Description: The Integrated Ocean Drilling Program (IODP) Expedition 307 was proposed to obtain evidence for understanding the origin and evolution of a 155 m high deepwater carbonate mound in the Porcupine Seabight. The major aim of this study is the reconstruction of environmental parameters using well-developed paleoceanographic proxies derived from calcareous tests and skeletons of benthic organisms based on sediment cores from this expedition. In particular, this study uses different archives such as scleractinian cold-water corals and calcitic foraminifers. The second chapter (published in Marine Geology 2011) reports on a high-resolution record of the mound base. Stable oxygen and carbon isotopes measured in several benthic and planktonic foraminifers as well as sortable silt analyses document the start-up phase of coral growth. Mound initiation and further development coincide with the intensification of Mediterranean Outflow Water (MOW) characterized by oceanographic conditions favourable for rapid cold-water coral growth. Furthermore excursions in foraminiferal δ13C values and increased flow conditions indicate erosional intervals, which overprinted probably diagenetically the original geochemical signals. The third chapter (to be submitted to Geology), also based on sediments from the mound base, shows that these ecosystems only thrive under specific oceanographic conditions. Based on core material, not only from Challenger Mound (IODP Expedition 307) but also from the Propeller Mound, we reconstructed paleo-seawater densities from oxygen isotope ratios in benthic foraminifera. Results clearly indicate results demonstrate that cold-water coral mound development occurred when a density window of sigma-theta (σΘ) = 27.35–27.55 kg m-3 was present in the ambient bottom water. Therefore we conclude that seawater density is reflecting one of the major controlling factors favoring mound growth and highlights the sensitivity of these ecosystems to environmental changes. The fourth chapter (submitted to Earth Planetary Science Letters) demonstrates the use of paleotemperature proxies in the scleractinian reef building cold-water coral Lophelia pertusa. Temperature calibrations are based on L. pertusa samples from temperature range of 5.9°- 13.65°C originating from the European continental margin and the Mediterranean Sea. 
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 Results could not confirm earlier findings of Rüggeberg et al. (2008) that δ88/86Sr in Lophelia skeleton is positively correlated with temperature and may serve as a potential paleotemperature proxy. Results rather show that δ88/86Sr is inversely correlated with temperature in samples from the North Atlantic. However, this temperature effect appears to be superimposed by changes in the ocean carbonate system. Furthermore, this sample set of L. pertusa clearly shows the temperature dependency of elemental ratios such as Mg/Li and Sr/Ca. The Mg/Li ratio may serve as a new paleotemperature proxy in scleractinian cold- water corals, whereas the Sr/Ca ratio needs more detailed research. The fifth chapter (to be submitted) focuses on the long-term controlling mechanisms of cold- water coral mound growth in the Porcupine Seabight. Here, different paleo-proxies such as Mg/Ca, δ13C and δ18O in foraminifera and Mg/Li, Ba/Ca and U/Ca in cold-water coral L. pertusa were used to reconstruct paleoenvironmental parameters. Based on existing and additional age determinations (87Sr/86Sr, Th/U) previous findings were supported. However, our data point to an earlier mound initiation at ~3 Ma coincidently with the intensification of the Mediterranean Ouflow Water (MOW). Foraminiferal temperature records reveal that early mound development occurred in glacial and interglacial conditions, whereas the recent mound decline was caused by high amplitude excursions of the last interglacial/glacial cycles. In particular, coral Mg/LiLophelia temperatures indicate that coral growth occurred within a temperature range of 8 to 10°C, comparable to the recent measured settings in the Porcupine Seabight. Hence, results imply that the variations in intermediate water masses (Mediterranean Outflow Water, Eastern North Atlantic Water,) are the main trigger for mound growth and decline in the Porcupine Seabight. Moreover prior to the mid-Pleistocene rapid Challenger Mound growth benefited from a stable boundary layer between the MOW and the Eastern North Atlantic Water (ENAW) at which organic matter and nutrients settled on.

