Bibliography - Richard D Slater

1. Marinov, I, M Follows, Anand Gnanadesikan, Jorge L Sarmiento, and Richard D Slater, 2008: How does ocean biology affect atmospheric pCO₂? Theory and models. Journal of Geophysical Research, 113, C07032, doi:10.1029/2007JC004598.
   [ Abstract ]

   This paper examines the sensitivity of atmospheric pCO₂ to changes in ocean biology that result in drawdown of nutrients at the ocean surface. We show that the global inventory of preformed nutrients is the key determinant of atmospheric pCO₂ and the oceanic carbon storage due to the soft-tissue pump (OCS_soft ). We develop a new theory showing that under conditions of perfect equilibrium between atmosphere and ocean, atmospheric pCO₂ can be written as a sum of exponential functions of OCS _soft . The theory also demonstrates how the sensitivity of atmospheric pCO₂to changes in the soft-tissue pump depends on the preformed nutrient inventory and on surface buffer chemistry. We validate our theory against simulations of nutrient depletion in a suite of realistic general circulation models (GCMs). The decrease in atmospheric pCO₂ following surface nutrient depletion depends on the oceanic circulation in the models. Increasing deep ocean ventilation by increasing vertical mixing or Southern Ocean winds increases the atmospheric pCO₂ sensitivity to surface nutrient forcing. Conversely, stratifying the Southern Ocean decreases the atmospheric CO₂ sensitivity to surface nutrient depletion. Surface CO₂ disequilibrium due to the slow gas exchange with the atmosphere acts to make atmospheric pCO₂ more sensitive to nutrient depletion in high-ventilation models and less sensitive to nutrient depletion in low-ventilation models. Our findings have potentially important implications for both past and future climates.

2. Najjar, R G., X Jin, F Louanchi, O Aumont, K Caldeira, S C Doney, J-C Dutay, M Follows, N Gruber, K Lindsay, E Maier-Reimer, R Matear, K Matsumoto, Patrick Monfray, A Mouchet, James C Orr, G-K Plattner, Jorge L Sarmiento, R Schlitzer, Richard D Slater, M-F Weirig, Y Yamanaka, and A Yool, 2007: Impact of circulation on export production, dissolved organic matter, and dissolved oxygen in the ocean: Results from Phase II of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Global Biogeochemical Cycles, 21, GB3007, doi:10.1029/2006GB002857.
   [ Abstract ]

   Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse-resolution ocean circulation models. The model mean (±1s) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr^-1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation-based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep-ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep-ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross-model means of several biogeochemical metrics show better agreement with observation-based estimates when restricted to those models that best simulate deep-ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

3. Mignone, B K., Anand Gnanadesikan, Jorge L Sarmiento, and Richard D Slater, 2006: Central role of Southern Hemisphere winds and eddies in modulating the oceanic uptake of anthropogenic carbon. Geophysical Research Letters, 33, L01604, doi:10.1029/2005GL024464.
   [ Abstract ]

   Although the world ocean is known to be a major sink of anthropogenic carbon dioxide, the exact processes governing the magnitude and regional distribution of carbon uptake remain poorly understood. Here we show that Southern Hemisphere winds, by altering the Ekman volume transport out of the Southern Ocean, strongly control the regional distribution of anthropogenic uptake in an ocean general circulation model, while winds and isopycnal thickness mixing together, by altering the volume of light, actively-ventilated ocean water, exert strong control over the absolute magnitude of anthropogenic uptake. These results are provocative in suggesting that climate-mediated changes in pycnocline volume may ultimately control changes in future carbon uptake.

4. Gnanadesikan, Anand, Richard D Slater, P S Swathi, and Geoffrey K Vallis, 2005: The energetics of ocean heat transport. Journal of Climate, 18(14), doi:10.1175/JCLI3436.1.
   [ Abstract ]

   A number of recent papers have argued that the mechanical energy budget of the ocean places constraints on how the thermohaline circulation is driven. These papers have been used to argue that climate models, which do not specifically account for the energy of mixing, potentially miss a very important feedback on climate change. This paper reexamines the question of what energetic arguments can teach us about the climate system and concludes that the relationship between energetics and climate is not straightforward. By analyzing the buoyancy transport equation, it is demonstrated that the large-scale transport of heat within the ocean requires an energy source of around 0.2 TW to accomplish vertical transport and around 0.4 TW (resulting from cabbeling) to accomplish horizontal transport. Within two general circulation models, this energy is almost entirely supplied by surface winds. It is also shown that there is no necessary relationship between heat transport and mechanical energy supply.

