| Summary: | Marine macroalgae are important foundation species on rocky shores. The large, habitat-forming species, in particular support a variety of associated flora and fauna. The Baltic Sea is naturally species-poor due to brackish water, and perennial, large macroalgae such as Fucus vesiculosus have high ecological importance and are characterized as foundation species in hard substrate bottoms. In the Baltic Sea, climate change has been predicted to result in elevated seawater temperatures, declining salinity, caused by increases in rainfall, coastal eutrophication and ocean acidification (OA). These changes may be harmful for macroalgae either directly or through interacting effects. This thesis investigates the potential effects of climate change on the Baltic macroalgae, focusing on the foundation species Fucus vesiculosus. Several ecosystem-level effects emerge from the results of Chapter I. The predicted changes brought about by climate change, declining salinity, increasing eutrophication and more frequent heat waves will likely be highly harmful for perennial foundation species such as brown (e.g. fucoid) and red algae, and favour fast-growing, green filamentous species. This can cause alterations in rocky shore ecosystems especially in the northern areas of the Baltic Sea. The experiments assessed in the systematic literature review of Chapter I allowed estimation of the fundamental abiotic niche of F. vesiculosus in relation to temperature and salinity. F. vesiculosus had a broad temperature optima for growth around 15 oC. Growth rate declined in salinities under 20, which are prevalent in the Baltic Sea. The experiments assessed covered temperature and salinity conditions which are not found in the Baltic under present climate, but may occur in the future, and thus yield important information on potential responses of F. vesiculosus under climate change. The experiments conducted in this thesis showed that the effects of short-term heat waves on F. vesiculosus were more severe under low salinity. Even short ...
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