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  • 1
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    Inter Research
    In:  Marine Ecology Progress Series, 182 . pp. 69-76.
    Publication Date: 2018-05-08
    Description: Sinking velocities of more than 300 Nitzschia closterium aggregates were determined during roller table incubation using digital image analysis. To examine the influence of transparent exopolymer particles (TEP) on aggregate settling speed, 3 experiments with different ratios of TEP to cell volume concentration were conducted. The results showed that, for N. closterium aggregates without TEP, sinking velocity (U) was significantly related to the equivalent spherical diameter (ESD) of the aggregates, yielding U (cm s-1) = 1.89 (ESD, cm)0.55. The higher was the specific TEP content of an aggregate, the lower was the sinking velocity and the less pronounced was the size versus velocity relationship. Excess densities (Δρ) of aggregates were derived from velocity measurements and 3-dimensional fractal dimensions (D3) of aggregates were calculated from scaling properties of Δρ. Values for D3 never exceeded 2 and fit well to values of the 2-dimensional fractal dimension (D2) attained from image analysis.
    Type: Article , PeerReviewed
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  • 2
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    Sears Foundation of Marine Research
    In:  Journal of Marine Research, 63 (2). pp. 335-358.
    Publication Date: 2018-03-21
    Description: A marine ecosystem model, that had previously been calibrated in a one-dimensional (1D) mode against observations at three time-series and process-study sites simultaneously, is coupled to a three-dimensional (3D) circulation model of the North and Equatorial Atlantic. Compared to an experiment with a previously employed subjectively tuned ecosystem model, the new 3D-model does not only reduce the model-data misfit at those locations at which observations entered the 1D optimization procedure, but also at an oligotrophic site in the subtropics that had not been considered in the 1D calibration. Basin-scale gridded climatological data sets of nitrate, surface chlorophyll, and satellite-derived primary production also reveal a generally lower model-data misfit for the optimized model. The most significant improvement is found in terms of simulated primary production: on average, primary production is about 2.5 times higher in the optimized model which primarily results from the inclusion of a phytoplankton recycling pathway back to dissolved inorganic nitrogen. This recycling pathway also allows for a successful reproduction of nonvanishing surface nitrate concentrations over large parts of the subpolar North Atlantic. Apart from primary production, the parameter optimization reduces root-mean-square misfits by merely 10–25% and remaining misfits are still much larger than observational error estimates. These residual misfits can be attributed both to errors in the physical model component and to errors in the structure of the ecosystem model, which an objective estimation of ecosystem model parameters by data assimilation alone cannot resolve.
    Type: Article , PeerReviewed
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  • 3
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    ICES
    In:  [Paper] In: ICES Annual Science Conference, 22.-26.09.2008, Halifax, Canada .
    Publication Date: 2017-01-06
    Description: A considerable amount of primary production by marine phytoplankton is released to seawater as dissolved organic matter (DOM) via exudation and leakage processes. The labile fraction of DOM can either directly serve as a source of energy and nutrients or is transformed to particulate matter by abiotic gel particle formation. Principally, both pathways induce diverse effects on higher trophic levels, as they: (i) affect the growth of bacteria and photo-autotrophic nanoplankton, which directly affects the microbial foodweb, and (ii) enhance the formation of aggregates, which provide pelagic microhabitats but also accelerate the export of organic matter to the benthos. Reliable biogeochemical flux estimates of these distinct pathways will crucially depend on our understanding of small-scale processes. Here, we show examples that address the microbial turnover of organic matter and how it is related to primary and secondary production in the North Atlantic and at sites in shelf regions. Recent findings on the sensitivity of microbial processes to changes in temperature and pH will be incorporated. Ecosystems in coastal and shelf regions are most sensitive to anthropogenic impacts, as they are susceptible not only to global changes but also to regional changes. We will therefore give an outlook on how to improve monitoring, experimental, and modelling strategies to better account for microbial foodweb dynamics when assessing climate change effects on ecosystems in coastal and shelf regions.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
    Publication Date: 2013-05-28
    Description: During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2012-02-23
    Description: The influence of seawater CO2 concentration on the abundance and size of suspended particles (2-40 μm) was investigated during a mesocosm experiment at the large scale facility (LFS) in Bergen. In nine outdoor enclosures the partial pressure of CO2 (pCO2) in the seawater was modified by an aeration system. The triplicate mesocosm treatments simulated low (~190 parts per million by volume (ppmV) CO2), present day (~370 ppmV CO2) and high (~700 ppmV CO2) CO2 conditions. The inorganic nutrients nitrate and phosphate were added initially to the mesocosms to induce phytoplankton blooms. Samples for suspended particles were collected daily over a period of 19 days and analysed with the Coulter Counter and by Flow Cytometry. First results indicate that the CO2 treatment significantly affected the size distribution of solid particles, and led to larger surface to volume ratios at lower pCO2. Size is important for diffusion-limited exchange processes at the cell surface as well as for gravitational settling of the solid particles. The observed changes in particle size distribution are therefore discussed with respect to organic matter production and potential sedimentation in the mesocosms during the bloom. An outlook on possible implications of our findings for the future carbon cycling and export in the ocean will be presented.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
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    In:  Journal of Plankton Research, 32 (8). pp. 1167-1184.
    Publication Date: 2013-05-28
    Description: Many critical processes of ecosystem function, including trophic relationships between predators and prey and maximum rates of photosynthesis and growth, are size-dependent. Size spectral data are therefore precious to modellers because they can constrain model predictions of size-dependent processes. Here we illustrate a multi-step statistical approach to create size spectra based on a reanalysis of plankton size data from the IronEx II experiment, where iron was added to a marked patch of water and changes in productivity and community structure were followed. First, bootstrapping was applied to resample original size measurements and cell counts. Kernel density estimation was then used to provide nonparametric descriptions of density versus size. Finally, parametric distributions were used to obtain parameter estimates that can more easily be applied in models. A major advantage of this approach is that it provides confidence envelopes for the density distributions. Our analyses suggest three basic distributional patterns of cell concentration versus logarithm of equivalent spherical diameter for individual taxa. Composite size-densities of heterotrophs and photoautotrophs reveal important aspects of the coupling between protist grazing and the phytoplankton community.
    Type: Article , PeerReviewed
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  • 7
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    In:  [Talk] In: Leipziger Meteorologisches Kolloquium, 10.12.2015, Leipzig, Germany .
    Publication Date: 2016-01-15
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    In:  [Poster] In: SOPRAN Annual Meeting 2014, 25.-26.03.2014, Bremen, Germany .
    Publication Date: 2014-12-05
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
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    In:  [Talk] In: IMBER Open Science Conference FUTURE OCEAN, 23.-27.06.2014, Bergen, Norway .
    Publication Date: 2014-12-09
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
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    In:  [Invited talk] In: Autumn School: Data Assimilation in Biogeochemical Cycles at Abdus Salam International Centre for Theoretical Physics (ICTP), 21.-27.09.2014, Trieste, Italy .
    Publication Date: 2014-12-09
    Type: Conference or Workshop Item , NonPeerReviewed
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