Turbidity is a measure of the transparency of water. It can be measured with a Secchi Disc. A secchi disc is a circular plate divided in quarters alternatively painted black and white. A secchi disc is usually mounted onto a rod that is marked off in centimeters or inches. For secchi disc measurement the disc is lowered into the water and when the disc first disappears from view this depth is recorded as the secchi disc measurement. Both phytoplankton and detrital turbidity can contribute to secchi disc turbidity.
In aquaculture ponds, it is important to maintain the phytoplankton density within a desirable range and, in general, between 30 to 60 cm is considered suitable. If phytoplankton are allowed to grow to very high density (e.g. high turbidity), a population þcrashþ is likely. Since plankton provide much of the oxygen and remove ammonia and CO2, a die-off can have profound effects on the health of the tilapia in the pond.
An abundance of plankton discolors water and reduces sunlight penetration. Plankton is the base of the pond food web and there is a strong relationship between plankton community health and fish production. Secchi disc turbidity measurement is a rapid means to check plankton status in pond or tank water. When monitored twice a week, secchi disc readings, along with visual inspection of the color and appearance of the water, provide a basis for setting water exchange rates. If plankton density increases to a potentially unstable level (< 30 cm), then steps can be taken to reduce density (e.g. increase water exchange; stop fertilizer addition, if being used or treat with an algicide).