Secchi Disk: The Scientific Frisbee

It is a humid, overcast evening when the Mystic Seaport van arrives at Buzzards Bay to greet the Morgan floating dockside in the Cape Cod Canal. The atmosphere is quiet, as the students on the Massachusetts Maritime Academy campus have dispersed for the summer. Seaport staff member Erik Ingmundson is a self-described “humanities person,” yet it is he who emerges from the van to perform the evening data collection for the scientific logbook, providing a reprieve for the 38th Voyagers to work on their own individual projects. In his hand is a white object that closely resembles a Frisbee.

The secchi disk can be either all-white or possess this alternating black and white pattern, depending on the waters in which it is used to measure visibility. Source: Wikimedia Commons

Ingmundson walks to the side of the pier, where he has easy access to the water. He lowers the disc, only 12 inches in diameter, with a rope marked at every half meter by different colors of electrical tape. As he slowly feeds out the line into the canal, he watches the white shape fade until it abruptly disappears. He calculates the distance from the surface of the water to the depth of the disc using the markings on the cord, and then repeats the process for good measure. Once certain of his estimate, Ingmundson records this number in the scientific logbook and returns to the van to store the scientific Frisbee until the next hourly entry.

Because it requires the user to remain relatively still for accurate measurement, the Secchi disk is only being used in port on this voyage. Though it appears simple, this device is used to measure water transparency. The Secchi reading, or depth of disappearance once lowered into the water, can be influenced by a variety of factors, both natural and anthropogenic. Suspended particles from erosion can make water cloudy, decreasing Secchi transparency in near-shore areas. Inorganic materials produced by pollution have the same effect. Perhaps more importantly for this voyage, the Secchi reading is also related to plankton density, biomass, and algal production. Its depth indicates where light level reaches 18%. Based on this measurement, a simple mathematical calculation can predict the depth of the 1% light level, at which photosynthesis is no longer possible.

In the absence of teeth, some species of whales feed using this material, known as baleen, to filter small organisms from the water. Source: Wikimedia Commons

This information would certainly have been of use to nineteenth-century whalers, particularly those who hunted baleen whales. Baleen whales have no teeth, but rather numerous plates of baleen that act as a filter system, allowing them to feed on massive amounts of small fish and microscopic zooplankton (animals) like krill as they swim. In waters that host large amounts of phytoplankton (plants), light doesn’t transmit as well due to the presence of millions of tiny organisms. However, the high rate of photosynthetic activity produced by the phytoplankton renders these areas the most productive. Such environments attract small prey species that are, in turn, followed by whales. Lower light transmission, which can be determined by the Secchi disk, is therefore a helpful indicator of whale presence.

Though the odds of a whale sighting while at port in Buzzards Bay were quite low at this time of year, the Secchi disk remains perhaps the most inexpensive and user-friendly method in monitoring general water quality. In fact, whether or not it was widely accessible, this tool was already in use in the days of the whalers. In 1865, a famous Italian astronomer and scientific advisor to the Pope, Father Pietro Angelo Secchi, lowered the first disk from a Papal Navy ship in the Mediterranean Sea to measure water transparency. Since then, this instrument has been used in multiple bodies of water, from local ponds to Buzzards Bay, to determine the health of the ecosystem, particularly in areas with the risk of high anthropogenic impact.