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.
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