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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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- Article
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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.
Biogeochemical cycling in polar, temperate and tropical coastal zones: similarities and differences
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- By David B. Nedwell, Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK
- Edited by Geoff Gadd, University of Dundee, Kirk Semple, Lancaster University
- Hilary Lappin-Scott, University of Exeter
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- Book:
- Micro-organisms and Earth Systems
- Published online:
- 06 July 2010
- Print publication:
- 13 October 2005, pp 173-200
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Summary
INTRODUCTION
This chapter will consider biogeochemical cycling in the coastal zone. This is defined as that area of estuarine and coastal, relatively shallow water where there is strong benthic-pelagic linkage and exchange between the water column and the underlying sediment. In deeper water this connection becomes increasingly tenuous as the exchange between the euphotic zone and the benthic layer declines. Longhurst et al. (1995) recognized the coastal boundary domain, divided into 22 provinces, as often bounded by a shelf-break front, and included coastal upwelling regions. The coastal zone generally exhibits high rates of primary production compared with the open ocean (Table 1), and there is the greatest impact from inputs from the land to the coastal sea through estuaries. Estuaries and coastal seas are highly heterotrophic systems which are net exporters of CO2 to the atmosphere due to the mineralization and recycling of both autochthonous and allochthonous organic matter (Borges, 2005).
PHYSICO-CHEMICAL DIFFERENCES BETWEEN LATITUDINAL REGIONS
The physical-biological interactions that influence marine phytoplankton production have been reviewed by Daly & Smith (1993). Because of the spherical shape of the Earth, more solar energy falls per unit area of surface in equatorial regions than at the poles (Fig. 1a), and the incidence of light at the equator is vertical to the surface, but oblique at the poles. Furthermore, the distance radiation travels through the atmosphere is longer at the poles, thus reducing the irradiation incident at the poles compared with equatorial regions.