The effects of brief periodic submergence on intertidal fucoid algae

Abstract

The growth rates of three species of intertidal algae were measured in nature on the Isle of Cumbrae, Scotland and in simulated tidal regimes in the laboratory. In culture the lower shore species Fucus serratus grew fastest when submerged for 11 h out of every 12, and progressively less well with shorter periods of submersion. Winter collected plants of the high shore dwellers Pelvetia canaliculata and Fucus spiralis responded similarly especially in nutrient-poor media, but plants collected in summer grew almost as well if submerged for only one hour out of every 12 as they did when submerged for 11 h. This implies a degree of seasonal adaptation to the low nutrient levels prevailing in the sea during summer. Further experiments emphasized the importance of nutrient shortage by showing that in culture the slow growth induced by only brief daily submergence could be accelerated by increasing the nutrient concentration of the medium in which the plants were submerged. In nature the growth of F. spiralis plants was clearly retarded at levels on the shore where P. canaliculata plants exhibited their fastest growth. There was nothing in their response to nutrient shortage to explain this difference. Nor could P. canaliculata grow more effectively whilst out of water. Both species grew equally slowly if illuminated only when exposed to air, and for rapid growth both require at least a short period of submergence in the light.