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  • 1
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    Global Marine N2 Fixation Estimates: from Observations to Models (2018)
    Frontiers
    In:  Frontiers in Microbiology, 9 . Art.Nr. 2112.
    Details
    Publication Date: 2021-03-19
    Description: Fixed nitrogen (N) limits productivity across much of the low-latitude ocean. The magnitude of its inventory results from the balance of N input and N loss, the latter largely occurring in regionally well-defined low-oxygen waters and sediments (denitrification and anammox). The rate and distribution of N input by biotic N2 fixation, the dominant N source, is not well known. Here we compile N2 fixation estimates from experimental measurements, tracer-based geochemical and modelling approaches, and discuss their limitations and uncertainties. The lack of adequate experimental data coverage and the unsufficient understanding of the controls of marine N2 fixation result in high uncertainties, which make the assessment of the current N-balance a challenge. We suggest that a more comprehensive understanding of the environmental and ecological interaction of marine N2 fixers is required to advance the field towards robust N2 fixation rates estimates and predictions.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3389/fmicb.2018.02112
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    14C-age tracers in global ocean circulation models (2015)
    Copernicus Publications (EGU)
    In:  Geoscientific Model Development, 8 . pp. 2079-2094.
    Details
    Publication Date: 2019-09-23
    Description: The natural abundance of 14C in total CO2 dissolved in seawater is a property applied to evaluate the water age structure and circulation in the ocean and in ocean models. In this study we use three different representations of the global ocean circulation augmented with a suite of idealised tracers to study the potential and limitations of using natural 14C to determine water age, the time elapsed since a body of water had contact with the atmosphere. We find that, globally, bulk 14C-age is dominated by two equally important components, one associated with aging, i.e. the time component of circulation and one associated with a "preformed 14C-age". This latter quantity exists because of the slow and incomplete atmosphere/ocean equilibration of 14C in particular in high latitudes where many water masses form. The relative contribution of the preformed component to bulk 14C-age varies regionally within a given model, but also between models. Regional variability, e.g. in the Atlantic Ocean is associated with the mixing of waters with very different end members of preformed 14C-age. In the Atlantic, variations in the preformed component over space and time mask the circulation component to an extent that its patterns are not detectable from bulk 14C-age alone. Between models the variability of age can also be considerable (factor of 2), related to the combinations of physical model parameters, which influence circulation dynamics, and gas exchange in the models. The preformed component was found to be very sensitive to gas exchange and moderately sensitive to ice cover. In our model evaluation exercise, the choice of the gas exchange constant from within the current range of uncertainty had such a strong influence on preformed and bulk 14C-age that if model evaluation would be based on bulk 14C-age it could easily impair the evaluation and tuning of a models circulation on global and regional scales. Based on the results of this study, we propose that considering preformed 14C-age is critical for a correct assessment of circulation in ocean models.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.5194/gmd-8-2079-2015
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    A new perspective on environmental controls of marine nitrogen fixation (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 (11). pp. 4482-4489.
    Details
    Publication Date: 2020-06-29
    Description: Growing slowly, marine N2 fixers are generally expected to be competitive only where nitrogen (N) supply is low relative to that of phosphorus (P) with respect to the cellular N:P ratio (R) of non-fixing phytoplankton. This is at odds with observed high N2 fixation rates in the oligotrophic North Atlantic where the ratio of nutrients supplied to the surface is elevated in N relative to the average R (16:1). In this study, we investigate several mechanisms to solve this puzzle: iron limitation, phosphorus enhancement by preferential remineralization or stoichiometric diversity of phytoplankton, and dissolved organic phosphorus (DOP) utilization. Combining resource competition theory and a global coupled ecosystem-circulation model we find that the additional N and energy investments required for exo-enzymatic break-down of DOP gives N2 fixers a competitive advantage in oligotrophic P-starved regions. Accounting for this mechanism expands the ecological niche of N2-fixers also to regions where the nutrient supply is high in N relative to R, yielding, in our model, a pattern consistent with the observed high N2-fixation rates in the oligotrophic North Atlantic.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2015GL063756
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Expanding the niche of marine N2 fixers (2015)
    In:  [Poster] In: ASLO Aquatic Sciences Meeting 2015, 22.-27.02.2015, Granada, Spain .
