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: 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.

Description: 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.

Description: 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.

Description: Glacial isostatic adjustment (GIA) is the ongoing response of the viscoelastic solid Earth, oceans and the gravitational field to the previous burden of the ice loads. The Earth’s surface was once covered with massive ice sheets, and melting of these ice sheets is still reshaping coastlines and affecting sea-level. To reconstruct former sea level and be able to predict future changes, it is necessary to constrain the rheological properties of the Earth’s structure. Widely used data to constrain Earth’s interior are sea-level indicators. In the first part of the thesis, we propose a statistical method that quantifies a relationship between the sea-level indicator and a relative sea level in order to compare it to GIA predictions. A statistical method is based on consideration of spatial and temporal probability density functions, derived from the age and elevation of each indicator. This method allows a more rigorous approach to validation with sea-level data and possibility to include low-quality data. We verified method performance in the Hudson Bay, Canada as a test run before applying it to the SW Fennoscandia. SW Fennoscandia identifies as an area where lateral heterogeneity is likely to exist. The south-western part of Fennoscandia lies on the crustal boundary called the Trans-European Suture Zone (TESZ), or the Tornquist Zone. GIA models have two representations of Earth’s structure; radially symmetric (1D), where the rheology only varies vertically, and lateral or 3D variations of viscosity structure. In this thesis, we compare glacial isostatic adjustment reconstructions with both representations of the rheology. Results from the 1D model show variations in the viscosity structure between the area near to the centre of the former ice sheet and the areas at the margin of the ice sheet. Hence, we verify the importance of including lateral variations in GIA models in this region. Application of 3D models displays the sensitivity of model parameters to crustal deformation. German Baltic coast yields thinner lithosphere than TESZ region and near-centre region. Additionally, in the TESZ region, we notice a steep increase in viscosity of the asthenosphere and upper-mantle. Furthermore, we compared two different global ice histories (ICE5G and ICE6G_C) and concluded that the marginal areas are more sensitive to different deglaciations, and we propose to use regional ice histories to constrain GIA models better. Apart from the new statistical method, this study sets a ground for future GIA studies in complex tectonic regions and demonstrates the importance of including laterally heterogeneous Earth structure in GIA models.

Description: The harmful algal bloom(HAB)-forming species Alexandrium ostenfeldii produces paralytic shellfish poisoning (PSP) toxins and the lesser known spiroimine shellfish poisoning (SSP) toxins. These toxins primarily belong to macrocyclic imine class, with cyclic imine moiety and macrocyclic nature as their distinct characteristics. By far, A. ostenfeldii is the only species identified to produce two types of cyclic imines—gymnodimines and spirolides. Studies on these toxins produced by different A. ostenfeldii strains revealed high structural diversity. The presence of highly varied structural types of cyclic imines that requires elaborate synthesis pathways implies its underlying function for the producing organism. However, known roles and mode of action (MOA) of cyclic imines are rather limited. That is, other than its “fasting-acting” toxicity and antagonistic activity against of nicotinic acetylcholine receptors, no other MOA of cyclic imines have been identified. Considering the increasing occurrence of HABs and the socio-economic impacts of toxins associated in A. ostenfeldii, it is crucial to elucidate not only the structural diversity of cyclic imines but also their respective modes of action and biological significance of their production. As such, this study aimed to isolate, characterize, and screen the bioactivity of the two types of cyclic imines (i.e., gymnodimines and spirolides) produced by A. ostenfeldii. Two strains of A. ostenfeldii (OKNL 48 and X-LF-19-F10) were mass cultivated to directly obtain purified samples of structurally known gymnodimines (i.e., gymnodimine A (GYM A)) and spirolides (i.e., 13-desmethyl spirolide C (SPX 1)). A total of 345 µg of GYM A and 559 µg of SPX 1 were purified (〉95% purity) from the large-scale microalgal culture. Three other A. ostenfeldii strains (AON 24, NX-56-10 and MX-S-B11) were identified to produce both structurally unknown spirolides. Isolation of the pure novel spirolides were not possible in quantities necessary for NMR structure elucidation work. Thus, mass-spectrometric (MS)-based techniques was sought as an alternative approach for the structural characterization of unknown compounds. Using the technique, this study revealed the presence of nine novel spirolides, eight of which have either a 5:5:6 (C-type spirolides) or 5:6:6 (G-type spirolides) triketal ring configuration. Another novel spirolide putatively belongs to a new spirolide subclass, which has a 6:5:6 triketal ring configuration that is only observed in pinnatoxins, another class of cyclic imines. The initial structural insights by mass spectral techniques demonstrate that structural variability of spirolides is not only limited to the presence or absence of certain functional groups (as observed in other eight novel spirolides) but also the triketal ring system. Moreover, the proposed 6:5:6 triketal ring system in one of the novel spirolides suggests that similar biosynthetic steps are involved in the synthesis of spirolides and pinnatoxins. To gain a better understanding on the potential role of cyclic imines, purified GYM A and SPX 1 were subjected into cell line-based bioassays to identify other MOA of the toxins. Intracellular calcium levels ([Ca]i) measured in rat pheochromocytoma (PC12) cells suggested that both toxins elicited differential effect on acetylcholine receptor (AChRs) subtypes. GYM A and SPX 1 activates nicotinic AChRs (nAChRs) while only GYM A activate muscarinic AChRs (mAChRs). The activation of AChRs and the subsequent influx of calcium ions into the cell illustrates the putative capacity of the toxins to mimic acetylcholine. These observations were possible as a result of determining the response of GYM A or SPX 1 on [Ca]i signaling in PC12 cells. Moreover, preliminary investigations on the effects of GYM A and SPX 1 to AREc32 and Nf-κB-bla THP-1 cell lines indicated that toxins potentially induce adaptive stress response pathways. SPX 1 likely activates both oxidative stress response and inflammation response pathways. GYM A, on the other hand, possibly initiates inflammation response pathways but lacks the capacity to induce oxidative stress response pathway. The ability of the toxins to potentially activate adaptive response pathways indicated that GYM A or SPX 1 might initiate cellular processes that will restore homeostasis suggesting possible medicinal applications of the toxins. Studies on the response of GYM A and SPX 1 to [Ca]i and adaptive stress response pathways provided more understanding on the potential MOA of the toxins. Further knowledge on structural diversity and MOA of cyclic imines could hopefully direct us to determine structure-activity relationship and, to some extent, purpose of the toxins to the producing organism.

