Spectral distribution of downward irradiance

The data in Fig. 6.2 show that attenuation of solar radiation in the whole photosynthetic waveband takes place at widely different rates in different parts of the spectrum. As a consequence, the spectral composition of the downwelling flux changes progressively with increasing depth. Figure 6.4a shows the changes down to 25 m depth in the clear oceanic water of the Gulf Stream. Most of the light below about 15 m is confined to the blue-green, 400 to 550 nm, waveband with the peak occurring in the blue region at about 440 to 490 nm. In the coastal/estuarine water of Batemans Bay, Australia, attenuation due to yellow substances in the blue region is comparable to that due to water at the red end of the spectrum, so that at 4 m depth, while there is still substantial radiant flux throughout the photosynthetic range, the distribution peaks markedly at about 570 nm (Fig. 6.4b). In European coastal waters, Halldal (1974) found spectral distributions rather similar to that in Fig. 6.4b, with a marked peak at about 570 nm.

In inland waters with their, usually, higher concentration of yellow substances, the rapid attenuation at the short-wavelength end of the spectrum means a virtually complete removal of blue light within quite shallow depths. In Lake Burley Griffin, Australia, for example, at a time of low water turbidity (Fig. 6.4c) there was essentially no blue light still present below about 2 m depth. In the lower half of the euphotic zone (defined in the following section) in such waters, the photosynthetically available flux typically consists of a broad band extending from the green to the red, often with a peak in the yellow at about 580 nm. Near the surface of the water there are significant levels of blue light available for photosynthesis, but taking the euphotic zone as a whole, the total amount of blue light available for photosynthesis is greatly reduced in such waters (see ยง6.5).

In those coloured, turbid waters in which there are high levels of humic material in the particulate fraction as well as in the soluble state, green light is also attenuated rapidly (Fig. 6.2d), with the result that the downward flux in the lower part of the, very shallow, euphotic zone consists largely of orange-red (600-700 nm) light. In the spectral distribution for Lake Burley Griffin under turbid conditions, shown in Fig. 6.4d, the peak is right over at the far-red end of the spectrum.

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