Author:
Priddle Julian,Nedwell David B.,Whitehouse Michael J.,Reay David S.,Savidge Graham,Gilpin Linda C.,Murphy Eugene J.,Ellis-Evans J. Cynan
Abstract
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.
Publisher
International Glaciological Society
Cited by
7 articles.
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