As we saw in Chapters 8 and 9, there are major differences between the main taxonomic groups of aquatic plants with respect to the kinds of photosynthetic pigment present and, as a consequence, major differences in the absorption spectra. Given the variation in intensity and spectral quality of the light field in the aquatic environment, we may reasonably suppose that for any given location within a water body there will be certain species that are well equipped to exploit the particular prevailing light field and others that are not. It thus seems likely that photosynthetic pigment composition could be a major factor determining which species of aquatic plant grow where.
In the case of the benthic algae of marine coastal waters it has been observed that the different major algal groups are not mixed at random. In some parts of the benthic environment browns predominate while in others the reds or the greens predominate, although domination by any type is not usually complete. Furthermore, progressive changes in the proportions of the different algal groups along the depth gradient can often be discerned. It has been a commonly held belief in marine biology since the nineteenth century that the most important factor determining algal zonation is the variation of the light field with depth. This theory has taken two forms, which have, perhaps needlessly, been seen as opposed rather than complementary. According to the chromatic adaptation theory of Engelmann (1883) it is the varying colour (spectral distribution) of the light with depth that determines the algal distribution, i.e. as the predominant colour changes due to selective absorption, those algae that have absorption bands corresponding best to the spectral distribution of the surviving light can photosynthesize most effectively and so predominate. Berthold (1882) and Oltmanns (1892), on the other hand, proposed that it is the varying intensity of the light with depth that determines the distribution of the different algal types.
In fact both the colour and the intensity of the light field change simultaneously with depth and the plants must adapt to both. For example, any plant growing near the bottom of the euphotic zone must be able to make use not only of a restricted spectral distribution, but also of a very low total irradiance. One possible form of adaptation to low irradiance is a lowering of the respiration rate; another would be an increase in the concentration of all the photosynthetic pigments present without alteration of the ratios. Harder (1923) suggested that both chromatic and intensity adaptation are involved: it is not in fact easy to disentangle the two. Nevertheless there is persuasive circumstantial evidence that chromatic adaptation plays an important role in determining the distribution of the different plant groups.
Before we examine this evidence, we must make clear the important distinction between phylogenetic and ontogenetic chromatic adaptation.1097 Phylogenetic adaptation is adaptation that has taken place during phylogeny, i.e. during the evolution of the species. In the present context it refers to the genetically determined differences in pigment composition between the different taxonomic groups of aquatic plants. Within a given species, while the nature of the pigments formed is fixed, the proportion of the different pigments can alter, with significant effects on absorption properties, in accordance with the environmental conditions prevailing during growth and development, i.e. during ontogeny. This is ontogenetic chromatic adaptation. We shall consider both kinds of chromatic adaptation.
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