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In this PhD thesis we investigated responses of natural Daphnia magna populations to recent climate change and the related increased occurrence of heat waves. We first gave an overview of the known responses in freshwater invertebrates and whether these responses could be attributed to climate change (Chapter 1). We studied differences in thermal tolerance in D. magna genotypes hatched from mesocosms populations exposed to two years of experimentalnbsp;(ambient and ambient +4°C). We took advantage of the production of dormant eggs in D. magna which form a historic archive in lake sediments. Using a resurrection ecology approach we tested adaptive responses in upper thermal tolerance in a natural population (Felbrigg Hall Lake, UK). To measure thermal tolerance to increasing temperatures, we used the maximum temperature for activity (CTmax), which has been commonly used when measuring how increasing temperatures affect fitness. Innbsp;resurrection ecology approach we compared CTmax of genotypes hatched from historic (1955-1965)nbsp;recent (1995-2005) sediment layers (Chapter 2). We broaden the scope of our research to study whether natural populations of D. magna differ in their upper thermal tolerance along a latitudinal gradient in Western Europe (Chapter 3). Using a space-for-time approach we found significant differences in CTmax. Southern populations (Camargue, France) had higher CTmax values compared with northern populations (Sweden and Denmark). To understand the mechanisms underlying thermal tolerance in D. magna, we measured differences in respiration rates in the same genotypes used in the resurrection ecology study. We found no significant differences in respiration rates (Chapter 4). We expected that clones with high CTmax values would show lower respiration rates than clonesnbsp;low CTmax values. This could not be confirmed with the current data. The main result from this thesis is that natural D. magna populations have evolutionary potential in the CTmax trait and that they have genetically tracked increasing temperaturesnbsp;the past 40 years (Chapter 2). We also foundnbsp;adaptation in thermal tolerance to average maximum summer temperatures across a latitudinal gradient across 6 populations from Western-Europe (Chapter 3). Our results represent the first evidence for evolution in thermal tolerance through time in response to climate change and the associated increases in heat wave occurrence. A second significant result concerning continued climate warming is the presence of evolutionary potential for the CTmax trait found within the current Felbrigg Hall Lake population and within latitudes in the space-for-time approach (Chapters 2-3). This suggests that upper thermal limits that would reduce survival fornbsp;magna, have not yet been reached and that there is potential for further increases in CTmax in response to climate warming. This, however, does not guarantee that these populations will be buffered from the continued effects of climate change. The third important resultnbsp;from the recurring significant effects of body size in explaining differences in thermal tolerance. Smaller genotypes from the resurrection ecology study had higher CTmax values compared with larger genotypes. The same result was found in the latitudinal study. Southern genotypes had higher CTmax and smaller body size compared with northern genotypes, which were generally larger. This result is possibly related to the intricate relationship between temperature, body size and oxygen in aquatic systems. Our research showed that natural populations of D. magna harbour the evolutionary potential to respond to rapid temperature increase and have done so during the past five decades. It also underscored the importance of studying the underlying mechanisms that influence thermal tolerance, such as body size and oxygen uptake. It is important to understand the physiological limits of organisms in order to predict their future responses and distribution. The rate of adaptation will alsonbsp;how species interact and compete, influencing the relative importance of local and regional processes and the outcome of competition with invasive species.