U.S.
JGOFS Synthesis & Modeling Project U.S.
JGOFS Synthesis & Modeling Project
| First U.S.
JGOFS Synthesis and Modeling Special Issue in Deep-Sea Research II
special editors: Scott Doney, Paul Falkowski, and Jorge Sarmiento The issue will be published as 49/1-3 which is scheduled to be despatched to subscribers on 11 December 2001. The issue should be available on ScienceDirect by the end of November. |
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OVERVIEW PAPER
Doney, S.C.,
J.A. Kleypas, J.L. Sarmiento, and P.G. Falkowski
The U.S. JGOFS Synthesis and
Modeling Project, an Introduction.
The field data collected as part of the international Joint Global
Ocean Flux Study (JGOFS) provides an unprecedented view of marine biogeochemistry
and the ocean carbon cycle. Following the completion of a series of regional
process studies, a global CO2 survey, and a decade
of sampling at two open-ocean time-series, U.S. JGOFS initiated in 1997
a final research phase, the Synthesis and Modeling Project (SMP). The objective
of the U.S. JGOFS SMP is to ``synthesize knowledge gained from the U.S.
JGOFS and related studies into a set of models that reflect our current
understanding of the oceanic carbon cycle.'' Here we present an overview
of the SMP and highlight the early scientific results from the project.
(link
to electronic version)
Armstrong,
Robert A.; Cindy Lee, John L. Hedges, Susumo Honjo, and Stuart G. Wakeham.
A new, mechanistic model for
organic carbon fluxes in the ocean, based on the quantitative association
of POC with ballast minerals.
In simulation studies of the ocean's role in the global carbon cycle,
predicting the depth-distribution for remineralization of particulate organic
carbon (POC) is of particular importance. Following Sarmiento et al. (1993),
most simulation models have used the power-law curve of Martin et al. (1987)
for this purpose. The Martin et al. curve is an empirical fit to data,
most of which is from shallow floating sediment traps. Making such a fit
implies that all the information necessary for prediction is contained
in the carbon flux itself, so that the organic carbon flux at any depth
z can be predicted from the flux of organic carbon at some near-surface
depth. Here we challenge this basic premise, arguing that fluxes of ballast
minerals (silicate and carbonate biominerals, and dust) determine deep-water
POC fluxes, so that a mechanism-based model of POC flux must simultaneously
predict fluxes of both POC and ballast minerals. This assertion is based
on the empirical observation that POC fluxes are tightly linked quantitatively
to fluxes of ballast minerals in the deep ocean. Here we develop a model
structure that incorporates this observation, and fit this model to U.S.
JGOFS EqPac data. This model structure, plus the preliminary parameter
estimates we have obtained, can be used in simulation studies of the ocean
carbon cycle.
Berelson,
William M.
Particle settling rates increase
with depth in the ocean.
Time and depth-scales of particulate organic carbon degradation and
CaCO3 and biogenic opal dissolution are critical to
understanding the depth distribution of CO2 and dissolved
nutrients in the ocean. The speed at which particles sink and the factors
which control sinking speed are of primary importance to the distribution
of oceanic nutrient concentrations and to the preservation of biogenic
material in the sediment record. Sequencing sediment trap collectors deployed
at U.S. JGOFS sites in the equatorial Pacific and Arabian Sea provided
a time series of particle fluxes from which particle setting rates were
estimated. A comparison of settling velocities obtained from 100-500 m
(Pilskaln et al., 1998) to settling velocities obtained for depths between
1000 and 3500 m indicate an increase of a factor of 2-10 between 100-2000
m and an increase of 15-60% between 2000-3500 m. The increase in settling
velocity in the deep ocean is generally correlated with the loss of Corg
with depth. Lithogenic content does not appear to impact particle settling
rate. Variability in particle setting rate is systematically related to
physical forcing of the surface ocean in the Equatorial Pacific, but not
in the Arabian Sea. The increase in particle settling rate with depth likely
influences the delivery of CaCO3 to the sea floor.
Carr, Mary-Elena.
Estimation of potential productivity
in Eastern Boundary Currents using remote sensing.
