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.

click on author's name to see abstract
Armstrong et al. Doney et al. (overview) Gregg Moore et al. Pt 1
Berelson Fennel et al. Hood et al. Moore et al. Pt 2
Carr Follows & Dutkiewicz Jackson & Burd Pätsch et al.
Christian et al. Pt 1 Friedrichs Keller et al. Roman et al.
Christian et al. Pt 2 Gnanadesikan et al. Lamb et al. Subramaniam et al.

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.

see also companion paper with Subramaniam et al.

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)

see also companion paper with Hood et al.

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