Description: Increasing human activities on land, such as intensive farming, fossil fuel burning and river flow modifications alter nutrient cycles with implications for both terrestrial and marine ecosystems. The nitrogen cycle has been particularly affected: the amount of nitrogen available for primary producers has doubled due to artificial atmospheric nitrogen fixation. Consequently, more reactive nitrogen is reaching oceans via river run-off and atmospheric deposition. The silicon cycle, on the other hand, has been affected in an opposite direction and to a lesser extent: with increasing river damming, more silicate is biologically fixed in dam reservoirs and less of it is reaching the coastal oceans in a reactive form. These changes result in a decline in silicon to nitrogen (Si:N) ratios and can alter the composition of phytoplankton - small, but numerous organisms, providing the base of pelagic marine food webs. Si:N ratios affect phytoplankton composition because nitrogen is required by all phytoplankton and silicon is essential only for certain groups, such as diatoms. Diatoms use silicate to build their porous cell walls and thus silicon availability can limit their growth. These organisms are abundant, especially in nutrient rich waters, and account for as much carbon fixed as the rainforests on land. Multiple experimental studies have shown that diatom proportion declines with decreasing Si:N ratios. Yet further knowledge of how this change in phytoplankton composition may affect the functioning of entire plankton communities is needed to ultimately understand and estimate the impacts of nutrient alterations on higher trophic levels and the marine carbon pump. In this thesis, I experimentally assessed the impacts of changes in Si:N ratios on the complex interactions in the lower pelagic food web. Two mesocosm experiments were conducted where natural Baltic Sea plankton communities were exposed to a range of Si:N ratios and varying copepod grazing pressure. The results showed that a lowered Si:N ratio not only lowers the proportion of diatoms within the phytoplankton community, but also increases the abundance and biomass of non-silicifying groups of plankton, with implications for the quality and quantity of food available for mesozooplankton. Two conceptual models were developed in Chapter I to illustrate food web structures in low and high Si:N environments, concluding that lowered Si:N ratios result in more complex plankton food webs, which are known to lower energy transfer efficiency. An unexpected finding was that some diatom species were not affected by grazing, indicating that the efficiency of the “diatom-copepod” food chain may be moderated by diatom edibility. In Chapter II, this aspect was investigated further, by assessing the effects of altered Si:N ratios on the nutritional value of plankton in terms of fatty acid and particulate nutrient indicators. The results showed that while high Si:N environments can be characterized by higher availability of essential fatty acids, ratios between particulate nutrients and selected fatty acids are more suitable for mesozooplankton when Si:N ratio is lowered. Changes in phytoplankton composition with declining Si:N ratios observed in this thesis are in line with Tilman’s Resource Ratio Theory, which states that ratios of limiting resources can determine the outcome of species competition. The applicability of this theory, however, has been questioned as it does not account for varying concentrations of resources. In Chapter III, this thesis presents evidence from natural communities that plankton composition responds to lowered Si:N ratios in a similar way both when nitrogen and when silicon concentrations are manipulated. However, while nutrient ratios are critical in determining community composition, absolute concentrations largely control the total biomass of phytoplankton. These findings contribute to the discussion on the ecological importance of nutrient ratios and concentrations and stress that resource ratios, in particular the ratio between silicon and nitrogen, can be used in predicting and modelling the outcome of species competition in natural phytoplankton communities. In conclusion, anthropogenic manipulations of nitrogen and silicon cycles can have strong effects on plankton composition, biomass and trophic interactions. This thesis underlines that implications of these changes on higher trophic levels and ecosystem functioning are complex and future studies are needed to understand the role of selective grazing, phytoplankton quality and defense mechanisms in marine food webs.