This study provides a satellite-based estimate of potential primary
production in the four Eastern Boundary Currents (EBCs), i.e. the California,
Humboldt, Canary, and Benguela Currents, from the first 24 months of the
Sea-Viewing Wide Field of View Sensor, SeaWiFS. Within each EBC, production
was estimated for the area of high chlorophyll concentration (> 1 mg m-3)
or active area, which is likely to determine the production level that
can be utilized by higher trophic levels. Primary production decreased
with latitude within each EBC while the extent of the active area was related
to the magnitude of offshore transport. The most productive EBC was the
Benguela Current (0.37 Gt C y-1), followed by the Canary (0.33
Gt C y-1), Humboldt (0.20 Gt C y-1), and California
(0.04 Gt C y-1) Currents. Interannual differences between the
nominal years 1997, 1998, and 1999 were largest for 1997 (measured by the
Ocean Color Temperature Scanner, OCTS), which may be due primarily to the
different sensor and algorithm. The Humboldt Current was more productive,
and the Canary much less, during 1997 than in the two following years.
The El Niño of 1997-1998 led to smaller annual production in 1998
in the Pacific EBCs. The upper bound of sustainable fish yield was estimated
assuming a food chain of 2.6 links and an average trophic efficiency of
10%. The resulting values are 4 to 150 times larger than the observed fish
catch from 1990 through 1997. Actual catch data in the Benguela Current
were 20 times smaller than in the Humboldt Current. The most likely explanations
for the differences in potential and observed fish catch are related to
differing trophic structure and spatial accessibility in different EBCs.
If the estimated yield is an upper bound which will be decreased to 10
or 20% by environmental accessibility, the small pelagic fishery in all
four EBCs is likely food-limited. (Benguela Current) were 20 times less
than in the Humboldt Current.
Christian,
J.R., M.A. Verschell, R. Murtugudde, A.J. Busalacchi, and C.R. McClain.
Biogeochemical modelling of
the tropical Pacific Ocean. I. Seasonal and interannual variability.
A coupled physical-biogeochemical model has been developed in order
to study physical-biological interactions in the tropical Pacific Ocean
on seasonal-to-interannual timescales. The model incorporates both iron-
and-nitrogen limited phytoplankton growth, and succession of phytoplankton
size classes in accordance with the ``ecumenical" iron hypothesis. The
model shows a strong El Niño Southern Oscillation component to phytoplankton
variability in the central equatorial Pacific over the period 1980-1998.
It is possible that this mode is more dominant in the model than in nature,
although the correlation of modelled and observed chlorophyll in this region
is strong. The model results show that interannual variability dominates
over the mean seasonal cycle for both physical and biogeochemical fields,
with the exception of undercurrent transport west of the dateline. Physical
and biogeochemical fields show consistent seasonal phasing among the four
El Niño events simulated, although there is variability in the magnitudes
and exact timing. Nutrient concentration anomalies at constant temperature
in the thermocline of the central equatorial Pacific appear to be largely
advected from the west, and the strong seasonal cycle of the equatorial
undercurrent in the west introduces a significant annual component to the
variability of nutrient concentrations at longitudes where the mean seasonal
cycle of the local physical forcing is negligible. The biogeochemical model
maintains realistic nutrient pools over the time scales required for interannual
simulation, and responds in a realistic fashion to changing upper ocean
hydrography and circulation. The model is quite sensitive to the temporal
resolution of the wind forcing, which introduces additional uncertainty
into the validation and prediction of biogeochemical fields.
(link
to electronic version)
Christian,
J.R., M.A. Verschell, R. Murtugudde, A.J. Busalacchi, and C.R. McClain.
Biogeochemical modelling of
the tropical Pacific Ocean. II. Iron biogeochemistry.
