Description: The statoliths of cephalopods are calcified bio mineral structures found in the gravity receptor system in the head of cuttlefish and squid. They grow throughout the lifetime of the animal and deposit microscopically visible daily increments similar to the annual rings in tree stems. Statolith growth and composition are linked to environmental factors. Thus, the statolith reveals the biological history of each individual cephalopod. If a relationship can be established between the composition of a growth layer of the statolith and the ambient water properties, then the statolith chemistry becomes a predictor of the surrounding water chemistry and/or temperature. Provided that statolith material is not altered or resorbed after deposition, the statolith becomes a permanent archive of environmental conditions and may provide information on habitat use, timing of exposure to a pollutant, and timing of migrations. Several micro analytical state-of-the-art techniques have been applied in this thesis to investigate the spatially resolved chemical composition of cephalopod statoliths. Recent applications of these methods include mainly geological samples, which do not contain organic compounds. Therefore a considerable part of this project focussed on adjusting and optimising the respective methods to the analyses of biogenic aragonite intergrown with organic compounds. In this thesis, the influence of different environmental factors on the chemical composition of cephalopod statoliths was investigated. On the basis of laboratory experiments under controlled conditions, it is now possible to qualify the influences of salinity, temperature and diet on the concentrations of several elements in the statoliths. Analytical results indicate that the incorporation of a number of elements is influenced by environmental factors. Barium and iodine appear to be the most suitable indicators for temperature. The incorporation of strontium into cephalopod statoliths, however seems to be influenced to a greater extent by diet than by the surrounding water. This is contradictory to results from corals and fish otoliths, where strontium is a well-established indicator for both temperature and salinity. The suitability of statolith micro-chemistry for field-studies has been proven as well. Statolith trace element compositional zoning reflects very well the life history and ontogenetic habitat-shifts of the boreoatlantic armhook squid Gonatus fabricii. Further, this thesis gives valuable insights into the microstructure of statoliths and the elemental nano-scale distribution in daily increments. For the first time, the application of NanoSIMS NS50 provided distribution patterns of calcium, strontium and sodium in cephalopod statoliths with a spatial resolution of 400nm. The results of this study provide an essential basis for future investigations in the field, probably leading into further understanding of yet unknown migration patterns and spawning grounds of various cephalopod species. Applying these future approaches could establish a consolidated biological knowledge on cephalopod species and stocks, and therefore may contribute to an effective and sustainable management of this both ecologically and economically valuable resource.

Description: Multiple global and local stressors threaten populations of the bladderwrack Fucus vesiculosus (Phaeophyceae). Baltic F. vesiculosus populations presumably have a lower genetic diversity compared to other populations. I investigated the adaptive potential under multifactorial environmental change in F. vesiculosus germlings. Effects of warming and acidification were crossed during one year at the two levels “present” and “future” (according to the year 2110) at the “Kiel Outdoor Benthocosms” by applying delta-treatments. Effects of warming varied with season while acidification showed generally weak effects. The two factors “ocean acidification and warming” (OAW) and nutrients were crossed showing that nutrient enrichment mitigated heat stress. Germlings previously treated under the OAW x nutrient experiment were subsequently exposed to a simulated hypoxic upwelling. Sensitivity to hypoxia was enhanced by the previous OAW conditions. Difference in the performance of genetically different sibling groups and diversity level were observed indicating an increased adaptive potential at higher genetic diversity. Different sibling groups were analysed under multiple factors to test correlations of genotypic sensitivities. Sensitivity towards warming, acidification and nutrient enrichment correlated positively while sensitivities towards OAW and hypoxia showed a negative correlation demonstrating that genotypes previously selected under OAW are sensitive to hypoxic upwelling. In a literature review, responses of marine organisms to climate change were analysed through different levels of biological organisation showing that climate change has different effects on each single level of biological organisation. This study highlights that global change research requires an upscaling approach with regard to multiple factors, seasons, natural fluctuations, different developmental stages and levels of biological organisation in the light of the adaptive potential.

