Ecosystem Functions in the Equatorial Pacific

1
Ecosystem Functions in the Equatorial
Pacific Richard Dugdale1, Fei Chai2, Dave
Nelson3, Chris Measures4, Mark Brzezinski5,
Frances Wilkerson1 1Romberg Tiburon Center, SFSU,
2University of Maine, 3Orgeon State University,
4University of Hawaii, 5UC Santa Barbara
Abstract
The equatorial Pacific Ocean is the largest
oceanic source of CO2 to the atmosphere, with
significant impacts on the global carbon cycle.
The elevated pCO2 and flux of upwelled CO2 to the
atmosphere at the equator is due to incomplete
use of the available NO3. The ecosystem of the
upwelling region approximates a chemostat, a type
of continuous culture systems in which one
nutrient is limiting and all others are in
excess. In the Pacific equatorial upwelling
system, the limiting nutrient is Si(OH)4 which
diatoms require for their shells. Diatoms are the
major users of NO3 in this system and the amount
they can assimilate is limited by the low amount
of Si(OH)4 available. As a consequence NO3 is
left in the surface waters along with unused CO2
This poster describes how the ecosystem functions
and discusses the question of how a minority
population of diatoms controls the
biogeochemistry of the surface equator,
especially the surface pCO2 concentration. The
source of the low Si(OH)4 in this system is found
in the Southern Ocean, where diatom growth
results in low Si(OH)4 Antarctic Mode Water
(AMW). These results suggest the importance of
Si(OH)4 and diatoms in the global carbon cycle.
The role of Fe in the equatorial upwelling system
is as a secondary limitation affecting the
Si(OH)4 uptake kinetics of the diatoms.
Diatoms make up a small proportion of the
autotrophic biomass in the equatorial Pacific but
carry out 75 of the NO3 uptake.
There is a good fit between modeled and field
data of diatom concentration (S2) as a function
of source Si(OH)4.
NO3 and Si(OH4 increase from 140º to 110ºW at the
equator with Fe showing the opposite trend
(decreasing). Any Fe effect will likely be
experienced at 110º W.
The flux of NO3, NH4, Si and C through the
euphotic zone components predicted by the CoSINE
model shows outward flux of C resulting from
un-used NO3 (upper panel), When the CoSINE model
is embedded in a Pacific Basin wide circulation
model, ROMS with 12.5 km resolution and forced
with daily air-sea fluxes it predicts an increase
in pCO2 of about 1.2 ppm between 1990 and 2006
lowr panel). This increasing trend is due to the
uptake of anthropogenic CO2.
There is high CO2 flux at the equator. Data from
JGOFS TT011 cruise shows concentration of Si(OH)4
to be less than NO3 in upper waters. Low Si(OH)4
input to the surface water of the equatorial
Pacific sets up a Si(OH)4 limited system for
diatom growth.
There are small changes in surface Si(OH)4, large
changes in NO3 and TCO2 with latitude, used here
as a proxy for source Si(OH)4. This follows
predictions of the CoSINE model.
Si(OH)4 uptake kinetics provide the positive
feedback that stabilizes the chemostat. This is
due to the positive relationship between Si(OH)4
concentration and uptake rate.
But Si(OH)4 uptake kinetics can be modified by
ambient Fe concentration, shown here ranging from
0,008 0.120 nmol L-1 .
The flow of NO3 into the equatorial ecosystem is
primarily due to diatoms
Summary

• Many predictions of the CoSINE model have been
  validated with data collected on the EB04 and
  EB05 Biocomplexity cruises.
• The equatorial upwelling ecosystem functions as a
  Si(OH)4 limited continuous culture system for
  diatoms with likely modulation by Fe.
• The amount of Si(OH)4 taken up determines the
  amount of NO3 taken up and therefore the amount
  of nitrate and TCO2 remaining at the surface.
• These results demonstrate the importance of
  understanding the role of diatoms and the Si
  cycle in atmosphere/ocean carbon flux of the
  equatorial Pacific.

The origin of the low Si(OH)4 in the Pacific
equatorial undercurrent is from Si0deficient
water originating in the Antarctic Circumploar
Current, advected as the AAIW.
Acknowledgements This research was supported by
the National Science Foundation (JGOFS SMP OCE
98-02060 and BioComplexity OCE 03-22074). We are
grateful to all the members of the
Dugdale/Wilkerson laboratory at RTC and the
scientists involved with the EB04 cruise for
useful discussions, and the assistance of the
crew of the R/V Revelle.
The CoSINE (Carbon, Silicate, Nitrogen Ecosystem)
biogeochemical model incorporates specifically
Si(OH)4 limitation and diatoms for the equatorial
Pacific. This 1-D model is driven by the vertical
circulation but after testing has been embedded
in 3-D GCM models and used to predict N and CO2
fluxes.