Of the factors that limit the rate of primary production in aquatic ecosystems – light, nutrients, carbon dioxide and temperature – the one that shows the most extreme variation within the aquatic medium is light. As we have seen (Chapter 6) the irradiance decreases with depth from intensities that are so high as to be damaging down to levels that cannot support photosynthesis, and the spectral distribution of the light also changes markedly. We have also seen that at any given depth the intensity and spectral quality of the light can vary greatly in accordance with the optical properties of the water. Furthermore, to a much greater extent than the other limiting factors, light availability varies with time: both within the day – from darkness to the full noon Sun, and as clouds pass across the Sun and with the seasons during the course of the year.

In this chapter we shall consider the ways in which the aquatic flora is adapted to this variability of the light climate.

Aquatic plant distribution in relation to light quality

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.