Apical length growth of five intertidal species of Fucales in relation to irradiance

The apical length growth of Pelvetia canaliculata (L.) Dec. et Thur., Fucus spiralis L., Fucus vesiculosus L., F. serratus L., and Ascophyllum nodosum (L.) Le Jolis from the Norwegian west coast (Lat. 63° N) was measured in darkness and in continuous illumination of irradiances from 1.7 to 130 Wm-2. At irradiances of 12.5 Wm-2 and below, all species showed a linear relationship between growth rate and irradiance. A light saturation was found for all species above 30 and 50 Wm-2. When algae, preconditioned to 12.5 Wm-2, were exposed to irradiances of more than 35 Wm-2 for 21 hours, subsequent growth was enhanced in P. canaliculata, while growth rate was reduced in Fucus, most for F. serratus, least for F. spiralis. No such effect was observed for A. nodosum.