5. Orr, James C., V J Fabry, O Aumont, L Bopp, S C Doney, R A Feely, Anand Gnanadesikan, N Gruber, A Ishida, F Joos, Robert M Key, K Lindsay, E Maier-Reimer, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, K B Rodgers, C L Sabine, Jorge L Sarmiento, R Schlitzer, Richard D Slater, I J Totterdell, M-F Weirig, Y Yamanaka, and A Yool, 2005: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437(7059), doi:10.1038/nature04095.
   [ Abstract ]

   Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms such as corals and some plankton will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean-carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

6. Doney, S C., Anand Gnanadesikan, Jorge L Sarmiento, and Richard D Slater, et al., 2004: Evaluating global ocean carbon models: The importance of realistic physics. Global Biogeochemical Cycles, 18, GB3017, doi:10.1029/2003GB002150.
   [ Abstract ]

   A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO2 flux, chlorofluorocarbon and anthropogenic CO2 uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO2. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

7. Gnanadesikan, Anand, John Dunne, Robert M Key, K Matsumoto, Jorge L Sarmiento, Richard D Slater, and P S Swathi, 2004: Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity. Global Biogeochemical Cycles, 18(4), GB4010, doi:10.1029/2003GB002097.
   [ Abstract ]

   Differing models of the ocean circulation support different rates of ventilation, which in turn produce different distributions of radiocarbon, oxygen, and export production. We examine these fields within a suite of general circulation models run to examine the sensitivity of the circulation to the parameterization of subgridscale mixing and surface forcing. We find that different models can explain relatively high fractions of the spatial variance in some fields such as radiocarbon, and that newer estimates of the rate of biological cycling are in better agreement with the models than previously published estimates. We consider how different models achieve such agreement and show that they can accomplish this in different ways. For example, models with high vertical diffusion move young surface waters into the Southern Ocean, while models with high winds move more young North Atlantic water into this region. The dependence on parameter values is not simple. Changes in the vertical diffusion coefficient, for example, can produce major changes in advective fluxes. In the coarse-resolution models studied here, lateral diffusion plays a major role in the tracer budget of the deep ocean, a somewhat worrisome fact as it is poorly constrained both observationally and theoretically.

8. Matsumoto, K, Jorge L Sarmiento, Robert M Key, O Aumont, J L Bullister, K Caldeira, J-M Campin, S C Doney, H Drange, J-C Dutay, M Follows, Y Gao, Anand Gnanadesikan, N Gruber, A Ishida, F Joos, K Lindsay, E Maier-Reimer, J Marshall, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, R Schlitzer, Richard D Slater, P S Swathi, I J Totterdell, M-F Weirig, Y Yamanaka, A Yool, and James C Orr, 2004: Evaluation of ocean carbon cycle models with data-based metrics. Geophysical Research Letters, 31, L07303, doi:10.1029/2003GL018970.
   [ Abstract ]

   New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data-based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.

9. Mignone, B K., Jorge L Sarmiento, Richard D Slater, and Anand Gnanadesikan, 2004: Sensitivity of sequestration efficiency to mixing processes in the global ocean. Energy, 29(9-10), 1467-1478.
   [ Abstract PDF ]