A coupled physical-biogeochemical model of the tropical Pacific Ocean
with simultaneous iron and nitrogen limitation was developed in order to
study questions of iron biogeochemistry, its effects on upper ocean production,
and ultimately the biogeochemical cycles of the other elements. The model
results suggest that iron limitation is ubiquitous in the equatorial Pacific,
and extends further west than is generally believed unless there are significant
inputs of geothermal iron at quite shallow depths. Most model parameters
(e.g., iron solubility, scavenging rates, Fe:N ratios) must be near the
limit of their generally accepted range of values in order to prevent elevated
surface nitrate concentrations from spreading further into the warm pool
than is observed. Transport of geothermal iron in the equatorial undercurrent
(EUC) provides a possible mechanism for limiting surface nitrate in the
warm pool, but the source must be near the upper boundary of the EUC to
provide iron to the surface west of the dateline. Accumulations of ammonium
in the western Pacific appear to result from the exhaustion of iron in
upwelled water before nitrogen. The realism of the simulation is limited
primarily by lack of information about the abundance and distribution of
dissolved iron; the assumption of constant Fe:N ratios and the magnitude,
distribution and solubility of the aeolian iron flux are also important
sources of uncertainty. The sensitivity of the simulation to the way that
iron is initialized in the western Pacific thermocline emphasizes the importance
of the equatorial undercurrent throughout the tropical Pacific and the
need for iron observations in this region.
(link
to electronic version)
Fennel, Katja,
Yvette H. Spitz, Ricardo M. Letelier, Mark R. Abbott, and David M. Karl.
A deterministic model for N2-fixation
at Station ALOHA in the subtropical North Pacific Ocean.
Marine N2-fixation by diazotrophic microorganisms
is a key process in biogeochemical cycling and yields an important input
of new nitrogen into the tropical and subtropical surface ocean. However,
it is only poorly accounted for in current numerical models. We present
a simple biological model that explicitly includes N2-fixation
by diazotrophic phytoplankton. The model employs a mechanistic parameterization
of N2-fixation based on physiological responses of
Trichodesmium
to physical conditions of the environment. The model is conceived to allow
shifts in nitrogen versus phosphorus control of the plankton community
by resolving the biogeochemical cycles of both elements. Typical N:P ratios
were assigned to the different functional groups to capture variations
in the N:P stoichiometries of inorganic and organic matter pools. The biological
model was coupled to a 1D physical model of the upper ocean. A simulation
was performed at stn. ALOHA (22°45'N, 158°W) in the subtropical
North Pacific Ocean where intense blooms of Trichodesmiumcurred
during the last decade. The model captures essential features of the biological
system including the vertical structure and seasonal course of chlorophyll,
the seasonal cycle and interannual differences in diazotrophic biomass,
the mean vertical particle flux, and an oscillation in the relative importance
of nitrogen versus phosphorus. We regard this model as a step towards a
mechanistic tool to assess the magnitude of marine N2-fixation
and to explore hypotheses on its effect on carbon sequestration from the
atmosphere.
(link
to electronic version)
Follows, Michael
and Stephanie Dutkiewicz.
Meteorological modulation of
the North Atlantic spring bloom.
Using ocean time-series observations and remote chlorophyll estimates
derived from SeaWiFS ocean color observations we examine and illustrate
the relationships between changes in the intensity of the spring bloom
and changes in weather patterns, mediated by upper ocean mixing. A simplified
two-layer model provides the conceptual framework, predicting regional
regimes of differing biological response to vertical mixing anomalies in
the ocean surface boundary layer. The meteorological anomalies may be derived
from re-analyzed meteorological data. We examine two regimes of regional
and interannual sensitivity to meteorological forcing, defined by the ratio
of the spring critical layer depth and the winter mixed layer depth, hc/hm.
Regions of large hc/hm
(subtropics) are characterized by an enhanced bloom in response to enhanced
mixing, both across the region and from year to year. The subtropics exhibit
consistent, interannual changes which are coordinated over large regions,
and local interannual changes are comparable in magnitude to the regional
variations in each bloom. In the low hc/hm
regime (subpolar), regional variations reflect retardation of the bloom
by enhanced mixing. Local interannual changes in the subpolar region, however,
are small relative to the regional variations and do not show a clear and
consistent response to interannual variability in the local meteorological
forcing. We infer that other factors, including changes in insolation,
local mesoscale variability, and grazing exert a stronger influence on
local interannual variability of the subpolar bloom. We discuss the implications
of these relationships for the implications of decadal climate changes
on biological productivity.
(link
to electronic version)
Friedrichs,
Marjorie A.M.
Assimilation of JGOFS EqPac
and SeaWiFS Data into a Marine Ecosystem Model of the Central Equatorial
Pacific Ocean.
