Description: Anthropogenic greenhouse gas emissions have been driving global climate change and they will continue to do so over the course of the 21st century. Most of the marine biosphere and especially coastal marine systems have suffered from high anthropogenic pressure per se and it is possible that the novel burden of very rapidly proceeding global climate change triggers shifts to alternative regimes and functioning in marine ecosystems. In the light of this background, my dissertation aims to contribute to the mechanistic understanding of global and local climate change effects on a common coastal marine seaweed (Fucus vesiculosus, Phaeophyceae) system of the Baltic Sea. The results of my experimental studies provide important mechanistic clues about the underlying direct and indirect effective pathways of environmental change in the studied seaweed system. To the best of my knowledge, it is one of the first studies which assess the seasonal variability of the same environmental factors on the same marine system over the course of one year. The detected context-dependency of global climate change effects within one ecosystem clearly shows that our understanding of the basic underlying ecosystem processes and patterns forms a prerequisite for testing, predicting and managing future ecological change in marine systems. Given that grazing forms a crucial ecological force in many coastal vegetated systems, the identified underlying mechanisms of change (top-down and bottom-up control) may allow reference to other similarly structured coastal systems. Importantly my findings point out, that ecological impacts of global climate change may be underestimated if local perturbation is disregarded and, thus, underline the chance and responsibility of local ecosystem management.

Description: The trace metal iron is considered to be the nutrient that limits marine primary production in one third of the global surface ocean (Martin, 1990; Boyd et al., 2007; Moore et al., 2013). It is also the nutrient that maintains future ocean fertility due to its irreplaceable role in the process of nitrogen fixation, which adds “new” nitrogen (another nutrient for phytoplankton) to the surface ocean (Raven, 1988; Kustka et al., 2003b; Zehr and Capone, 2020). Due to iron’s importance, it is not surprising that the demand for incorporating iron into global biogeochemical models is high. However, including iron in an earth system model has been shown to have no clear benefits with respect to model misfit against observational data (Nickelsen et al., 2015) . How smart is it then to introduce iron models into global biogeochemical models, when the benefits are not clearly identifiable? Especially, when the iron models perform poorly at reproducing observed iron patterns in the ocean (Tagliabue et al., 2016). The poor performance of iron models, coupled with their failure to improve biogeochemical tracer representation of the ocean, inspired this additional effort to identify the advantages of including iron in a global biogeochemical model, both for the preindustrial state and under conditions of a changing climate. The working hypothesis was that the relatively poor performance of iron models might come from inadequate model calibration. A first sensitivity study on biogeochemical model parameter values was conducted in order to identify key parameters for model calibration. It was found that while some of the parameters influence simulated nitrogen, phosphorus, and oxygen concentrations, few parameters influence simulated iron concentrations. This suggests that our modelling skill of the iron cycle is still limited and/or that the observational data base is insufficient for comprehensive model calibration so far. Thus it was decided not to include iron data in further model calibration. A model calibration framework (Kriest et al., 2017) was next applied to a hierarchy of global models with different implementations of iron; one without iron, one with prescribed iron concentrations, and another one with a dynamic iron cycle. Using calibration against global data sets of nitrogen, phosphorus, and oxygen, the misfit of each model was pushed to its minimum. It was found that under an assumed preindustrial steady state, the calibrated model with a full dynamic iron cycle has the lowest model misfit against observations (thus confirming the working hypothesis). It was also found that the calibrated model with a fully dynamic iron cycle has 50% less net primary production (which is closer to empirical estimations) compared to the calibrated model without iron. Finally, transient simulations for all calibrated models were integrated from their pre- industrial state until the end of the 21st century using an atmospheric CO2 concentration pathway consistent with a ’business-as-usual’ CO2 emission scenario. It was found that nitrogen fixation trends diverge among models. This divergence is caused by whether iron limits the productivity of the upwelling regions, e.g. in the eastern tropical Pacific. The export production in the eastern tropical Pacific (and other tropical upwelling regions) reacts differently to warming, depending on whether iron is a limiting nutrient. These different responses trigger a divergent chain of downstream responses that affect nitrogen fixation across the tropical oligotrophic regions in the model. Through the comparison between calibrated models, this thesis quantifies the advantages of including iron in a global biogeochemistry model and reveals how important iron is for future nitrogen fixation trends. It furthermore illustrates the interconnection between tropical upwelling and oligotrophic regions.

