Light, Secchi disks, and trophic states1

Megard, Robert O.; Settles, J. C.; Boyer, Haley; Combs, W. S.

doi:10.4319/lo.1980.25.2.0373

Cited by 79 publications

(60 citation statements)

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“…9a), is broadly occurring in lacustrine systems (Davies-Colley et al 2003). Earlier lake studies focused on phytoplankton as the primary regulator of Z SD (Megard et al 1980;Tilzer 1983); recent studies, however, have identified the importance of minerogenic particles for a number of systems (Peng and Effler 2005;Swift et al 2006), as depicted for Cayuga Lake. The dominance of clay mineral particles in this regard links this important component of b p to watershed sources.…”

Section: Discussionmentioning

confidence: 99%

Long‐term study of minerogenic particle optics in Cayuga Lake, New York

Effler

Peng

2014

Limnology & Oceanography

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The dynamics of light scattering by minerogenic particles in the upper waters of Cayuga Lake, New York, were characterized for the spring-autumn interval of 8 yr (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) at pelagic and nearshore sites with a scanning electron microscope interfaced with automated image and x-ray analyses (SAX). SAX results were used to estimate the minerogenic scattering coefficient ( Light scattering by particles, a fundamental process regulating radiative transfer in water (Kirk 2011), is important in determining apparent optical properties (AOPs, depend on geometry of the light field) of interest, including clarity (Preisendorfer 1986) and the remote sensing signal (Woźniak and Stramski 2004). The total scattering coefficient (b; m 21 ), corresponding to the integration of the volume-scattering function over all directions (Kirk 2011), quantifies a central feature of the light-scattering regime. b is an inherent optical property, independent of the geometry of the light field. The magnitude and spectral features of the particulate component of scattering, b p (which greatly exceeds that due to water), depend on multiple attributes of a particle population, including the number concentration (N), the particle size distribution (PSD), the composition of the individual particles and their shapes (Babin et al. 2003).The particle populations of aquatic ecosystems are heterogeneous, varying in time and space and differing greatly among systems in response to an array of drivers (Stramski et al. , 2007Peng and Effler 2011). Protocols for resolving the various components of light scattering are needed to understand these differences and dynamics, and to identify their origins and drivers, objectives that are consistent with the reductionist approach advocated by Stramski and co-workers (Stramski et al. 2001(Stramski et al. , 2007 where V is the sample volume, N m is the number of minerogenic particles in a sampled volume of water, and PA m,i is the projected area (m 2 ) of minerogenic particle i. Q bm,i depends on the complex refractive index (m i , function of composition) and size (d i ) of the particle, and the wavelength of light (l). This SAX-Mie approach supports further partitioning of b m into contributions according to size and particularly composition (e.g., clay minerals, quartz, and calcite) of particles Effler 2010, 2011). Early research with the SAX-Mie approach had appropriately focused first on testing the credibility of the forward estimates of b m for a range of particle assemblage conditions (see

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Section: Discussionmentioning

confidence: 99%

Long‐term study of minerogenic particle optics in Cayuga Lake, New York

Effler

Peng

2014

Limnology & Oceanography

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“…1, p. 137) and Gordon and Wouters (1978).] For recent attempts to use regression techniques combined with Secchi disk procedures, see Carlson (1977) and comments on this by Lorenzen (1980) and Megard et al (1980), and finally Carlson (1980). Caution: The regression line results are functions of season, location, local recent meteorological events, and ephemeral lighting conditions, among other things.…”

Section: Ten Laws Of the Secchi Diskmentioning

confidence: 99%

Secchi disk science: Visual optics of natural waters1

Preisendorfer

1986

Limnology & Oceanography

459

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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“…In our studies of the pelagic waters of the eastern Mediterranean (Berman et al 1984a) we found significant correlations between the inverse Secchi depth and both the diffuse downwelling attenuation coefficient and the chlorophyll concentrations in the euphotic zone (Megard et al 1980). We suggested that, within defined oceanic areas, the use of the Secchi disk could facilitate the rapid acquisition of sea-truth data for calibration of airborne remote color sensors.…”

mentioning

confidence: 79%