Description: Results of the of the present study provide a strong indication that reproductive periods of the bladderwrack Fucus vesiculosus is tuned by environmental conditions, such as day length, although it cannot be entirely ruled out that genetic constitution may play a role, as well. Furthermore results of the present study identified high temperatures as the most challenging condition for alga recruitment. Sea surface temperature rise could therefore be one of the reasons for the decline of F. vesiculosus populations in the Baltic Sea over the last few decades, particularly in the marginal environments (〈 7 psu). Additionally, fertility of F. vesiculosus from the marginal region, in contrast to all other regions, was very low, which also indicates towards a lower capacity to deal with environmental changes. A rather high germination success of some sibling groups (F. vesiculosus) under various environmental conditions, however, is promising in the light of adaptation to climate change.

Description: Small to meso-scale distribution of Baltic cod (Gadus morhua L.) as resolved by hydroacoustics: Habitat preferences, environmental limits, and resulting implications for stock development

Description: A central question in ecology is how organisms react to changing environmental conditions induced by global climate change. This is particularly important for ecosystem engineering species, as the fate of whole ecosystems is depending upon their performance and survival. In coastal marine habitats, seagrasses are of outstanding importance as ecosystem builders. Eelgrass, the study species of this thesis, is the most widespread and locally abundant seagrass along soft-sediment coasts of the northern hemisphere. In this thesis I assessed variation among and within eelgrass populations in response to heat stress. I conducted heat stress experiments in a “common stress garden”, simulating a summer heat wave of three weeks followed by a recovery phase. I measured various physiological parameters and assessed the expression profile of selected heat stress associated genes with qPCR as well as the whole transcriptome with next generation sequencing using eelgrass with differing thermal history (a southern population from the Mediterranean Sea and northern populations from the Kattegat and Limfjord, Baltic Sea). To assess variation within populations, I used genotypes originating from a Baltic population. I found that different genotypes showed varying growth rates in control and heat treatment at acute heat stress, but that all populations lost shoots in response to the heat wave, irrespective of their thermal pre-adaptation. While populations diverged in their expression profiles of selected heat stress associated genes already at the onset of heat stress, subsequent global transcription profiling revealed that those effects were of relatively minor importance compared to massive differences in gene expression during the recovery phase between two of the populations. This is in line with findings on the genotype level within one population which showed differences in the expression profiles of selected stress-associated genes between replicated individuals only in the recovery phase. This thesis provides a basis for investigating the potential for microevolution of eelgrass populations in the face of global climate change. Both, cold- as well as warm adapted eelgrass populations responded to heat stress with shoot reduction, a finding that is in line with worldwide records of seagrass decline. On the other hand, there is considerable variation for heat stress-related gene expression within populations, a trait that is likely to be important under global change. As this variation among genotypes is the prerequisite for natural selection and adaptation, populations may succeed to persist.

Description: 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.

Description: Originating from East Asia, the perennial red macroalga Gracilaria vermiculophylla (Ohmi) Papenfuss has successfully invaded several temperate areas of the Northern hemisphere and continues to spread. In its new range, the seaweed tends to form local mass appearances and to dominate the native community. A high tolerance towards both abiotic and biotic environmental stressors could explain the invasion success of this species. I therefore compared the stress resistance of G. vermiculophylla from six native populations from South Korea and China and eight invasive populations from Europe and NW-Mexico. In short-term experiments G. vermiculophylla individuals were exposed to 1) heat shock, 2) UV-C-radiation and 3) elevated copper concentrations in the water. In a long-term experiment the seaweed had to cope with depletion stress (darkness in combination with low temperature and dryness) for several months. All experiments were carried out twice - one time in the native range in Qingdao, China and one time in the invaded range in Kiel, Germany - to rule out local acclimation effects. In order to compare the resistance against herbivory individuals of native and invasive G. vermiculophylla populations were fed to snails from the native (Littorina brevicula) and the invasive (Littorina littorea) range. In the Baltic Sea, G. vermiculophylla might threaten the habitat-forming native brown alga Fucus vesiculosus through direct competition for resources and by providing a shelter for mesograzers, which prefer to feed on F. vesiculosus. Mesocosm-experiments were conducted over one year in the Kiel Fjord in order to test the direct and indirect effects of G. vermiculophylla on F. vesiculosus.

Description: The present thesis investigates the potential suitability of surface water dissolved titanium (Ti) concentrations as complementary dust tracer to dissolved aluminium (Al). Both metals reflect variations in the dust deposition into the surface ocean over different time scales, with seasonal timescales for Al and longer temporal scales for Ti. In order to lay the basis for future estimations of dust deposition from surface water concentrations of both metals, the behavior of Al and Ti is examined in the upper water column of the Atlantic Ocean. Detailed insights are given into the size distribution, removal processes and residence times of Al and Ti, in regions that largely differ in atmospheric inputs and biological productivity.

Description: Salt expelled during the formation of ice in polynyas leads to a downward precipitation of brine that causes thermohaline convection and erodes the density stratification of the water column. In this thesis we investigate by means of flux models and satellite data the ability of the Western New Siberian (WNS) flaw polynya to modify the stratification of the water column and to form saline bottom water. The accuracy of existent microwave satellite-based polynya monitoring methods is assessed by a comparison of derived estimates with airborne electromagnetic ice thickness measurements and aerial photographs taken across the polynya. The cross-validation indicates that in the narrow flaw polynyas of the Laptev Sea the coarse resolution of commonly used microwave channel combinations provokes errors through mixed signals at the fast and pack ice edges. Likewise, the accuracy of flux models is tested by comparing model results to ice thickness and ice production estimates derived from high-resolution thermal infrared satellite observations. We find that if a realistic fast ice boundary and parameterization of the collection depth H is used and if the movement of the pack ice edge is prescribed correctly, the model is an appropriate tool for studying polynya dynamics and estimating associated fluxes. Hence, a flux model is used to examine the effect of ice production on the stratification of the water column. The ability of the polynya to form dense shelf bottom water is investigated by adding the brine released during an except ionally strong WNS polynya event in 2004 to the average winter density stratification of the water body. Owing to the strong density stratification and the apparent lack of extreme polynya events in the eastern Laptev Sea, we find the likelihood of convective mixing down to the bottom to be extremely low. We conclude that the recently observed breakdown of the stratification during polynya events is therefore predominantly related to wind- and tidally-driven turbulent mixing.