Secchi Disk Station

The Secchi Disk is a tool that measures water clarity and color. Both of these can give an indication of the plankton in the water. The disk is dropped overboard and lowered to a depth to where it can just barely be seen. The depth of visibility (in meters) gives a measure of water clarity. We will then also read the water color as shown against the white of the disk, and record that color using a standard color scale called the Forel-Ule Scale (see picture below).

• Blue water color and high clarity typically indicate little or no plankton in the water. This reading could also indicate mostly zooplankton, since zooplankton are usually not abundant enough to change the color or clarity of the water much. Common local types of zooplankton include copepods, fish eggs, and various types of larvae on benthic (bottom-dwelling) animals such as clams, barnacles, crabs, etc.

o Green water color and low to moderate clarity typically indicate a large amount of phytoplankton in the water, mostly diatoms. The chlorophyll in diatoms (the molecule that lets them capture the Sun’s energy during photosynthesis) gives them a green color, and even though each individual diatom is microscopic and invisible to your naked eye, millions of them can color the water green and reduce the water’s clarity.

• Brown or reddish water color and low clarity also typically indicate a large amount of phytoplankton in the water, but in this case mostly dinoflagellates instead of diatoms. These blooms of dinoflagellates are commonly called ”red tides,” but this is a misnomer because they have nothing to do with tides and are more often brown, not red. The proper term is a harmful algal bloom (HAB), because some species of dinoflagellates are toxic, and their presence in great numbers can be harmful to other ocean life.

Whatever readings you get at the Secchi Disk Station, you can verify what you find when you go to the Plankton Station.

Secchi Disk onboard and ready to be deployed. Be sure the end of the rope is tied to the boat BEFORE you throw it overboard!

The Secchi Disk lowered into the water. Note the green color. What type of plankton might this indicate?

The Forel-Ule Scale: the standard color scale we use to record the Secchi Disk color.

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Modified from pages developed by John Turbeville.