The Secchi disk was created in 1865 by an Italian priest and scientist, Pietro Angelo Secchi, to measure water turbidity (water cloudiness) in oceans, lakes and ponds. Traditionally, plain white Secchi disks are used for ocean work, and black and white for inland waters.

Activity steps - making the Secchi disk

1. Draw a circle (c. 20cm across) on the plastic lid and cut around the line, so that you have a plain disk,
2. Draw a cross on the topside of your disk and fill in two quarters, to match the pattern shown in the video.
3. Cut a slot in the middle of the disk and push a ruler or marked stick through it, taping or sticking it in place.

Activity steps - measuring turbidity

1. Fill the bucket or rubbish bin with water.
2. Add one spoonful of soil to the bucket and stir in, making sure that the soil is suspended in the water, rather than sitting at the bottom.
3. Place the Secchi disk in the bucket and measure the depth at which you can no longer see the disk. It may be that you can touch the bottom of the bucket and still see the disk.
4. Make a note of this depth. Write 'bottom' if you can still see the disk if it touches the bottom.
5. Repeat this process, adding a total of 10 spoonfuls of soil, making a note of the visible depth of the disk for each spoonful added.

Reflection

• How does the amount of soil affect turbidity?
• How might increased turbidity affect coral? Why do you think this might be the case?

• Piece of white plastic, e.g. the lid of a margarine or ice cream tub
• Scissors
• Waterproof tape
• Marker pen
• Ruler
• Large container, e.g. bucket or trash can
• Some sediment, e.g. soil

Problems with land run-off

This activity helps to understand the impact of increased turbidity from land run-off on coral reefs. Land run-off is the process where soil sediments, nutrients, and chemicals are washed into the sea from the land. The amount of land run-off has increased because of changes in land use, including deforestation, land clearing for agriculture, and urbanisation.

An example of this is the increase of cattle farming in eastern Australia. Trees are cut down to make way for cattle. Their hooves break down the ground into dust, which is worsened by dry weather. All this dusty soil is then washed away during periods of sudden rainfall. The change in the weather patterns in Australia have also contributed to land run-off. Australia is experiencing periods of drought, followed by periods of intense rain.

There are several problems associated with land run-off that affect the Great Barrier Reef. The first is connected to the amount of soil in the water being washed out over the reef. This increases turbidity and sedimentation. Turbidity is the measure of the cloudiness of the water, and if the water is cloudy, less light can get through to organisms that need sunlight for energy. This includes the coral zooxanthellae, which provides corals with up to 90% of their energy. Sedimentation is when the soil settles on the bottom of the reef and can smother coral and other reef organisms.

To find out more about how corals get their energy, see the Coral feeding game.