An Investigation of Drought Avoidance in Intertidal Fucoid Algae

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Abstract
The possibility that fucoid algae inhabiting the upper shore might possess physiological or morphological mechanisms for avoiding drought, was investigated experimentally. The 4 spp. chiefly investigated characteristically occupy different levels on the shore with Pelvetia canaliculata (L.) Dcne. et Thur. the highest, followed by Fucus spiralis L., Ascophyllum nodosum (L.) Le Jol. and finally Fucus serratus L. Under controlled conditions dehydration proceeded as a roughly log function with time in all species. Pelvetia lost water at a slightly higher flux than the Fucus species, but this did not reflect a difference in epidermal resistance to water loss, for rates of water loss from surfaces composed either of Pelvetia or of F. spiralis plants were identical and about 84% as fast as that from a free water surface of the same area. When dried at water potentials between -395 and -150 bars F. spiralis retained significantly more water per 100 g dry matter than either Pelvetia or F. serratus, but in drier conditions all 3 spp. retained similar amounts. There was a close correlation between percentage dry matter of the thallus and the amount of bound water retained. Pelvetia consistently had the highest dry matter content, and the most bound water, but it did not have the greatest hygroscopic capacity at any of the water potentials tested. The 5 intertidal spp. investigated had very similar hygroscopic capacities, but 2 spp. inhabiting the lowest levels on the shore had lower capacities. In young fucoid algae there was a progressive decrease in the ratio of surface area to mass from lower shore species to upper shore species, but this trend was reversed in the adult plants. The effects of thallus shape were investigated. The flux of water vapor from the more or less cylindrical stripes of Fucus and axes of Ascophyllum was estimated to be almost 3 times that from the flattened laminae of Fucus. In spite of its channelled thallus, Pelvetia lost water faster than Fucus laminae of similar surface to mass ratio. Branch overlap in natural stands was estimated to reduce the area of thalli exposed to the air to only about 20% of that exposed by isolated plants. F. spiralis receives at least as much protection from aggregation as does Pelvetia and since it grows much more rapidly, it attains this protection earlier in its life. Drought avoidance is probably not the primary adaptive mechanism in intertidal fucoids. The tissues of upper shore species can apparently tolerate periodic dehydration to air dryness. Both Pelvetia and F. spiralis survived after losing almost 96% of their water content.