Downward and upward radiation in natural and simulated algal canopies is analysed. The light field is characterized in terms of specific spectral light proportions, i.e. red:far-red, blue:red, green:red and blue:green light. These ratios were selected because they can act as photomorphogenic signals or affect the relative composition of pigments associated with photosystem II and photosystem I. Both downward and upward radiation in algal canopies was enriched by green and far-red light. The red:far-red ratio decreased in algal canopies. In natural shade cuvettes with macroalgae, al1 light ratios decreased compared to that in underwater habitats not colonized with algae. Growth rate of Porphyra 1eucosticta decreased beneath an algal screen produced with Porphyra. In these algal canopies (Porphyra screen), the red:far-red ratio decreased and the green:red and b1ue:red ratios increased. Due to the fact that the photosynthetic active radiation (400-700 nm) was identical beneath the algal screen compared to the control, the decrease in the growth rate was attributed to the changes in the light quality. The thickness of the thallus in algal canopies decreased. This produced an increase in the light absorption per unit of biomass by developing greater photosynthetic surface per unit of carbon fixed. The increase of green:red and b1ue:red ratios, which favour the light absorbed by photosystem II, determined a decrease in the pigment ratios phycoerythrinlphycocyanin and phycoerythrin/chlorophyll. This result is interpreted as an acclimation of the photosynthetic equipment to the light quality. The possible role of the red/far-red ratio as photomorphogenic signals controlling growth rate and thallus thickness, and the possible effect of the changes on the green:red and b1ue:red ratios in the photosynthetic pigment composition in algal canopies is discussed.