Description: Comb jellies were a relatively obscure group of zooplankton, until Mnemiopsis leidyi invaded the Black Sea in the 1980’s with cascading effects on several ecosystem levels including commercial fisheries. This native to the east coasts of America triggered large public and scientific attention as a result of this invasion and its ecological and economic impacts. In 2005, when M. leidyi was sighted in Northern Europe for the first time, similar consequences were feared. The aim of my PhD project was to understand the potential impact of M. leidyi on the Baltic Sea ecosystem and constrains on its dispersal. Specifically, the project investigated (i) direct and indirect effects of M. leidyi on the Baltic cod population in its most important spawning ground, (ii) factors governing the spatial and temporal distribution of M. leidyi eggs, larvae and adults in the Baltic and, (iii) M. leidyi reproduction and its effect on population development. The approach involved 13 monthly monitoring cruises from high saline Skagerrak to low saline northern Baltic regions, in situ and laboratory controlled reproduction and feeding experiments, molecular analysis for species verification, and statistical modeling. The low feeding rates and passive negative selection of cod eggs in experiments demonstrate that M. leidyi does not pose a direct threat to the Baltic cod population at the environmental conditions characteristic for its spawning ground. Furthermore, the drastically reduced reproduction rates observed under low salinities suggest M. leidyi is not likely to compete with cod recruits prey. Spatial and temporal surveys show highest abundances during October, with a consistent absence of adult and larval M. leidyi in the northern Baltic. Abundances in the Kattegat were 60 times higher than in the central Baltic, suggesting that the M. leidyi population in the central Baltic is dependent on advection from high saline areas. This interpretation is consistent with the low reproduction rates measured and a low fraction of up-growing animals in the central Baltic. While adults were not observed from April to June in high saline areas, M. leidyi larvae were present throughout the year. It remains unclear where M. leidyi overwinters but high saline areas appear to be important in the annual establishment of the population. Laboratory and in situ reproduction experiments confirmed that fecundity is a major contributor to M. leidyi’s invasion success, although salinity is regulating, and possibly restricting, its range expansion in Northern Europe. Maximum reproduction rates are shown to be attained at low food concentrations, helping reconcile the high population densities observed in localized areas despite low food concentrations. An unexpected discovery was that the arctic relict ctenophore Mertensia ovum, thought to be restricted to the northern Baltic, also occurs in the high saline Kattegat/Skagerrak during winter and spring. Interestingly, in the northern Baltic the M. ovum population consists exclusively of larval-sized animals that are actively reproducing and maintaining a self-sustained population. Natural selection can favor early maturation at small size when mortality rates are high, and our results are consistent with this hypothesis. Currently, M. leidyi has established itself permanently in high and intermediate saline areas in Northern Europe. While the ecological impact of M. leidyi in the central Baltic appears to be limited concern, the environment in other European waters should be more favourable to their populations. In these areas, it is suggested that M. leidyi constitutes a potential threat to fisheries through resource competition with fishes

Description: The aim of the present thesis was to assess the potential and limitations of ozonation in marine recirculating aquaculture systems (RAS) while particularly focussing on the toxicity, formation and removal of ozone-produced oxidants (OPO) in order to develop guidelines and thresholds for a reasonable and safe ozone application. In the first two chapters the toxicity of OPO was investigated for Pacific white shrimp (Litopenaeus vannamei) and turbot (Psetta maxima) and maximum safe exposure levels were determined for both species. Despite their strong differences in biology, both investigated species possess a similar sensitivity towards OPO. Results demonstrate that OPO concentrations ≥ 0.10 mg/l cause adverse effects in both species. An OPO concentration of 0.06 mg/l was determined as the maximum safe exposure level for rearing juvenile L. vannamei and P. maxima. Furthermore, we proved this safe level to be sufficient to control and reduce bacterial biomass in the recirculating process water. To improve the control of toxic OPO, the removal performance of activated carbon filtration was tested for different oxidant species (free bromine, bromamines, free chlorine and chloramines). Results proved activated carbon filtration to be very efficient in removing the dominating oxidant species free bromine and bromamines formed during the ozonation of natural and most artificial seawaters. In contrast, removability of chloramines, sometimes present in ozonated bromide-free artificial seawater, was shown to be significantly lower. Finally the suitability of ozone for water quality improvement was evaluated by investigating the ozone-based removal of nitrite, ammonia, yellow substances and total bacterial biomass with regard to feasibility, efficiency as well as safety for the cultivated organisms. Results demonstrate that ozone can be efficiently utilized to simultaneously remove nitrite and yellow substances from process water in RAS without risking the formation of toxic OPO concentrations. Although ammonia oxidation in seawater by ozonation is independent from pH and enables almost the complete removal of ammonia-nitrogen from the aquaculture system with nitrogen gas as the primary end product, it presupposes an initial accumulation of OPO to highly toxic amounts, restricting a safe application in aquaculture.

