Description: Climate change driven by anthropogenic utilization of fossil fuels and deforestation over the past 250 years is leading to ongoing changes in sea surface temperature (i.e. ocean warming) and seawater carbonate chemistry speciation (i.e. ocean acidification, OA) at an unprecedented pace. Both of these environmental stressors are expected to impact marine ecosystem functioning in the near future with consequences for marine biogeochemical cycling. In the context of this doctoral thesis, phytoplankton physiology and biogeochemical dynamics were investigated under the individual and combined effects of OA and warming through experimental work. Chapter I of this thesis presents data on the individual and synergistic effects of OA and warming on coccolithophore physiology. In order to test for possible synergistic effects, two coccolithophore species, Emiliania huxleyi and Gephyrocapsa oceanica, were exposed to a broad range in CO2 concentrations at three different temperatures. The results from this study showed that both species displayed optimum-curve responses for key metabolic processes (i.e. growth, photosynthesis and calcification) at all temperatures, with species-specific sensitivities. Most importantly, increasing temperature modulated the optimum CO2 concentration and sensitivity of metabolic processes. Our results enabled us to propose a conceptual model showing that the temperature sensitivity of metabolic processes in these organisms could help explain the discrepancies found in the literature on coccolithophore physiology in response to OA. Interested by the results from experiments in Chapter I with single species, mesocosm experiments were carried out in Chapters II and III with natural plankton communities. Since most of the literature with natural communities has focused on effects of individual environmental factors, experiments in Chapters II and III investigated the combined effects of OA and warming during a natural spring bloom (Kiel Bight) and a nutrient-induced summer bloom (Thau lagoon, France). During experiments in Chapter II a shift in phytoplankton community composition towards larger diatoms under combined OA and warming conditions (i.e. ‘Greenhouse’ scenario) was observed. Possible explanations for the observed shift in size are discussed in detailed and compared with results in the literature. Furthermore, the shift in species composition significantly increased losses of organic matter at the end of the experiment in the Greenhouse treatment were larger species dominated. Chapter III focused on the temporal development of phytoplankton derived particulate and dissolved organic matter (i.e. POM and DOM, respectively). Increased CO2, individually and in combination with warming, enhanced biomass build-up and modulated the negative effects of warming (i.e. decreased biomass build-up). In summary, the experimental data from the work presented in this doctoral thesis shows the importance of investigating the synergistic effects of changing environmental factors when trying to understand the response of marine ecosystems to climate change and its importance when assessing the future of marine ecosystem functioning. Some suggestions for experimental work are proposed to follow up on the results from experiments presented in this doctoral thesis.

Description: Climate change is expected to impact oceanic ecosystem functioning in the upcoming decades, ocean acidification and global warming being the most important factors which will shape the future ocean dynamics (Jackson 2008). In the experiments presented here we used an innovative approach that consisted of extending the number of treatments across a wide range of carbon dioxide partial pressure (pC02) levels and two temperatures which allowed to test lower and upper thresholds of biological production. Growth, calcification and POC production rates showed an optimum curve response to increasing pC02, with an almost doubling in production rates at 20CC and opt imum rates for pC02 levels of ~290-599 μatm at 15CC and ~488-1052 μatm pC02 at 20CC. PIC and POC cellular quotas showed changes to increasing pC02 but no effect from increasing temperature. Results obtained from the combined effect of global warming and increasing pC02 from these experiments might contribute to shape biogeochemical modeling formulations in the near future.