Water clarity, or transparency, is commonly measured with a
Secchi disk. The clarity of lake water is reduced by the presence
of suspended sediment and Marl (CaCO3), bits of organic matter,
free-floating algae, and zooplankton. Algae are often the dominant influence on transparency of lake water. Therefore, SDT
is related to the free-floating algae concentration or the primary
biological productivity of a lake and is used as a trophic state
indicator