This thesis provides new insight concerning drivers behind differences in arthropod diversity and abundance in Baltic shore ecosystems and how the arthropod communities might be affected when the conditions in the Baltic Sea are altered due to climate change.  The focus has been on climate related changes that are unique for coastal ecosystems, especially sea level rise and changes in the inflow of marine nutrients. As sea levels rise, features in coastal landscapes will be altered, islands and habitats will be flooded and diminished, and structural connectivity within the island landscape will therefore change. This thesis shows that arthropod diversity within the two arthropod groups, spiders and beetles, increases with island size but also that diversity is positively influenced by a high number of islands in the surroundings. A changed distribution and occurrence of marine species, due to climate change or eutrophication, can also affect terrestrial organisms on the shore.  In the Baltic Sea the new conditions following climate change will decrease the prevalence of bladder-wrack and benefit filamentous algae. Algal deposits on shores reflect the marine species composition and a decreased prevalence of bladder-wrack in the Baltic Sea will also be visible on the shores.  This thesis shows that a lower proportion of bladder-wrack in the algal deposits will decrease the diversity and abundance of arthropods in these deposits. Changes in the marine environment may also affect the inflow of insects with aquatic life stages and terrestrial adult stages.  On Baltic shores, prey species with aquatic life stages, especially chironomids, constitute a large proportion of the diet of the terrestrial predatory group, wolf spiders. In freshwater system, the inflow of chironomids is known to decrease with elevated water temperatures if this is true in the Baltic Sea prey availability of wolf spiders would decrease.  This thesis supports the importance of chironomids as a prey for coastal wolf spiders, but also shows that the diet varies over season with dominance of terrestrial prey in early summer shifting to a dominance of marine prey in late summer and autumn. This seasonal variation is primarily due to a gradual increase in the consumption of chironomids over season. Climate change has the potential to alter the biogeographical conditions in coastal landscapes as well as the density and quality of marine nutrient inflow. Sea level rise will diminish and flood islands and this thesis shows that a moderate sea level rise of 0.5 meters would make the total number of islands in the outer part of Stockholm archipelago decrease with about 25 %. Sea level rise could thus have consequences for arthropod diversity in Baltic shore meadows in the near future. The combined effects of sea level rise and changed prevalence of marine species in the Baltic Sea will affect the abundance and diversity of arthropods substantially. The abundance and diversity of spiders and beetles will decrease on shores that today have a high occurrence of bladder-wrack and prey availability for coastal predators might decrease due to a decreased inflow of chironomids. Changes in the arthropod communities could have consequences also further up in the food chain, such as for shore birds feeding on these arthropods. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.</p>

The purpose of this study was to identify if terrestrial arthropod predators on Baltic Sea shores vary in their use of marine versus terrestrial food items, and to construct a bottom-up food web for Baltic Sea shores. The inflow of marine nutrients in the area consists mainly of marine algal detritus and emerging aquatic insects (e.g. phantom midges, Chironomidae). Diets of coastal arthropods were examined using carbon and nitrogen stable isotope analysis, and a two source mixing model was used to examine proportions of marine carbon to diets. The results suggest that spiders are the terrestrial predators mainly utilizing nutrients and energy of marine origin on Baltic Sea shores, while insect predators such as beetles and hemipterans mainly utilize nutrients and energy derived from terrestrial sources, possibly due to differences in hunting behaviour. That spiders are the predators that benefit the most from the marine inflow suggest that eventual effects of marine subsidies for the coastal ecosystem as a whole are likely mediated by spiders.