[L&O Featured Article] L&O Featured Article, Vol. 49 (4) July 2004

[lo-feature at aslo.org]

The Featured Article in the July 2004 issue of L&O is:

Hansell, Dennis A., Hugh W. Ducklow, Alison M. Macdonald, and Molly O'Neil 
Baringer. 2004. Metabolic poise in the North Atlantic Ocean diagnosed from 
organic matter transports. Limnol. Oceanogr. 49: 1084-1094.

The article is freely available at:

          http://aslo.org/lo/toc/vol_49/issue_4/1084.pdf

Instructions for reading PDF files are located on the ASLO web page: 

          http://aslo.org/help/loonline.html 

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Introductory comments by Mary Scranton (L&O Associate Editor)

For the past decade, researchers have argued about whether the ocean is "net 
heterotrophic" or, in other words, whether carbon remineralization exceeds 
carbon fixation on a basin-wide scale. Workers have gotten quite different 
results, perhaps because they have used many different techniques (carbon 
budgets, comparisons of oxygen respiration and production, geochemical tracers, 
and sediment flux measurements) to approach the problem.  Previous studies 
have not, however, estimated fluxes of dissolved organic carbon, which 
dominates the organic carbon reservoir and is likely to be of considerable 
importance in supporting respiration. This paper addresses the metabolic balance 
of the North Atlantic using a budget of organic carbon across the boundaries of 
that system.

Data for the present study come from the North Atlantic and are derived from 
highly detailed NOAA transect on 24.5°N.  DOC data were combined with 
model calculations of the velocity field across this transect, as well as estimates 
of transport of DOC from the Arctic into the North Atlantic, from the North 
Atlantic further south, as well as riverine and atmospheric supply of organic 
matter.  As previously predicted, the North Atlantic does indeed receive more 
organic matter from outside than it exports and is thus net heterotrophic.  
However, the excess carbon supply is quite small relative to the rates of primary 
production and the system seems to be closely balanced.

The authors also compare estimates of allochthonous inputs of carbon to the gyre 
with published estimates of net carbon mineralization rates. Their conclusion is 
that, even if high estimates are used for the carbon fluxes, estimates of net 
heterotrophy greatly exceed (by an order of magnitude) plausible estimates of 
organic carbon supply. This imbalance suggests either that carbon fluxes are 
underestimated (unlikely since the authors have tried to use generous estimates of 
flux terms) or that prior estimates of net heterotrophy may be too high.

Although many aspects of this budget have been assumed, the authors do an 
excellent job of clearly stating their uncertainties and assumptions so it should be 
relatively easy for the budget to be improved as more data become available. 
Proponents of basin-wide heterotrophy will need to consider this paper carefully!



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