    Details
    Publication Date: 2019-09-23
    Description: Nitrogen fixation is essential for maintaining the marine fixed nitrogen (N) inventory which regulates ocean productivity. Still, environmental controls of marine N2 fixation are not well understood. Growing slowly, N2 fixers are expected to be competitive only where N supply is low relative to phosphorus (P) with respect to the cellular N:P ratio of non-fixing phytoplankton (R). This is at odds with observed high N2 fixation rates in the oligotrophic North Atlantic where the surface nutrient supply is elevated in N relative to P. Using resource competition theory and a global coupled ecosystem-circulation model, we show that the ability of N2 fixers to invest additional N into the exo-enzymatic break-down of dissolved organic phosphorus (DOP) gives them a competitive advantage in oligotrophic regions of low P supply. Accounting for this mechanism expands the modeled ecological niche of marine N2 fixers and explains the observed pattern of N2 fixation in the oligotrophic North Atlantic.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Poster
  • 5
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    Oxygen utilization rate (OUR) underestimates ocean respiration – a model study (2016)
    Details
    Publication Date: 2022-02-18
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Talk
  • 6
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    Feedbacks in the cycles of N and P in anoxic waters - negative or positive? (2015)
    In:  [Talk] In: ASLO Aquatic Sciences Meeting 2015, 22.-27.02.2015, Granada, Spain .
    Details
    Publication Date: 2019-09-23
    Description: In marine environments, negative and positive feedbacks associated with anoxic conditions have been described for nitrate (removal by denitrification) and phosphate (liberation from iron oxides as they are reduced), respectively. In both feedbacks, the supply of organic matter to deep water from surface production, controlled by the respective element, is essential for establishing or maintaining deep-water anoxia. The Baltic and Black Seas, both with large anoxic deep-water bodies are examined for the existence of these feedbacks using extensive datasets collected during the past decades. For both the Baltic Sea with its deep water alternating between oxic and anoxic conditions, and the Black Sea with permanently anoxic deep water, these feedbacks are shown to be not important. The removal of nitrate by denitrification, which requires the absence of oxygen depends also on the presence of nitrate, which itself requires oxygen to form. In the Baltic Sea, the overwhelming share of denitrification occurs in sediments where nitrate - and thus indirectly oxygen - availability limits denitrification. The liberation of phosphate from iron oxides being reduced can significantly increase deep water phosphate concentration, but not necessarily that in surface water, which would be necessary for the proposed feedback loop. On the contrary, surface phosphate concentrations are especially low in the Black Sea. This is most likely due to the re-precipitation of metal oxides, their re-adsorption of phosphate and subsequent sinking from surface water. In oceanic oxygen minimum zones (OMZ) N and P feedbacks work differently since there sediments are involved to a much lesser degree.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Talk
  • 7
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    A Puzzle of Oceanic Oxygen Utilization (2015)
    In:  [Talk] In: ASLO Aquatic Sciences Meeting 2015, 22.-27.02.2015, Granada, Spain .
    Details
    Publication Date: 2019-09-23
    Description: Large uncertainties exist in the quantification of the biological carbon pump of today’s ocean. The export of organic matter from the ocean’s euphotic zone is a critical benchmark number to describe the pump. Local measurements of the export flux are highly uncertain due to severe methodological issues and undersampling of the ocean. To this uncertainties the contribution of dissolved organic matter adds. The vertical integral of oxygen utilization in the interior of the ocean is considered an estimate of export production accounting for the export of both particular and dissolved organic matter. Regional oxygen utilization rates (OUR) are computed from apparent oxygen utilization (AOU) and an estimate of the time elapsed since the last contact with the atmosphere. Surprisingly the assumptions underlying this concept have not been tested rigorously. Using global ocean biogeochemical models we compare OUR computed from AOU and an ideal age tracer with an independent and perfect estimate of ocean respiration available in the model. In three different global models OUR underestimates true respiration by a factor of about three. Most of the difference is in the upper 1000m of the ocean. There are also important qualitative differences between the two independent approaches. For example, the contribution of dissolved organic matter driving oxygen utilization is largely underestimated when based on bulk tracer concentrations (AOU, DOC), the usual approach applied to observations.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Talk
  • 8
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    Loss of fixed nitrogen causes net oxygen gain in a warmer future ocean (2019)
    Nature Research
    In:  Nature Communications, 10 (1). Art.Nr. 2805.
