On the relationships between primary, net community, and export production in subtropical gyres

Holger Brix, and Nicolas Gruber, IGPP and Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles
David M. Karl, Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI
Nicholas R. Bates, Bermuda Biological Station for Research, Bermuda

It has been proposed that net primary production (NPP), net community
production (NCP), particulate organic carbon export (ΦPOC) and the
relationships among them are governed by local environmental
conditions that favor either a microbially dominated assemblage
leading to a regeneration loop (low ratio of ΦPOC to NPP) or a system
dominated by large plankton with export pathway characteristics (high
ratio of ΦPOC to NPP). We analyze more than 10 years of data from two
subtropical time-series stations (Hawaii Ocean Times-series (HOT) in
the North Pacific, and Bermuda Atlantic Time-Series (BATS) in the North
Atlantic) in order to investigate this regeneration loop versus export
pathway hypothesis and in particular to test the idea that the switch
between the two is controlled by enhanced input of nutrients. In the
decadal long-term mean, the relationships between NPP, ΦPOC and NCP,
which we take here as a proxy for export production, reveal export
pathway characteristics at BATS, while HOT is dominated by the
regeneration loop. This difference is consistent with the stronger
seasonal forcing at BATS and the resulting higher new nutrient input.
However, these characteristics are only valid for parts of the year.
Especially at BATS the export pathway exists only in spring and the
system reverts to a regeneration loop in summer and fall, consistent
with our hypothesis given the strong summer-time stratification and
the resulting low levels of new nutrient input. On interannual
time-scales we find little evidence for statistically significant
alterations of the long-term mean characteristics, a finding we
ascribe to a combination of limited magnitude of forcing, length of
the data records, and possibly an inherent lack of predictability.  A
comparison of our results for the ratio between NCP and NPP (e-ratio)
and the ratio between ΦPOC and NPP (pe-ratio) with those predicted by
the models of Laws et al. [2000] and Dunne et al. [2005], respectively, show
reasonable agreement for the long-term mean, but these models fail to
capture the observed interannual variability in these ratios.

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