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  • OceanRep  (270)
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  • OceanRep  (270)
  • 1
    Publication Date: 2022-01-17
    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.
    Type: Thesis , NonPeerReviewed
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  • 2
    Publication Date: 2023-01-19
    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.
    Type: Thesis , NonPeerReviewed
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  • 3
    Publication Date: 2024-08-26
    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.
    Type: Thesis , NonPeerReviewed
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  • 4
    Publication Date: 2024-02-07
    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.
    Type: Thesis , NonPeerReviewed
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  • 5
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    In:  (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 169 pp
    Publication Date: 2021-02-08
    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.
    Type: Thesis , NonPeerReviewed
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  • 6
    Publication Date: 2024-08-30
    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.
    Type: Thesis , NonPeerReviewed
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  • 7
    Publication Date: 2023-07-26
    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.
    Type: Thesis , NonPeerReviewed
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  • 8
    Publication Date: 2024-08-14
    Description: The aim of this thesis is to study the small-scale vertical distribution patterns, habitat preferences and limits for Baltic cod in its major spawning area, the Bornholm Basin. The study is based on hydroacoustic single-target measurements and fish tracking algorithms, which allow to identify individual cod in their environment and to allocate hydrographic parameters to each individual. Additionally, distribution patterns were evaluated to identify the possible influence of climate driven long term trends in abiotic conditions on the Eastern Baltic cod stock. In Chapter I, the applicability of the hydroacoustic single-fish tracking method was confirmed by comparing the results with in-situ distribution data obtained concurrently in other studies from data-storage-tags (DST) attached to cod. It could be shown that hydroacoustics can enhance the datasets provided by DST when it comes to scaling of individual data to population level. Distribution patterns were analyzed for the peak spawning period in two years with contrasting hydrographic conditions (post inflow and stagnation). In both years, vertical distribution of cod was controlled by salinity as upper and oxygen concentration (during stagnation) as lower boundary level, marking suitable and preferred habitat. Temperature had no significant effect on vertical distribution. Spawning cod in both years avoided salinities 〈11 psu and oxygen depleted layers. A small fraction of cod was distributed in oxygen concentrations of less than 1 ml l-1. It is assumed that these cod undertake short feeding excursions into deeper layers where their prey organisms are distributed. Seasonal changes in vertical cod distribution were analyzed and related to ambient hydrographic conditions in Chapter II to identify the driving mechanisms. In all years observed, cod experienced a loss of suitable habitat with deteriorating oxygen conditions in deep layers during the course of the year. This induced an upward movement of cod. The upper distribution boundary in the upper layers of the halocline remained nearly constant. A GLM model revealed that apart from a significant year-month effect, salinity and oxygen concentrations were the most significant factors affecting cod vertical distribution. This increasingly pelagic distribution is not taken into account in surveys contributing to stock assessment of (Eastern) Baltic cod. Thus, indices derived from bottom trawl surveys in the area might be biased. In Chapter III, climate driven long-term trends in Baltic Sea oxygen concentrations and potential consequences for the Eastern Baltic cod stock were evaluated. Effects of an observed decline in ambient oxygen concentrations in the central Baltic were studied for all life stages of cod from 1951-2007. Based on field distribution data of different life stages and on laboratory experiments it could be shown that negative effects of proceeding oxygen depletion affect all life stages. Cod eggs were and possibly will continue to be frequently exposed to lethal oxygen conditions. Larvae hatching from deeper floating eggs will possibly not be able to undertake vertical migration activities required for initiation of first feeding. Potential settling areas for juveniles deteriorated over 25 % in size during the time the series is representative for. Based on distribution patterns of adult cod and their corresponding abiotic environment, an oxygen induced reduction in gastric evacuation rates was evident leading to reduced overall conditions. Generally, the expected trend in long-term oxygen development will negatively affect all life stages of cod.
