PHOTOSYNTHETIC ABILITIES OF FUCOID ALGAE DURING LOW TIDE: DIFFERING RESPONSES OF ADULTS AND YOUNG STAGES

Lamote, M. 1 , Johnson, L. E. 1 & Lemoine, Y. 21 G.I.R.O.Q., Université Laval, Québec, G1K 7P4, Canada; 2 Laboratoire de cytophysiologie végétale et Phycologie, Université de Lille I, 59575 Villeneuve d'Ascq, France On the intertidal rocky shores of the Gaspé Peninsula (Québec), three species of fucoid algae (Ascophyllum nodosum, Fucus vesiculosus, Fucus distichus) form distinct zones. One possible underlying cause of this pattern is that the upper limit of each species determined by physiological stresses, especially that experienced by younger stages which are likely to be more susceptible. To assess this possibility, we investigated the photosynthesis activity of these macroalgae using chlorophyll a fluorescence as measured by pulse‐amplitude modulated fluorometry (PAM). Measurements of the responses of both adult and early stages (3‐wk old) were made in the field during low tide under different meteorological regimes, including sunny and windy conditions. In general, algae became more photoinhibited (i.e., reduced photosynthetic efficiency, Fv/Fm) with increasing levels of insolation and desiccation, and early stages showed a more dramatic response than did the adults. Recovery was rapid, especially for the embryos. During sunny, calm conditions adults of the species found lowest on the shore (F. distichus) became photoinhibited more rapidly than those at higher levels. In contrast, the embryos of the highest species (A. nodosum) were the most susceptible to stresses experienced during emersion, especially high levels of insolation. Thus, whereas the physiological characteristics of the adults were consistent with their patterns of vertical distribution, those of the early post‐settlement stage were not, suggesting possible ontogenetic shifts in their capacities to response to environmental conditions.