Light Extinction

The solar radiation just modelled refers to the energy reaching the earth surface. In aquatic ecosystems, the solar radiation of interest for primary productivity is that penetrating the water surface. Light is one of the main factors affecting plant growth. Because many of the materials frequently dissolved or suspended in aquatic systems absorb or scatter light, light entering at the surface is attenuated as it penetrates the water. Light intensity is therefore a function of depth and of water content and it is essentially defined by the Lambert-Beer law

where I is the light intensity at depth z below the surface. /„ is the light intensity at the surface and y is the light extinction coefficient (L '). The surface light intensity photosynthetically active, used in the algal growth formulations, corresponds only to the visible range, which is typically about 50% of the total solar radiation provided by relation (3.20). Almost all non-visible radiation is absorbed within the first metre below the surface (Orlob, 1977).

The light extinction coefficient is usually defined as a linear sum of several extinction coefficients representing each component of light absorption (water, colour, particulate turbidity due to non-living and living matter such as phyto-plankton). If in the model we can assume that the major cause of the extinction is the self-shading effect of the algal bloom, a linear or quadratic relation to the phyto-plankton concentration of the living matter extinction coefficient can be assumed

Y=Yo + a¡A or y = yü + a/4 + aAh where ya is the extinction coefficient due to all the other factors, A is the algae concentration and a,, a„ b are coefficients related to the self-shading effect.

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