Secchi disc

A Secchi disc (named after its creator) is a tool used to measure water clarity. It is lowered into the water while attached to a tape measure or pole with marked measurements. As the disc is lowered from a vessel, the depth at which it can no longer be seen is called the Secchi depth – a measure of water clarity. This can provide an estimation of the amount of phytoplankton at the sea surface. Living at the surface means phytoplankton are vulnerable to changes in sea surface temperature as a result of global climate change. Declines in phytoplankton could cause widespread change in the marine ecosystem as phytoplankton are primary producers – the basis of many marine food webs.

How would scientists estimate phytoplankton populations in relation to Secchi disc measurements?

Join a global citizen science project using Secchi discs to study marine phytoplankton concentrations.

Image acknowledgement: Public domain

Different kinds of Secchi disks. A marine style on the left and the freshwater version on the right.