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Water fleas (Daphnia) are small, planktonic crustaceans that inhabit different habitats all over the world. They are small, well studied, able to reproduce parthenogenetically (asexual reproduction by the formation of clones) and sexually with the formation of resistant resting eggs. This is the reason why they are very suited for research on their physiology, evolution and ecology. We wanted to determine the causes for the population dynamics of one particular species, Daphnia magna, in rock pools on several Swedish islands in the Baltic Sea. We did this by genetic analysis of the similarities and differentiations between these populations. To see if the population structure that we discovered is influenced in any way by the saline conditions that are common in a coastal region, a so-called life-table experiment was conducted. We followed up several parameters of the life history of the species, related to survival, maturation and broods, in a gradient of salt concentrations. The population structure that came out of the genetic analysis seemed to be caused by limited dispersal capabilities, rather than adaptation to local environmental conditions. This can be explained by the characteristics of the studied rock pool habitats. Wind dynamics, wave action and the frequency of dry periods decrease the stability of the environmental variables in rock pools substantially. It is difficult to adapt to local conditions if they change continually and at high rates. Still, not all observations could be explained by dispersal limitation, since some of the populations were genetically very different in comparison to nearby populations and more closely related to populations from other islands. Other processes, such as colonisation history, where a subset of one population and therefore a subset of the gene pool forms the foundation of another, should be taken into account. Salt concentrations, differing from one rock pool to another, did not seem to be important for the species in their habitat selection. Adaptation to the local salt concentration in the rock pool could not be concluded from the results. Elevated salt concentrations, however, do have significant effects on the performances of Daphnia magna and can slow down population growth. Some of the parameters we observed increased in the highest salt condition of our experiment, which was not completely in line with the literature on the subject, but this can be explained by the fact that all populations in our experiment have a significant history of living in saline conditions, unlike those from other experiments. The Baltic Sea is also a brackish sea, with very low salt concentrations in comparison to other seas, which makes it easier for freshwater species to colonise its waters. The research presented in this thesis was meant to provide some suggestions and prospects for the execution of an experiment in the field, where individuals from one population are brought in another rock pool to study their performance outside of the original environment and in competition with the residential population. By means of an analysis of the current population structure and its causal drivers, we hope that this transplant experiment will be an opportunity for interesting research on the dynamics of populations in this environment. Even more so because of the changing climate, which will have a certain influence on coastal areas by rising sea levels, altered weather conditions and invasive species.