Disclaimer & Copyright Notices; Optimized for the MS Edge

Secchi disk (SD)

Updated: September 23, 2020           

(............. in memory of a valued deceased member, Lcdr. Tom Willdey of Cadets Canada; photo on right when he was sampling Paces Lake along the eastern HRM; and a link embedded in it)

Contents:

• Introduction
  • Value of Secchi disk data
• History
  • Record Secchi depths
• A proper method of taking a Secchi Disk reading
• The Relationship between Secchi Depth and Solar Altitude
  • The Observations
  • Secchi Depths and Path Lengths
  • Normalization of Secchi Depth data
  • Comparison between geographical locations

Introduction

Value of Secchi disk data:

Davies-Colley et. al. are quoted by Carlson as having found that measurements taken with the disk are no less precise than other water quality measurements. Secchi depth is even a more precise measurement than turbidity, which is the preferred substitute measure.

"In bringing attention to the Secchi disk, volunteer programs have forced a thoughtful reconsideration of the utility of the disk in limnology. It is a one hundred and thirty year old instrument, but it is enjoying a scientific renaissance, largely thanks to volunteer monitoring." (Carlson, 1995).

History

The Secchi disk is perhaps the oldest, the most durable, the most controversial, and potentially indispensable tool of the contemporary limnologist. "Secchi" rhymes with etch-e, not ekki. Secchi (1818-1878) was not an oceanographer, but an astrophysicist. The first Secchi disk was lowered from the papal steam yacht, l'Immacolata Concezione in the Mediterranean Sea on April, 20, 1865. On that April day, Fr. Peitro Angelo Secchi, the scientific advisor to the Pope initiated a series of seven experiments over a six week period at the request of Commander Cialdi, head of the Papal Navy. Marine disks are 40 or 50 cm (16 to 20 in) in diameter, but, in lakes the standard has become 20 cm (8 in). (Carlson, 1995)

Record Secchi depths:

The record for the greatest number of Secchi readings in a single 24 hour period probably is held by volunteer monitor, Gerrit L. Verschuur PhD, who has taken 532 measurements on May 20, 1996 at Garner Lake, Tennessee before sunset (cf. Verschuur, 1997).

A proper method of taking A Secchi disk reading

(Davies-Colley et. al., 1993)

Use a disk of the appropriate size for the clarity range (20 mm for 0.15-0.5 m, 60 mm for 0.5-1.5 m, 200 mm for 1.5-5 m, 600 mm for 5-15 m), painted matte white or in black and white quadrants. Use a graduated line, and attach a weight to hold the line vertical. (Some studies found that, in very clear waters, the disk disappeared, not because of loss of contrast, but because the disk became too small to see. Hence the authors are suggesting that by changing the size of the disk, the apparent size of the disk when it disappears would remain approximately the same).

• Lower the disk on the sunny side of the boat. A underwater viewer (viewscope) is desirable.
• Allow sufficient time (preferably 2 min) when looking at the disk near its extinction point for the eyes to adapt completely to the prevailing luminance level.
• Record the depth at which the disk disappears.
• Slowly raise the disk and record its depth of reappearance.
• The Secchi depth is the average of the depth of disappearance and reappearance. The readings should be made as near to mid-day as possible. The water depth should be at least 50% greater than the Secchi depth so that the disk is viewed against the water background, not bottom-reflected light.

The Relationship between Secchi Depth and Solar Altitude

The data showed that on any given day the apparent (measured) Secchi depth depends on solar altitude. This relationship has been modeled. A key point in the model is that the magnitude of the observed Secchi depth is determined by the trajectory of a beam of sunlight through the water. The model calculations allow the Secchi depth observed at any date, time, or geographical location to be normalized to a standard, the value that would be found with the sun in the zenith. Use of such a normalization offers the opportunity to accurately quantify and study Secchi depth variations over time, or differences between lakes in different geographical locations. Based on the experience gained in this experiment, several suggestions are offered on how to obtain the most accurate measurements on any given day.

If one is interested in accuracy of a few percent, which is achievable if from 6 to 10 measurements are made at any time and averaged, then solar altitude corrections between lakes must be made before comparsions can be undertaken in a meaningful manner (cf., Table 4).

The Observations:

The solar elevation for each cluster of measurements was determined using Voyager, an astronomy program for the Macintosh that allows one to set the astronomical sky at any location, date, and time and then to find the altitude of the sun with respect to the local horizon.

Secchi Depths and Path Lengths:

Normalization of Secchi Depth data:

The shape of this scale factor as a function of solar altitude is shown in Fig. 10 below and the numerical values are given in Table 3 following.

Comparison between geographical locations:

We salute the Chebucto Community Net (CCN) of Halifax, Nova Scotia, Canada for hosting our web site, and we applaud its volunteers for their devotion in making `CCN' the best community net in the world!