Water Transparency
Secchi Disk being submerged in pond
However, plankton, both phytoplankton and zooplankton, tends to die and disappear when water is close to freezing. Warmer water supports plankton growth, leading to a loss of water transparency. Therefore transparency measurements are complicated by these strong seasonal variations on top of nutrient variations.
A Secchi disk is a very simple gadget used to estimate the transparency of pond water. It is a disk of 8” diameter, with its quadrants painted alternately in white and black. A string is attached to its center. It is lowered into a pond until it disappears from sight. Then it is raised again. The depths at which it disappears and reappears are averaged and called the Secchi depth. Measurements typically are somewhere between 1 m and 12 m.
Secchi depth data can be interpreted as follows:
- Low amount of nutrients, oligotrophic pond -- Secchi depth greater than 5 m.
- Moderate amount of nutrients, mesotrophic pond – 2.5 to 5 m.
- High amount of nutrients, eutrophic pond -- Less than 2.5 m.
The Secchi disk has a colorful history, Fr. Pietro Angelo Secchi, in the mid- eighteenhundreds, was an astrophysicist, and scientific advisor to the Pope. He was requested to measure water transparency in the Mediterranean Sea by the head of the Papal Navy. Yes, the Pope at that time had a navy. Secchi used white disks to measure the clarity of water in the Mediterranean in April of l865. I must admit, I don’t have the foggiest idea why the papal navy needed this information. But there is no question that Fr. Secchi came up with an ingeniously simple but reliable way to deliver on what he had been asked to do.
We started Secchi depth measurements in 2010, and they are now collected routinely. As our data indicate, there are huge variations in the results, pond water being very clear in winter and early spring, and then rapidly losing transparency in summer and fall. These data agree with the direct nutrient measurements.