   A number of large-scale sequestration strategies have been considered to help mitigate rising levels of atmospheric carbon dioxide (CO2). Here, we use an ocean general circulation model (OGCM) to evaluate the efficiency of one such strategy currently receiving much attention, the direct injection of liquid CO2 into selected regions of the abyssal ocean. We find that currents typically transport the injected plumes quite far before they are able to return to the surface and release CO2 through air–sea gas exchange. When injected at sufficient depth (well within or below the main thermocline), most of the injected CO2 outgasses in high latitudes (mainly in the Southern Ocean) where vertical exchange is most favored. Virtually all OGCMs that have performed similar simulations confirm these global patterns, but regional differences are significant, leading efficiency estimates to vary widely among models even when identical protocols are followed. In this paper, we make a first attempt at reconciling some of these differences by performing a sensitivity analysis in one OGCM, the Princeton Modular Ocean Model. Using techniques we have developed to maintain both the modeled density structure and the absolute magnitude of the overturning circulation while varying important mixing parameters, we estimate the sensitivity of sequestration efficiency to the magnitude of vertical exchange within the low-latitude pycnocline. Combining these model results with available tracer data permits us to narrow the range of model behavior, which in turn places important constraints on sequestration efficiency.

10. Sarmiento, Jorge L., Richard D Slater, R T Barber, L Bopp, S C Doney, A C Hirst, J Kieypas, R Matear, U Mikolajewicz, Patrick Monfray, V Soldatov, S A Spall, and Ronald J Stouffer, 2004: Response of ocean ecosystems to climate warming. Global Biogeochemical Cycles, 18, GB3003, doi:10.1029/2003/GB002134.
    [ Abstract ]

    We examine six different coupled climate model simulations to determine the ocean biological response to climate warming between the beginning of the industrial revolution and 2050. We use vertical velocity, maximum winter mixed layer depth, and sea ice cover to define six biomes. Climate warming leads to a contraction of the highly productive marginal sea ice biome by 42% in the Northern Hemisphere and 17% in the Southern Hemisphere, and leads to an expansion of the low productivity permanently stratified subtropical gyre biome by 4.0% in the Northern Hemisphere and 9.4% in the Southern Hemisphere. In between these, the subpolar gyre biome expands by 16% in the Northern Hemisphere and 7% in the Southern Hemisphere, and the seasonally stratified subtropical gyre contracts by 11% in both hemispheres. The low-latitude (mostly coastal) upwelling biome area changes only modestly. Vertical stratification increases, which would be expected to decrease nutrient supply everywhere, but increase the growing season length in high latitudes. We use satellite ocean color and climatological observations to develop an empirical model for predicting chlorophyll from the physical properties of the global warming simulations. Four features stand out in the response to global warming: (1) a drop in chlorophyll in the North Pacific due primarily to retreat of the marginal sea ice biome, (2) a tendency toward an increase in chlorophyll in the North Atlantic due to a complex combination of factors, (3) an increase in chlorophyll in the Southern Ocean due primarily to the retreat of and changes at the northern boundary of the marginal sea ice zone, and (4) a tendency toward a decrease in chlorophyll adjacent to the Antarctic continent due primarily to freshening within the marginal sea ice zone. We use three different primary production algorithms to estimate the response of primary production to climate warming based on our estimated chlorophyll concentrations. The three algorithms give a global increase in primary production of 0.7% at the low end to 8.1% at the high end, with very large regional differences. The main cause of both the response to warming and the variation between algorithms is the temperature sensitivity of the primary production algorithms. We also show results for the period between the industrial revolution and 2050 and 2090.

11. Gnanadesikan, Anand, Jorge L Sarmiento, and Richard D Slater, 2003: Effects of patchy ocean fertilization on atmospheric carbon dioxide and biological production. Global Biogeochemical Cycles, 17(2), 1050, doi:10.1029/2002GB001940.
    [ Abstract ]

    Increasing oceanic productivity by fertilizing nutrient-rich regions with iron has been proposed as a mechanism to offset anthropogenic emissions of carbon dioxide. Earlier studies examined the impact of large-scale fertilization of vast reaches of the ocean for long periods of time. We use an ocean general circulation model to consider more realistic scenarios involving fertilizing small regions (a few hundred kilometers on a side) for limited periods of time (of order 1 month). A century after such a fertilization event, the reduction of atmospheric carbon dioxide is between 2% and 44% of the initial pulse of organic carbon export to the abyssal ocean. The fraction depends on how rapidly the surface nutrient and carbon fields recover from the fertilization event. The modeled recovery is very sensitive to the representation of biological productivity and remineralization. Direct verification of the uptake would be nearly impossible since changes in the air-sea flux due to fertilization would be much smaller than those resulting from natural spatial variability. Because of the sensitivity of the uptake to the long-term fate of the iron and organic matter, indirect verification by measurement of the organic matter flux would require high vertical resolution and long-term monitoring. Finally, the downward displacement of the nutrient profile resulting from an iron-induced productivity spurt may paradoxically lead to a long-term reduction in biological productivity. In the worst-case scenario, removing 1 ton of carbon from the atmosphere for a century is associated with a 30-ton reduction in biological export of carbon.

