A crude visual method of estimating Kd, commonly used before the ready availability of photoelectric instruments but not used today, is based on the device known as the Secchi disc. A white disc, of diameter 20 to 30 cm, is lowered into the water and the depth at which it just disappears from view is noted. This is referred to as the Secchi disc transparency, or as the Secchi depth, ZSD. On the basis of their measurements in marine waters, Poole and Atkins (1929) made the empirical observation that the Secchi depth is approximately inversely proportional to the vertical attenuation coefficient for downwelling irradiance and pointed out that ZSD could therefore be used to estimate Kd: the value obtained will be that applicable to a rather broad waveband corresponding roughly to the spectral sensitivity curve of the human eye. To calculate Kd the relation Kd = 1.44/ ZSD may be used.580
Using the contrast transmittance theory of Duntley and Preisendorfer, Tyler (1968) concluded that the reciprocal of the Secchi depth is proportional to (c + Kd), the sum of the beam attenuation and vertical attenuation coefficients, rather than to Kd alone. On theoretical grounds, Tyler arrived at the relation (c + Kd) = 8.69/ ZSD; from measurements in turbid coastal sea water, Holmes (1970) obtained (c + Kd) = 9.42/ ZSD. For a number of New Zealand lakes covering a wide range of optical properties, Vant and Davies-Colley (1984, 1988) have also found an approximately linear relationship between the reciprocal of Secchi depth and (c + Kd).
Since, in natural waters, c is usually substantially greater than Kd, the Secchi depth is determined more by c than by Kd. Within some waters, for example in much of the ocean, variations in attenuation and transparency are predominantly due to changes in one component of the system, such as the phytoplankton. As a consequence, Kd and c will tend roughly to covary, and this will account for the approximate constancy of the inverse relationship between ZSD and Kd that is sometimes observed in such cases. A priori considerations suggest, however, that use of the Secchi disc could sometimes give highly inaccurate values for Kd. If, for example, as a consequence of increased levels of particles, light scattering in a water body increased much more than absorption, then c would increase much more than Kd and use of the relation Kd = 1.44/ ZSD would overestimate Kd. There is, in fact, now ample field evidence that the product, KdZSD, is very far from constant, especially in inland waters: it has been found to vary as much as five-fold in New Zealand lakes,286 and seven-fold in Alaskan lakes.736 In agreement with theory, Secchi depth is particularly sensitive to turbidity.
A comprehensive account of the physical and physiological basis of the Secchi disc procedure was given by Preisendorfer (1986a, b), and the subject has also been reviewed by H0jerslev (1986). Preisendorfer concluded that the primary function of the Secchi disc, and indeed its only legitimate raison d'être, is to provide a simple visual index of water clarity in terms of ZSD, or the inferred quantity (c + Kd) (which is ~9/ZSD, and which could reasonably be referred to as the contrast attenuation coefficient707,1403). In the same vein, Davies-Colley and Vant (1988), on the basis of their own extensive studies on the relationship between Secchi depths and other optical properties in 27 New Zealand lakes, propose that this device should be recognized as measuring image attenuation, as distinct from diffuse light attenuation.
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