"Intraspecific genetic diversity is the ‘raw material’ that allows species to adjust to a changing world. For predictions of the effects of Global Change, it is crucial to know if low genetic diversity is a result of local adaptation (i.e. if a species has experienced strong selection), or if other processes are responsible. The Baltic Sea is an extreme marine environment and its extreme temperature and salinity gradients are likely to be strong selective forces. A recent review revealed that for many Baltic Sea species, intraspecific genetic diversity was lower than in the North Sea or Atlantic. It remains unknown, however, if genetic diversity declines with increases in environmental stress and/or if marginal populations (assumed to experience highest selection) show the lowest genetic diversity. The proposed research focuses on the selective forces behind diversity loss in the bladder-wrack, Fucus vesiculosus L., a canopy forming keystone species in the Baltic Sea. On the other hand, ecological consequences (i.e. population adaptability) of a changed diversity at distributional edges will be examined. We will study changes in genetic diversity associated with various stresses by first examining the population genetic diversity and structure of F. vesiculosus along the environmental gradients in the Baltic Sea, focusing on latitudinal edges, edges within estuaries and edges on slopes. Secondly, we will conduct laboratory experiments that challenge germlings of F. vesiculosus with different stresses (temperature, salinity, light and nutrient levels). The fitness of different genotypes will be examined under different environments and interactions between factors will be tested. Additionally, genetic diversity of germling populations will be varied to test whether more diverse populations are more tolerant to stress. Finally, in collaboration with other laboratories we will search for genetic polymorphisms that are directly linked to genes with putative function."