Light requirements and depth zonation of marine macroalgae

Light compensation points (I,) for growth were low (0.3 to 2.5 pmol n r 2 S-') for the temperate marine macroalgae Chondrus crispus, Fucus serratus, Petalonia fascia, Porphyra purpurea and Ulva lactuca measured at 7 'C.These I,-values corresponded to those estimated by a physiological model including light absorption and quantum yield for growth to describe carbon gain, and weight specific dark respiration, dark loss rate and thallus specific carbon (m01 C m-* thallus) to describe carbon loss.Absorption and quantum yield were close to the theoretical maximum for all species and could not explain differences in I,.Respiration and thallus specific carbon varied more than 15-fold and were the main factors responsible for variations in I,.Experimental &-values correspond to 0.12 to 0.61 % of the yearly surface light dose in Denmark (56" N).These values agree with the K of surface light (%SI) available at the depth limits of leathery and foliose macroalgae at different latitudes.Hence, there is no surplus of energy to balance grazing and mechanical losses, and these factors must be of minor importance for macroalgae growing at great depths.A literature review of depth limits for marine macroalgae reveals an upper zone of mainly leathery algae with depth limits of about 0.5 %) SI, an intermediate zone of foliose and delicate algae with depth limits at about 0.10 % SI, and a lower zone of encrusted algae extending down to about 0.01 % SI.This zonation pattern is accompanied by a decrease in thallus specific carbon (i.e.thinner thalli) with increasing depth.The inverse relationship between growth rate at low light and thallus specific carbon suggests that a thin thallus is essential for growth and survival of marine macroalgae at great depths.' calculated from a mean attenuation coefficient of 0.052 m-' for the water column as stated in the paper; a lower attenuation coefficient due to the spectral changes at great depths may increase this value to about 0.0005 %