Light and photosynthesis in aquatic ecosystems: Absorption of light within the aquatic medium

Having discussed how the solar radiation gets through the water surface, we shall now consider what happens to it within the water. Sooner or later most of the photons are absorbed: how this happens, and which components of the medium are responsible, form the subjects of this chapter. The absorption process The energy of a molecule can be considered to be part rotational, part vibrational, part electronic. A molecule can only have one of a discrete series of energy values. Energy increments corresponding to changes in a molecule's electronic energy are large, those corresponding to changes in vibrational energy are intermediate in size, and those corresponding to changes in rotational energy are small. This is indicated diagrammatically in Fig. 3.1. When molecules collide with each other in the liquid or gaseous state, or are in contact with each other in the solid state, there can be transfer of rotational or vibrational energy between molecules and this is accompanied by transitions from one rotational or vibrational energy level to another within each molecule. Molecules can obtain energy from radiation as well as from other molecules. When a photon passes within the vicinity of a molecule, there is a finite probability that it will be captured by that molecule, i.e. be absorbed. If the photon is captured, then the energy of the molecule must increase by an amount corresponding to the energy of the photon. If the photon is of long wavelength (> 20 μm), in the far infrared/microwave region of the spectrum, then its energy is low and its absorption can only cause a transition in the energy of the molecule from one rotational energy level to another.