Description: Over a long time, marine organism have adapted to their biotic and abiotic environment. Currently, anthropogenic induced climate change is rapidly altering the environment with unpredictable consequences for marine ecosystems. To predict how organisms will cope with those changes in a future ocean, research mainly focused on exposing individual species to elevated water temperatures and ocean acidification scenarios. In the Baltic Sea, a decrease in salinity due to increased rainfall is predicted to be an additional stressor for marine life. However, the impact of low salinity levels on marine organisms has been ignored. By studying the interaction between a filamentous phage and an opportunistic bacterium (Vibrio alginolyticus) as well as the interaction between V. alginolyticus and the pipefish Syngnathus typhle, this thesis provides empirical data on the ecological and coevolutionary consequences of altered salinity levels on species interactions. Filamentous phages can infect and integrate in the genome of Vibrio bacteria. Whether filamentous phages are detrimental or beneficial for the bacterium depends not only on the costs they are causing for the bacterium, but also on the additional genes they are carrying and the environment. The genes introduced by the phage can provide the bacterium with additional properties which help the bacterium to infect marine animals. The results of chapter I, show that filamentous phages predominantly infect Vibrio bacteria within one clade. Infections and thus potential transfer of genes across bacterial clades occur at lower frequencies. In chapter II, I showed that reduced salinity levels made the bacteria more susceptible for phage infections, which may result in an increased transfer of genes between bacteria and facilitate the spread of virulence and antibiotic resistant genes in the future Baltic Sea. In chapter III, I used an evolution experiment to find out that Vibrio bacteria can quickly become resistant against filamentous phages and that resistance evolution is delayed a low salinity level. At low salinity levels, phage-infected bacteria persisted longer in the populations compared to coevolving populations at high salinity levels suggesting that phage-infected bacteria are more competitive in predicted salinity conditions. In chapter IV, I investigated how the pipefish S. typhle copes with reduced salinity levels and the associated shift in the microbial community. By taking advantage of the natural salinity gradient of the Baltic Sea, I investigated the role of genetic adaptation, transgenerational plasticity, developmental plasticity and their interactions in responding towards a shift in biotic and abiotic conditions. Pipefish males collected at high salinity and exposed to low salinity levels in the lab were infected by a fungus in the brood pouch, which points out that rapid salinity changes can impair the immune system resulting in a higher susceptibility to ambient pathogens. Gene expression analysis, survival measurements of juveniles and resistance to a fungus infection suggest that pipefish collected at low salinity are locally adapted but retain the phenotypic plasticity to cope with ancestral salinity levels and associated pathogens. This PhD thesis provides evidence that a rapid salinity reduction in the Baltic Sea has negative effects on individual organisms ranging from microbes to teleost fish. The negative impact of an altered environment can be amplified by an additional organism. On evolutionary time scales changing environmental conditions in combination with threatening biotic species can be overcome via resistance evolution and local adaptation to salinity conditions. The results suggest that conclusions drawn from single species studies fail to capture fundamental interactions between organisms which constrains our ability to predict the future of any species in a rapidly changing ocean.