JOHNSON, L.E.; LAMOTE, M.; LEMOINE, Y.: Photosynthesis of fucoid algae during periods of emersion: ontogenetic and interspecific responses to varying levels of environmental stress.

Physical conditions experienced during low tide may affect the performance and survival of intertidal algae thereby contributing to patterns of distribution. Qu�bec rocky shores are dominated by three fucoid algae (Ascophyllum nodosum, Fucus vesiculosus, Fucus distichus), which can form distinct zones. We investigated the photosynthetic activity of various stages of these algae as measured by pulse-amplitude modulated fluorometry (PAM) under a variety of different meteorological regimes, including the sunny and windy conditions that exacerbate thallus dehydration. For adult plants, photosynthesis continued for several hours after emersion before photoinhibition (i.e., reduced photosynthetic efficiency, Fv/Fm) was observed. Differences among species corresponded with their position on the shore with adults of the lowest distributed species, F. distichus, showing the quickest and most dramatic evidence of photoinhibition, especially under sunny-windy conditions. Early stages (3 wk-old) of all species responded more quickly than adult stages, and no differences were seen among the different species, suggesting possible ontogenetic shifts in their response to environmental conditions. More fine-scale measurements on F. distichus showed that photosynthesis is sensitive to vertical position on the shore and appears to be determined by rates of evaporation. Because desiccation is controlled by environmental conditions, differences in recruitment observed between different tidal heights and different years may depend on the variation in the frequency, timing, and duration of particularly stressful meteorological conditions that reduce growth and survival.