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| Two-Decade Decrease of Particulate Phosphorus Inventory Revealed by
Hawaii Ocean Time-series |
| Ya-Wei Luo |
| Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, USA |
| *Corresponding author: |
Ya-Wei Luo
Department of Marine Chemistry and
Geochemistry
Woods Hole Oceanographic Institution
Woods Hole, Massachusetts
02543, USA
Tel: 508-289-1818
E-mail: yluo@whoi.edu |
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| Received April 24, 2012; Accepted April 25, 2012; Published April 27, 2012 |
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| Citation: Luo YW (2012) Two-Decade Decrease of Particulate Phosphorus
Inventory Revealed by Hawaii Ocean Time-series. J Marine Sci Res Dev 2:e109.
doi:10.4172/2155-9910.1000e109 |
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| Copyright: © 2012 Luo YW. This is an open-access article distributed under the
terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
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| Since the Hawaii Ocean Time-series (HOT) program was
initialized in 1988, repeated observations of a series of physical,
chemical and biological parameters have been made on a monthly
basis at a deep-ocean station (~4,700 m) locating in the North Pacific
subtropical gyre [1]. The two-decade data have revealed inter annual
variations in many parameters. Most of these inter annual variations
were not sustained toward one direction in the two decades. The
surface particulate Phosphorus (P) inventory, however, was one of the
few exceptions whose annual averages changed monotonically in this
period. The annual average particulate P inventory in the surface 100
m has significantly decreased (Spearman rank correlation to year: ρ =
-0.95, p <0.001) from 21 nM in 1990 to 8.6 nM in 2010, a 60% drop!
Meanwhile, the particulate carbon (C) and nitrogen (N) did not show
sustained decrease. This decrease may indicate a profound change at
the ecosystem level, considering P as an essential nutrient utilized by
all living organisms for genetic materials (DNA and RNA), membranes
and energy storage (ATP and ADP). |
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| This phenomenon was noticed by the scientists in the first decade
of the HOT program [2,3]. In around 2000, it was hypothesized that
the development of dinitrogen (N2) fixation had increased the N to
P stoichiometry in this region and had shifted the ecosystem from
N-limiting to P-limiting [4,5]. A consequence of the shift was that
the smaller prokaryotes with better P assimilation efficiency, such as
Prochlorococcus, whose cellular C:P and N:P were also higher, would be
selected against eukaryotic phytoplankton cells [3]. Thus the particulate
P was decreasing even the particulate C and N was basically unchanged. |
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| However, this hypothesis is not supported by up-to-date HOT
data. With one more decade of HOT data, the decrease of particulate
P continued. But the abundance of Prochlorococcus did not show an
increase and the abundance of eukaryotes even slightly increased. The
hypothesis could still be true if the average size of Prochlorococcus
became larger so that the total Prochlorococcus biomass increased and
the average size of eukaryotes became smaller so that their biomass
decreases. Unfortunately, the cell size was not measured by the HOT
program. |
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| So what is the real cause for the decadal decrease of the particulate
P? The only other parameter measured by the HOT program that
showed a sustained change over the last two decades was the dissolved
inorganic C (and therefore the decreasing pH), which increased with
the atmospheric CO2. We do not have any evidence that can possibly
relate the high CO2 and ocean acidification to the cellular elemental
compositions of marine microorganisms. Also note that the particulate
P is only a small portion of the total P pool at the HOT station. It is
less than 10% of the total dissolve P in the surface 100 m. In 1990-
2000 when both particulate P and total dissolved P were measured by
the HOT program, it was interesting that the total P pool (particulate
plus total dissolved P) did not decrease and even slightly increased
(not significant though, p = 0.3). Does that indicate that the utilization
of dissolved organic P were changing so that less dissolved organic P
was recycled to support phytoplankton growth and less particulate P
was synthesized? However, the HOT low-level phosphorus analysis, a precise measurement for dissolve inorganic P, did not show a sustained
decrease in the surface 100 m over the past two decades, which does
not support that the cycling from organic P to inorganic P was
slowed down. The whole mystery appears that the P continued to be
redistributed from the particulate pool to the dissolved organic pool
and we do not know the underlying mechanism. |
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| Long-term observatories are very valuable, but sometimes are also
very challenge, for the marine sciences. As shown by this example,
the HOT data have revealed an important dynamics which is hard to
be explained with current available measurements. In order to solve
the problem of the continuous decrease of particulate P, the diversity
of microorganisms, their cell size distribution and their elemental
compositions have to be tracked in a long term, which will not be a
trivial work and may limited by financial, technological and human
resources. Even those measurements can be conducted in a near future,
it could still have been too late. The particulate P cannot drop infinitely.
We may have already missed an opportunity. |
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| It is still exciting to track the particulate P at the HOT station to see
where the trend would stop or turn around. |
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| References |
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- Karl DM, Dore JE, Lukas R, Michaels AF, Bates NR, et al. (2001) Building the long-term picture: the U.S. JGOFS time-series programs. Oceanography 14: 6-17.
- Hebel DV, Karl DM (2001) Seasonal, interannual and decadal variations in particulate matter concentrations and composition in the subtropical North Pacific Ocean. Deep Sea Res Part 2 Top Stud Oceanogr 48: 1669-1695.
- Karl DM, Bjoerkman KM, Dore JE, Fujieki L, Hebel DV, et al. (2001) Ecological nitrogen-to-phosphorus stoichiometry at station ALOHA. Deep Sea Res Part 2 Top Stud Oceanogr 48: 1529-1566.
- Michaels AF, Olson D, Sarmiento JL, Ammerman JW, Fanning K, et al. (1996) Inputs, losses and transformations of nitrogen and phosphorus in the pelagic North Atlantic Ocean. Biogeochemistry 35: 181-226.
- Karl D, Letelier R, Tupas L, Dore J, Christian J, et al. (1997) The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific Ocean. Nature 388: 533-538.
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