Showing papers on "Secchi disk published in 1986"

Secchi disk science: Visual optics of natural waters1

Abstract: The Secchi disk is a circular white disk that is lowered into a natural body of water by a human observer until it disappears from view. The depth of disappearance is a visual measure of the clarity of the water. This review examines the physical and physiological basis of the Secchi disk procedure. The theory of the white disk is detailed to show the underlying assumptions and the consequent strengths and limitations of the procedure. The theory shows how to use a calibrated Secchi disk to predict illuminance levels as a function of depth. In particular it is shown how to predict the euphotic depth of a medium. Ten laws of the Secchi disk are stated verbally and in mathematical form. The laws show how variations in properties of the disk and the surrounding light field affect the depth of disappearance of the disk. Theory and examples lead to the following three main conclusions of this paper: (i) the Secchi disk reading zs,, (in meters) yields a quantitative estimate of a single apparent optical property (CX + K) (in meter-') of a natural hydrosol, where (Y is the (photopic) beam attenuation coefficient and K the (photopic) diffuse attenuation coefficient of the medium; (ii) the primary function of a Secchi disk is to provide a simple visual index of water clarity via z,, or a! + K, (iii) to extend the use of the Secchi disk by auxiliary objective electronic measurements of a or of K, or both, is to risk obviating or abusing this primary function.

The role of biomanipulation in fisheries management

Abstract: Biomanipulation is a relatively new approach toward managing lakes. As a result, few data support its use, and many fisheries managers are reluctant to apply this technology. However, the results of biomanipulation and fisheries renovation programs are often the same: reduced populations of nuisance algae and improved water transparency. In this paper, the relative roles of rough fish elimination and restructuring the plankton–fish mix (biomanipulation) are compared as mechanisms in improving water transparency in lakes. Fisheries survey records from 13 renovated or winterkilled Indiana lakes are used to quantify these relationships. A statistically significant inverse relationship between Secchi disk transparency and percent of planktivorous fish exists for the lakes studied. Possible mechanisms for change are discussed, and recommendations for fisheries management are advanced.

A comparison of water quality in chautauqua lake, new york: 1937–1983

Abstract: Limnological studies undertaken on Chautauqua Lake, N.Y., between 1974 and 1983 indicated that lake water quality can be assessed by monitoring Secchi disk transparency and phytoplankton chlorophyll a. These measurements of phytoplankton abundance were observed to vary from year to year depending on the amount of precipitation; high chlorophyll a and poor transparency correlated with high precipitation, presumably due to increased loading of nutrients to the lake. Comparison of the data collected between 1974 and 1983 with comparable measurements made in the 1930s revealed that Chautauqua Lake water quality has not been substantially altered over the last 50 years. Transparency and the occurrence of blue-green algae were similar, although diatoms may have been more abundant in the 1930s. The results of these limnological studies reinforce the concept that long-term monitoring of algal abundance and species composition can be an appropriate means of detecting changes in lake water quality.