What is a Secchi Disk?
AWI Field Technician lowering a Secchi disk into the water column.
If you’re familiar with oceanography or limnology, perhaps you’ve heard of a Secchi Disk. Otherwise you’re probably imagining something like a CD or frisbee, and you wouldn’t be too far off. A Secchi is a flat, weighted, disk like instrument, lowered on a tether to measure light penetration and transparency, a proxy for turbidity, in bodies of water. There are many different factors that can affect turbidity and thus affect water clarity, such as suspended sediments, algae, and dissolved organic matter, which can scatter light and alter water color. Low turbidity, or clearer water, means more sunlight can penetrate, while high turbidity, or cloudier water, does not allow as much sunlight to penetrate.
Secchi was named for Father Angelo Secchi, who was an Italian priest, astronomer, and scientific adviser to Pope Pius IX. In fact, the first Secchi Disk was lowered from the papal Yacht “The Immaculate Conception” on the Mediterranean Sea in 1865. In 1865, Secchi used a plain white disk 12” (30 cm) in diameter which remains the standard for marine research. However, in 1899 American Civil Engineer Geroge C. Whipple modified the original disk to one 8” (20 cm) in size and alternating white and black quadrants. This type of Secchi Disk allows for better contrast and is used today in freshwater ecosystems and research efforts like the Adirondack Lake Assessment Program (ALAP). The Secchi Disk is easy to use and a cost-effective tool for assessing water quality and ecosystem health.
So how does it work? Simply lower the disk into the water until it can no longer be seen. Record the depth at which it disappears. Then lower the disk another 30-40 cm and slowly raise the disk until it is just visible again. Record this second depth. The average of these two measurements is your Secchi depth, which helps us to understand how deep light can penetrate the water column. Light penetration is valuable for Limnologists to study, because light is necessary for photosynthesis and plant and algal growth which can tell us how productive a lake or pond is. Photic zones in lakes and ponds, where sunlight can penetrate, are typically double the depth of Secchi Disk transparency.
If you’re planning to grab a Secchi and hit the water, here’s a few things to remember for getting the best results:
Because visibility can be affected by cloud cover and time of day, it is best to take your Secchi measurement around midday and when the sun is not obscured by clouds. Keep your face close to the water while making a reading and remember to take your sunglasses off.
Water conditions can also influence the accuracy of Secchi measurements. Measurements are best taken on calm days and preferably on the shaded side of the boat, to minimize interference from waves and glare.
Have fun!