Coral Cam Web Site

Water Clarity                                        Mr. Cesare Filice

OBJECTIVE:   To determine the relative clarity of different bodies of water.

AGE GROUP:   Students' age 10 and up 


Corals form symbiotic relationships with colourful algae. The relationship is mutualistic.  The coral provides the algae with a protected constant environment, while the coral receives nutrients in the form of photosynthetic products.  In order for photosynthesis to occur, sufficient light must reach and be absorbed by the algae.  As a result, corals live near the surface of the ocean.  However, the marine environment may change allowing little light to reach the algae for photosynthesis.

An indirect way to measure the amount of light reaching the coral is to determine the relative penetration of light

(transmittance) through the water using different light sensitive instruments. 

Here we will simulate the a field method for relative light penetration as well as obtain quantitative data for further study. 


bottom of a white plastic cup        string                              ruler   

small weight                                  milk carton                     food colouring

spectrophotometer                        black fine tip permanent marker                                        


1.Using a black permanent marker draw a cross on the cup bottom so that there will be four equal sections ("traingles"). 

2.Using the marker fill in two triangles that are diagonally opposite to each other. 

3.Thread the string through a tiny hole in the center of the cup bottom and tie it off.  The string should be knotted on the unmarked side of the cup bottom. 

4.Attach the small weight to the unmarked side of the cup bottom. 

5.Fill the milk carton with water. 

6.Students will lower the cup bottom and weight (our "Secchi Disk") into the milk carton until the division between the black and white "triangles" becomes blurred. 

7.When the division between "triangles" blurrs, the students should take note of the point on the string at the surface of the water.  They can pinch the string at that point. 

8.Students will then measure the length of the string from that point to the surface of the cup bottom. 

9.Results may then be recorded in table form. 

10.Students then would add a drop of food colouring to the water in the milk carton and stir. 

11.They would then take another depth  reading, and record their data in the table. 

12.Using the spectrophotometer the percent light transmittance of a sample of the diluted dye could be measured and recorded. 

13.Steps 10 to 12 could be repeated many times with many drops of food colouring being added to the water. 

                        SAMPLE DATA TABLE 

                Number           Depth             Percent
                of Drops            (cm)          Transmittance 

                      0                   30                   100
                      1                   28                     93
                      2                   22                     73
                      3                   22                     73
                      4                   22                     73
                      5                   20                     67
                      6                   18                     60
                      7                   16                     53
                      8                   15                     50
                      9                   10                     33
                      10                 10                     33 

14.The data could be plotted and results discussed. 

15.Class discussions should include benefits of each method, the relevance (importance) of this type of investigation, and what are the causes of water turbidity.


Make a large Secchi Disk, 20 cm diameter (the bottom of a bleach or liquid laundry softener jug bottom), with a large weight and long strong rope.  Measure the relative clarity of different bodies of water in the neighbourhood, such as ponds, lakes and streams.

BBSR and TCOE Coral Web Site Team 1999
Funded by a grant from the Goldman Foundation