On Celebrating Simplicity: The Secchi Disk

The following article was written by Joseph Hudson, Watershed Specialist with the Erie County Conservation District, and was originally included in the Spring 2020 edition of the "Tracking Invasive Species with Pennsylvania iMapInvasives" newsletter.

Pictured above: Secchi disk. Credit: Mps197

Earlier this year, the Pennsylvania iMapInvasives program announced two initiatives to employ the general public in new and innovative ways to track the occurrence and spread of invasive species. The Water Chestnut Chasers Challenge returns in July, and a new Invasive Species Scavenger Hunt will take place in August. Both programs rely on the participation of citizen scientists, and the utilization of the iMapInvasives mobile phone app. The mobile app allows for the grassroots gathering of scientific information, and reminds us of some of the history and results of similar efforts.

This past April marks the 155th birthday of the Secchi disk (pronounced sek-ee), created by Father Angelo Secchi. The modest black and white patterned circle has been used to interpret basic water quality ever since, and has become a hallmark tool of water and natural resource conservation. The first Secchi disk was 43 centimeters in diameter and plain white. In 1899, engineer and microbiologist George Chandler Whipple standardized the size and added the familiar black and white quarters to the design.

Pictured above: Father Pietro Angelo Secchi. Credit: Reggioreport.it

Pietro Angelo Secchi was born in Reggio Emilia in northern Italy, June 28th of 1818. He entered the seminary and was ordained in 1847. Due to the Roman Revolution of 1848, he and the rest of his order were forced to flee the country, which provided him the opportunity to teach both in the United Kingdom as well as in the United States. In the U.S., he became a student and lifelong friend to Matthew Fontaine Maury, the first Director of the United States Naval Observatory in Washington. Following his return to Rome in 1850, Secchi served as the head of the Observatory of the College at the Pontifical Gregorian University for the rest of his life.

Secchi’s contributions to astronomy cannot be understated. Secchi is sometimes called the “Jesuit father of Astrophysics”, and is better known for his work in astronomy than in water quality. He made the first survey of the spectra of stars and suggested that they be classified according to their spectral type. He discovered several comets and mapped portions of both the moon as well as Mars.

His chief interest was the sun, and he made many important discoveries about its behavior and structure. Indeed, Secchi was one of the first scientists to assert that the sun itself was a star. He was extremely talented in his studies of climate and weather as well. His meteorological work led him to create the first daily weather service in Italy, a service that he would later expand on in France.

After even just a few of these contributions, the Secchi disk seems almost a petty footnote in Secchi’s career. On April 20th, 1865, Pope Pius the IX boarded his personal yacht, the Immacolata Concezione (last vessel of the papal navy, normally used for fighting pirates) for a special demonstration. One of his astronomers had, in his study of the stellar spectra, made an interesting observation on the behavior of light transmission and dispersion through water. Allegedly to the pope’s delight, Secchi lowered the disk from the side of the Immacolata Concezione until it disappeared from sight, and then elevated it until it appeared again. The first Secchi depth had been taken, and the disk has been used to read water turbidity ever since.

The Secchi disk today is still a simple and low-cost method of measuring water clarity. It can be quickly and easily explained to citizen scientists, which directly resulted in the creation of the Secchi Dip-In program at Kent State University. Now managed by the North American Lake Management Society, the Secchi Dip-In has gathered nearly 40 years of data on more than 7,000 different bodies of water. The use of the simple disk has put relatively standardized testing in the hands of people around the world. The image of the Secchi disk is internationally recognized as a symbol of conservation science.

Pictured above: Citizen scientists participate in the annual Great American Secchi-Dip-In. Credit: Coastal Watershed Council

For those not already familiar with the Secchi disk, it may come as a surprise to know there is a distinct relationship between water clarity and the presence of invasive species. The impact of sedimentation on water quality and the respiration of aquatic organisms is generally well understood. Further, particles in water also absorb sunlight, causing an increase in water temperature, which results in lowered dissolved oxygen levels. Bodies of water with increased turbidity therefore pose a greater threat to native species by subjecting them to further habitat stresses – which put them at a greater disadvantage in the competition with invasive species.

Beyond that, recent research shows a direct link between increasing turbidity and the spread of invasive species, relating to the capacity of invaders to transform habitats and increase eutrophication. Secchi depth readings can aid in monitoring the spread of various invasive species at a citizen science level, so long as the relationship between specific organisms and water clarity is understood. A boom in phytoplankton might result in greater turbidity, while a boom in zooplankton might result in greater water clarity. Certainly, the introduction of invasive zebra and quagga mussels (Dreissena polymorpha and D. bugensis) to the Great Lakes watershed has had a widespread impact on Secchi depths. Increased turbidity in water bodies has even been linked to the spread of terrestrial invasive species.

One wonders, now, what Secchi might think of the internet, of global positioning satellites, or smartphones. iMapInvasives has been successfully used to track the introduction, occurrence, and spread of various invasive aquatic and terrestrial species. Much like the Secchi disk of 1865, it is simple and can be explained and used without difficulty by citizen scientists. It puts standardized monitoring in the hands of the general public, creating a virtual army of observers. The impact of successful widespread application of measurable scientific observations cannot be understated. This concept is essential to the success of iMapInvasives, and it falls to us to see it to fruition.

Secchi was certainly not the first person to drop something into the water and watch it disappear. But the amount of research he put into it, and the rigorous testing standards he developed for its use, would forever put his name on the method. It’s certainly worth taking a few moments to celebrate the birthday of a simple flat disk, and to purposefully apply ourselves to the success of its descendants.


About the Author:

Joseph Hudson is the Watershed Specialist at the Erie County Conservation District. A conservation scientist and engineer, Joseph has worked for more than 20 years to improve natural resources management in Pennsylvania.

Joseph is an adjunct professor at the graduate level, traveling lecturer, and has contributed to a number of scholarly articles and programs. He also serves on the board of directors of the Pennsylvania Lake Management Society. Professional interests include Pennsylvania ecology, habitat management, and native species protection. You can contact Joseph by email at jhudson@erieconservation.com.


The Pennsylvania iMapInvasives Program is a partnership of the Western Pennsylvania Conservancy, the Pennsylvania Natural Heritage Program, and NatureServe.

Funding for Pennsylvania iMapInvasives is provided by the Great Lakes Restoration Initiative.

NatureServe logo, iMapInvasives partner
Pennsylvania Natural Heritage Program, iMapInvasives partner
Great Lakes Restoration Initiative logo, iMapInvasives funding source