Lesson Title: Light and Water: The Secchi Disk Experiment
Authors: David Palandro and Lucia Odriozola
Grade: 3-6
Subject areas: Marine Science, Mathematics, Environmental education.
SSS: SC.D.2.2, SC.H.1.2, MA.A.4.2, MA.B.3.2, MA.E.1.2
Objective: To understand the importance of water column transparency for life in the
water, and its uses as assessment of water quality and health of the environment.
Duration of activity: One period of 45 minutes
Introduction
Water quality is important for plants, animals and humans, who may eventually swim or
drink that water. The water quality can be inferred from how much light can penetrate
into the water column, or how transparent the water is. If there is a decline in the
transparency of the water, it may be a sign of degrading water quality. A Secchi (rhymes
with Becky) Disk (figure 1) is a white and black, quarters reversed disk. Secchi disks
come in different sizes depending on the environment where they will be used: lakes use
a disk with a diameter of 20 centimeter (8 inches), whereas a larger disk is used in the
marine environment (40-50 cm). The disk is named after father Pietro Angelo Secchi,
the science advisor to the Pope during the mid-nineteenth century. The Papal Navy
wished to test a new device to measure the transparency of water. The first test was
done by Secchi himself, on board the Pope’s yacht in the Mediterranean Sea in 1865.
Today, the Secchi disk is still used in the same way; it offers an inexpensive manner for
scientists to measure the transparency of a water body. This data is important, as it
helps the scientist make certain inferences about the area they are working in. The
Secchi Depth is defined as the depth in the water at which the disk is no longer visible. It
is usually the average between the dept at which the disk was no longer visible
descending the water column and when it reappeared ascending it. Knowing this depth it
possible to calculate the amount of colored substances (i.e., phytoplankton, algae,
detritus) in the water. By taking many measurements over a period of time scientists
can study changes in the water, due to sediment run-off from land or increased
phytoplankton populations. This can give an excellent reference as to the quality of the
water. In this activity, the students will be asked to build Secchi disks and test them. We
suggest a minimum of two but no more than four students per group. Be sure that they
exchange jobs within the group (i.e., Secchi Disk operator, note-taker, etc.). The preand post-experiment discussion allows for as much leeway as desired, and has allowed
the experiment to take up to 80 minutes. The building of the Secchi Disk should take no
more than 15 minutes.
Materials
•
•
•
•
•
•
•
•
Large containers with water (1000 mL beakers*)
Instant coffee
Ruler (with centimeters or inches)
String
Pennies
Permanent black marker
Pencil and datasheet
Duct tape
•
White plastic bottle caps for building the Secchi Disks (instructions below)
*The number of beakers depends on the number of groups designated by the teacher.
This is the preferred option, but the experiment can be done with several or even one
liter or more glass container.
Description of activities:
a) As an introduction, share with the students the history of the Secchi disk, what is
b)
c)
d)
e)
f)
g)
the Secchi Depth, its uses and the importance of good water quality for people
and the environment.
Building a Secchi Disk: Students should have on their tables the white caps,
string, a piece of duct tape, pennies and markers. Find the center of a white
plastic bottle cap and puncture a hole (using a sharp pencil or nail). It is
advisable that the teacher aid the students in this step. Use a ruler to divide the
cap into four quarters, color two of the opposite quarters black using the
permanent marker (use figure 2A and 2B as reference for the students). Slide
the string through the hole and tape the penny, directly under the cap, to the
string. The penny will serve as a weight and as a stopper for the string. Using
the ruler, measure and mark the string every two centimeters up to the height of
the water container being used. The final result should look something like figure
2B.
Preparing the environment: This experiment requires the students to take several
Secchi Disk measurements in different water types. These water types will be:
(1) clear water (2) brown water and (3) dark water. Everyone will start with the
clear water type. Have one of the students in each group fill the designed
container with warm water. Each table should also have a small container with
the instant coffee. If wanted, blue coloring can be added to the water, mimicking
the color of the ocean.
