Secchi Disk Transparency

by Mary Rust and Katie McArthur

Introduction

The secchi disk was invented by the Italian Astronomer Pietro Angelo Secchi circa 1860. In 1865, Pietro Secchi was asked to assist the commander of the papal ship S.S. I'Immacolata Concezione in measuring the transparency of the coastal waters of the Mediterranean Sea (Horne). Secchi experimented with two types of disks, a 43 cm disk of white clay and a 60 cm diameter disk of sailcloth painted white and stretched over an iron ring. Secchi lowered the disks until they became invisible; this depth was thereafter known as the secchi depth (Horne). Secchi tested brown and yellow disks; occasionally he would shade them from the sun with umbrellas or hats. He also tested variations in the secchi depth in calm and stormy waters (Horne).

Pietro Secchi was well aware of the reasons for variations in transparent depth. After his studies on the papal ship, he concluded that the critical factors in the estimation of the secchi depth were the diameter of the disk, the spectral reflectance of the disk, a calm or stormy sea, reflections of the sun and the sky on the water surface, shadows on the submerged light path, and the amount of plankton in the water (Horne).

Modern Limnology Today

The secchi disk remains one of the most valuable tools in the study of Limnology. The secchi disk is one of the most inexpensive methods to measure the transparency of water for lake quality studies. Scientifically, the depth of transparency is the path length in the Beer's law equation through which light is scattered and absorbed as a function of the concentration of the particles in the water (Cooke). According to Wetzel, the secchi disk transparency is a function of the reflection of light from the water's surface. The secchi depth is thus influenced by the absorption characteristics of water and its dissolved and particulate matter. Although a general parabolic relationship exists between dissolved organic matter and transparency, theoretical analyses and empirical observations have shown that the secchi depth is more associated with the particulate suspended matter than dissolved organics (Wetzel). The particulate suspended matter may be algae or other suspended solids.

Secchi depth values can range anywhere from a few centimeters to over 40 meters in very rare lakes (Wetzel). The percentage of surface light intensity occurring at the secchi depth has been given much attention over the last 50 years. The noted boundaries of the depths of observation of secchi disk transparency can fall between 1 to 15% of light transmission (Wetzel).

Description of Disk

In Limnological studies, the average size of a secchi disk is 20 cm. The disk consists of a weighted circular plate painted black and white in contrasting quarters. A calibrated line hangs from a ring in the center of the disk so that the disk hangs horizontally when lowered in the water (Lind).

Figure 1: A Schematic of a Secchi Disk (From Wetzel, Limnology)

Secchi Disk Method

1. Slowly lower the secchi disk into the water until it disappears, and note the depth.

2. Lower the disk further in the water and the raise the disk until it reappears. Note depth.

3. The average of the two readings is the secchi disk visibility depth, Zsd (Lind).

Secchi Depth Measurement Video

When to Take a Secchi Disk Measurement

The secchi disk method is a very useful way of comparing the visibility of different waters, so the method has been standardized. A secchi disk measurement should always be taken off the shady side of the boat between 9 A.M. and 3 P.M. (Lind). According to Cole, the time interval is between 10 A.M. and 2 P.M. The same observer should always take secchi depth measurements in the same manner every time. For example, readings should be taken while standing or sitting, or with or without glasses (Lind). Figure 2 illustrates the changes in secchi depth after sunset, hence the need for standardized methods.

Figure 2: Changes in the Secchi Depth Transparency vs. Light Intensity at the Surface (From Wetzel, Limnology)

References