The photosynthetic activity during resubmergence, following various degrees of desiccation, was measure for Fucus spiralis L., F. vesiculosus L. and F. serratus L., three marine macroalgae forming a distinct vertical zonation pattern along the Swedish west coast. It was found that F. serratus, the species growing in the upper subtidal, desiccated slower than the other two species, rehydrated slower and could not rehydrate fully after the water loss exceeded 70%. Differences in the net photosynthetic capacity among the species during resubmergence were apparent after desiccation to water contents below 30%. Fucus spiralis, growing high up in the intertidal zone, could regain most of its full photosynthetic activity quickly following dehydration to 5% water while F. serratus showed a slower recovery and reduced photosynthetic rates when dehydrating to water contents of 30% or less, and its net gas exchange remained negative (net respiration) during resubmergence following desiccation to water contents below 12%. Fucus vesiculosus, which occupies an intermediate stand in the intertidal zone, showed initial negative gas exchange rates upon resubmergence following dehydration to 5-10% water content but, unlike F. serratus, regained positive rates after rehydrating maximally. In summary, our results indicate that desiccation resulting in critically low (less than 30%) water contents dictates the photosynthetic performance during resubmergence. In concurrence with Dring and Brown (1982), we conclude that the degree of photosynthetic recovery upon resubmergence may be one of the most important responses underlying the conspicuous zonation of Fucus species along tidal gradients.
