Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: The Cabo Verde Archipelago in the Atlantic Ocean exhibits seamounts and islands in different stages of evolution. Mature islands are well-eroded, exposing early stages of volcanism. However, little is known about the submarine realm of the archipelago. To gain a better understanding of the geological and geodynamic evolution of the archipelago, data and observations from seamounts and islands were combined and different phases of magmatic-tectonic evolution investigated. The thesis provides the first geochronological data for the Cabo Verde Seamounts. A pillow lava from the extinct Nova Holanda (Senghor) Seamount in the NE yields a 40Ar-39Ar weighted mean age of 14.872 ± 0.027 Ma (2sigma). Lava samples from the phonolitic Cadamosto Seamount (SW) were 40Ar-39Ar dated, revealing young eruption ages with a combined weighted mean age of 21.14 ± 0.62 ka (2sigma). This volcanic eruption period can be linked to rapid crustal unloading due to sea level lowstands in the Last Glacial Maximum. Combined with petrological observations a long-lived magmatic plumbing system below Cadamosto is revealed. The evolution of the mature Maio Island was investigated. Uplifted mid-ocean ridge basalts exposed on Maio and overlying deep-sea sedimentary sequences contain Early Cretaceous fossil assemblages and record information on the young Atlantic Ocean. Based on field observations, 40Ar-39Ar geochronology and structural geology the Miocene evolution of Maio was refined, confirming a period of intense igneous growth between ~16 and 8.7 Ma. Extensive polymict conglomerates below Late Miocene lavas are re-interpreted as landslide deposits, confining an erosional period to between 8.7 to 6.7 Ma. A Pleistocene large-scale fossil dune located in the E of Maio records recent uplift and changes in the paleo-environmental conditions. The thesis provides important insights to different phases of the magmatic-tectonic evolution of the archipelago and ocean island evolution worldwide.

Beschreibung: My work developed a kernel density estimator that well resolves typical structures of probability densities, which was demonstrated on a newly compiled marine data set of organic carbon-13 isotope ratios (δ13CPOC). All work was conducted within the emerging field of marine data science. I identified classical data science, a general understanding of ocean science, communication skills, and confidence as requirements for marine data scientists. In the beginning of my work, the existing δ13CPOC data consisted of about 500 data points in the global ocean. I expanded the existing data set to 4732 data points in a first version, and additionally to 6952 in a second. Both are published at PANGAEA along with meta information such as measurement location, time, and method, and interpolations. I have published a description of the temporal and geographic distribution of the first version at Earth System Science Data. I designed the development of the kernel density estimator algorithm on the existing concept of computing it as a solution of the diffusion equation. My algorithm uses finite differences in space and equidistant time steps with an implicit Euler method, and approximates the optimal smoothing parameter by two pilot steps. Compared to other well-known kernel density estimators, my algorithm produces reliable approximations of multimodal and boundary-close distributions on artificial and real marine data and is robust to noise. My implementation is published as a Python package on Zenodo, its description is submitted to Geoscientific Model Development. I was able to show that my kernel density estimator reliably evalu- ates ocean data and thus lays a foundation for calibrating Earth system models. At the same time, I was able to contribute to the definition and establishment of the field of Marine Data Science.

Beschreibung: The communication of current scientific topics with societal relevance to young people is of great importance in order to prepare them adequately for present and future life and to provide them with a solid basis for a sustainable development of society. The scientific, didactic and technical demands for communicating such complex topics in an authentic learning environment in a way that is appropriate to the subject matter and the target audience pose great challenges for scientists and educators allone. This doctoral thesis therefore presents the interdisciplinary development and evaluation of two out-reach activities for secondary grade students in a student laboratory. The students worked out the effects of future changes in the Baltic Sea with hands-on experiments and an interactive computer simulation. Computer simulations and experiments are important learning methods in modern sci-ence education. However, existing research has not sufficiently identified the educational and di-dactical advantages and disadvantages of both methods and often lacks appropriate comparability. The effects of the methods were investigated on knowledge gain, situational interest and beliefs about science first in a direct comparison (NStudy I = 443) and second in a combined approach (NStudy II = 367). The comparative study showed that a simulation conveyed more content knowledge and experi-ments caused higher situational interest. The combined approach indicated that two methods con-veyed more knowledge than one, and the combination of the two methods positively influenced the perceptions of scientific predictions. The results also showed that the students had a fundamentally high level of trust in science. Combinations of experiments and simulation can contribute to a comprehensive understanding of, and interest in com-plex current issues in science, and combine the potential of both media and methods.