    Type: Thesis , NonPeerReviewed
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  • 9
    Publication Date: 2023-07-17
    Description: Over the past decade large carbonate mound structures, situated in several provinces along the European continental margin have been discovered in 600 to 1200 m water depths. These structures appear as single, conical or ridge-like features, which are often colonised by a deep-water coral ecosystem. Cold-water corals Lophelia pertusa and Madrepora oculata dominate the coral community. Associated sessile and vagile fauna, like sponges, soft corals, bivalves, gastrodpods, crinoids, brachiopods, and fish make these bioherms as diverse as their tropical counterparts. The objective of this study is to reconstruct past environmental settings locked in the sediments of a carbonate mound in the northern Porcupine Seabight, west off Ireland. Detailed investigations were concentrated on seven 3.5 to 6 m long sediment cores, retrieved from Propeller Mound and its closer vicinity. This mound is a ~150 m high structure within the Hovland Mound province, a cluster of several sea-floor protruding carbonate mounds. A first detailed analysis on benthic foraminiferal assemblages in the vicinity of a carbonate mound was performed on two sediment cores, one located on top of the mound and a second one from an off-mound position further north as control site. The off-mound samples reveal two different assemblages: (1) an Interglacial group dominated by infaunal species reflecting present-day environmental and oceanographic conditions with high nutrient flux to the sea-floor and low sediment accumulation under a strong hydrodynamic regime, and (2) a Glacial group, dominated by cassidulinid species, which describe an influence of polar conditions with low nutrient supply and cold intermediate waters. A single species, Elphidium excavatum, dominates the lower core section and is described here as a species displaced from shallow shelf areas. This removal of sediments from the shelf is related to a first advance of the Irish Ice Sheet onto the Irish Mainland shelf, inducing high sedimentation rates of ~28 cm/kyr with a coeval sea-level drop of ~50 m during Late Marine Isotope Stage (MIS) 3. Due to an incomplete stratigraphic record in the on-mound core, the same species of the off-mound assemblages have been grouped in on-mound samples. The results indicate a dominance of the Interglacial group, whereas the Glacial group is less abundant throughout the entire core. This pattern indicates the lack of glacial time intervals in the on-mound core, which is coherent with stable oxygen isotope data and U/Th dates on coral fragments, presenting only interglacial/-stadial values and ages. A third assemblage is abundant in samples of the on-mound core showing elevated epibenthic species not occurring in offmound samples or only to a minor degree. This Mound group shows a great affinity to strong currents, high nutrient availability and is supposed to indicate Mediterranean Outflow Water in the northern Porcupine Seabight, as well as a higher coral cover on Propeller Mound in an earlier interglacial period. A Late Pleistocene decline in mound growth for Propeller Mound is suggested by a decrease of the Mound group towards the Holocene, which might face its complete burial in the future as this already happened to the buried mounds of the Magellan Mound province further north. Detailed information on off-mound sediment structures and contents from visual core description and the interpretation of Computer Tomographic images were used to evaluate sedimentary processes in glacial and interglacial periods. The sediments portray the depositional history of the past ~31 kyr BP, mainly controlled by sea-level fluctuations and the climate regime with the advance and retreat of the Irish Ice Sheet onto the Irish Mainland Shelf. A first advance of glaciers is indicated by a turbiditic release slightly older than 31 kyr BP, coherent with Heinrich event 3. During MIS 3 and MIS 2 shelf erosion prevailed with abundant gravity flows and turbidity currents. A change from glaciomarine to hemipelagic contourite sedimentation during the onset of the Holocene indicates the establishment of the strong, present-day hydrodynamic regime at intermediate depths. A general decrease in sediment accumulation with decreasing distance towards Propeller Mound was discovered throughout the entire core sections. This suggests that currents (turbidity currents, gravity flows, bottom currents) have had a strong impact on sediment accumulation at the mound base for the past ~31 kyr BP. Finally, the reconstructed environmental setting deduced from sedimentary and micropaleontological analyses portrays the boundary conditions of the habitable range for the cold-water coral Lophelia pertusa. The growth of this ecosystem occurs during interglacial and interstadial periods, whereas a glacial retreat of corals is documented in the absence of glacial sediments in the onmound core. These conclusions are summarised in a model – the Mound Factory – which efficiently accounts for the moundd development covering the period of the Northern Hemisphere Glaciation (past 3.1–2.5 Ma).
    Type: Thesis , NonPeerReviewed
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  • 10
    Publication Date: 2024-04-11
    Description: This cruise was conducted as part of the educational training of fishers in the framework of the transdisciplinary SeaRanger program which is scientifically accompanied by the Institute of marine ecosystem and fisheries Science (IMF) at the University of Hamburg (UHAM), the Christian-Albrechts University Kiel, Centre for Ocean and Society (CeOS), the Thuenen-Institute for Baltic Sea fisheries (TI-OF), and the German Centre for Integrative Biodiversity Research (iDiv) in the framework of the joint project SpaCeParti (Coastal Fishery, Biodiversity, Spatial Use and Climate Change: A Participative Approach to navigate the Western Baltic Sea into a Sustainable Future; Grant no. 03F0914) funded by the BMBF. In order to give the fishermen as realistic an application of the standard monitoring techniques as possible, the trip was planned in such a way that the training part was integrated into a scientific monitoring programme focussing on the spawning activity of fish in the Belt Sea. By sampling a standardised station grid contributing to the joint long-term sampling efforts in the Western Baltic Sea which are internationally coordinated by the WBCF (Western Baltic cod Forum), the fishers learned how plankton, fish and water samples are taken, preserved, and analysed and gained a comprehensive insight into the hydrography and fauna of the western Baltic. Similar to the previous cruise AL606 in January 2024 conducted by the IMF no cod larvae and generally less larvae compared to previous years were observed in the Bongo 500 μm net samples from the Plankton grid stations, potentially indicating a delayed spawning activity of fish in the Belt Sea potentially related to the comparably low water temperatures in winter 2023/24.
    Type: Report , NonPeerReviewed , info:eu-repo/semantics/book
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