John T. O. Kirk

Effects of suspensoids (turbidity) on penetration of solar radiation in aquatic ecosystems

In mainland Australia and in southern Africa, the aridity of the climate and sparse vegetative cover increase the susceptibility of the soils to erosion, and as a consequence surface waters are usually turbid. The inanimate suspensoids in such waters, the ‘tripton’ fraction of the limnologist, are responsible for virtually all the light scattering, and also, by virtue of the yellow-brown humic materials adsorbed on their surface, for a substantial part of the light absorption. Spectral absorption data for suspensoids in terms of theirin situ absorption coefficient values, and the contribution of suspensoids to absorption of photosynthetically available radiation (PAR) are given for certain Australian water bodies.To understand the effect of suspensoids on attenuation of the solar flux with depth, the scattering coefficient must also be known, and this can be determined from the nephelometric turbidity or from up- and down-welling irradiance measurements. The effect of particle size on scattering efficiency is discussed.An equation expressing the vertical attenuation coefficient for downward irradiance as a function of absorption coefficient, scattering coefficient and solar altitude is presented, and is used to explore the effects of absorption due to dissolved colour and suspensoids, and the effects of scattering by suspensoids, on the penetration of PAR.Suspensoids, by increasing the rate of attenuation of the solar flux with depth, can greatly diminish the euphotic depth of a water body, with a consequent decrease in the ratio of the euphotic to the mixed depth: thus turbidity can reduce productivity of a water body substantially below that which might be expected on the basis of nutrient availability. Shallow turbid waters of low intrinsic colour can, however, be highly productive. By diminishing the depth of the layer within which solar energy is dissipated as heat, suspensoids can greatly modify the hydrodynamic behaviour of water bodies, and this also has far-reaching ecological consequences.Suspensoids drastically impair the visual clarity of water, a fact of major significance for the aquatic fauna, as well of aesthetic significance for humanity. The reciprocal of the Secchi depth is more correctly thought of as a guide to the vertical contrast attenuation coefficient rather than to the vertical attenuation coefficient for irradiance. The reflectivity of a water body, being at any wavelength proportional to the backscattering coefficient divided by the absorption coefficient, is highly dependent on the concentration, and optical character, of the suspensoids present. This has implications not only for the appearance (colour, muddiness) of the water to an observer, but also for the remote sensing of water composition by air- or satellite-borne radiometric sensors.

Estimation of the absorption and the scattering coefficients of natural waters by use of underwater irradiance measurements

The graphic presentation of relationships between a, b, R, and µ¯ in a previously published nomogram on calculation of inherent optical properties [Aust. J. Mar. Freshwater Res. 32, 533 (1981)] are replaced by explicit mathematical expressions.

Point-source integrating-cavity absorption meter: theoretical principles and numerical modeling

A mathematical model has been developed for photon behavior within a spherical integrating-cavity absorption meter with a point light source at the center of the cavity. Explicit expressions for the average number of collisions with the wall per photon, the average path length per photon, and the transmittance of the cavity containing a water sample relative to that containing pure water are derived for an absorbing nonscattering medium. Monte Carlo modeling shows that the operation of the point-source integrating-cavity absorption meter is essentially unaffected by scattering. Calculations of the performance of the absorption meter as a function of the cavity diameter, absorption coefficient of the medium, and the reflectivity of cavity are presented.

1-Cyano-3,4-epithiobutane: A major product of glucosinolate hydrolysis in seeds from certain varieties of Brassica campestris

Abstract A new hydrolysis product derived from 3-butenylglucosinolate in seeds of certain strains of Brassica campestris Yellow Sarson is described. The structure, 1-cyano-3,4-epithiobutane is proposed. If the seeds are heated at 115° for 30 min before hydrolysis, 3-butenyl isothiocyanate is the main product.

The Nature and Measurement of the Light Environment in the Ocean

The nature — that is, the characteristics, and the properties — of the light environment in the ocean is determined by two things: first, by the nature of the light flux incident on the surface of the ocean from above, and second, by the optical properties of the oceanic water itself. The underwater light environment is what results from the operation of the latter on the former. In this paper we shall consider (a) the nature of the incident solar flux; (b) the inherent optical properties of the ocean, how they are measured and what components of the aquatic medium they are due to; and (c) how the characteristics of the underwater light field are measured, and what these measurements reveal about the light environment in the ocean.

Multiple scattering of a photon flux: implications for the integral average cosine of the underwater light field

It is shown that where micros is the average cosine of scattering, then for any set of photons that undergoes exactly n scatterings per photon, the average cosine after scattering is micro(0)micro(s)(n), where micro(0) is the average cosine of the photon flux before scattering. For a set of photons that has traversed distance d through a medium with scattering coefficient b, the average cosine is micro(0) exp[-bd(1 - micro(s))]. For water bodies in which loss of upward-scattered photons through the surface is small enough to be disregarded, the value of micro(c) (the average cosine of all the photons instantaneously present in the water column) for any given incoming flux of photons with average cosine micro(0) is determined entirely by the inherent optical properties of the water in accordance with micro(c)= micro(0)/[1 + (b/a)(1 - micro(s))], where a and b are the absorption and scattering coefficients.

Effect of scattering and absorption on solar pond efficiency

Abstract This article outlines the minimum set of concepts of hydrological optics required for an understanding of solar energy penetration into bodies of water. The practical application of these concepts to solar ponds, especially by means of the Monte Carlo modeling procedure, is discussed, and an account is given of the optical measurements that need to be made in order to arrive at an understanding of radiation transfer within any given solar pond. The results are presented of a series of Monte Carlo calculations of the behavior of solar radiation within idealized but realistic solar ponds with optical properties covering a wide range of values. The effect on energy collection efficiency of varying the concentration of colored substances, the scattering coefficient, and the albedo of the bottom are explored in detail. The optical criteria that must be satisfied for successful solar pond operation are briefly discussed.

Absorption coefficients of the ocean: their measurement and implications for remote sensing

The spectral energy distribution of the emergent radiant flux from the ocean is determined by the inherent optical properties of the water, especially the absorption coefficients. To interpret the remotely sensed spectral distribution in terms of ocean composition we need a database relating seawater absorption spectra to composition, especially in terms of phytoplankton concentration and type, and soluble and detrital color. Measurement of seawater absorption spectra is difficult because absorption is so low, and because of the confounding effects of scattering. The use of the integrating cavity absorption meter to overcome these problems is discussed, and an account is also given of the potential and limitations of seeking to estimate absorption coefficient values form in-water spectral irradiance data.