Beschreibung: Some of the best studied fish species with the longest monitored stock time series worldwide occur in the North Atlantic. Over the decades numerous publications added knowledge to our understanding of ecology and behaviour of these fish species and stocks. However, there are also gaps in knowledge, obvious uncertainties in ecological interactions and potential biases. These are rarely profoundly questioned unless environmental changes or inconsistent stock assessment results challenge our established points of view. In the Baltic Sea, Atlantic cod (Gadus morhua L.) has been intensively surveyed and investigated for decades. Yet, our understanding of this marine species living in a semi-enclosed, brackish environment is still incomplete. For example, despite numerous otolith exchanges to improve the age reading and large scientific efforts to understand the ecology of Baltic cod, the analytical stock assessment of the Eastern Baltic cod (EBC) stock was suspended in 2014; obviously age data were uncertain and the aut- and synecological knowledge was incomplete to explain the changes occurring in this stock in distress. For instance, spreading hypoxic areas in the central Baltic basins forced EBC to contract in the southern Baltic Sea but behaviour and movements of cod in response to the consequences are poorly understood. This dissertation aims at providing a more thorough understanding of cod ecology in the southern Baltic Sea, with a particular focus on patterns in movements of wild, free-ranging cod and validation of ring patterns in otoliths. Tagging performance, data storage tags recording depth and ambient temperature and otoliths of recaptured cod were analysed. Tagged cod had been released in the southern Baltic Sea within the scope of the international mark-recapture project TABACOD (“Tagging Baltic cod”). In chapter 1 (“Short-term tagging mortality of Baltic cod (Gadus morhua)”) the post-release short-term mortality of cod after tagging with T-bar tags and tetracycline was assessed. The mortality associated with catching and on-board handling was estimated to be 16% with no significant influence of the tagging process itself. While no effect of tagging month was observed, mortality rate was decreasing with increasing fish length. The low mortality rate confirmed the appropriateness of the tagging technique and potentially explained the lack of smaller recaptures. In chapter 2 (“Validation approaches of a geolocation framework to reconstruct movements of demersal fish equipped with data storage tags in a stratified environment“) a geolocation framework was adapted to reconstruct the movements of cod in the southern Baltic Sea tagged with temperature-depth data storage tags (DSTs). The adapted geolocation framework was tested with five validation experiments including i) simulated tracks, ii) stationary, nearshore moored DSTs, iii) temperature data from an offshore measuring station, iv) a temperature-depth probe attached to an otter board of a commercial vessel, and v) DSTs mounted on the CTD and otter boards of a survey vessel. The difference between true and modelled positions was on average between 2 and 19 km and generally better when there was a pronounced stratification of the water column and simulated individuals regularly conducted vertical movements. In chapter 3 (“Movement of cod (Gadus morhua L.) in the southern Baltic Sea: evidence from data storage tags”) the adapted and validated geolocation model was applied to the temperature-depth DSTs from 28 recaptured Baltic cod assigned to the EBC or Western Baltic cod (WBC) stock by genetics or otolith shape analysis to reconstruct daily positions. The temperature and depth profiles were supplemented with information on salinity and oxygen estimates from the regional ocean model also used for geolocation. Individual movements could be classified into three behavioural types: 1) coastal, shallow-water WBC, 2) resident EBC, and 3) migratory EBC. Unlike WBC, EBC generally occupied deeper waters, were exposed to higher salinities and regularly spent short period in hypoxic waters. While resident EBC stayed within the Bornholm Basin year-round, migratory EBC moved between spawning grounds in the Bornholm Basin during summer and coastal feeding grounds during autumn and spring. This study highlights the importance of coastal shallow-water feeding grounds, especially in autumn and spring which are underrepresented in the current bottom trawl survey. In addition, the temperature-depth profiles of all EBC revealed daily vertical movements in the water column which were triggered by twilight and partly followed the lunar cycle. Regular defaecation below the thermocline of small pelagics ingested during nightly feeding excursions above the thermocline may exacerbate hypoxia in the stagnant water bodies of the deeper basins. In chapter 4 (“Validation of otolith zone formation and otolith growth of adult wild cod (Gadus morhua L.) in the southern Baltic Sea through mark-recapture and tetracycline marking“) the chemically time-stamped otoliths of 258 T-bar tagged cod assigned to the WBCor EBC stock were analysed. WBC in the southern Baltic Sea confirmed the recent age validation result from the Belt Sea that the translucent zone is formed during summer when cod are stressed by adverse temperatures and reduced feeding. The translucent zone of EBC was also laid down during summer, but under very different conditions, i.e. during spawning coinciding with hypoxic conditions and food limitation. The faster otolith growth of WBC were in line with previous findings showing that EBC exhibit slower somatic length growth and, hence, severely reduced productivity of the EBC stock. Through this dissertation, it was possible to interlink the environmental conditions experienced by cod tagged with data storage tags to the growth and patterns in ring formation in the otoliths of cod tagged with T-bar tags. The evaluation of movement patterns in the present centre of cod distribution thus provided new insights into the behaviour and ecology of cod inhabiting the permanently stratified brackish-water ecosystem of the southern Baltic Sea.

