Inside the Water Column: The Secchi Disk

By Dr. Claude E. Boyd

Pond owners, especially in the south and parts of the Midwest, are advised to apply fertilizer to promote adequate phytoplankton blooms that serve as the base of the food web for fish production. Turbidity from plankton blooms also helps prevent underwater aquatic weed infestations. Of course, excessive plankton can lead to nighttime dissolved oxygen depletion, and north of the Mason-Dixon Line, winterkill during ice covered winter months. A general recommendation for pond fertilization is to apply enough fertilizer to create phytoplankton turbidity to restrict underwater visibility to between 12 and 18 inches (30-45 centimeters). If the water has underwater visibility of 12 to 15 inches or less the scheduled day for fertilization, the fertilizer application should be delayed.

There are various ways of determining underwater visibility. The distance from one’s fingertips to elbow is usually about 18 inches. Thus, the arm can be extended into the water to obtain a rough assessment of turbidity. Others may extend a yard stick in the water until its end disappears. A white object may be placed on the tip of the yardstick to make its end more visible. However, the best way for determining underwater visibility is to measure the Secchi disk visibility.

The Secchi disk is an 8-inch (20-cm) diameter disk with alternate black and white quadrants on its upper face (Fig. 1). There are those Secchi disks that have an entirely white upper face, but the black and white version is more common. The disk is attached to a calibrated line by way of an eyelet in the center of the upper face, and a weight is attached under the disk. The disk is lowered very slowly into the water until it just disappears from view and this depth recorded (in inches or centimeters) from the calibrated line. The disk is then slowly raised and the depth at which it first reappears is recorded. The average of the two readings is the Secchi disk visibility.

The Secchi disk apparently was named for an Italian ship captain who in the early 1800s used the depth of disappearance of a white disk as an index of water clarity. Some reports claim that this individual was the captain of the Papal vessel. The Secchi disk has a long history of use in limnology for comparing the clarity of lakes. It may be purchased from several Pond Boss advertisers or from scientific supply houses or forestry suppliers for a reasonable price—$30 to $75 depending upon quality.

Secchi disk visibility decreases with greater turbidity. This allows it to be used in fish ponds to determine whether light penetration is great enough to favor the growth of aquatic weeds on the bottom. Usually, large aquatic plants will not grow well at depths greater than about twice the Secchi disk visibility.

The more common use of the Secchi disk in fish ponds is to determine the adequacy of the plankton bloom and serve as an indicator of the need for fertilizer applications. Plankton often is the major source of turbidity in pond water, but particles of mineral soils and suspended nonliving or organic matter also create turbidity. Thus, Secchi disk visibility is not a good index of plankton density in all ponds. This is a key take-home point in your management strategy.

Plankton usually gives water a greenish-yellow, blue-green, olive, or brown color. Suspended mineral particles impart a color similar to that of surface soil in the area – usually brown, yellow, or red. Thus, plankton and nonliving particles sometimes give water similar colors. However, when viewed in a clear glass held in bright light, particles of nonliving matter can be distinguished because they are smaller than plankton and more regular in shape. Thus, it is possible to make a rough assessment of the proportion of the particle matter resulting from plankton.

Most water is turbid because of a mixture of mineral particles, organic particles, and plankton. Nevertheless, in a properly designed and managed pond, the main source of turbidity is plankton and dead organic particles (detritus), and Secchi disk visibility is related to plankton abundance.

Water used to fill a particular pond has a certain level of background turbidity. In some ponds, source water may be very clear, while in others it may be noticeably turbid. The addition of nutrients usually stimulates plankton abundance and causes turbidity to increase with a corresponding decrease in Secchi disk visibility below the background level. Secchi disk visibility guidelines for a specific location, such as those presented in Table 1, are not applicable to all situations. Secchi disk visibility associated with plankton levels differs with background turbidity. The guidelines in Table 1 are for water where the background turbidity usually limits Secchi disk visibility to about 60 inches. If background turbidity is greater or less, the criteria in Table 1 should be adjusted. However, the Secchi disk visibility necessary to shade pond bottoms and prevent underwater weed infestations is similar almost everywhere.

Where Secchi disk data are used to evaluate plankton abundance, the change in Secchi disk visibility over time is more telling than the visibility reading itself. When Secchi disk visibility increases as the result of declining turbidity, the change may be caused by declining plankton abundance or the settling of nonliving particles. If observation of the water suggests the declining turbidity is caused by a diminishing plankton bloom, fertilizer should be applied to encourage more phytoplankton. Following a successful fertilizer application, Secchi disk visibility would be expected to decline in response to greater plankton abundance.

Because Secchi disk visibility is used to compare clarity among water bodies, or in the same water body over time, a standard procedure must be followed in its measurement or serious errors in interpretation can occur. Guidelines for correct Secchi disk visibility measurement follow:

• The disk should be slowly lowered until it just disappears from view and raised until it just reappears. The average of the two measurements should be used as the Secchi disk visibility.
• The measurement should be made on clear or partly cloudy days when the sun is not obscured by clouds.
• The reading should be taken at the same time of day for determining changes in water clarity with time in a pond.
• The reading should be taken with the sun behind the observer.
• The observer’s face should be within 25-50 cm of the water surface while making the reading.
• The observer should not wear sunglasses while making the measurement.
• If reading the disk from a boat, the boat should not be moving.

The measurement of Secchi disk visibility is quite useful in evaluating the condition of pond water—especially for timing fertilizer applications. I encourage pond owners to purchase a Secchi disk, but a reasonable facsimile can be made from simple materials.

Dr. Claude Boyd is a renowned aquaculture water chemistry expert retired from the School of Fisheries, Aquaculture and Aquatic Sciences, Auburn University, Auburn, Alabama 36849. Dr. Boyd’s latest book, Handbook for Aquaculture Water Quality, is available in the Pond Boss online store, or you can order one by calling Pond Boss, (800)687-6075 or (903)564-6144.

Table 1. Interpretation of Secchi disk visibility based on experience with sportfish ponds in Alabama.
Secchi disk reading (inches)	Comments
Less than 8	Pond too turbid. If pond is turbid with phytoplankton, there are likely to be problems with low dissolved oxygen concentrations in the early morning. When turbidity is from suspended soil particles, productivity will be low.
8-12	Turbidity becoming excessive
12-18	If turbidity is from plankton, this suggests an adequate phytoplankton bloom.
18-24	Phytoplankton becoming scarce.
More than 24	Water is too clear. Inadequate productivity and danger of aquatic weed problems.