    Details
    Publication Date: 2022-01-31
    Description: Oceanic anoxic events have been associated with warm climates in Earth history, and there are concerns that current ocean deoxygenation may eventually lead to anoxia. Here we show results of a multi-millennial global-warming simulation that reveal, after a transitory deoxygenation, a marine oxygen inventory 6% higher than preindustrial despite an average 3 °C ocean warming. An interior-ocean oxygen source unaccounted for in previous studies explains two thirds of the oxygen excess reached after a few thousand years. It results from enhanced denitrification replacing part of today’s ocean’s aerobic respiration in expanding oxygen-deficient regions: The resulting loss of fixed nitrogen is equivalent to an oceanic oxygen gain and depends on an incomplete compensation of denitrification by nitrogen fixation. Elevated total oxygen in a warmer ocean with larger oxygen-deficient regions poses a new challenge for explaining global oceanic anoxic events and calls for an improved understanding of environmental controls on nitrogen fixation.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    Format: text
    DOI: 10.1038/s41467-019-10813-w
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Oxygen utilization rate (OUR) underestimates ocean respiration: A model study (2016)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-11-08
    Description: We use a simple 1-D model representing an isolated density surface in the ocean and 3-D global ocean biogeochemical models to evaluate the concept of computing the subsurface oceanic oxygen utilization rate (OUR) from the changes of apparent oxygen utilization (AOU) and water age. The distribution of AOU in the ocean is not only the imprint of respiration in the ocean's interior but is strongly influenced by transport processes and eventually loss at the ocean surface. Since AOU and water age are subject to advection and diffusive mixing, it is only when they are affected both in the same way that OUR represents the correct rate of oxygen consumption. This is the case only when advection prevails or with uniform respiration rates, when the proportions of AOU and age are not changed by transport. In experiments with the 1-D tube model, OUR underestimates respiration when maximum respiration rates occur near the outcrops of isopycnals and overestimates when maxima occur far from the outcrops. Given the distribution of respiration in the ocean, i.e., elevated rates near high-latitude outcrops of isopycnals and low rates below the oligotrophic gyres, underestimates are the rule. Integrating these effects globally in three coupled ocean biogeochemical and circulation models, we find that AOU-over-age based calculations underestimate true model respiration by a factor of 3. Most of this difference is observed in the upper 1000 m of the ocean with the discrepancies increasing toward the surface where OUR underestimates respiration by as much as factor of 4.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Model experiment on heterotrophic denitrification vs. autotrophic anammox - quantifying collateral effects on the oceanic carbon cycle (2015)
    PANGAEA
    In:  Supplement to: Koeve, Wolfgang; Kähler, Paul (2010): Heterotrophic denitrification vs. autotrophic anammox – quantifying collateral effects on the oceanic carbon cycle. Biogeosciences, 7(8), 2327-2337, https://doi.org/10.5194/bg-7-2327-2010
    Details
    Publication Date: 2023-02-24
    Description: The conversion of fixed nitrogen to N2 in suboxic waters is estimated to contribute roughly a third to total oceanic losses of fixed nitrogen and is hence understood to be of major importance to global oceanic production and, therefore, to the role of the ocean as a sink of atmospheric CO2. At present heterotrophic denitrification and autotrophic anammox are considered the dominant sinks of fixed nitrogen. Recently, it has been suggested that the trophic nature of pelagic N2-production may have additional, "collateral" effects on the carbon cycle, where heterotrophic denitrification provides a shallow source of CO2 and autotrophic anammox a shallow sink. Here, we analyse the stoichiometries of nitrogen and associated carbon conversions in marine oxygen minimum zones (OMZ) focusing on heterotrophic denitrification, autotrophic anammox, and dissimilatory nitrate reduction to nitrite and ammonium in order to test this hypothesis quantitatively. For open ocean OMZs the combined effects of these processes turn out to be clearly heterotrophic, even with high shares of the autotrophic anammox reaction in total N2-production and including various combinations of dissimilatory processes which provide the substrates to anammox. In such systems, the degree of heterotrophy (deltaCO2:deltaN2), varying between 1.7 and 6.5, is a function of the efficiency of nitrogen conversion. On the contrary, in systems like the Black Sea, where suboxic N-conversions are supported by diffusive fluxes of NH4+ originating from neighbouring waters with sulphate reduction, much lower values of Delta CO2:Delta N2 can be found. However, accounting for concomitant diffusive fluxes of CO2, the ratio approaches higher values similar to those computed for open ocean OMZs. Based on this analysis, we question the significance of collateral effects concerning the trophic nature of suboxic N-conversions on the marine carbon cycle.
    Keywords: BIOACID; Biological Impacts of Ocean Acidification
    Type: Dataset
    Format: application/zip, 19.9 kBytes
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    DATA PANGAEA
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