A five-component (phytoplankton, zooplankton, ammonium, nitrate and
detritus) ecosystem model developed for the central equatorial Pacific
(Friedrichs and Hofmann, 2001), is reformulated in a data assimilative
mode, using the variational adjoint technique. This method minimizes model/data
misfits by adjusting six model parameters that were selected by assessing
parameter codependencies and model sensitivity to parameter variations.
Through the assimilation of cruise data from the U. S. Joint Global Ocean
Flux Study (JGOFS) Equatorial Pacific Process Study (EqPac), and ocean
color data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), it
is possible to reduce model/data misfit by estimating optimal parameters
governing processes such as phytoplankton and zooplankton mortality, zooplankton
grazing, phytoplankton growth, and the recycling of nutrients from detritus
remineralization. The success of this approach is evident in that similar
parameter sets are obtained even when independent data sets are assimilated.
For example, the assimilation of in situ EqPac (depth-resolved) data from
the 1991-92 El Niño produces a parameter set which is nearly identical
to that estimated via the assimilation of remotely-sensed (surface) SeaWiFS
data collected during the 1997-98 El Niño. The assimilation of biological
data also allows objective determination of whether or not a given model
structure is consistent with a specific set of observations. For example,
the assimilation process demonstrates that data collected during and after
the 1991-1992 El Niño are consistent with the same single-species
ecosystem model, thereby suggesting that El Niño conditions may
not necessarily be associated with shifts in species composition. In contrast,
the increased abundance of diatoms associated with the passage of a tropical
instability wave in October 1992 (Bidigare and Ondrusek, 1996) as well
as a brief period of macronutrient limitation observed from November 1997
through January 1998 (Chavez et al., 1998) violate key assumptions of the
model. Assimilation of data that include these dynamics results in unrealistic
simulations of the lower trophic levels. The successful simulation of these
particular data sets will require that the model dynamics allow for species
composition changes and alternation between macro- and micronutrient limitation.
In this way, assimilation of biological data into marine ecosystem models
cannot necessarily overcome inappropriate model dynamics and structure;
rather, it can serve to guide model reformulation.
(link
to electronic version)
Gnanadesikan,
Anand, Richard D. Slater, Nicolas P. Gruber, and Jorge Sarmiento.
Oceanic vertical exchange and
new production: A comparison between models and observations.
This paper explores the relationship between large-scale vertical exchange
and the cycling of biologically active nutrients within the ocean. It considers
how the parameterization of vertical and lateral mixing affects estimates
of new production (defined as the net uptake of phosphate). A baseline
case is run with low vertical mixing in the pycnocline and a relatively
low lateral diffusion coefficient. The magnitude of the diapycnal diffusion
coefficient is then increased within the pycnocline, within the pycnocline
of the Southern Ocean, and in the top 50m, while the lateral diffusion
coefficient is increased throughout the ocean. It is shown that it is possible
to change lateral and vertical diffusion coefficients so as to basically
preserve the structure of the pycnocline while changing the pathways of
vertical exchange and hence the cycling of nutrients. Comparisons between
the different models reveal that new production is very sensitive to the
level of vertical mixing within the pycnocline, but only weakly sensitive
to the level of lateral and upper ocean diffusion. The results are compared
with two estimates of new production based on ocean color and one based
on the annual cycle of nutrients. On a global scale, the observational
estimates are most consistent with the circulation produced with a low
diffusion coefficient within the pycnocline, resulting in a new production
of around 10 GtC yr-1. On a regional level, however, large differences
appear between observational and model based estimates. In the tropics,
the models yield systematically higher levels of new production than the
observational estimates. We present evidence from the Eastern Equatorial
Pacific that this is due both to biases in the data used to generate the
observational estimates and problems with the models. In the North Atlantic,
the observational estimates vary more than the models, due in part to the
methodology by within the nutrient-based climatology is constructed. In
the North Pacific, the modelled values of new production are all much lower
than the observational estimates, probably as a result of the failure to
form intermediate water with the right properties. The results demonstrate
the potential usefulness of new production for evaluating circulation models.
(link
to electronic version)
Gregg, Watson
W.
Tracking the SeaWiFS record
with a coupled physical/biogeochemical/ radiative model of the global oceans.
