Making and using a Secchi disk

  Taking the measurments   Interpreting your measurements   Secchi depth and water quality

Making the Secchi Disk

A Secchi disk is easy to make, and you can use it to measure the clarity of the water in ponds, rivers, lakes and the sea.

You will need:

• 5-10 m length of string (for use in the sea or clear lakes 20-25m rope may be required)
• Plastic or metal disk about 25-30 cm in diameter (you could use an old CD, but they are a bit on the small side).
• Black and white waterproof paint or permanent marker.
• A weight such as a fishing weight, diving weight or even a brick or stone.
• Tape measure or ruler
• Red, green and blue filters (sweet wrappers or acetate printed with red, green and blue work well).

What to do

1. Drill four holes in the disk and fasten it to the rope and weight as shown in the drawing.
2. Use the plastic tape or marker pens to mark the rope at 0.5 m intervals. It's a good idea to use three different colours - one for 5m marks, one for 1m marks and one for the 0.5 m intervals between the 1m marks.

Now you are ready to go Secchi-dipping!

Making the measurements

Measuring water clarity

Take your Secchi disk to a local body of water - the sea, or a lake, pond or river. (If you are making your measurements from the quayside or a boat, and the water is deep, remember to wear a life jacket - even if you can swim.)

1. Lower the disk into the water using the rope (don't let go!)
2. Watch to see when the disk disappears. You may need to move it up and down a couple of times to determine the exact point at which you stop being able to see it.
3. Pull the disk out and count the number of markers that appear as you haul the rope back in. This gives you the Secchi depth (the distance between the surface and the disk when it disappeared).

The clearer the water, the further you can see below the surface, and the greater the Secchi depth. But other things also affect it. Ripples on the surfae may make it harder to see, for instance. So make a note, not just of the Secchi depth, but also of the conditions you're in.   Is it cloudy or sunny?   Are there waves?.

Measuring water colour

You can do another experiment with your Secchi disk to see how water absorbs different wavelengths (colours) of light.

Lower the disk again, but this time hold a coloured transparent material to your eye so that you are looking through it at the Secchi disk.

Repeat this experiment with each of the red, green and blue filters, and make a note of your Secchi depth for each.

1. Which colour has the greatest Secchi depth?
2. Which colour has the shortest Secchi depth?
3. How do these Secchi depths compare with the Secchi depth for white light (i.e. without filters?)
4. What colour is the water?

Interpreting your measurements

Water clarity

In clear water the Secchi depth will be large. This is often the case in the open ocean, along calm, rocky coasts, and in mountain lakes.

If the water contains a lot of particles, all four Secchi depths - white, red, green and blue will be shallow. The particles scatter light and make it more diffuse - it's a bit like trying too look at your Secchi disk through thick fog.

Water colour

The filter removes all wavelengths of light except those that correspond to the filter colour. So if you are looking through a red filter you are studying what is happening to the red light. The Secchi disk will disappear at the depth where nearly all red light has been lost on the 2-way journey from the water surface to the disk and back up again. Similarly green and blue filters allow you to see how the water affects light at green and blue wavelengths.

	Order of disappearance: red, green, blue Pure water absorbs red light more strongly than green and blue, so in many cases you will find that the red Secchi depth is shorter than the other two. In clear water containing little phytoplankton the blue Secchi depth is greater than either of the other two.
	Order of disappearance: red, blue, green If some phytoplankton cells are present in the water you may find that the green secchi depth is the longest. This is because chlorophyll in the plankton absorbs blue light.   However, blue will also be absorbed if the water contains yellow substance leached from rotting plants and soil.
	Order of disappearance: blue, red or green In some muddy rivers and ponds, blue will disappear quite fast, and red and green may both disappear soon afterwards, at about the same depth. Sometimes - when the water contains a lot of organic material, both green and blue may even disappear before red!

Using Secchi measurement in water quality studies

The Secchi depth can be used to calculate how deep plants can photosynthesis and grow - in UK waters this is about 3 times the Secchi depth.

Secchi disks are still used to measure the optical properties of water as part of ecological studies or the monitoring of water quality. In the USA volunteers with Secchi disks take part in an annual Secchi dip-in, helping to keep an eye on ecologically important lakes, rivers and estuaries.

Clear, clean water may have Secchi depths of more than 30-40m, while in some polluted waters the depth may be 1 m or less.

But a low Secchi depth does not necessarily mean that the water is polluted. All your measurements really tell you is how fast the light is lost because it is scattered by particles (sand, clay, mud or plankton) and absorbed by chlorophyll in phytoplankton or by or organic matter dissolved in the water. To draw further conclusions from your Secchi disk measurement, you need to gather as much extra information as you can about the area and the water you are investigating.

Last update: 17 February 2009

Contact: o4s@noc.soton.ac.uk