Optical classification of natural waters

Natural waters vary greatly in the extent to which they transmit solar radiation and it is useful to have some broad indication of the optical character of a water without having to fully specify all the inherent optical properties. Jerlov (1951, 1976) has classified marine waters into a number of different categories on the basis of the curve of per cent transmittance of downward irradiance against wavelength. He recognized three basic types of oceanic water (I, II and III) and nine types of coastal water (1 to 9), in order of decreasing transmittance: the spectral variation of percentage transmittance for some of these water types is shown in Fig. 3.14.

Jerlov's pioneering measurements were, however, made with broadband colour filters, and the curves obtained with modern submersible spectroradiometers are in some cases in poor agreement with his. Pelevin and Rutkovskaya (1977) proposed that, instead, ocean waters be classified in terms of the vertical attenuation coefficient (base 10 logarithm) for

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300 350 flOtl «0 500 faSG too GH? 7nn Wartlengih InmJ

Fig. 3.14 Transmittance per metre of various optical types of marine water for downward irradiance. The waters are Jerlov's oceanic types I and III, and coastal types 1, 5 and 9 (data from Table XXVI of Jerlov, 1976).

irradiance at 500 nm, multiplied by 100. Since the Kd versus l curves vary in a fairly systematic manner from one oceanic water to another, the value of Kd (500 nm) would indeed convey quite a lot of information about any such water. However, given that all definitions of optical properties for natural waters are now standardized to use logarithms to the base e rather than base 10,402 it would be preferable to modify Pelevin and Rutkovskaya's proposal so that the water type was specified by 100 Kd (500 nm), where Kd is based on loge, in accordance with the definition in eqn 1.20. This has the advantage that the number obtained is approximately equal (if attenuation is not too intense) to the percentage of downwelling light that is removed. For example, Jerlov's oceanic waters I and III, with irradiance transmittance values at 500 nm of 97.3% and 89%, corresponding to irradiance diminution of 2.7% and 11%, have 100Kd(500 nm) values of 2.7 and 11.6. One of the oceanic parameters that is now commonly mapped by ocean colour remote sensing is Kd(490) (Chapter 7, § 7.5), which is close enough to Kd(500) for Pelevin and Rutkovskaya's classification system to be put into effect.

Smith and Baker (1978b), on the basis of their measurements of the spectral variation of the vertical attenuation coefficient (Kd) for irradiance in various ocean waters, have concluded that in regions away from terrigenous influences, the attenuation (apart from that due to water) is mainly due to the phytoplankton and the various pigmented detrital

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300 350 flOtl «0 500 faSG too GH? 7nn Wartlengih InmJ

Fig. 3.14 Transmittance per metre of various optical types of marine water for downward irradiance. The waters are Jerlov's oceanic types I and III, and coastal types 1, 5 and 9 (data from Table XXVI of Jerlov, 1976).

products that covary with it. They suggest that for such ocean waters, the total content of chlorophyll-like pigments provides a sufficient basis for optical classification, since on the basis of the pigment content, the curve of Kd against wavelength can be calculated. A classification that has been found useful in the context of remote sensing of the ocean is that into 'Case l' and 'Case 2' waters, put forward by Morel and Prieur (1977), and further refined by Gordon and Morel (1983). Case 1 waters are those for which phytoplankton and their derivative products (organic detritus and dissolved yellow colour, arising by zooplankton grazing, or natural decay of the algal cells) play a dominant role in determining the optical properties of the ocean. Case 2 waters are those for which an important or dominant contribution to the optical properties comes from resuspended sediments from the continental shelf, or from particles and/or dissolved colour in river run-off or urban/industrial discharge. Case 1 waters can range from oligotrophic to eutrophic, provided only that the particulate and coloured materials characteristic of Case 2 waters do not play a significant role. In Case 2 waters, phytoplankton and their derivative products may or may not also be present in significant amount.

A crude optical classification applicable mainly to inland waters was proposed by Kirk (1980b) on the basis of measurements of the absorption spectra of the soluble and particulate fractions from Australian water bodies. In type G waters, the dissolved colour (gilvin), at all wavelengths in the photosynthetic range, absorbs light more strongly than the particu-late fraction: examples are shown in Figs 3.6a, b and e. In type GA waters, gilvin absorbs more strongly than the particulate fraction throughout the shorter wavelength part of the spectrum, but the absorption coefficients of the particulate fraction exceed those of the soluble fraction at the red end of the spectrum due to the presence of substantial levels of algal chlorophyll: Figs 3.6c and f are examples. In type T waters, the particulate fraction, consisting mainly of inanimate material (tripton), absorbs light more strongly than the soluble fraction at all wavelengths: turbid waters with large amounts of suspended silt particles (Figs 3.6d and g) are in this category. In type GT waters, absorption by the soluble and particulate fractions is roughly comparable throughout the photosynthetic range: the waters in certain billabongs in tropical Australia have been observed to be in this category. In highly productive waters, the absorption coefficients due to algal biomass may exceed those due to dissolved colour (and water itself). Measurements by Talling (1970) of the spectral variation of the vertical attenuation coefficient of water from Loch Leven, Scotland, suggest that this eutrophic lake, dominated by Synechococcus sp., was in this category (type A) at the time: the waters of the upwelling area off West Africa, studied by Morel and Prieur (1975), also appear to be of type A. Most oceanic, and some coastal, waters (Fig. 3.12e) are so non-productive, and so free of silt and dissolved colour, that water itself is the dominant light absorber: they may be categorized as type W. Such waters are rare inland: one example is Crater Lake, Oregon, USA.1253 In certain estuarine and the more coloured coastal waters, the gilvin absorption at the blue end of the spectrum can be roughly similar in magnitude to the water absorption at the red end. Such waters, for example that in Fig. 3.8, may be classified as WG.

A water body can change from one optical type to another. Heavy rain in the catchment with consequent soil erosion could, for example, quickly change a type G water (gilvin dominated) to a type T water (tripton dominated); development of an algal bloom could change it to a type GA water. Nevertheless, some water bodies have water of a particular type most of the time. Bog lakes are typically type G all the time. Shallow, wind-exposed lakes with unconsolidated sediments are likely to be of type T all the time. Marine waters are, apart from the effects of the yearly phytoplankton cycle (in non-tropical areas), generally constant in their optical properties.

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