The Sea-Viewing Wide Field-of-view Sensor (SeaWiFS) has observed multiple
years of routine global chlorophyll observations from space. The mission
was launched into a record El Niño event, which eventually gave
way to one of the most intense and longest-lasting La Niña events
ever recorded. The SeaWiFS chlorophyll record captured the response of
ocean phytoplankton to these significant events in the tropical Indo-Pacific
basins, but also indicated significant interannual variability unrelated
to the El Niño/La Niña events. This included large variability
in the North Atlantic and Pacific basins, in the North Central and equatorial
Atlantic, and milder patterns in the North Central Pacific. This SeaWiFS
record was tracked with a coupled physical/biogeochemical/ radiative model
of the global oceans using near-real-time forcing data such as wind stresses,
sea surface temperatures, and sea ice. This provided an opportunity to
offer physically and biogeochemically meaningful explanations of the variability
observed in the SeaWiFS data set, since the causal mechanisms and interrelationships
of the model are completely understood. The coupled model was able to represent
the seasonal distributions of chlorophyll during the SeaWiFS era, and was
capable of differentiating among the widely different processes and dynamics
occurring in the global oceans. The model was also reasonably successful
in representing the interannual signal, especially when it was large, such
as the El Niño and La Niña events in the tropical Pacific
and Indian Oceans. The model provided different phytoplankton group responses
for the different events in these regions: diatoms were predominant in
the
tropical Pacific during the La Niña but other groups were predominant
during El Niño. The opposite condition occurred in the tropical
Indian Ocean. Both situations were due to the different responses of the
basins to El Niño. Interannual variability in the North Pacific
was exhibited as an increase in the spring bloom in 1999 and 2000 relative
to 1998. This resulted in the model from a shallower and more rapidly shoaling
mixed layer, producing more average irradiance in the water column and
preventing herbivore populations to keep pace with increasing phytoplankton
populations. However, several aspects of the interannual cycle were not
well-represented by the model. Explanations ranged from inherent model
deficiencies, to monthly averaging of forcing fields, to biases in SeaWiFS
atmospheric correction procedures.
Hood, Raleigh
R., Ajit Subramaniam, Linda R. May, Edward J. Carpenter, and Douglas G.
Capone.
Remote estimation of nitrogen
fixation by Trichodesmium.
In this paper a non-spectral model is described that can be used to
calculate N2-fixation rate from remote estimates of
Trichodesmium
biomass.
This model, which is similar to formulations that have been developed for
estimating primary production from satellite-derived phytoplankton chlorophyll
concentrations, is parameterized using measured
Trichodesmium N2-fixation
vs. irradiance data and observed subsurface
Trichodesmium biomass
profiles from the Tropical Atlantic Ocean. These data reveal that the N2-fixation
vs. I responses and subsurface distributions of Trichodesmium vary
substantially in tropical waters. Analyses show that the calculated rates
are sensitive to only one of three forcing variables: the remotely sensed
Trichodesmium
chlorophyll concentration, BTsat, and two of the
model parameters: the maximum N2-fixation rate, PmaxBT,
and the depth of the subsurface
Trichodesmium biomass maximum, Zm.
The model is particularly sensitive to the latter. These results suggest
that in order to generate N2-fixation rate estimates
with reasonable confidence limits with this model, means must be sought
to account for in situ variablity in PmaxBT
and Zm. A series of correlation analyses are presented which
reveal statistically significant correlations between the diffuse attenuation
coefficient, Kpar, and PmaxBT,
and between wind speed and Zm. These relationships are suggested
as potential means of accounting for natural variability in PmaxBT
and Zm. An example remote sensing-based rate calculation is
made using SeaWiFS-derived Trichodesmium chlorophyll concentration
in the South Atlantic Bight described in Subramaniam et al. (this
issue). Although the optical conditions in the Bight were not all within
the range used to derive the model parameters, the model gives rates that
are consistent with direct rate measurements in Trichodesmium blooms.
Because Trichodesmium biomass can only be detected remotely at relatively
high concentrations, efforts to estimate global rates with this model will
require the use of both shipboard and satellite data.
Jackson, George
A. and Adrian B. Burd.
A model for the distribution
of particle flux in the mid-water column controlled by subsurface biotic
interactions.