Beschreibung: Understanding the processes affecting the distribution of an avian (and every other) species in time and space necessitates the inclusion of a variety of factors. Choice and utilisation of a habitat should be seen in the context of energetic costs and benefits, as organisms are believed to distribute in order to enhance their survival. In this thesis, the distribution and habitat selection of the Common Scoter Melanitta nigra (Linneaus, 1758) a benthic-feeding sea duck spending much of its non-breeding season in the eastern German Bight, southeastern North Sea, was investigated with regard to seasonal differences. As Common Scoters are located in German Waters predominantly during their moulting and wintering season and as these periods reflect different energetic needs, this seasonal variability is expected to have an influence on how they distribute themselves relative to their food and disturbance. To analyse distribution patterns and habitat selection, Common Scoter count data from Seabirds at Sea surveys collected in 2006-2017 were used, either from a long term data base (FTZ) or obtained from additional surveys conducted during the PhD. To relate bird distributions to their prey base, benthos and sediment samples were collected in separate moulting and wintering areas to investigate benthic community structures. In addition to sediment samples, information on supplementary environmental parameters (such as water depth or bed shear stress) was gathered during surveys or taken from the literature. To determine, which parameter(s) might have the greatest influence on Scoter distribution, Generalised Additive Models (GAMs) with Integrated Nested Laplace Approximation (INLA) were performed for the moulting and wintering season.

Beschreibung: The Eastern Tropical South Pacific (ETSP), hosts an extensive Oxygen Minimum Zone (OMZ) in the water column which has a major imprint on local and global marine biogeochemistry. Due to the low oxygen conditions within the OMZ, microbial processes of nitrogen (N) loss, such as anammox and denitrification are sustained in the water column. These processes result in a pronounced N deficit which reduces bioavailable N for primary productivity and thus influences fisheries production in the region. To maintain a balanced marine N inventory regionally in ETSP, the N deficit would have to be compensated by N inputs via upwelling or N2 fixation. A classical assumption is that N2 fixation is favoured by iron (Fe) availability and a surplus of inorganic phosphate relative to inorganic nitrogen (this relativity is defined as P*), both conditions are present in the ETSP. Over the past decades, this assumption has been integrated into most coupled circulation and N-cycle biogeochemical models. These models indicate that there is a close spatial link between areas of high N loss, generally confined to OMZs and N2 fixation. On the contrary, other biogeochemical models have revealed that a close spatial link between N loss and N2 fixation in OMZ areas may give rise to run-away loss of fixed N in the ETSP, ultimately destabilizing the regional marine N inventory. While N loss processes are relatively well understood in the ETSP, the lack of a comprehensive dataset that resolves N2 fixation rates in both space and time constraints an accurate assessment of the regional marine N inventory, potential feedback mechanisms, and their impact on N turnover and productivity. Therefore, the main objective of this doctoral dissertation was to investigate the spatial distribution of N2 fixation relative to N loss in the ETSP, in order to understand potential feedbacks in the regional N cycle.