A secchi disk is a universal tool for measuring water clarity, this is otherwise known as turbidity. 

This disk is attached to a rope and slowly lowered into the water column until the disk is no longer visible to the naked eye.  The resulting reading is the Secchi depth or transparency of the water. 

What does this tell us?

The depth at which we stop being able to see the disk would be taken as a measurement of water clarity, this distance may change over time based on number of factors, including;

Presence of suspended sediment, algae or other organic matter.

The variation in these factors is influenced by abiotic factors such as wind, tides and pollutents. 

Using the secchi depth as well as knowledge of past environmental conditions we are able to make an inference as to what may contribiute to the overall clarity of the water.

On the other side of our disks we have a rainbow painted, this is to show the effects of the absorption of light at various depths. this can be used to explain that as the disk descend we are not able to see red after approx. 5 m.  it is difficult to see any other results as the coastlline is rather shallow in long bay unless you paddle a great distance out.