Of the light energy incident on the water surface, only a small fraction is converted to chemical energy in the form of aquatic plant biomass. We shall now consider the reasons why this is so.
The first mode by which energy is lost is reflection at the surface. As we saw earlier (§2.5, Table 2.1), however, such losses are small. For that range of solar angles at which most aquatic primary production takes place, only 2 to 6% of the incident light is lost by surface reflection. Thus the main causes of inefficiency of light utilization are to be found beneath the water surface.
In shallow water bodies (very shallow in coloured and/or turbid waters; moderately shallow in clear waters) substantial amounts of light reach the bottom. Some is absorbed, some reflected, the proportion depending on the optical characteristics of the substrate. Of the bottom-reflected light, a fraction will succeed in passing up through the water column again and escaping through the surface. Thus in shallow waters, bottom absorption and bottom reflection, followed by surface escape, are mechanisms preventing utilization of some of the light in photosynthesis. The light lost can be anything from a trivial proportion up to nearly 100% in, say, very shallow clear water over a white sandy bottom. Our main concern here, however, is with optically deep waters in which the fraction of the incident light that penetrates to the bottom is negligible. In such waters, most of the light that penetrates the surface is absorbed within the aquatic medium. A fraction of the light, however, usually small, is back-scattered upwards within the water (see §6.4) and some of this succeeds in passing up to, and out through, the surface. Combining data for the irradiance reflectance just beneath the surface (§6.4) with the fact that about half the upwelling flux is reflected down again at the water-air boundary,41 we may conclude that the amount of incident PAR lost in this way is 1 to 2.5% in oceanic waters, 1 to 10% in inland waters of low to high turbidity and as little as 0.1 to 0.6% in waters with intense colour but low scattering.
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