Recent evolution in Baltic Fucus vesiculosus: reduced tolerance to emersion stresses compared to intertidal (North Sea) populations

The Baltic is a young, brackish and non-tidal sea, supporting an impoverish ed marine flora compared with adjacent open coastal areas. Populations of t he normally intertidal brown alga Fucus vesiculosus L. are permanently subm erged in the Baltic. We tested the hypothesis that these populations have e volved a reduced ability to withstand water-stresses caused by aerial expos ure (desiccation and freezing), relative to adjacent intertidal populations in the North Sea. Desiccation and freezing tolerance were compared using c hlorophyll fluorescence to monitor photosynthetic status during stress and recovery. To control for the influence of growth salinity on stress toleran ce, the experimental material consisted of either adult algae cross-acclima ted at Baltic and North Sea salinities (6.5 and 20 to 24 practical salinity units [psu], respectively), or juveniles from both populations grown in th e Baltic from embryos (submersed, 6.5 psu). Baltic algae were less able to recover maximum photochemical yield (F-v/F-m) after freezing at -15 degrees C than North Sea algae, and neither acclimation (adults) or growth salinit y (juveniles) accounted for between-population differences. During desiccat ion at 5 degrees C, differences in the response of variable fluorescence (F -v), as well as in initial fluorescence (F-0) and F-v during recovery, indi cated that impaired photoprotective processes may contribute to the inabili ty of Baltic algae to fully recover F-v/F-m after stress, in contrast to No rth Sea algae which displayed dynamic and rapidly recoverable reductions of F-v/F-m. Subsequent desiccation experiments during the summer (at 25 degre es C) showed that, relative to North Sea algae, the effective photochemical yield (Delta F/F-m') of Baltic algae started to decline at lower tissue-wa ter content (TWC) and recovered less completely after a return to seawater. A critical TWC of ca 10% for Baltic populations was identified, below whic h Delta F/F-m' did not fully recover. In addition, Baltic algae were less a ble to regain initial TWC during recovery. These results indicate that, in ca 7500 yr since the recruitment of the present marine flora to the Baltic, F. vesiculosus has evolved reduced tolerance to emersion stresses compared to adjacent intertidal populations.