Modern environmental science often involves a lot of expensive gadgets and measurement tools. But when scientists want to test the visibility in an ocean or lake, they still usually start with a 150-year-old tool called a Secchi disk.
The white or black-and-white disk is roughly one foot in diameter. A researcher slowly lowers the disk into the water. When the disk is no longer visible to an observer standing at the surface, the researcher records that “Secchi disk depth.” Scientists can then use that number to quantify water clarity. Secchi disks have remained popular for so many years because they are so inexpensive and simple to use.
According to ZhongPing Lee, professor of optical oceanography in the School for the Environment, the way that Secchi disk depth is interpreted is simple, easy, and also wrong. In a new publication in Remote Sensing of Environment, Lee and his collaborators showed that the way that Secchi disk depth is interpreted does not take into account the ways in which visibility under water is different than visibility in the air. The interpretation theory that has been in use for over 60 years is based on the Law of Contrast Reduction, which accurately describes how humans see things in the atmosphere, but doesn’t help scientists predict how visible objects will be underwater.
In the new article, Lee and his colleagues proposed a new Law of Contrast Reduction based on radiative transfer, and developed a new theoretical model to interpret and predict underwater visibility. This research not only improves our understanding of underwater visibility and visibility in general, but also significantly advances scientists’ capability to monitor water clarity of aquatic environments via satellite and remote sensing.
“It isn’t a simple thing to peer beneath the surface of the ocean from space. Up until Dr. Lee’s work we used a Secchi disk to tell us how deeply light penetrates into the ocean. And we could only do it one place at a time,” said School for the Environment Dean Robyn Hannigan. “Now we can quantify light penetration depth across vast areas of the ocean using satellite data, meaning that we can for the first time quantify the interactions of light with phytoplankton, and understand how biological productivity works in the ocean — not only at the surface, but at depth. And most importantly we can retire our Frisbees on a string!’”
Though he has upended a longstanding theory in his field, Lee says that the scientific community has responded very positively to this new development.
“My colleagues are very excited,” Lee said. “This new theory will enable the remote sensing of water clarity of global oceans and lakes from satellite observations, which has been a long elusive goal of the community.”