Description: Seagrass is a foundation species within shallow water ecosystems because it provides habitat and food and thereby supports biodiversity. It has a function as atmospheric CO2 storage and improves the water quality by filtering nutrients (Greiner et al. 2013). Currently, seagrass meadows are facing multiple challenges such as ocean warming, reduced light caused by increasing nutrient input and more frequent disturbance events or direct anthropogenic impact (Unsworth et al. 2019). All these factors affect the performance of seagrass and thereby impair the ecosystem services seagrass meadows provide. This thesis represents a systematic review and meta-analysis of the physiological effects of temperature change on seagrass to provide a better understanding of the effect of rising temperatures on seagrass meadows worldwide. In this thesis, 766 papers were reviewed and a subsequent meta-analysis of 43 papers including 407 control-treatment temperature combinations matching the inclusion criteria were conducted. The log response ratio (lnR) was used for calculating the effect sizes, because it is more intuitive to interpret. Hedges’ g was further used to verify the results. It was tested for effects of the physiological parameters measured, the treatment type, the temperature direction, the experiment duration, the control temperature, latitude and longitude of the source population and the genus on relative seagrass performance (lnR). The key results of the meta-analysis showed that (I) plant physiological performance was reduced by an average of 39% by temperature change across all studies; (II) per 1°C experimental ocean warming a reduction in seagrass performance of 11% was observed; (III) the measured performance parameters (growth, biomass, photosynthesis and survival) showed differential susceptibility to warming, with survival being most affected and photosynthesis least affected; (IV) seagrass genera did not differ significantly in their response to experimental ocean warming but varied between locations. There was a strong geographic bias in this meta-analysis since most case studies were conducted in developed countries including Europe, the US and Australia. Thus, many species were underrepresented while also some climate conditions were not covered. Further, it was also not possible to make a statement about the recovery after experimental temperature stress had ceased, as there were too few studies focusing on recovery. Altogether, this thesis identified two profound knowledge gaps, which should be addressed by future studies. In conclusion, more frequent and intense heat waves are an increasing threat to seagrass meadows in the future. As seagrass provides important ecological services, it needs to be protected. It is particularly striking that every degree Celsius of temperature change matters for seagrass as it means a reduction in physiological and morphological performance, which is another indication that global warming should be kept below 2 degrees Celsius

Description: Coastal ecosystems worldwide are experiencing increasing anthropogenic pressure, mainly caused by growing human populations in near-shore urban areas and by the rising number of megacities. One of the consequences of this process is the eutrophication of marine habitats that lie in the vicinity of rivers carrying high loads of nutrients that come from agriculture and human sewage. The capital of the Republic of Indonesia, Jakarta, is an example of a megacity impacting the adjacent marine ecosystems: In Jakarta Bay excessive loads of nutrients cause frequent phytoplankton blooms and the resulting microbial activity causes hypoxia events. One of the few species that copes well with these conditions is the Asian green mussel Perna viridis. It forms dense aggregations on bamboo settlement stakes in the bay located within the native distributional range of the mussel that is also a well-known invader of coastal habitats. Non-native populations of this species exist in southern Japan, at some Pacific islands and in the West Atlantic. In Indonesia, P. viridis is native to the western parts of the archipelago but non-native to the eastern parts and was found in the non-native range as fouling on ships that cross the Indonesian archipelago from west to east. One of the reasons for its invasion success is the ability of P. viridis to tolerate large fluctuations in abiotic environmental conditions. Therefore, understanding the factors influencing the mussel’s tolerance to environmental stress, should help to understand their invasion success. To address this question, I conducted three studies in which I exposed mussels to hypoxia in the laboratory under different scenarios. In the first study, I compared the hypoxia tolerance and nutritional status of mussels collected from a ship hull in the non-native range to those of mussels from Jakarta Bay in the native range. I found that the mussels collected from the ship hull were in a very poor nutritional status and tolerated hypoxia in the laboratory only half as long as mussels from the eutrophic Jakarta Bay. The finding suggests that transport on a ship hull may reduce the invasion potential of the species if the journey leads through areas of low food supply. The other two studies that comprise this thesis aim at assessing the potential roles of local adaptations (i.e. an irreversible modification that is manifested in the gene pool of a population), acclimation to stress (i.e. a reversible modification that is not genetically manifested) and a good nutritional status (caused by ample planktonic food supply in a eutrophic habitat) in determining the degree of tolerance to environmental stress in mussels. The idea of investigating this closer had arisen from a previous study, which found that individuals from Jakarta Bay are more tolerant to environmental stress (i.e. salinity, thermal and oxygen stress) than conspecifics from a more natural habitat in Indonesia. However, it remained unknown which mechanisms led to this difference. I approached this question by conducting a reciprocal transplantation experiment and subsequent hypoxia tests in the laboratory with P. viridis from the eutrophic Jakarta Bay and an oligotrophic habitat in West Java. The experiment showed that tolerance to hypoxia was rather determined by the conditions in the habitat where the mussels had lived for two months after transplantation before exposure to stress and not by the characteristics of the habitat where they originated from. This suggests that local adaptations to stress did not occur in Jakarta Bay mussels - although they have a long history of experiencing adverse conditions – or that they have been overwritten by other determinants of tolerance to hypoxia. The main determinant of stress tolerance again was the nutritional status. In the third study of this thesis, I conducted experiments that allowed establishing a causal relationship between a high nutritional status and hypoxia tolerance. Jakarta Bay mussels that had obtained more food supply in the laboratory had a better hypoxia tolerance than Jakarta Bay mussels that had obtained less food and were in a poor nutritional status. Furthermore, acclimation to low, non-lethal concentrations of dissolved oxygen enhanced hypoxia tolerance in mussels with low nutritional states. Taken together, these results show that a good nutritional status is the most relevant determinant of tolerance to environmental stress in P. viridis, which implies that the mussel can benefit from eutrophication caused by anthropogenic impact. Perna viridis may, therefore, be a species that can extend its distributional range if anthropogenic pressure in urban, near-shore areas is increasing and contributing to eutrophication. However, it may not succeed and establish in more non-native areas if conservation efforts apply that keep tropical and subtropical coastal ecosystems in an oligotrophic state and maintain high levels of biodiversity.