The sub-euphotic zone water column is important in controlling the
downward transport of material falling from the surface waters. Descriptions
of the carbon flux as a function of depth have focused on empirical relationships
that neglect biological processes which might control them. We develop
here a series of simple models of the region which describe changes in
flux in terms of the population dynamics of a particle feeder and its predator.
One model predicts that the flux and predator concentration at steady state
decrease exponentially with depth while the concentration of the particle
feeders is constant; a second predicts that flux, particle feeder, and
predator concentrations are proportional and decrease at rates that are
approximately inversely proportional to depth. Away from steady state,
variations in particle flux leaving the surface can induce oscillations
in the near-surface animal populations but not the deeper populations.
As a result of the animal oscillations associated with the surface flux
variations, there can be large swings in the deep vertical particle flux
that are not synchronized to the surface variations for one model formulation;
a second formulation predicts that fluctuations in surface flux are damped
out near the bottom. The differences in predictions for the various models
make it possible to verify the utility of one or the other formulation.
(link
to electronic version)
Keller, Klaus,
Richard D. Slater, Michael Bender, and Robert M. Key.
Possible biological or physical
explanations for decadal scale trends in North Pacific nutrient concentrations
and oxygen utilization.
We analyze North Pacific GEOSECS (1970's) and WOCE (1990's) observations
to examine potential decadal trends of the marine biological carbon pump.
Nitrate concentrations (NO3) and apparent oxygen utilization
(AOU) decreased significantly in intermediate waters (by -2.9 and -0.6
micromoles kg-1, respectively at sigma theta=27.4 kg m-3,
corresponding to approx. 1050 m). In shallow waters (above roughly 750m)
NO3 and AOU increased, though the changes are not
statistically significant. A sensitivity study with an ocean general circulation
model indicates that reasonable perturbations of the biological carbon
pump due to changes in export production or remineralization efficiency
are insufficient to account for the intermediate water tracer trends. However,
changes in water ventilation rates could explain the intermediate water
tracer trends and would be consistent with trends of water age derived
from radiocarbon. Trends in AOU and NO3 provide relatively
poor constraints on decadal scale trends in the marine biological carbon
pump for two reasons. First, most of the expected changes due to decadal
scale perturbations of the marine biota occur in shallow waters, where
the available data are typically too sparse to account for the strong spatial
and temporal variability. Second, alternative explanations for the observed
tracer trends (e.g., changes in the water ventilation rates) cannot be
firmly rejected. Our data analysis does not disprove the null-hypothesis
of an unchanged biological carbon pump in the North Pacific.
(link
to electronic version)
Lamb, M. F.,
C.L. Sabine, R.A. Feely, R. Wanninkhof, R.M. Key, G.C. Johnson, F.J. Millero,
K. Lee, T.-H. Peng, A. Kozyr, J.L. Bullister, D. Greeley, R.H. Byrne, D.W.
Chipman, A.G. Dickson, C. Goyet, P.R. Guenther, M. Ishii, K.M. Johnson,
C.D. Keeling, T. Ono, K. Shitashima, B. Tilbrook, T. Takahashi, D.W.R.
Wallace, Y.W. Watanabe, Y. Watanabe, C. Winn, and C.S. Wong.
Consistency and synthesis of
Pacific Ocean CO2 survey data.
Between 1991 and 19996, carbon measurements were made on twenty-five
WOCE/JGOFS/OACES cruises in the Pacific Ocean. Investigators from 15 different
laboratories and 4 countries have analyzed at least two of the four measurable
ocean carbon parameters (DIC, TAlk, fCO2, and pH)
on almost all cruises. The goal of this work is to assess the quality of
the Pacific carbon survey data and to make recommendations for generating
a unified data set that is consistent between cruises. Several different
lines of evidence were used to examine the consistency including comparison
of calibration techniques, results from certified reference material analyses,
precision of at-sea replicate analyses, agreement between shipboard analyses
and replicate shore based analyses, comparison of deep water values at
locations where two or more cruises overlapped or crossed, consistency
with other hydrographic parameters, and internal consistency with multiple
carbon parameter measurements. With the adjustments proposed here, the
data can be combined to generate a Pacific Ocean data set with over 36,0000
unique sample locations analyzed for at least two carbon parameters in
most cases. The best data coverage was for DIC, which has an estimated
overall accuracy of ~3 mmol kg-1. TAlk, the second most common
carbon parameter analyzed, had an estimated overall accuracy of ~5 mmol
kg-1. To obtain additional details on this study including detailed
crossover plots and information on the availability of the compiled, adjusted
data set, visit the Global Data Analysis Project web site at: http://cdiac.esd.ornl.gov/oceans/glodap.
