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.

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.