Chesapeake Quarterly
Now You See It, Now You Don't
Student lowering a secchi disk - photo by Sandy Rodgers
The simple Secchi disk is an important tool for studying how light penetrates the Chesapeake. Credit: Sandy Rodgers.

IT'S NOT MUCH MORE THAN a plastic circle attached to the end of a rope. But despite its humble structure (or perhaps because of it), the Secchi disk holds its own among oceanographic instruments.

"It's one of the simplest tools we use, says University of Maryland Center for Environmental Science ecologist Walter Boynton. "And it's an important one."

He should know. Boynton's been dropping the disks overboard for more than three decades of studying water quality in the Chesapeake Bay — following a protocol that has roots in 19th century Europe.

In 1865, Italian Pietro Angelo Secchi, a professor and Jesuit priest, conducted a series of experiments aboard a papal ship at the request of the commander of the Vatican Navy. The commander had read an account by a captain who observed a dish caught in a net 40 meters underwater. He wondered: Could this simple observation apply to studies of the transparency of the sea?

On board the SS L'Immacolata Concezione, Secchi observed how sunlight and shadow influenced when he could and couldn't see a disk lowered into the water below. His experiments and writings led to an established procedure for observing water clarity using the instrument now named for him. Oceanography is not the only field upon which Secchi left his mark. He trained originally as an astrophysicist, and craters on the moon and Mars also bear his name.

The Secchi disks used today by Walter Boynton and others are no doubt similar to the ones first dropped beside the Vatican's steam sloop. Gradations on the line used to lower the disk allow researchers to record the depth at which it first becomes invisible in the water. After lowering the rope a bit more, they then slowly raise it and record when the disk comes back into view. The average of the two measurements yields the Secchi depth.

The Secchi depth provides a measure of water clarity. Waters rich in suspended matter — things like plankton, detritus, and sediment — generally have a shallow Secchi depth reading. The particular mix of sediment and organic material in the Chesapeake has led to increasingly shallow readings in the Bay in recent years (see Shadow on the Chesapeake).

The earliest reliable Secchi measurements for the Chesapeake date back to the 1950s. This affords a historical perspective that Boynton says is useful in an era of high-tech instrumentation. While today's sophisticated tools gather more accurate and precise water clarity data than the comparatively crude Secchi disk, many of these instruments didn't come into use until the 1980s or after, Boynton notes. The Secchi disk's longevity allows a look at trends for particular sampling sites over a longer period of time.

Even so, Boynton wishes he could find regular Secchi measurements from earlier in the 20th century. He suspects this data lie scattered among forgotten notebooks of scientists from long ago and would require considerable sleuthing to piece together.

But it's detective work that could be worth the effort. "I think it would really be a help in trying to understand how turbid the Bay and its tributaries have gotten over a 100-year period," he says.

As Boynton continues to work to understand the Chesapeake's water clarity woes, he seems sure to remain faithful to the easy-to-use, inexpensive Secchi disk.

"It's the one device that my colleagues and I routinely use, but haven't been able to break," he quips.

"It's pretty daggone dependable."

September 2008
vol. 7, no. 3
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