Silicate Regulation on Diatom Growth in the Eastern Equatorial Pacific Evaluated with SilicateNitrat

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Silicate Regulation on Diatom Growth in the
Eastern Equatorial Pacific Evaluated with
Silicate/Nitrate/TCO2 Distribution Al Marchi,
Richard Dugdale, Frances Wilkerson (Romberg
Tiburon Center - SFSU, 3152 Paradise Drive,
Tiburon, CA 94920 Fai Chai (University of Maine,
School of Marine Sciences, Orono, ME 04469) Mark
Brzezinski (University of California at Santa
Barbara, Department of Ecology, Evolution, and
Marine Biology, Santa Barbara, CA 93106) Richard
Feely (Pacific Marine Environmental Lab/NOAA,
7600 Sand Point Way NE, Seattle, WA 98115 )
Abstract The equatorial Pacific Ocean upwelling
system is characterized as HNLSiLC (high NO3, low
Si(OH)4, low chlorophyll), and exhibits high
surface pCO2 with a net flux of CO2 to the
atmosphere. Low Si(OH)4 concentrations relative
to NO3 in the equatorial undercurrent, the source
water for upwelling, results in Si(OH)4 limiting
diatom production. Diatoms take up NO3 at Si(OH)4
controlled rates, while the small picoplankton
utilize primarily NH4, the main regenerated form
of inorganic nitrogen. In this quasi-continuous
culture system, surface Si(OH)4 concentrations
vary only within a small range, while NO3 surface
values reflect the portion of upwelled NO3 not
used by the diatom population. In the same way,
TCO2 levels at the surface are high. The nutrient
and TCO2 profiles from the top of the
undercurrent to the surface are controlled
primarily by diatom activity. As nutrient
drawdown occurs through phytoplankton activity,
the slope of the ratio Si(OH)4NO3 versus depth
changes as a result of variations in the initial
silicate concentration in the equatorial
undercurrent. Breakpoints occur in the
Si(OH)4NO3 and TCO2NO3 ratios plotted against
depth where the transition from source water to
euphotic zone takes place. These breakpoints
appear also in the outputs of the one-dimensional
(1D) CoSINE (Carbon, Si(OH)4, Nitrogen Ecosystem)
model, and they vary with source water Si(OH)4
concentration. Research cruises to the eastern
equatorial Pacific were made during December 2004
(EB04) and September 2005 (EB05) as part of an
NSF-funded Biocomplexity program. North-south and
zonal sections were sampled in each cruise (110W
from 4N-4S in 2004 and 140W from 4N-4S in 2005).
Vertical plots of nutrients and inorganic carbon
data from the 2004 zonal section are examined
here for breakpoints and slopes of Si(OH)4NO3
and TCO2NO3 ratios. The distributions of
Si(OH)4, NO3, and TCO2, along with the CoSINE
model results, yield information about the source
water concentration of Si(OH)4, NO3 and TCO2,
their variations across and along the equator,
and potential biological assimilation in the
euphotic zone.
EB04 Cruise Track Si(OH)4 (mmol/m3)
One-dimensional Equatorial Ecosystem Model
Comparing the 1-D Model to 2004 Equatorial Data
Preliminary conclusions suggest that our range of
measurements during EB04 fit the mid-range (7.5
mmol/m3) of the predicted 1-D equatorial model
Si(OH)4 concentrations, indicating that the
undercurrent source nutrient supply is stable in
NO3Si(OH)4 being upwelled, and that source
silicate concentrations fit the model (Graph K,
Table 1). The slopes of NO3Si(OH)4, TCO2NO3,
and TCO2Si(OH)4 all suggest that the eastern
equatorial Pacific generally provides a source
Si(OH)4 concentration in a range 7.5 mmol/m3
(Graphs L and M). Finally, when comparing the
intercept vs. slope values of NO3 and Si(OH)4
between 0-200m, the data fall within model
parameters (Graph N) (slope mean0.85).