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Re-examining the Antarctic Paradox: speculation on the Southern Ocean as a nutrient-limited system
- Julian Priddle, David B. Nedwell, Michael J. Whitehouse, David S. Reay, Graham Savidge, Linda C. Gilpin, Eugene J. Murphy, J. Cynan Ellis-Evans
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- Journal:
- Annals of Glaciology / Volume 27 / 1998
- Published online by Cambridge University Press:
- 20 January 2017, pp. 661-668
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The Southern Ocean is the largest of the high-nutrient, low-chlorophyll (HNLC) regions of the world ocean. Phytoplankton production fails to utilise completely the pool of inorganic nutrients in the euphotic zone, giving rise to low phytoplankton bio-mass and leaving relatively high summer nutrient concentrations. This enigma is of considerable significance for our understanding of the role of the oceans in the global carbon cycle. Various limiting factors have been considered: low light, low temperature, absence of necessary trace elements, grazing pressure and other means of biomass removal.
The dynamics of nitrogen uptake by phytoplankton are of particular importance. Classically, nitrate mixed into the surface layer during winter provides the nitrogen pool for growth in the spring bloom. Some organic material is exported to depth, whilst the remainder is recycled, providing ammonium and other reduced species as nitrogenous substrates for growth during the remainder of the season. The oxidation state of the inorganic nitrogen supply thus identifies new and recycled carbon fixation. Whilst this is convenient “shorthand” for the nitrogen nutrition of carbon export in much of the ocean, it is an inappropriate model for the Southern Ocean. Here, nitrate and ammonium use are simultaneous, and nitrate is never exhausted by the annual phytoplankton production.
We speculate that a range of environmental factors combine to make the large pool of nitrate partially inaccessible to phytoplankton. in addition to the documented effects of low iron availability and high ammonium concentrations, the low temperatures characteristic of the Southern Ocean may decrease nitrate availability because of the increased energetic overheads in its uptake and reduction. This in turn makes ammonium an important nitrogenous substrate, and its production by zooplankton and heterotrophic microorganisms is an important component of the plankton nitrogen cycle. There is some evidence that ammonium production by large grazing animals may stimulate phytoplankton growth. Microbial removal of nitrogen from sedimenting phytoplankton cells may result in local decoupling between the carbon and nitrogen cycles, allowing some reduced nitrogen to remain in the euphotic zone whilst carbon is exported to depth.
7 - Nitrogen processes in aquatic ecosystems
- from Part II - Nitrogen processing in the biosphere
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- By Patrick Durand, INRA, France, Lutz Breuer, Institute for Landscape Ecology and Resources Management, Penny J. Johnes, University of Reading, Gilles Billen, University Pierre & Marie Curie, Andrea Butturini, University of Barcelona, Gilles Pinay, University of Birmingham, Hans van Grinsven, Netherlands Environmental Assessment Agency, Josette Garnier, UMR Sisyphe UPMC ' CNRS, Michael Rivett, University of Birmingham, David S. Reay, University of Edinburgh, Chris Curtis, University College London Environmental Change Research Centre, Jan Siemens, University of Bonn Institute of Crop Science and Resource Conservation – Soil Sciences, Stephen Maberly, Centre for Ecology and Hydrology, Øyvind Kaste, Norwegian Institute for Water Research, Christoph Humborg, Stockholm University, Roos Loeb, B-ware Research Centre, Jeroen de Klein, Wageningen University and Research Centre, Josef Hejzlar, Institute of Hydrobiology, Nikos Skoulikidis, Pirkko Kortelainen, Finnish Environment Institute, Ahti Lepistö, Finnish Environment Institute, Richard Wright, Norwegian Institute for Water Research
- Edited by Mark A. Sutton, NERC Centre for Ecology and Hydrology, UK, Clare M. Howard, NERC Centre for Ecology and Hydrology, UK, Jan Willem Erisman, Gilles Billen, Albert Bleeker, Peringe Grennfelt, Hans van Grinsven, Bruna Grizzetti
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- Book:
- The European Nitrogen Assessment
- Published online:
- 16 May 2011
- Print publication:
- 14 April 2011, pp 126-146
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Summary
Executive summary
Nature of the problem
Freshwater ecosystems play a key role in the European nitrogen (N) cycle, both as a reactive agent that transfers, stores and processes N loadings from the atmosphere and terrestrial ecosystems, and as a natural environment severely impacted by the increase of these loadings.
Approaches
This chapter is a review of major processes and factors controlling N transport and transformations for running waters, standing waters, groundwaters and riparian wetlands.
Key findings/state of knowledge
The major factor controlling N processes in freshwater ecosystems is the residence time of water, which varies widely both in space and in time, and which is sensitive to changes in climate, land use and management.
The effects of increased N loadings to European freshwaters include acidification in semi-natural environments, and eutrophication in more disturbed ecosystems, with associated loss of biodiversity in both cases.
An important part of the nitrogen transferred by surface waters is in the form of organic N, as dissolved organic N (DON) and particulate organic N (PON). This part is dominant in semi-natural catchments throughout Europe and remains a significant component of the total N load even in nitrate enriched rivers.
In eutrophicated standing freshwaters N can be a factor limiting or co-limiting biological production, and control of both N and phosphorus (P) loading is often needed in impacted areas, if ecological quality is to be restored.