A study of the environmental factors governing the vertical distribution of intertidal fucoids

The aim of this investigation was to determine the environmental factors that control the limits of vertical distribution of five intertidal species of fucoid algae which form a characteristic sequence of zones on the shore. The death of algae transplanted above their normal limits, together with field observations of the seasonal truncation of the uppermost plants in natural stands, clearly indicated that the upper limits of Pelvetia, Fucus spiralis and Ascophyllum are determined by their ability to survive the prolonged desiccation occasioned by warm dry weather during neap tides. Measurements made under controlled laboratory conditions showed that the upshore species Pelvetia and F. spiralis do not avoid tissue dehydration more effectively than the lower shore species F. serratus. Plants of all three species evaporated water at similar rates, with Felvetia marginally the fastest owing to its peculiar thallus shape. All three species also retained similar amounts of residual water when air-dried to a low water potential. In nature, mutual protection from desiccation appears to be less in mature stands of Pelvetia and F. spiralis than in those of the larger midshore species. However, the vulnerable zyoqoies and young qermlings of the upshore species enjoy protection from extremely rapid dehydration by settling in sheltered microhabitats and growing in dense -carpet-like stands. Drought tolerance was assessed in the laboratory by comparing oxygen evolution rates before drying and on resubmersion after drying, and by measuring growth in culture over a 10 to 15 day period after an experimental stress. All except the low-shore species F. serratus survived dehydration to air-dryness, but the maximum duration for which each species tolerated this stress correlated precisely with its position in the zonation sequence: Pelvetia > F. spiralis > Ascophyllum > F. vesiculosus > F. serratus. Felvetia demonstrated great drought tolerance all year round, and could be air-dried at any temperature and relative humidity which occurs on British shores without immediate harm. F. spiralis often suffered suhlethal, damage upon "being air-dried, and its drought tolerance showed a pronounced seasonal variation. It was found that substantial drought hardening could he induced in this species by daily exposures to a warm dry atmosphere. Other physical conditions during tidal exposure can modify the physiological effects of dehydration. High temperature and high humidity accelerated time-dependent injury in experimentally dried plants, possibly by increasing the rate of adverse metabolic changes. Hence it is actually advantageous for Pelvetia and F. spiralis to dry quickly during exposure to hot weather. Sudden simulated rainfall upon air-dry plants also aggravated drought damage, although rain by itself seemed to exert little effect on fucoids, either in the laboratory or on the shore. With regard to lower limits, both Pelvetia and F. spiralis survived and grew when transplanted to levels below their normal zones. However, Pelvetia, unlike other fucoids, seems to have a physiologically determined lower limit on the shore, as it decayed in winter when transferred to the midshore, and became necrotic when kept constantly submerged in culture. F. spiralis grows much more rapidly in length than Pelvetia and forms a shading canopy beneath which Pelvetia apparently cannot grow. Pelvetia zyqotes settled, germinated and grew normally within the F. spiralis zone when the latter species was experimentally weeded out. However, Pelvetia was never observed to develop into macroscopic plants within natural stands of F. spiralis even where the canopy was broken. There is no evidence that F. spiralis could directly eliminate Pelvetia by either whiplash effect or by shading. Indeed