Description: The overall aim of my thesis was to improve our understanding of environmental drivers causing the dynamics of the Eastern Baltic cod reproduction habitat and to assess their relevance for a possible application in the sustainable stock management. A novel approach to map the reproductive habitat of Eastern Baltic cod, the Buoyancy Depending Reproductive Layers (BDRL), was developed and used to propose an alternative stock indicator, the effective Spawning Stock Biomass (eSSB). The eSSB was found to improve the fit of a recruitment model compared to the model using the conventional Spawning Stock Biomass. Oxygen depletion was also negatively impacting the available size of nursery areas for juveniles. The mechanism was able to partly explain the observed decline of the condition of juveniles in the nursery areas, because a high population density in the remaining habitat could increase the impact of density depending effects. Furthermore, by the application of the novel approach of BDRLs the spawning habitat was shown to be sensitive to eutrophication and that this sensitivity is strongly depending on the size of the female spawner using the habitat. As predicted, the BDRL approach was superior to the “classic” approach, the Reproductive Volume (RV) because it was more sensitive to environmental change, able to incorporate stock structure, was not overestimating the spawning habitat in the eastern spawning areas and could provide estimates of other stressors depending on the female spawner size. Due to a permanently installed measurement platform in the Arkona Basin, the new methodology could be used to establish a new environmental indicator on the spawning habitat conditions. It was recommended to be used in future stock assessments.

Description: Oceanic emissions of sulfur containing trace gases alter global atmospheric chemistry. The gases act for example as aerosol precursors and change the radiative budget of the Earth, with a significant impact on climate. Large uncertainties exist in the amount of sulfur gases emitted from the ocean, and a gap in the atmospheric budget of carbonyl sulfide – the most abundant sulfur gas in the atmosphere – has been suggested to result from tropical ocean emissions. This thesis uses new shipbased measurements from the tropical Pacific and Indian Ocean together with models to quantify these emissions. Three studies were performed: 1) A 3D model study to test how oceanic emissions can be represented in atmospheric chemistry climate models, 2) A combination of new shipbased data and box model calculation to derive a global emission estimate of carbonyl sulfide and 3) a detailed process study of production processes and their drivers for the gases carbonyl sulfide and carbon disulfide in the Eastern tropical South Pacific. Together, the results yield a new temporally and spatially resolved emission climatology of the three gases.

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: 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: 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: 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: Plastics enter the environment via different sources and are transported and deposited there. They vary regarding polymer, density, colour, shape and size. Concerning the size, plastics are distinguished by their diameter in macroplastics, d ≥ 5 mm, and microplastics, d 〈 5 mm. Macroplastics, that enter the environment, are often the origin for microplastics due to degradation and fragmentation. Based on numerous environmental sampling and numeric modelling the fate of macro- and microplastics in the environment can be understood. Thereby, the entry is caused exclusively by anthropogenic action and the following transport is mainly by freshwater systems. Plastics in the environment accumulate due to the material’s durability on water surfaces and in soils and sediments which are therefore considered as temporary sinks. The final sink for plastic in the environment is the sea bed. To better understand the accumulation processes, more environmental sampling is necessary. For the following sample preparation, a separation method was developed based on the density independent extraction with canola oil in an efficient and cost-effective way using a plastic free separation unit. This method was extensively validated and could thus be identified as an equivalent separation technique which was applied on two different environmental areas. First, samples from marine water and sediment in the Northeast Atlantic were taken not only to prove the applicability of the separation method with canola oil but also to identify microplastic concentration there with microscopic analysis and polymer identification. The results showed a microplastic accumulation and furthermore an increase in microplastic concentration with increasing water depths and therefore distance to the coast. Second, fluvial sediments from a regional river catchment in North Rhine Westphalia were taken and analysed by microscope and infrared spectroscopy. The sampling included depth profiles in the river’s floodplains, composite samples from the river bed and surface samples outside the flooding area. The microplastic concentration was highest within the depth profile samples, followed by the river bed samples and the surface samples. Concerning the grain size, microplastic accumulated predominantly within fine sediment fraction. Furthermore, microplastic detection was set in a sedimentary context for the first time by using it to determine sedimentation rates. Additionally, a connection could be drawn between the polymers of the detected microplastic and the depth of the related sediment layer: the older the polymer, the deeper the layer in which it was found.With the knowledge about a temporal connection between microplastics and sediment deposition, a dating method for recent sediment layers can be developed in the future. In general, the detection of plastics can be seen as an indicator for a deposition after 1950, where the plastic mass production has started and enabled extensive environmental input. The understanding of entry, transport and accumulation of macro- and microplastics as well as the method validation of canola oil extraction and following application in marine and fluvial environments can be used variously as basics especially in upcoming microplastic research. With the consideration of microplastic detection as temporal marker for sediment deposition an additional groundwork for the development of a sediment dating method was set.