The Experiment: Place the Secchi disk in the clear water and lower slowly; note
at what depth the Secchi Disk disappears. In this case, the students will not see
the disk disappear; most likely the Secchi Disk will hit the bottom and still be
visible. Students should note these observations.
Recording the experiment: Each student should have a datasheet. The datasheet
should include: date, time, lighting in the room during experiment, as well as the
three Secchi measurements taken for each of the different water states. Also, for
the brown and dark water experiment, the number of scoops of instant coffee
added prior to taking the Secchi Depth. An example of such data sheet is shown
below.
For the brown water experiment, have students add five scoops of the instant
coffee to the water, and stir until mixed. Repeat the experiment, lowering the
Secchi Disk in the brown water. Students should note the depth at which it
disappears. After it disappears, bring it back up slowly. Note at what depth the
Secchi disk reappears. The average of these two numbers is the Secchi Depth.
For the dark water experiment, add enough instant coffee until the Secchi Depth
is less than 5 cm, making sure to do this in a methodical manner and noting how
many spoons of coffee were added. Ask the students, as they proceed, to think
about how this would affect the plant and animal life in a lake or ocean, if the
instant coffee were runoff from the land. What would happen to plants and
animals that live on the bottom and cannot move?
h) Conclusions and closure: Have the students present their findings to the class
and explain what would be the causes of dark water in lakes and oceans, and
their impact on the environment and humans as well. This last part can also be a
take home assignment, where they are asked to research this information. Some
questions that can be used for a guided discussion or homework are below.
1. How did the Secchi Depth measurements change with the addition of more
mud? Did the aquarium water become more or less transparent?
2. What else in the water, besides mud, can contribute to lowering the Secchi
Depth? What are the sources of these things?
3. Do you think that each member of the research team lost and then regained
sight of the Secchi Disk at the same time for each water state?
4. How long did it take you to get ten Secchi Depth measurements? Do you
think you were faster towards the end?
5. What does this mean in a real world setting (i.e., lake or ocean)?
6. How can the ‘water quality’ (less transparent water infers lower water quality)
affect the plant and animal life? Both at the surface and at the bottom.
7. What can be done to improve the water quality?
Extension: To build upon this experiment and to introduce a clear ‘cause and effect’ type
experiment, you can have the students graph (by computer or by hand) the amount of
instant coffee in the water on the x-axis and the Secchi Depth on the y-axis. The final
graph will show that, as the amount of instant coffee increases, the Secchi Depth
decreases, showing a measurable cause and effect. In the interest of time, you can
perform this experiment in front of the class, having the students keep track of the
number of scoops of instant coffee added as you go.
If the school is located in close proximity to a lake, slow moving stream, pond or ocean,
students can measure the Secchi Depth of the body of water. An actual Secchi Disk can
be constructed quite easily by using the instructions available at The Great American
Dip-in website (link below). This would allow a class the opportunity to perform and
ongoing experiment that requires discipline in scientific procedure, data collection and
analysis.
Related Links
•
•
•
•
The Great American Dip-in: http://dipin.kent.edu/secchi.htm
Massachusetts Water Watch: http://www.umass.edu/tei/mwwp/secchi.html
Lake George Association:
http://www.lakegeorgeassociation.org/html/secchi_disk.htm
Ocean Voyagers - Sea Scholars
http://voyager.snc.edu/voyager2/summer2001/pictures/secchi/
Figures
Figure 1. Secchi
Disk being used in
the field
2 cm
B
A
height of
beaker
Duct tape
1¢
Figure 2. Top view (a) and side view (b) of Secchi Disk to be made.
Example Datasheet
Group members: One, Two and Three.
Type of water
Clear water
Date and
Time
July 22, 1959
Brown water
July 22, 1959
Dark water
July 22, 1959
Lighting in the
room
Lots of light
coming from
the window
Light from the
lamps above
Little light in the
room
Secchi Depth
No Secchi
Depth
4 cm + 4cm /2
= 4 cm
1.5 cm + 2 cm
= 1.75 cm
Number of
coffee scoops
0
5
18