(link
to electronic version)
Moore, J. Keith,
Scott C. Doney, Joan A. Kleypas, David M. Glover, and Inez Y. Fung.
An intermediate complexity marine
ecosystem model for the global domain.
A new marine ecosystem model designed for the global domain is presented,
and model output is compared with field data from nine different locations.
Field data were collected as part of the international JGOFS (Joint Global
Ocean Flux Study) program, and from historical time series stations. The
field data include a wide variety of marine ecosystem types, including
nitrogen- and iron-limited systems, and different physical environments
from high latitudes to the mid-ocean gyres. Model output is generally in
good agreement with field data from these diverse ecosystems. These results
imply that the ecosystem model presented here can be reliably applied over
the global domain. The model includes multiple potentially limiting nutrients
that regulate phytoplankton growth rates. There are three phytoplankton
classes, diatoms, diazotrophs, and a generic small phytoplankton class.
Growth rates can be limited by available nitrogen, phosphorus, iron, and/or
light levels. The diatoms can also be limited by silicon. The diazotrophs
are capable of nitrogen fixation of N2 gas and cannot
be nitrogen limited. Calcification by phytoplankton is parameterized as
a variable fraction of primary production by the small phytoplankton group.
There is one zooplankton class which grazes the three phytoplankton groups
and a large detrital pool. The large detrital pool sinks out of the mixed
layer, while a smaller detrital pool representing dissolved organic matter
and very small particulates does not sink. Remineralization of the detrital
pools is parameterized with a temperature dependent function. We explicitly
model the dissolved iron cycle in marine surface waters including inputs
of iron from subsurface sources and from atmospheric dust deposition.
(link
to electronic version)
Moore, J. Keith,
Scott C. Doney, David M. Glover, and Inez Y. Fung.
Iron cycling and nutrient limitation
patterns in surface waters of the world ocean.
A global marine ecosystem mixed layer model is used to study iron cycling
and nutrient limitation patterns in surface waters of the world ocean.
The ecosystem model has a small phytoplankton size class whose growth can
be limited by N, P, Fe, and/or light, a diatom class which can also be
Si limited and a diazotroph phytoplankton class whose growth rates can
be limited by P, Fe, and/or light levels. The model also includes a parameteriza
tion of calcification by phytoplankton and is described in detail by Moore
et al. (this issue). The model reproduces the observed high nitrate, low
chlorophyll (HNLC) conditions in the Southern Ocean, subarctic Northeast
Pacific, and equatorial Pacific, and realistic global patterns of primary
production, biogenic silica production, nitrogen fixation, particulate
organic carbon export, calcium carbonate export, and surface chlorophyll
concentrations. Phytoplankton cellular Fe/C ratios and surface layer dissolved
iron concentrations are also in general agreement with the limited field
data. Primary production, community structure, and the sinking carbon flux
are quite sensitive to large variations in the atmospheric iron source,
particularly in the HNLC regions. This supports the Iron Hypothesis of
Martin (1990). Nitrogen fixation is also strongly influenced by atmospheric
iron deposition. Nitrogen limits phytoplankton growth rates over less than
half of the world ocean during summer months. Export of biogenic carbon
is dominated by the sinking particulate flux, but detrainment and turbulent
mixing account for 30% of global carbon export. Our results in conjunction
with other recent studies suggest the familiar paradigm that nitrate inputs
to the surface layer can be equated with particulate carbon export needs
to be expanded to include multiple limiting nutrients and modes of export.
(link
to electronic version)
Pätsch,
J., W. Kühn, G. Radach, J.M. Santana Casiano, M. Gonzalez Davila,
S. Neuer, T. Freudenthal, and O. Llinas.
Interannual variability of carbon
fluxes at the North Atlantic station ESTOC.