12. Gnanadesikan, Anand, Richard D Slater, and Bonita L Samuels, 2003: Sensitivity of water mass transformation and heat transport to subgridscale mixing in coarse-resolution ocean models. Geophysical Research Abstracts, 30(18), 1967, doi:10.1029/2003GL018036.
    [ Abstract PDF ]

    This paper considers the impact of the parameterization of subgridscale mixing on ocean heat transport in coarse-resolution ocean models of the type used in coupled climate models. Increasing the vertical diffusion increases poleward heat transport in both hemispheres. Increasing lateral diffusion associated with transient eddies increases poleward heat transport in the southern hemisphere while decreasing it in the northern hemisphere. The results are interpreted in the context of a simple analytical model.

13. Watson, A J., James C Orr, Anand Gnanadesikan, Robert M Key, Jorge L Sarmiento, and Richard D Slater, 2003: Carbon dioxide fluxes in the global ocean In Ocean Biogeochemistry: A Synthesis of the Joint Global Ocean Flux Study (JGOFS), Berlin, Germany, Springer-Verlag, 123-143.
14. Dutay, J-C, J L Bullister, S C Doney, James C Orr, R G Najjar, Jorge L Sarmiento, and Richard D Slater, et al., 2002: Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models. Ocean Modelling, 4(2), 89-120.
    [ Abstract PDF ]

    We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO2 uptake. We found a large range in the modeled global inventory (±30%), mainly due to differences in ventilation from the high latitudes. In the Southern Ocean, models differ particularly in the longitudinal distribution of the CFC uptake in the intermediate water, whereas the latitudinal distribution is mainly controlled by the subgrid-scale parameterization. Models with isopycnal diffusion and eddy-induced velocity parameterization produce more realistic intermediate water ventilation. Deep and bottom water ventilation also varies substantially between the models. Models coupled to a sea-ice model systematically provide more realistic AABW formation source region; however these same models also largely overestimate AABW ventilation if no specific parameterization of brine rejection during sea-ice formation is included. In the North Pacific Ocean, all models exhibit a systematic large underestimation of the CFC uptake in the thermocline of the subtropical gyre, while no systematic difference toward the observations is found in the subpolar gyre. In the North Atlantic Ocean, the CFC uptake is globally underestimated in subsurface. In the deep ocean, all but the adjoint model failed to produce the two recently ventilated branches observed in the North Atlantic Deep Water (NADW). Furthermore, simulated transport in the Deep Western Boundary Current (DWBC) is too sluggish in all but the isopycnal model, where it is too rapid.

15. Gnanadesikan, Anand, Richard D Slater, N Gruber, and Jorge L Sarmiento, 2002: Oceanic vertical exchange and new production: a comparison between models and observations. Deep-Sea Research, Part II, 49(1-3), 363-401.
    [ Abstract PDF ]

    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 effects 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 50 m, 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 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 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. Evidence from the Eastern Equatorial Pacific suggests that this is due to both 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 which 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.

16. Keller, K, Richard D Slater, M Bender, and Robert M Key, 2002: Possible biological or physical explanations for decadal scale trends in North Pacific nutrient concentrations and oxygen utilization. Deep-Sea Research, Part II, 49(1-3), 345-362.
    [ Abstract PDF ]

    We analyze North Pacific GEOSECS (1970s) and WOCE (1990s) 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 -0.6 and -2.9 μmol kg-1, respectively, at = 27.4 kg m-3, corresponding to 1050 m). In shallow waters (above roughly 750 m) [NO3] and AOU increased, though the changes were 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.

