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Instrument: SECCHI DISKS
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Description
The Secchi (rhymes with etch-e) disk originated with Fr. Pietro Angelo
Secchi, an astrophysicist, who was requested to measure transparency
in the Mediterranean Sea by Commander Cialdi, head of the Papal Navy. Secchi
was the scientific advisor to the Pope. Secchi used some white disks
to measure the clarity of water in the Mediterranean in April of l865. Various
sizes of disks have been used since that time, but the most frequently
used disk is an 8 inch diameter metal disk painted in alternate black and white
quadrants.

The Secchi disk is used to measure how deep a person can see into the
water. It is lowered into a body of water until the observer loses
sight of it. The disk is then raised until it reappears. The depth of
the water where the disk vanishes and reappears is the Secchi disk
reading.

Even though the Secchi disc measurement of water clarity is an
approximate evaluation of the transparency of water, it is used
primarily for its simplicity. A more accurate measurement of
underwater irradiance can be made by the use of photometer.

A Secchi Disk measures water clarity. Water clarity may be affected by
three different factors -- algae, sediment and / or water color.
One of the major reasons why Secchi disc measurements decrease from
spring to fall, is due to the increase of plankton-- both
phytoplankton and zooplankton. Since zooplankton graze on
phytoplankton, (algae i.e.), the Secchi disc readings may increase
during the summer by the reduction of algae in
the water. Algae blooms may occur when the amount of available
nutrients increases faster than the macrophytes (plants rooted in the
bottom of a lake)can absorb them.

This summary has been adapted from the MSLA page on
"THE SECCHI DISK - WHAT IS IT?"

Additional information on Secchi Disk available at
http://www.pollardwater.com/emarket/pages/L0921002disks.asp