The impact of sea surface temperature and wind stress on primary production,
export production and CO2 air-sea exchange at the
ESTOC station (29°N, 15.5°W) north of the Canary Islands is the
focus of our investigations. A one-dimensional carbon and nitrogen cycling
model was applied for the 10-year period 1987-1996. The simulation results
compare well with upper layer observations for the years 1994-1996. Our
simulated deep water particle fluxes mostly overestimate the originally
observed values for 1992-1996. On the other hand the simulated fluxes underestimate
the 230Th corrected particle fluxes (Scholten et al., 2000). Identifying
the original observations as lower and the corrected values as upper estimate
for the particle flux the simulation results fall into the range between
these estimates. The large simulated interannual variability of carbon
fluxes is in apparent contrast to the low interannual variability of the
meteorological forcing typical for this subtropical regime. The key to
this phenomenon lies in the sensitivity of this ecosystem to nutrient supply:
depending on the meteorological situation, in different years the mixed-layer
depth can or cannot reach the nitracline.
(download
estoc.zip from ftp directory)
Roman, M.R.,
H. A. Adolf, M.R. Landry, L.P. Madin, D.K. Steinberg, and X. Zhang.
Estimates of oceanic mesozooplankton
production: A comparison using the Bermuda and Hawaii time-series data.
Mesozooplankton growth rates were estimated for the Hawaiian (HOT)
and Bermuda (BATS) ocean time-series stations using the empirical model
of Hirst and Lampitt (1998) which predicts copepod growth rate from temperature
and body size. Using this approach we derived seasonal and annual estimates
of mesozooplankton production as well as rates of mesozooplankton ingestion
and egestion using assumed growth and assimilation efficiencies for the
period 1994 - 1997. Annual mesozooplankton production estimates at HOT
(average 0.79 mol C m-2 y-1) were higher than production
estimates at BATS (average 0.33 mol C m-2 y-1) due
to both higher mesozooplankton biomass and higher estimated mesozooplankton
individual growth rates. Annual primary production at the two sites was
similar (average 14.92 mol C m-2 y-1 at HOT and 13.43
mol C m-2 y-1 at BATS). Thus, mesozooplankton production
was a greater fraction of primary production at HOT (0.05) as compared
to BATS (0.02). Mesozooplankton potentially contributed more to the gravitational
flux of carbon at HOT, where the ratio of the average annual estimate of
mesozooplankton fecal pellet carbon production / annual estimate of carbon
flux at the base of the euphotic zone was 1.03 compared to the same ratio
of 0.39 at BATS. Mortality estimates were similar to estimates of mesozooplankton
production when compared over the entire study period. The higher mesozooplankton
biomass and derived rate parameters at HOT compared to BATS may be due
to the more episodic nature of nutrient inputs at BATS which could result
in mis-matches between increases in phytoplankton production and the grazing/production
response by mesozooplankton. In addition, there is evidence to suggest
that there are periodic blooms of gelatinous macrozooplankton (salps) at
BATS which may not be captured sufficiently by the monthly sampling program.
Thus the gelatinous zooplankton would add to the overall grazing impact
on the phytoplankton at BATS as well as the contribution of zooplankton
to the gravitational flux of biogenic material via fecal pellet production.
Subramaniam,
Ajit, Christopher W. Brown, Raleigh R. Hood, Edward J. Carpenter, and Douglas
G. Capone
Detecting Trichodesmium blooms
in SeaWiFS imagery.
A multispectral classification scheme was developed to detect the cyanobacteria
Trichodesmium
sp. in satellite data of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS).
The criteria for this scheme were established from spectral characteristics
derived from (1) SeaWiFS imagery of a Trichodesmium bloom located
in the South Atlantic Bight and (2) modeled remote sensing reflectances
of Trichodesmium and other phytoplankton. The classification scheme,
which is valid for moderate chlorophyll concentrations of Trichodesmium
in coastal waters, is based on the magnitude of the 490-channel reflectance
and the spectral shape of remote sensing reflectance at 443, 490 and 555
nm. Analysis suggests that the spatial structure of Trichodesmium
populations at sub-pixel scales must be considered when employing spectral
characteristics to detect their presence in satellite imagery. This study
demonstrates the potential of mapping Trichodesmium from space using
spectral observations, even in waters as optically complex as the South
Atlantic Bight. Future efforts, which incorporate ancillary data such as
wind speeds and water temperature, will improve the likelihood of correct
identification.
(link
to electronic version)