Secchi Disk

The secchi disk measures water clarity and color. Both of these can give an indication of the microbiology found in the water. The disk is dropped overboard and lowered to a depth of 12 meters or less if the water visibility is not very good. The color of the disk at 12 meters (or at the lowest visible depth) is observed and compared with a standardized test kit shown below.

o Green secchi disk readings may indicate a large amount of phytoplankton in the water. Phytoplankton are small floating plants that are green in color. This may be combined with poorer water visibilities. Common local types of phytoplankton include various forms of diatoms.

• Blue/clear secchi disk readings may indicate little or no plankton in the water. This reading could also indicate zooplankton as most zooplankton are clear. Large amounts of zooplankton may limit water visibility to some degree. Common local types of zooplankton include copepods, fish eggs, and various types of larva.

• Red/brown secchi disk readings may indicate a dinoflagellate bloom. These types of blooms called ”red tides” but they have nothing to do with tides. The proper term is a harmful algal bloom (HAB). These blooms severely limit water visibility.

These plankton hypotheses can be verified when we get to the plankton station.

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

This is a slightly green secchi disk reading.

Heres the color scale we use to standardize our color observations.