Taucher, Jan (2013) Effects of increasing temperatures and CO2 on phytoplankton and marine biogeochemical cycles - Combining experimental work and numerical modeling. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 193 pp.

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Abstract

The ongoing increase in atmospheric carbon dioxide (CO2) leads to a global increase in temperatures and its subsequent uptake by the ocean considerably alters the carbonate chemistry of seawater, a phenomenon generally referred to as “ocean acidification”. Both ocean warming and acidification occur at a pace unprecedented in recent geological history and are expected to significantly affect marine biota. In the present thesis, the sensitivity of marine ecosystems and biogeochemical cycling to increasing temperatures and CO2 was investigated in a combined approach of numerical modeling and experimental work. In a first step, the role of direct temperature effects in the response of marine ecosystems to ocean warming was investigated by simulating climate change in a global earth system model, based on emission scenarios for the 21st century. The study revealed fundamental uncertainties in our knowledge about temperature sensitivities of marine ecosystems and biogeochemical cycling. Depending on whether biological processes were assumed temperature sensitive or not, simulated marine NPP increased or decreased under projected climate change. Motivated by the outcome of this modeling study, a mesocosm experiment was carried out to specifically investigate the temperature sensitivity of biogeochemically important processes in diatom-dominated plankton communities.The results from this mesocosm study suggested a pronounced increase in carbon uptake and production of organic matter in response to elevated temperatures, which was contrary to results from similar experiments. A major difference to previous mesocosm studies was the dominant phytoplankton species, suggesting that the physiological response of this species determined the biogeochemical response of the entire plankton community. In order to test this hypothesis, culture experiments were conducted to compare the sensitivities of two globally important diatom species (Thalassiosira weissflogii and Dactyliosolen fragilissimus)to temperature and CO2.The results of these experiments revealed a pronounced effect of temperature and CO2 on carbon uptake and partitioning into particulate and dissolved organic matter, and especially the phenomenon of carbon overconsumption and the associated decoupling of carbon and nitrogen cycling. Furthermore, the experiments could show that the sensitivity of these processes to temperature and CO2 varies substantially between species, thereby confirming the hypothesis derived from the preceding mesocosm study. The findings from these various laboratory experiments were the basis for the development of a novel biogeochemical ecosystem model. Most models do not account for carbon overconsumption and dynamic stoichiometry, and sensitivities of associated processes to temperature and pCO2, as observed in these experimental studies. Consequently, a model was constructed that simulates carbon overconsumption and its sensitivity to temperature and pCO2. Application of this model may help to understand how carbon overconsumption and associated processes affect marine biogeochemical cycling. Further work investigated how the warming-induced decrease seawater viscosity under global warming might affect sinking velocity of marine particles and the carbon flux to the deep ocean. Application of a global earth system model demonstrated that this previously overlooked 'viscosity effect' could have profound impacts on marine biogeochemical cycling and oceanic carbon uptake over the next centuries to millennia. In the model experiment, the viscosity effect significantly accelerated particle sinking, thereby effectively reducing the portion of organic matter that is respired in the surface ocean and enhancing the long-term sequestration of atmospheric CO2 in the ocean. The representation of particle sinking in biogeochemical models was investigated in more detail in an additional sensitivity analysis. Results of this study demonstrated that the inherent structure of commonly used ecosystem models sets an upper limit to the flux of organic matter from the euphotic zone to the deep ocean, even under light-saturated and nutrient-replete conditions. This upper limit is determined by the functional form of the various process descriptions in the simulated ecosystem, as well as their respective parameter settings. These findings indicate that, even though such relatively simple ecosystem models may show good skill in reproducing observed current distributions of biogeochemical tracers, it is questionable whether such models can realistically simulate the sensitivity of biogeochemical cycles to environmental change. Altogether, this doctoral thesis revealed substantial sensitivities of marine carbon fluxes to increases in temperature and CO2, which should be considered when assessing the impact of climate change on marine ecosystems and feedbacks on the global carbon cycle.

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