Factors controlling the upper limits of fucoid algae on the shore

Year-round observations on the condition of intertidal seaweeds growing in situ on the shore, show that the upper limits of the zones characterized by Pelvetia canaliculata (L.) Done et Thur., Fucus spiralis L. and Ascophyllum nodosum (L.) Le Jol. were periodically pruned back by environmental conditions. The uppermost plants of each species showed clear signs of tissue damage 21 to 28 days after a time when drying conditions coincided with neap tides which exposed the plant to aerial conditions for long periods. High air temperatures aggravated the damage, but neither frost nor prolonged rain had any obvious adverse effects. On spring tides the plants were wetted every day and no damage resulted regardless of the weather.These species clearly all reach up to their physiological limits on the shore investigated, but presumably Fucus vesiculosus L. and F. serratus L. do not, for they were never observed to show signs of tissue damage attributable to exposure to air. Transplant experiments did, however, prove that F. serratus cannot survive in the F. spiralis zone and nor can F. spiralis persist in the Pelvetia canaliculata zone.Laboratory experiments also demonstrated that the ability to tolerate desiccation and then to resume photosynthesis and growth when re-submerged was greatest in P. canaliculata, the species found highest on the shore, and was progressively less in species inhabiting successively lower levels.