The Secchi Disk in Turbid Coastal Waters1

Holmes, Robert W.

doi:10.4319/lo.1970.15.5.0688

Cited by 256 publications

(179 citation statements)

References 4 publications

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“…Poole andAtkins (1926, 1929), who were the first to compare measurements of underwater light with transparency measurements, assumed that transparency is inversely proportional to the vertical attenuation coefficient for diffuse underwater sunlight, as predicted by the Lambert-Bouguer law. Many subsequent workers have adopted this assumption, but others (Tyler 1968;Holmes 1970) assumed that transparency depends on the sum of diffuse attenuation and beam attenuation, as proposed by Duntley (1963) and Preisendorfer ( 19 8 6). Apparently there has been no test of which model better describes transparency.…”

Section: Acknowledgmentsmentioning

confidence: 99%

Effects of algae on the Secchi transparency of the southeastern Mediterranean Sea

Megard

Berman

1989

Limnology & Oceanography

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The mean transparency (37.5 m) measured by Secchi disk at "blue-water" pelagic sampling stations in the southeastern Mediterranean Sea during 3 yr was greater than in most other seawater. Our measurements in the Mediterranean and those by others in the English Channel, Atlantic Ocean, and Pacific Ocean obey the Lambert-Bouguer law if it is specified that transparency depends on the vertical attenuation coefficient for downward irradiance in the waveband of underwater sunlight that has lowest attenuance. Mean downward irradiance (+95% C.L.) at the depth where a Secchi disk disappears, calculated from 187 paired estimates of transparency and attenuation coefficients from these oceanic regions, is 22-t 1% of surface irradiance in this waveband. This value is significantly larger than the value (18%) that has been used widely since an early study by Poole and Atkins, who calculated attenuation coefficients incorrectly.As predicted by the Lambert-Bouguer law, reciprocal transparency was proportional to the attenuation coefficient for the most-penetrating waveband. Attenuance of this waveband and reciprocal transparency both increase linearly with increasing concentration of phytoplankton chlorophyll in pelagic water. Chlorophyll concentrations in pelagic Mediterranean water were very low (0.02-O. 14 mg m-3), but attenuance by units of phytoplankton chlorophyll was about 10 times larger than in more productive seawater, probably because blue light was predominant and many of the phytoplankton were very small coccolithophorids (cell diameters < 10 pm) with optically dispersive cell surfaces.Measurements of transparency by Secchi disk are as accurate and precise as estimates of attenuation coefficients calculated from irradiances measured at sea with photoelectric sensors. The Poole-Atkins equation for transparency in terms of the waveband with lowest attenuance describes the variation of transparency as well as the Duntley-Preiscndorfer equation does. Much of the information needed for interpreting satellite ocean-color imagery could be obtained very efficiently with closely spaced measurements of transparency from surface vessels during synoptic cruises.Planktonic algae can cause large variations in the transparency of seawater, especially in the open ocean. It is becoming increasingly important to understand how algae can cause such variability, because there is a global pattern of marine transparency. The transparency map compiled recently by Lewis et al. ( 1988) shows a pattern that is very similar to those of phytoplankton chlorophyll concentrations in- AcknowledgmentsContribution 308 from the University of Minnesota Limnological Research Center and also a contribution from Israel Oceanographic and Limnological Research, Ltd. Financial support provided by the U.S. Aid Program and the University of Minnesota Graduate School.We thank Y. Azov and A. Schneller for their assistance and J. T. 0. Kirk and A. Morel for critical comments and valuable suggestions on the manuscript.

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Section: Acknowledgmentsmentioning

confidence: 99%

Effects of algae on the Secchi transparency of the southeastern Mediterranean Sea

Megard

Berman

1989

Limnology & Oceanography

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“…It is possible to do fieldwork with the disappearing disk and thereby obtain fresh estimates of C, beyond Blackwell's (1946) tables. For example, Holmes (1970) made the first of such estimates and found CT = 0.0014 + 0.0013. Hojerslev (1986) found CY.…”

Section: Air-water Surface Eflectsmentioning

confidence: 99%

“…Obtain a linear regression line K = ~a + r. This yields the slope E and residual r. For example, Holmes (1970) found that E = 0.136, r = 0.114 m-l. [See also Tyler (1968, figure 1) for the range of possible E values.] Such empirical attempts at estimates of E are not unreasonable.…”

Section: Ten Laws Of the Secchi Diskmentioning

confidence: 99%

Secchi disk science: Visual optics of natural waters1

Preisendorfer

1986

Limnology & Oceanography

443

292

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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.

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“…κ and ZSD are the attenuation coefficient at 490 nm and the estimated Secchi depth respectively, κZSD = 1.44 [49]. Here, ZSD was used as a qualitative and readily interpretable measure of water clarity.…”

Section: Depth and Water Column Specific Spectral Librariesmentioning

confidence: 99%

A Method to Analyze the Potential of Optical Remote Sensing for Benthic Habitat Mapping

Garcia

Hedley²,

Tín

et al. 2015

Remote Sensing

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Quantifying the number and type of benthic classes that are able to be spectrally identified in shallow water remote sensing is important in understanding its potential for habitat mapping. Factors that impact the effectiveness of shallow water habitat mapping include water column turbidity, depth, sensor and environmental noise, spectral resolution of the sensor and spectral variability of the benthic classes. In this paper, we present a simple hierarchical clustering method coupled with a shallow water forward model to generate water-column specific spectral libraries. This technique requires no prior decision on the number of classes to output: the resultant classes are optically separable above the spectral noise introduced by the sensor, image based radiometric corrections, the benthos' natural spectral variability and the attenuating properties of a variable water column at depth. The modeling reveals the effect reducing the spectral resolution has on the number and type of classes that are optically distinct. We illustrate the potential of this clustering algorithm in an analysis of the conditions, including clustering accuracy, sensor spectral resolution and water column optical properties and depth that enabled the spectral distinction of the seagrass Amphibolis antartica from benthic algae. OPEN ACCESSRemote Sens. 2015, 7 13158

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The Secchi Disk in Turbid Coastal Waters1

Cited by 256 publications

References 4 publications

Effects of algae on the Secchi transparency of the southeastern Mediterranean Sea

Effects of algae on the Secchi transparency of the southeastern Mediterranean Sea

Secchi disk science: Visual optics of natural waters1

A Method to Analyze the Potential of Optical Remote Sensing for Benthic Habitat Mapping

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