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: Leaf litter decomposition constitutes an important source of energy in many aquatic environments that is controlled by the joint action of microbial decomposers such as bacteria and fungi and also animal detritivores. In view of current scenarios of global environmental change, it is predicted that rapid temperature increases could directly affect most ecosystems including freshwaters. Additionally, human activities and industrial development have impacted water quality of many streams and rivers. In freshwater systems, eutrophication is a process, whereby excessive receive of inorganic nutrients, especially N and P, that may effect on leaf litter processing. In the present study, I investigated how warming, nutrient-addition (N and P) and detritivores, interact to affect multiple parameters associated with leaf decomposition. Investigations were carried out in the laboratory in two sets. For the studies presented here leaf litter Betula pendula (Birch) and the detritivore Gammarus pulex (Amphipoda) were chosen because of their numerical importance in northern temperate ecosystem. In the first set of experiments (Chapter I), I investigated the synergistic effects of warming and nutrient-addition (N and P) on the impact of amphipods on density and community composition of leaf litter-colonizing bacteria. I found that warming significantly exhibit stronger effects on the composition of litter-associated bacterial communities, irrespective of nutrient load but amphipods mediated warming-effects on bacterial community composition by selective feeding. In addition, Short-term effects of nutrient-addition on bacterial biofilm density were stronger than warming-effects but less pronounced so at increased temperatures. Alongside, Long-term effects of nutrient-addition on bacterial density were strongest, irrespective of environmental temperature. Additionally, nutrient-addition effectively compensated for biofilm reduction upon grazing by amphipods. In second set of experiments (chapter II), I proceeded to improve understanding of leaf litter decomposition process by comprehensive experiments to investigate how warming, nutrient-addition (N and P) and detritivores, interact to affect multiple parameters associated with leaf decomposition. These parameters included microbial (bacteria and fungi) biomass and community structure, decomposition rate and detritivore growth. I found that detritivores and nutrient-addition have strong effects on leaf litter decomposition rate but relative growth of detritivores does not increase with warming and nutrient addition. Additionally, bacterial biofilm density increases by both warming and nutrient-addition, but reduced by amphipod grazing-pressure and fungal biomass also appears to be stimulated by warming and nutrient-addition but also by amphipod presence. Moreover, litter-associated fungal composition were only slightly affected by warming or nutrient-addition, but strongly responded to selective feeding by amphipods and the community composition of bacterial colonizers on birch litter was also influenced by grazing pressure of amphipods and warming. In summary, this study provides new insights into the effect of simultaneous change in temperature and nutrient-load on microbial decomposers and also helps in better understanding of selective role of detritivores on bacterial and fungal communities on litter surfaces in freshwaters.

Description: In this thesis, population genetic structures and evolutionary aspects of speciation in notothenioid fishes from the Southern Ocean have been investigated. Special emphasis was given to the role of pelagic larval dispersal on gene flow between geographically separated populations, since notothenioids are characterized by unusually long pelagic larval durations of up to one year, which is assumed to counteract genetic divergence of populations and ultimately allopatric speciation processes. By elucidating the population genetic structures of selected notothenioid species, inferences were made about the level of gene flow between populations. The major goal of this thesis was to compare the population genetic structures of sympatric species within the Atlantic sector of the Southern Ocean to determine factors responsible for regulating gene flow. Species-specific traits, such as larval durations, were expected to result in differences between the observed patterns, while environmental factors, such as oceanographic currents or frontal systems, were assumed to influence multiple species in a similar way. In addition, it has been tested whether the evolution of notothenioids fulfills the criteria of an adaptive radiation, of which only a few examples are known from the marine realm.