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 ever-increasing complexity of in silico experiments in computational science is reflected in the growing complexity of the simulation software enabling these experiments. However, computational scientists rarely employ state-of-the-art software engineering methods, which negatively affects their productivity as well as the reliability of their scientific results. To tackle this challenge, this book introduces the Sprat Approach, which hierarchically integrates multiple domain-specific languages to facilitate the cooperation of scientists from different disciplines and to support them in creating well-engineered software without extensive software engineering training. To evaluate the Sprat Approach, it is applied to the implementation of the Sprat Marine Ecosystem Model in an exploratory case study. The Sprat Marine Ecosystem Model is a novel end-to-end ecosystem model based on population balance equations. In order to evaluate the Sprat Model, it is parametrized for the eastern Scotian Shelf ecosystem with its intertwined direct and indirect fish stock interactions, which previously could not be modeled satisfactorily. The simulation results described in this book provide new insights into the main drivers of regime shifts in marine ecosystems.

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: In an era of biodiversity loss caused by anthropogenic impacts, it appears essential to improve our understanding of how ecological filters interact with regional species pools, in order to obtain valuable information on the process of community assembly as well as for biodiversity conservation. Especially in the Baltic Sea, which is characterized by strong environmental gradients and far reaching human-mediated pressures, baseline information provided by monitoring approaches are needed to disentangle community shifts from natural background variability. In the frame of this doctoral thesis, the role of ecological filters on the richness and community structure of hard-bottom assemblages in the southwestern Baltic Sea was investigated and the variability of important environmental drivers described. In the southwestern Baltic Sea, hard-bottom communities are mainly found on boulders and stones left by the last glaciation. The characteristics of these substrates are thought as an important driver of the benthic assemblages living in these boulder fields. Thus, the relationship between geological and biological diversity was examined at the local and regional scale. In a multidisciplinary approach, geological seafloor mappings were combined with biological samplings of hard-bottom communities. At the local scale, the size of boulders was found to positively correlate with taxonomic and functional richness, and negatively correlate with the β diversity of the communities. At the regional scale, differences in taxonomic community composition and β diversity were suggested to be the result of site-specific factors like boulder densities and sediment distribution. Whether of natural or anthropogenic origin, the shallow waters of the Baltic Sea are subject to strong environmental fluctuations, sometimes within short timeframes. Temporally highly resolved in-situ measurements of important water parameters can therefore help to understand the environmental dynamics biological communities are facing in coastal waters. Thus, a monitoring network along the southwestern Baltic coast was established, to measure temperature, salinity and oxygen concentration at 10 min interval as well as nutrient concentrations twice a month. The obtained recordings revealed strong temporal and spatial variabilities, highlighting the need to consider such fluctuations in experimental scenarios, as predictors of biodiversity patterns or within environmental assessments. Long-term records of community composition are crucial to distinguish directional regime shifts from random fluctuations. The monitoring of hard-bottom communities established on standardized settlement panels over a period of 11 years showed regional differences in community development. Multivariate analyses revealed the decline of the foundational species Mytilus sp. to be responsible for the observed community changes over time. In a modeling approach, the decline was explained by changes in sea surface temperature, current speed and chlorophyll a content. Moreover, since the mussels recovered only in stations of Lübeck Bight, regional factors like limitations in dispersal and population connectivity were suggested as significant driving forces. To summarize, this doctoral project demonstrated the effects and variabilities of ecological filters in hard-bottom communities of the southwestern Baltic Sea. In all studies, monitoring approaches were of central importance to detect the presented patterns, underlining the strategic need of these efforts in order to improve our understanding of community assembly and persistence, in times when biodiversity management is more vital than ever.

