![]() Plankton usually give water a green, yellow, blue-green, or brown color. Thus, a small Secchi disk visibility means a water mass is turbid but does not necessarily indicate there is a heavy plankton bloom. Plankton often are the major source of turbidity in pond water, but particles of mineral soils and suspended nonliving organic matter also create turbidity. A more common use of the Secchi disk in aquaculture is to determine the adequacy of the plankton bloom for fish or shrimp culture and serve as an indicator of the need for fertilizer applications. It is excellent for this purpose because Secchi disk visibility is correlated strongly with turbidity. Secchi disk visibility can be used in aquaculture to determine whether light conditions are suitable for the growth of underwater weeds on pond bottoms. The observer should not wear sunglasses while making the measurement.The observer’s face should be within 25-50 cm of the water surface while making the reading.The reading should be taken with the sun behind the observer.The measurement should be made on clear or partly cloudy days when the sun is not obscured by clouds.The average of the two measurements should be used as the Secchi disk visibility. The disk should be slowly lowered until it just disappears from view and raised until it just reappears.Guidelines for correct Secchi disk visibility measurement follow. Guidelines for useīecause Secchi disk visibility is used to compare clarity among water bodies, a standard procedure must be followed in its measurement or serious errors in interpretation can occur. Thus, twice the Secchi disk visibility is a rough estimate of the depth of the photic zone in lakes, ponds, and other water bodies. Generally, there is enough light for plant growth down to about twice the Secchi disk visibility. The greater the Secchi disk visibility, the deeper plant growth can occur in a water body. Limnologists are interested in light penetration because light is needed for photosynthesis. Thus, Secchi disk visibility provides a reasonable way of comparing the ability of different bodies of water to transmit light. ![]() It has been shown that the extinction coefficient is related closely to Secchi disk visibility in meters: K = the extinction coefficient, and z = depth in meters. Where e = base of the natural logarithm (2.303), ![]() Light at depth z = Incident light x e -kz The passage of light through a column of water is described by the equation: Secchi disk visibility usually is reported in centimeters, and in natural waters, the value may vary from a few centimeters to several meters. It is attached to a calibrated line and fitted with a weight so it will sink rapidly. The standard Secchi disk is a 20-cm-diameter disk with alternate black and white quadrants. The Secchi disk since has become a popular limnological instrument for determining the clarity of water. The Secchi disk apparently was named after an Italian sea captain who in the early 1800s used the degree of disappearance of a white disk as an index of water clarity. Still, this test is often executed incorrectly, and the limitations of its interpretation are often not fully understood. It would be difficult to find a pond aquaculture worker who has not measured Secchi disk visibility or at least seen someone measure it. Secchi disk visibility testing likely is the most frequent measurement of water quality made in pond aquaculture. Limnological instrument determines water clarity Although simple in nature, accurate secchi disk visibility testing results can be improved by following a few guidelines. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |