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Secchi disk (SD)
Soil & Water Conservation Society of Metro Halifax (SWCSMH)
July 26, 2006
The Secchi disk transparency measurement is perhaps one of the oldest and simplest of all measurements. But there is grave danger of errors in such measurements where a water telescope is not utilized, as well as in the presence of water color and inorganic turbidity (Vollenweider and Kerekes, 1982).
Value of Secchi disk data:
Relationships between Secchi depth and the concentration of algal chlorophyll pigments do exist. However these relationships are not universal. If silt or dissolved color, not algae, causes the turbidity in the lake, the algal abundance will be overestimated.
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).
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:
In absolutely pure water, the theoretical maximum value would be between 70-80 meters (230-262 ft). The deepest published Secchi disk reading was 66 meters (217 ft) in the Sargasso Sea.. In lakes,, a record depth of 44 m (144 ft) was obtained in Crater Lake using a 1 m diameter disk, and at the same time, a reading of only 39 meters (128 ft) was recorded with a 20 cm disk. The record for the smallest Secchi depth recording was 1-2 cm (0.4-0.8 in) in Spirit Lake, Oregon, after the eruption of Mt. St. Helens. Perhaps the strangest unofficial record for shallow Secchi depths should be given to Bob Kortmann, who, lowering his disk through foam generated by a blue-green die-off, reported a Secchi depth of minus 5 inches; the disk disappeared before ever reaching the water!
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
(.... with salutations to Verschuur, 1997)
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).
Measurements of Secchi depth were undertaken using a standard 20-cm disk divided into quadrants, with observations being made from the deck of a pontoon boat about 45 cm above the surface of the water on the sunny side of the boat. The Secchi depth was taken to be the depth at which the disk was just visible after having been moved up and down several times to establish this position. Data were collected in clusters of 8 measurements spaced at roughly 15- to 20- minute intervals, both with and without the aid of a viewing tube (hence 16 measurements per cluster). The tube was a 10-cm-diameter white PVC pipe open at both ends and inserted in the water to a depth of 30 cm. The upper end was 57 cm above water level. The experiment was confined to calm, sunny days. It was also determined that no statistical difference in apparent Secchi depth existed whether sunglasses or regular spectacles were worn.
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 normalization involves multiplying the observed value (ASD) by a scale factor determined from Eqn. 12 above, given by (1+F)/(2F), which then allows the zenith value, ZSD, to be calculated.
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:
As an example, Table 4 below shows that significant differences due only to local solar altitude will occur between lakes at different latitudes. If the latitude is not taken into account, such differences might be attributed to the properties of the lake waters.
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