The quality of the radiation, i.e. the wave length of the partial radiation, is affected by the density of the layers run by the light. In seawater both infra-red and ultra-violet radiation are absorbed in the surface layers while blue penetrates to the maximum depth (See figure 2).

Different categories of plant make use of different ranges of visual radiation for their photosynthesis, and thus have the ability to colonize different depths of the water column. However, there is a depth beyond which photosynthesis will not be sufficient to cover the energy demands of the plants. This depth, known as compensation depth, is a very important limiting factor from the ecological point of view. Below that depth, organisms are completely dependent on the energy produced in the upper layers.

A problem related to eutrophication processes in coastal waters is the observed increase in light attenuation in surface waters that leads to shifts in distribution patterns of algae as revealed by historical comparisons in Swedish coastal waters .
(See figure 3)

Another ecological aspect related to the drastic decrease in the radiation reaching the deeper layers (reduced tenfold with almost every 75 m of depth) is the problem of vision. Those organisms which live in shallow waters are obliged to cope with problems such as:

• the avoidance of predators during the day
• the finding and identifying of food items
• the recognition of individuals of the other sex for mating

In the deeper layers, reduced radiation leads to specific adaptations, such as:

• enlarged eyes (although there are also species with degenerated eyes)
• increased sensitivity pigments
• over-development of other senses
• the bioluminescence phenomenon.

The latter has been detected in representatives of various animal groups and has obviously occurred through different evolutionary paths. Bioluminescence functions are not completely understood except for some rather obvious ones (attraction of the opposite sex, of prey etc.).