Description: The biological composition of most of the earth’s major ecosystems is being dramatically changed by human activities. The breakdown of natural barriers, as a consequence of an increasingly connected world, has contributed to a rise in biological invasions worldwide with thousands of non-indigenous species established in freshwater, brackish, and marine ecosystems. Identifying traits correlated with invasion success is a central goal in invasion ecology to predict and prevent future invasions. This dissertation is divided into five chapters. Chapter 1 gives a general introduction to the main topic of the thesis, including invasion ecology and possible determinant factors that might influence invasion success such as geographic origin and life history stages. Furthermore, it also explores the influence of experimental design on results in ecology. In Chapter 2, I question the role of geographic origin on invasion success, specifically, whether Ponto-Caspian species are better able to acclimatize to and colonize habitats across a range of salinities than taxa from Northern European and North American regions. The experiments, using eight gammarid species native to those three regions, demonstrated that although species from all three tested regions indicated high tolerance to a wide range of salinities, significant differences in the direction of salinity tolerance were observed among the regions, with Northern European species having a better survival in higher salinities, and Ponto-Caspian species in lower salinities. Therefore, it is important to consider geographic origin as a predictor of invasion success because it might foresee pre-adaptation of certain species due to its evolutionary history. Following these findings, in Chapter 3, I further compare the salinity tolerance of adults and juveniles of three gammarid species originating from Northern European, the Ponto-Caspian and North American regions to determine whether juveniles tolerate salinity changes equally well as adults. During the invasion process, individuals must overcome several challenges and be able to survive and reproduce to establish a successful population. Thus, the role of life history stages in the context of invasion ecology is important to consider. While experimental results determined that both adults and juveniles of all three species endured wide ranges of salinity, juveniles tolerated a narrower salinity range than their parents. The evidence from this study emphasizes the importance of testing several life history stages when constructing models to predict future invasions. In Chapter 4, bearing in mind that the approaches used to test scientific questions may differ not only in spatial scale but also in ecological complexity, I explored how the type of experiment (i.e., scale and ecological complexity) affects the outcome and to what extent the two types of experiments are comparable. Two experiments differing in size and ecological-complexity (i.e. outdoor large-scale community-level mesocosm vs. indoor small-scale two-species laboratory experiment), were conducted to assess the effects of marine heatwaves on two gammarid species. The results revealed that while for one species the population growth was similar independently of the size and ecological-complexity, for the other species, the inclusion of the community seemed to have benefited the species’ growth rate, demonstrating stronger performance in the mesocosm than in the laboratory experiment. These results suggest the importance of biotic interactions and complexity of natural environments in buffering or boosting the effects of environmental stress on organisms while carrying out ecological experiments. Finally, Chapter 5 summarizes the findings from all experiments and concludes that not only geographic origin and life history stages need to be considered in invasion ecology, but also the approach when selecting our experimental designs to answer research questions.

Description: After the closure of the Central American Seaway around 3.6 Ma, the benthic carbonate ecosystems developed differently in the Caribbean and on the Pacific side of the Isthmus of Panama. In this thesis, fossil and recent carbonate systems were studied and a comparison was made between fossil and present-day carbonate ecosystems from the same paleolatitude. This opens up the possibility to document the evolution of these sedimentation systems through time.