Climate change is challenging marine ecosystems worldwide, severely straining 
the tolerance of marine species and likely leading to distributional shifts. In the 
brackish-water Baltic Sea, there is a strong salinity gradient and pronounced 
seasonality, which together are responsible for its low biodiversity. These 
communities are dominated by very few species which fulfil the key ecosystem 
functions. Therefore, to predict how Baltic Sea communities might change in the 
future, it is first necessary to understand the effects of future changes on these 
key species, particularly with regard to their potential for adaptation. 
I studied the consequences of future climate change, specifically in terms of 
simultaneous hyposalinity and warming, on three of the most important species 
in Baltic rocky littoral communities: Fucus vesiculosus, Fucus radicans, and 
Idotea balthica. Using indoor experiments, I exposed several populations of F. 
vesiculosus and I. balthica (from entrance, central, and marginal regions of the 
Baltic) and one population of F. radicans (marginal region) to both current 
ambient conditions and simulations of future climate (salinity and temperature). 
For both Fucus species, I replicated individuals in order to study variation within 
populations and within clonal lineages in tolerance to the future conditions. 
Furthermore, I analysed how short-term hyposalinity exposure affects gene 
expression in two populations of F. vesiculosus, to reveal the mechanisms 
behind acclimation to low salinity in this species. 
The results of my thesis suggest that the effects of future conditions on F. 
vesiculosus and I. balthica will vary among and within Baltic regions. I found that 
hyposalinity and warming had the strongest effects on populations from the 
northern margin, as indicated by reductions in the survival and growth rate of F. 
vesiculosus and in the survival of I. balthica. These results may suggest that 
future conditions are likely to drive southward the distributional limits of F. 
vesiculosus and I. balthica in the Baltic Sea. Future conditions likewise hampered 
the survival of F. radicans, but actually enhanced the growth rate of the 
survivors. I show that the most tolerant individuals of F. radicans may benefit 
from the future conditions, and thus the species is likely to maintain its 
distributional range and possibly even increase in abundance in the marginal 
region. 
Furthermore, I found both among-population (F. vesiculosus and I. balthica) and 
within-population (both Fucus species) variation in tolerance to climate change, 
indicating the existence of genetic variation in plasticity with respect to future 
conditions. Marginal populations of F. vesiculosus also varied in gene expression 
when exposed to hyposalinity, although in general, the stress response to 
hyposaline conditions included an acute oxidative-stress response, inhibition of 
photosynthetic activity, and higher metabolic rate. Finally, I found that members 
of the same clonal lineage of F. radicans varied in their responses to the climate 
conditions. This result indicates that there may be variation in phenotypic 
plasticity within haplotype lineages in traits responsible for tolerance to 
environmental shifts, despite the putative lack of genetic variation. Standing 
genetic variation in phenotypic plasticity is an important component of 
adaptation, because it provides the variation upon which natural selection can 
act to pass on the successful traits to the next generation. Thus, this potential 
for adaptation may enable the future persistence of these key species, especially 
in the northern Baltic Sea.