17. Sarmiento, Jorge L., John Dunne, Anand Gnanadesikan, Robert M Key, K Matsumoto, and Richard D Slater, 2002: A new estimate of the CaCO3 to organic carbon export ratio. Global Biogeochemical Cycles, 16(4), doi:10.1029/2002/GB001919.
    [ Abstract ]

    We use an ocean biogeochemical-transport box model of the top 100 m of the water column to estimate the CaCO3 to organic carbon export ratio from observations of the vertical gradients of potential alkalinity and nitrate. We find a global average molar export ratio of 0.06 ± 0.03. This is substantially smaller than earlier estimates of 0.25 on which a majority of ocean biogeochemical models had based their parameterization of CaCO3 production. Contrary to the pattern of coccolithophore blooms determined from satellite observations, which show high latitude predominance, we find maximum export ratios in the equatorial region and generally smaller ratios in the subtropical and subpolar gyres. Our results suggest a dominant contribution to global calcification by low-latitude nonbloom forming coccolithophores or other organisms such as foraminifera and pteropods.

18. Fasham, M J., Jorge L Sarmiento, Richard D Slater, H W Ducklow, and R G Williams, 1993: Ecosystem behavior at Bermuda Station "S" and Ocean Weather Station "India": A general circulation model and observational analysis. Global Biogeochemical Cycles, 7(2), 379-415.
    [ Abstract ]

    A model of biological production in the euphotic zone of the North Atlantic has been developed by coupling a seven-compartment nitrogen-based ecosystem model with a three-dimensional seasonal general circulation model. The predicted seasonal cycles of phytoplankton, zooplankton, bacteria, nitrate, ammonium, primary production, and particle flux have been compared to data from Bermuda Station "S" and Ocean Weather Station "India". Bearing in mind the simplicity of the model and the paucity of data, the results are encouraging. However, deficiencies in the physical model lead to winter nitrate values at Bermuda being overestimated, and at both positions the predicted magnitude of the spring phytoplankton bloom was too high. Simulations were carried out with different detrital sinking rates and it was found that a sinking rate of 10 m d^-1 gave the best agreement with observations. The model was used to investigate the factors affecting the population growth of phytoplankton and it was found that the model supported the generally held theory that the spring bloom is initiated by the cessation of physical mixing. After the bloom, phytoplankton are controlled by zooplankton grazing. At Ocean Weather Station "India" the model reproduced the observed high summer nitrate levels and suggested that these high values are caused by a combination of high vertical nitrate transport, ammonium inhibition of nitrate uptake, and zooplankton grazing control. The model demonstrated the critical importance of zooplankton in understanding ecosystem dynamics and highlights the need for more observational data on the seasonal cycles of zooplankton biomass and growth rates.

19. Sarmiento, Jorge L., Richard D Slater, M J Fasham, H W Ducklow, J Robert Toggweiler, and G T Evans, 1993: A seasonal three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone. Global Biogeochemical Cycles, 7(2), 417-450.
    [ Abstract ]

    A seven-component upper ocean ecosystem model of nitrogen cycling calibrated with observations at Bermuda Station "S" has been coupled to a three-dimensional seasonal general circulation model (GCM) of the North Atlantic Ocean. The aim of this project is to improve our understanding of the role of upper ocean biological processes in controlling surface chemical distributions, and to develop approaches for assimilating large data sets relevant to this problem. A comparison of model predicted chlorophyll with satellite coastal zone color scanner observations shows that the ecosystem model is capable of responding realistically to a variety of physical forcing environments. Most of the discrepancies identified are due to problems with the GCM model. The new production predicted by the model is equivalent to 2 to 2.8 mol m^-2 yr^-1 of carbon uptake, or 8 to 12 GtC/yr on a global scale. The southern half of the subtropical gyre is the only major region of the model with almost complete surface nitrate removal (nitrate<0.1 mmol m-3). Despite this, almost the entire model is nitrate limited in the sense that any addition of nitrate supply would go predominately into photosynthesis. The only exceptions are some coastal upwelling regions and the high latitudes during winter, where nitrate goes as high as ~10 mmol m^-3 .

20. Slater, Richard D., Jorge L Sarmiento, and M J Fasham, 1993: Some parametric and structural simulations with a three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone In Towards a Model of Ocean Biogeochemical Processes, NATO Series I, Vol. 10, Berlin, Germany, Springer-Verlag, 261-294.

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