XVI International Botanical Congess

Since 7500-3000 BP the Baltic Sea has been brackish and atidal. Fucus vesiculosus occurs in the intertidal zone throughout most of its distribution, but is permanently submerged in the Baltic Sea. Baltic and Swedish west coast (Skagerrak) algae were cross-acclimated in culture at salinities of 6.5 and 20-24 psu. Zygotes of both populations were also grown in the field at 6.5 psu in the Baltic for > 2 years. Chlorophyll fluorescence kinetics monitored in laboratory experiments during and/or following freezing (winter) and desiccation stress (winter and summer) indicated that tolerance to emersion is considerably reduced in Baltic versus Skagerrak algae. This was true of juveniles grown in the Baltic (and therefore in the absence of emersion), and suggests that rapid genetic divergence has occurred between the two populations. Furthermore, the higher growth rate of Baltic juveniles suggests a possible trade-off between stress-tolerance and growth rate.