When light rays penetrate into the water column, they are absorbed, and three phenomena occur that are closely related to each other: a decrease in illumination, a change in the spectral composition of light, and a decrease in the duration of illumination during the day. A significant loss of energy, estimated at about 15%, occurs already at the surface; it is caused, on the one hand, by reflection, and, on the other, by scattering by particles located here. The indicated loss value is given, however, in a rather rough approximation: to a large extent it depends on the state of the sea surface. When the sea is completely calm, losses can be reduced to 5%, and with strong winds — exceed 30%. The amount of light decreases very quickly with depth. If we consider the average data and assume 100% illumination at the level of 2 cm below the surface, then at a depth of 10 m it will be approximately 10%, at a depth of 50 m — 1.5%, at a depth of 100 m — 0.5%. It should probably be assumed, although it is quite approximate, that the illuminated zone in which benthic algae live is limited to a depth where the illumination is on the order of 1% of the surface. As for the relationship between lighting and plant life itself, the main concept here is the compensation depth, which we mentioned in connection with planktonic algae; this depth corresponds to an average illumination at which the values of respiration gas exchange and photosynthesis are equivalent. Algae cannot develop below the compensation depth, as the processes of catabolism take precedence over the possibilities of anabolism.


The average annual compensation depth or the average for a given season is taken into account; it is determined for one plant species or for several. Studies conducted by various authors allow us to give the following estimates: in very turbid waters (in the area of ports) — 6-7 m, in the Baltic Sea — 20 m, in the English Channel - 45 m, in various tropical waters — at least 96-120 m, in the eastern Mediterranean Sea — up to 160 m (and for individual calcareous red algae, there may be even more). Apparently, as a rule, the compensation depth at noon is slightly higher than the average daily value (at most by 4-5 m). The duration of lighting during the day should also be taken into account, and it is shorter in water than in air. transescort.org/trans/india/delhi/