Instructor's Manual - Water Transparency
What is water transparency?
Transparency of water relates to the depth that light will penetrate water. The transmission of light into a body of water is extremely important since the sun is the primary source of energy for all biological phenomena. Light is necessary for photosynthesis, a process that produces oxygen and food for consumers. It is common practice for biologists to consider the depth of the euphotic zone (the upper layers of a body of water into which sufficient light penetrates to permit growth of green plants) to be 2.7 times (roughly 3 times) the limit of visibility. As light penetrates water, it becomes attenuated and altered in its spectral composition. The change that occurs is from predominantly yellow light at the surface to blue-green at depth in clear water or yellow-green in waters having a high concentration of dissolved organic material.
What is a Secchi disk?
The Secchi disk is a very simple device about 20 cm in diameter made of metal or plastic. It is based on the work of Father Pietro Angelo Secchi of Rome, Italy who devised a method for studying the transparency of the waters of the Mediterranean Sea in 1865. Secchi was a science advisor to the Pope and head of the Roman Observatory.
The Secchi disk provides a means for determining the limit of visibility that is based on contrast. The upper surface of the Secchi disk is divided into four quadrants that are alternately black or white. An eyebolt is located at the center of the disk on the upper side so that a line can be tied to the disk. This makes it possible to lower the disk into the water from a boat or dock. A weight is attached to the underside of the disk so that the equipment will sink below the surface. This line is marked every 0.5 meter making it possible to determine the depth at which the Secchi disk disappears from sight as it is lowered into the water.
The Secchi readings are a semi-quantitative measure of water transparency since a variety of factors such as the time of day, sky and water surface conditions, and differences between observers will give varying depths for the same location. It is possible that each person will have a different opinion of the depth at which the disk disappears from sight. That is why it is important that Secchi disk records contain information about the conditions under which the readings were taken.
Standard conditions for Secchi disk measurements include a clear sky, sun directly overhead, and minimal waves or ripples. These measurements must be taken on the shaded and protected side of the vessel. Any deviations from these conditions should be clearly stated in the data. It is interesting to note that visibility in water is roughly twice the Secchi depth since the light must travel twice through a column of water equal in length to the Secchi depth from the surface to the disk and then back up again after being reflected from the disk.
What is the significance of Secchi disk readings?
Secchi disks are standard tools for inland lake monitoring along with measurements of chlorophyll a and phosphorus. Volunteer groups throughout the state and country take Secchi disk readings to indicate the current status of their lake and to compare with data from previous years. The Secchi disk provides a measure of the amount of suspended inorganic and organic matter in the water.
Transparency readings in oligotrophic or low nutrient lakes are often greater than 15 feet (5 meters) whereas eutrophic or nutrient rich lakes have readings less than 7.5 feet (2.5 meters). Water clarity is related to amounts of suspended particles (turbidity) as well as amounts of phytoplankton and zooplankton.
Instructions for use of a Secchi disk:
- A Science Instructor will take selected members of the group to the shaded side of the vessel to perform the Secchi disk readings. If working from a dock, provide some kind of sunshade.
- Make certain that the line on the Secchi Disk is free of tangles.
- Hook or tie the free end of the line or line holder to the railing to prevent losing the Secchi disk overboard.
- Lower the disk into the water. While lowering it, count the number of half-meter marks on the rope as they go below the surface. Lower the disk until it disappears. A team of two or three people working together on this makes it easier. One person handles the line and counts the half-meter marks while the others concentrate on the disappearance and reappearance of the disk.
- Lower the disk an additional half meter, then slowly lift it until it reappears. As the disk is retrieved, count the number of half-meter marks on the rope as they emerge from the water.
- Average the two depths, the depth at which the Secchi disk disappeared and the depth at which it reappeared. Record this value as the Secchi depth. The Secchi depth is also known as the Secchi transparency.
- For GLOBE trips, repeat this procedure two more times. These data can then be entered into the GLOBE hydrology database on the GLOBE Internet site (www.globe.gov).
Page last modified March 20, 2012