Photosynthesis of Intertidal Brown Algae During and After Periods of Emersion: A Renewed Search for Physiological Causes of Zonation

CO2-exchange of emersed plants and 0,-exchange of submersed plants were measured in 5 species of brown algae from different tidal heights on shores in Ireland and Helgoland (southern North Sea). The photosynthesis of emersed fucoids and Laminaria digitata increased as up to 25 % of tissue water was lost, but then declined with further desiccation. The relationship between decrease in photosynthesis and loss of tissue water was similar in 3 species of Fucus, and the photosynthetic apparatus of F. spiralisappeared to be no more resistant to desiccation than that of F. serratus. Recovery from severe desiccation took about 2 h in all species, regardless of their typical position on the shore, but the extent of recovery from a given degree of desiccation was greater in upper shore species. Pelvetia canaliculata and F. spiralis showed complete recovery from 80 to 90% water loss, F. vesiculosus from about 70 %, F. serratus from 60 %, and Laminaria digitata from 55 % water loss. The photosynthetic rate of each species after full recovery decreased linearly as water loss increased beyond these values. It is in the extent of recovery of photosynthesis after desiccation that intertidal brown algae show the clearest correlation with their heights in the zonation pattern on European shores.