Scalar irradiance

For randomly oriented phytoplankton cells, photons are equally useful in photosynthesis regardless of the direction from which they come. Scalar irradiance, E0, is therefore the best all-round measure of the availability of light for photosynthesis at a given depth. For monochromatic light scalar irradiance, like downward irradiance, typically diminishes with depth in an approximately exponential manner, as the linear graph (Fig. 6.9) of log E0 against depth for green light in the Danish Sound570 shows. When scalar irradiance is measured with a quanta meter that responds to the

Scalar irradiance (%) 0.1 1 10 100

Scalar irradiance (%) 0.1 1 10 100

Fig. 6.9 Attenuation of scalar irradiance with depth. The data for Burrinjuck Dam (an Australian inland impoundment) were obtained with a scalar quantum irradiance sensor measuring the full 400 to 700 nm waveband (Kirk, unpublished measurements). The data for the Danish Sound (Baltic Sea) are measurements by Hojerslev (1975) at 532 nm.

Fig. 6.9 Attenuation of scalar irradiance with depth. The data for Burrinjuck Dam (an Australian inland impoundment) were obtained with a scalar quantum irradiance sensor measuring the full 400 to 700 nm waveband (Kirk, unpublished measurements). The data for the Danish Sound (Baltic Sea) are measurements by Hojerslev (1975) at 532 nm.

whole photosynthetic waveband, the variation of log E0 with depth in fairly clear water is biphasic, the rate of attenuation being higher near the surface than lower down. This can be seen in the data for Burrinjuck Dam, Australia, in Fig. 6.9. The explanation is the same as that for the analogous phenomenon observed with downward irradiance (Fig. 6.5): the more strongly absorbed wavelengths are being removed in the upper layer, leaving the weakly absorbed wavelengths to penetrate lower down. Using numerical modelling, Leathers and McCormick (1998) have shown that in highly scattering waters (high b/a ratio) under near-vertically incident light, it is possible for scalar irradiance to at first increase slightly with depth, near the surface, before commencing to decrease. Such behaviour has not yet been described in the field.

Although scalar irradiance is the best parameter in terms of which to express the availability of light for photosynthesis, the most commonly measured parameter is downward irradiance, Ed. It is of interest to examine the relation between them. As might be expected, since scalar irradiance includes both the upwelling and the downwelling light, and since it represents all angular directions equally (whereas downward irradiance, in accordance with the cosine law, is progressively less affected

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