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: Commercially exploited stocks that have experienced declines in population abundance have responded by altering life history traits of growth and maturation. Cod is not only becoming mature at an earlier age but also, the majority of the stock comprises fishes with no previous spawning experience. Actual fisheries management does not take in account qualitative differences within the spawning stock. If the stock responds to continued exploitation by shifting maturity to an earlier stage, fish will spawn at smaller sizes. They will produce smaller eggs, and consequently small and less viable larvae, so that the contribution to the spawning stock biomass will be less than expected. There are many advantages for delaying maturation: Larger and heavier fish will be better conditioned for spawning, have higher fecundity and larger eggs that are more viable. Harvesting at delayed recruitment enables the stock to maintain a larger SSB with an expanded age structure while supporting a sustainable fishery.

Description: Humans are altering the composition of biological communities through a variety of activities at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a streng potential to alter ecosystem properties and the goods and services they provide to humanity. Since the industrial revolution, atmospheric carbon dioxide (CO2) increased from 280 to 380 μatm and is expected to further increase to 700 μatm by the year 2100. Ocean acidification is the consequence of increasing atmospheric CO2, which dissolves in seawater and subsequently increases seawater acidity and decreases carbonate ion concentration. Changes in carbonate chemistry can act both as fertilizer in case CO2 is a limiting resource and as stressor, particularly for calcifying organisms. Ocean acidification represents a pervasive environmental change that is predicted to affect a wide range of species, yet our understanding of the emergent ecosystem impacts is very limited. Two most challenging questions largely remain uncertain. Firstly, how much of the expected change in community functioning due to elevated CO2 is owing to either changes in the physiology of individual species or in the relative abundance of species or is there a hint towards evolutionary adaptation? Secondly, how da effects of community composition on ecosystem functioning compare to direct effects of ocean acidification? In chapter 1, I tested whether varying initial dominance scenarios lead to different competitive outcomes and subsequently translate into altered community functioning. I used experimental communities consisting of four naturally co-occuring coccolithophore species and manipulated initial community structure by creating five different dominance scenarios: (1) all species contributing evenly to initial biomass, and (2-5) one of each species contributing 4x that of the remaining three species to total initial biomass. I was able to show that priority effects in the communities caused the initially dominant species to remain dominant during the stationary phase in three out of four cases. However, despite varying carrying capacities when species were grown in monocultures and different dominant species, community functioning was unaffected. I suggest that selective and facilitative effects were responsible for the equalization of community functioning. In chapter II, I used three of the four coccolithophore species used in chapter I and explored the effect of initial community composition in combination with ocean acidification on community biomass. In particular, I tackled the question of how much of the expected change in community functioning due to elevated CO2 is owing to either direct changes in the physiology of species or indirect ecological changes in the relative abundance of species. In order to complete the picture, I additionally indirectly tested for evolutionary adaptation to elevated CO2. Contrary to my expectation I found neither a significant physiological effect nor an ecological effect of elevated CO2 on biomass at bloom peak. 1 concluded that the lacking effect on ecosystem functioning in this particular model system in response to elevated CO2 was likely caused by community reorganization due to evolutionary adaptation. In chapter I and II, community functioning at bloom peak was affected neither by initial community composition nor ocean acidification. The communities in both studies however, consisted only of coccolithophores. In order to overcome this limitation, in chapter III, I used communities harboring a variety of functional groups and tested the hypothesis that initial community composition and elevated C02 are equally important to the regulation of phytoplankton biomass. 1 was able to show that initial community composition had a significantly greater impact than elevated CO2 on phytoplankton biomass, which varied largely among communities. Furthermore, I showed that depending on initial community composition, elevated CO2 selected for larger sized diatoms, which led to increased total phytoplankton biomass. Overall, the results suggest that when looking at more than one functional group, initial community composition can have a much greater effect on biomass than elevated CO2. Consequently, the importance of ocean acidification hitherto appears to be overestimated whereas the effect of community composition has been largely overlooked, although it is among the dominant drivers of changes in ecosystem functioning. Because phytoplankton functioning depends on trait composition, it remains a major challenge to understand how phytoplankton communities will reorganize in response to climate change in order to predict the impact on future oceans' ecosystems. lnherently, using independent natural communities, instead of directly manipulating biodiversity, limits the possibility for mechanistic explanation. For future research I suggest to overcome this problem by using one known source-community in which biodiversity (i.e. the loss or distribution of given traits) is manipulated in a non-random approach.

Description: From the establishment of proper cultivation conditions of phototrophic sulfur bacteria 50 years ago up to today significant improvements have been made to systematically treat the phototrophic green and purple sulfur bacteria and identify them in environmental communities. Important steps for these improvements were first of all the description of a large number of pure cultures representing a proper fraction of environmental diversity, their correct taxonomic treatment and the clear definition of the taxa. Further important steps were the establishment of a phylogenetics-based taxonomy supported by 16S rRNA gene sequences and the demonstration of congruence between phylogenies based on 16S rRNA genes and functional genes. The formation of a large database of fmoA genes of green sulfur bacteria and of pufLM genes of purple sulfur bacteria and their obvious phylogenetic congruence with the 16S rRNA gene enabled detailed studies of environmental communities of these bacteria and the recognition of species and genera in natural habitats. The comprehensive studies of selected habitats yielded promising results and demonstrated the potential of this approach for the systematic characterization of environmental communities.

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: Results of the of the present study provide a strong indication that reproductive periods of the bladderwrack Fucus vesiculosus is tuned by environmental conditions, such as day length, although it cannot be entirely ruled out that genetic constitution may play a role, as well. Furthermore results of the present study identified high temperatures as the most challenging condition for alga recruitment. Sea surface temperature rise could therefore be one of the reasons for the decline of F. vesiculosus populations in the Baltic Sea over the last few decades, particularly in the marginal environments (〈 7 psu). Additionally, fertility of F. vesiculosus from the marginal region, in contrast to all other regions, was very low, which also indicates towards a lower capacity to deal with environmental changes. A rather high germination success of some sibling groups (F. vesiculosus) under various environmental conditions, however, is promising in the light of adaptation to climate change.