Measuring Turbidity

Background

What do plants and animals need for growth? One answer is light. Because underwater plants need light too, water clarity is an important factor in wetland habitats. Fewer plants means less food for many animals, and less production of oxygen from plants to help keep the water environment healthy. Young creatures find protection in beds of underwater plants. When water is cloudy, it is said to be turbid. Turbidity is caused when sediment and other particles are stirred up in the water. When loose soil washes into a wetland sediment levels rise. Excess nutrients in runoff promote the growth of algae, which also clouds the water. Turbidity blocks the sunlight so important to aquatic life. Suspended particles absorb heat from sunlight as well, warming the water and decreasing the DO.

Materials

• lids from a tin can at least 15 to 20 cm in diameter
• black and white waterproof paint
• drill or ice pick
• large eye bolt and nut
• heavy string
• tape measurer
• permanent marker
• glass jars

Procedure

1. Make the Secchi discs as follows: paint the can lid white, then put a large black X on the top; drill or punch a small hole in the middle of the X and attach the string to the lid with the eye bolt and nut; and mark off 0.5 meter increments on the string with the marker.
2. At the edge of the water lower the disc straight down into the water until you can no longer see the X. Then, bring the disc up until you can barely see the X and hold the disc there. Reach down and grasp the string right at the surface of the water, and hold the string there until you pull the disc back out. Read the marking on the string to see at what depth to which light could penetrate. Compare readings from several places in the wetland.
3. The GPs readings will be the same, but mark on the data sheet at which locations you were able to take a turbidity reading.