Description: Marine hypoxia has become one of the major concerns of the world, as oceanic dead zones continue expanding horizontally and vertically, a phenomenon primarily caused by global warming and anthropogenic eutrophication. As consequence, drastic changes in community structures, predator-prey relationships (i.e. uncoupling) and/or habitat compression are expected followed by severe impacts on food-webs, ecosystems and fisheries. Moreover, habitat compression is aggravated by the synergistic effects of climate change, as elevated temperature and PCO2 will narrow the habitat from above. The jumbo squid, Dosidicus gigas, undergoes diel vertical migrations into oxygen minimum zones (OMZs) off the Eastern Tropical Pacific, where he plays an important ecological role both as predator and prey. In fact, this species can easily remove more than 4 million tons of food per year from the pelagic food web and is an important component in the diets of birds, fishes, and mammals. Besides its ecological role, the jumbo squid also plays an important economically role being target of the world’s largest cephalopod fishing industry with around 14% of world’s total squid catch and landings estimated at 818,000 tons in 2006. However, the main problem that arises with hypoxia is a reduced gradient that drives O2 uptake via diffusion pathways. At some point, the critical O2 partial pressure (Pcrit), the reduced diffusion gradient cannot support the metabolic demand fully aerobically, and has to be supplemented by anaerobic pathways and/or compensated by a reduction in metabolic rate. Commonly, aquatic animals respond to hypoxia by first attempting to maintain O2 delivery, as aerobic metabolism is much more efficient, followed by conserving energy expenditure and reducing energy turn over and finally by enhancing energetic efficiency of those metabolic processes that remain and derive energy from anaerobic sources. A further problem that vertical migrators of OMZs have to face is the elevated production of radical oxygen species (ROS) during the reoxygenation phase while ascending, as non-neutralized ROS formation can damage biological macromolecules (i.e. lipids, proteins and DNA) resulting in severe functional alterations in cells and tissues. To determine the cost and benefits of such diel vertical migrations, I investigated biochemical and physiological mechanisms in juvenile D. gigas off the Gulf of California with a focus on ventilation, locomotion, metabolism and antioxidant defense. The respiratory regulation in D. gigas was unpredictably high and is mirrored in maximized oxygen extraction efficiencies (EO2) at early (EH, 〈 160 min, 1 kPa O2) and late hypoxia (LH, 〉 180 min, 1 kPa O2). EO2 at EH was maximum 82% and achieved via (1) deep-breathing mechanism with more powerful contractions and an enlarged inflation period, and (2) reduction in the relaxed mantle diameter to favor diffusion. At LH, EO2 was still 40%, despite all other ventilatory mechanisms were drastically reduced, probably by using the collar-flap system (uncoupling of locomotory and ventilatory mechanisms) and a further reduction in the relaxed mantle diameter. Moreover, the drastic change in locomotion between EH and LH (onset of lethargy) was accompanied by a switch in the energy source of anaerobic pathways. At EH, anaerobic energy equivalents (AEE) primarily arrived via rapid energy reserve depletion (ATP, phospho-L-arginine), and, under LH, was mainly obtained via fermentative pathways (mainly octopine). As octopine formation simultaneously creates protons, intracellular acidosis and acid-base disturbances under progressing hypoxia are expected, which might negatively impact squid’s energy household and expenditures from locomotion towards more important cellular processes (i.e. ion regulation). Energy reserve depletion might even trigger lethargic behavior to conserve energy and extend hypoxia residence time. At EH, in contrast, deep-breathing behavior enabled D. gigas to pass the same amount of water through the mantle cavity per period of time and thereby could maintain a stable ventilatory volume per min, which explains its high level of activity observed under such extreme conditions. Moreover, D. gigas suppressed its metabolism (45-60%) at severe hypoxia (below Pcrit), as the reduction in O2 consumption rate (70-80%) could not be compensated by an upregulation in anaerobic energy production (70%). Cephalopods primarily feed on proteins and their glycogen storage potential is low (〈 0.4% of body weight). Therefore anaerobic protein degradation came into focus as strategy in hypoxia tolerant species. Yet, total protein concentration in muscle tissue of D. gigas did not vary significantly under severe hypoxia, but the reduced protein expression of heat shock protein 90 (Hsp90) and α- actinin indicates that, at least under progressing hypoxia, jumbo squids might degrade specific muscle proteins anaerobically. Moreover, the lower α-actinin expression at LH might be related to a decreased protection via the Hsp90 chaperon machinery resulting in increased ubiquitination and subsequent degradation. Therefore, the ubiquitin-proteasome system seems to play an important role in hypoxia tolerance, but further investigations are necessary to discover its full potential and pathways. Antioxidant enzyme activities in D. gigas were generally low and in the range of other squid species, but malondialdehyde concentrations (indicative of cellular damage) did not significantly change between normoxic and hypoxic conditions, demonstrating an efficient antioxidant defense system. Moreover, superoxide dismutase and catalase activities were enhanced under normoxia that seem to constitute an integrated stress response at shallower depths by buffering increased ROS formation, and, in addition, might even be a strategy to cope with the reoxygenation/recovery process. Moreover, heat shock protein 70 concentration was significantly increased under severe hypoxia (1 kPa O2), which may constitute a preparation for the reoxygenation phase during squid’s upward migration. Accordingly, the present thesis demonstrates that D. gigas evolved a variety of adaptive mechanisms and strategies to cope with hypoxia and the imposed challenges of diel vertical migrations. D. gigas might even actively descent into OMZs to suppress metabolism and escape from high metabolic demands at surface waters. Especially the high O2 uptake capacity and respiratory regulation were surprising taking into account cephalopods physiological and anatomical restraints. Therefore, D. gigas seems well-adapted to hypoxic conditions and might even out-compete less hypoxia tolerant species under hypoxia expansion, but the synergistic impacts of climate change, in turn, might endanger its survival.