in shallow waters--or perhaps a movement of turbid water from another area, set in motion by a passing storm front.

The results so far are "very encouraging," says Miller, but there's more to do. For example, each pixel in the images from these satellites represents one square kilometer on the ground, so the application of this remote sensing technology is currently limited to large bodies of water.

The research team is now starting a new phase of field trials that incorporates a different satellite sensor that has better resolution. Called the MODerate-resolution Imaging Spectro-radiometer (MODIS), the pixels in images from this sensor are only 1/16th of a square kilometer on the ground. MODIS rides aboard two NASA satellites--Terra and the recently launched Aqua -- which together will provide two snapshots per day, one in the morning and one in the afternoon.

see captionThe field trials have also expanded to a new site at Pamlico Sound in North Carolina. In collaboration with Reide Corbett of East Carolina University, this phase of the trials will focus on the effects of fishing trawls or bottom nets.

Right:  A thin line of land separates North Carolina's Pamlico Sound from the Atlantic Ocean. The wetland communities of this area are vital to productive fisheries and water quality. Image credit: Terra/MISR. [more]

Ultimately, the researchers want to construct a system for delivering an executive-summary version of the satellites' observations to the regulators and decision makers who need it. Miller says that his team's goal is to collaborate with decision makers in the region to design a system to suit their needs. He expects that the project could be producing these executive reports in six months' time.

Putting this knowledge into the hands of decision makers will help keep our waterways clean, so that fishers in the future can safely make a meal of the day's catch ... not just a trophy.

Web Links

Earth Science Applications Directorate -- (NASA/Stennis)

Testing the waters: using satellites to monitor lake water quality -- (EarthObservatory.com)

Satellite links: Terra; MODIS; SeaWIFS; MISR

Lake Pontchartrain Basin -- information about bottom sediments and pollutants in Lake Pontchartrain, from the USGS


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