SUBLETHAL STRESS IN THE INTERTIDAL ZONE - TIDAL EMERSION INHIBITS PHOTOSYNTHESIS AND RETARDS DEVELOPMENT IN EMBRYOS OF THE BROWN ALGA PELVETIA-FASTIGIATA

The effect of tidal emersion on survivorship, photosynthesis and embry onic development was studied in 8 h old zygotes and 7 d old embryos of the intertidal brown alga Pelvetia fastigiata (J. Ag.) DeToni. Zygote s and embryos were outplanted for single low tides in the intertidal z one on the central coast of California (U.S.A.) during June, 1990. Bot h zygotes and embryos exhibited close to 100% survival when outplanted beneath the canopy of adult P. fastigiata. Embryos (7 d old) also exh ibited high survival when outplanted in;a red algal turf, the microhab itat where most successful recruitment occurs. However, zygotes (8 h o ld) experienced high mortality (65-90%) when outplanted in the turf mi crohabitat. Embryos and zygotes that survived emersion experienced sub -lethal stress that temporarily impaired light-saturated photosynthesi s when plants were reimmersed in seawater. The effects of sub-lethal s tress were more pronounced in 8 h old zygotes than 7 d embryos, and mo re severe in the turf microhabitat than beneath the adult Pelvetia can opy. Zygotes outplanted in the red algal turf did not re-establish net photosynthesis until at least 6 h after re-immersion. Photosynthesis was less inhibited in 8 h old zygotes outplanted beneath the adult Pel vetia canopy, and recovered to control (non-emersed) levels within 3 h of re-immersion. Embryos (7 d old) were able to achieve positive net photosynthesis immediately on re-immersion after emersion in the turf or canopy microhabitats. Emersion also retarded the rate of embryonic development in 8 h old zygotes, delaying the formation of primary rhiz oids, which help to attach the plant to the substrate. For example, at 19 h post-fertilization, 75% of control (non-emersed) zygotes had dev eloped rhizoids, compared to 3% and 30% for zygotes outplanted in the turf and canopy microhabitats. The different emersion responses of 8 h old zygotes and 7 d old embryos appeared to be related to their abili ty to tolerate cellular dehydration. Overall, our data suggest that th e effects of sub-lethal stresses may have been underestimated in studi es of intertidal ecology.