A secchi disk is an instrument used for measuring the clarity of water, especially seawater. It consists of a circular plate divided into alternating black and white quadrants and attached to a long measuring tape. The plate is lowered into the water, and the depth at which it is no longer visible from the surface is recorded.
Unfortunately, Secchi disk data is not always the most reliable. This method can be misinterpreted in monitoring programs because people often directly equate Secchi disk readings with algal density. There are, however, many other factors found both inside and outside the lake that affect how deep a person can see into the water.
Inside the lake, water transparency can be reduced by:
microscopic organisms other than algae;
natural or unnatural dissolved materials that color or stain the water; and
Factors outside the lake can also affect a Secchi disk reading. These outside factors can include:
the observer's eyesight and other sources of human error;
the angle of the sun (time of day, latitude, season of the year);
weather conditions (cloud cover, rain); and
Water surface conditions (waves, sun glare, and surface scum).
In sum, the Secchi disk should always be considered simply as an instrument to measure water transparency. Algae can play an important role in reducing transparency; however, this assumption must be proven by measuring a parameter directly associated with the algal population.
A number of programs use a marked line. Volunteers simply read the Secchi depth based on the markings on the line. The advantage of this method is simplicity, but there are several disadvantages to marking the line. Shrinkage could introduce a significant error in the measurement. A number of programs report shrinkage in the line. The possible psychological effect of having markers may be a third, as yet undocumented, error. If the volunteer has a priori knowledge about the Secchi depth based on values obtained during their last visit or if they take replicate samples, this knowledge, combined with the visual clues of depth given by the markings, could bias their readings. Wilson (Personal communication).
The human factor must also be considered. The observer must accurately see and record the Secchi depth. The human eye does not respond exactly to the spectrum of light being reflected by the disk, and, therefore, the disk reading would be different in waters of different color (Preisendorfer, 1986). To correct for this, the observer should view the disk through a Wratten #61 green filter to standardize the color of light entering the eye (Williams, 1970). However, apparently no one has adopted standardized filter glasses.
The chlorophyll A concentration cannot be considered a precise measurement of algal density, however, because the amount of chlorophyll a found in living cells varies among algal species. Thus, two lakes can have identical densities of algae yet have significantly different concentrations of chlorophyll a because they are dominated by different species. Direct comparability, even within a single lake, is further complicated by the fact that the amount of chlorophyll a in an algal cell varies with light conditions. Healthy algal cells constantly try to maintain chlorophyll concentrations at a level for maximum photosynthetic efficiency. Chlorophyll in a cell usually decreases during high light conditions and increases during the night or low light condition.