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We report on performance tests of pair interaction lattice gas automata in two and three dimensions coded in FORTRAN and C. The programs have been run on ALLIANT/FX-80, ALLIANT/FX-2800, CONVEX C2, CRAY-YMP, NEC/SX3, and SUN/IPC. The maximum update rates are 200 million site updates per second on the NEC/SX3 (FORTRAN), 117 (2D version) and 29 (3D version) on the CRAY-YMP (C). As a byproduct we give results for the performance of integer arithmetic and bit operations. Usually the C-programs were somewhat faster than the FORTRAN-programs except on the NEC/SX3 where the C-compiler was not able to vectorize the main loops.

Since large, rapidly-sinking particles account for most of the vertical flux in the ocean, mechanisms responsible for particle aggregation largely control the transport of carbon to depth. The particle flux resulting from a variety of different phytoplankton bloom conditions was simulated with a numerical model in which phytoplankton growth dynamics were combined with physical aggregation, particle size-dependent sedimentation and degradation. Model results demonstrated that particle flux to the deep ocean be generated by solely invoking physical aggregation during phytoplankton blooms. Sensitivity of the model in response to variations of both physico-chemical and biological paramters was tested. The model outcome, described as the fraction of export production leaving the upper ocean carbon pool, proved to be most sensitive to biological variables such as phytoplankton cell size, stickness, and growth characteristics (i.e. solitary vs chain-forming). Changes in these factors strongly affect the efficiency of the “biological pump” and could be explain interannual and geographic variance in deep-ocean flux.

A one-dimensional model of Fe speciation and biogeochemistry, coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model, is applied for the Tropical Eastern North Atlantic Time-Series Observatory (TENATSO) site. Among diverse processes affecting Fe speciation, this study is focusing on investigating the role of dust particles in removing dissolved iron (DFe) by a more complex description of particle aggregation and sinking, and explaining the abundance of organic Fe-binding ligands by modelling their origin and fate. The vertical distribution of different particle classes in the model shows high sensitivity to changing aggregation rates. Using the aggregation rates from the sensitivity study in this work, modelled particle fluxes are close to observations, with dust particles dominating near the surface and aggregates deeper in the water column. POC export at 1000 m is a little higher than regional sediment trap measurements, suggesting further improvement of modelling particle aggregation, sinking or remineralisation. Modelled strong ligands have a high abundance near the surface and decline rapidly below the deep chlorophyll maximum, showing qualitative similarity to observations. Without production of strong ligands, phytoplankton concentration falls to 0 within the first 2 years in the model integration, caused by strong Fe-limitation. A nudging of total weak ligands towards a constant value is required for reproducing the observed nutrient-like profiles, assuming a decay time of 7 years for weak ligands. This indicates that weak ligands have a longer decay time and therefore cannot be modelled adequately in a one-dimensional model. The modelled DFe profile is strongly influenced by particle concentration and vertical distribution, because the most important removal of DFe in deeper waters is colloid formation and aggregation. Redissolution of particulate iron is required to reproduce an observed DFe profile at TENATSO site. Assuming colloidal iron is mainly composed of inorganic colloids, the modelled colloidal to soluble iron ratio is lower that observations, indicating the importance of organic colloids.

[1] We report on the discovery of the mineral ikaite (CaCO3·6H2O) in sea-ice from the Southern Ocean. The precipitation of CaCO3 during the freezing of seawater has previously been predicted from thermodynamic modelling, indirect measurements, and has been documented in artificial sea ice during laboratory experiments but has not been reported for natural sea-ice. It is assumed that CaCO3 formation in sea ice may be important for a sea ice-driven carbon pump in ice-covered oceanic waters. Without direct evidence of CaCO3 precipitation in sea ice, its role in this and other processes has remained speculative. The discovery of CaCO3·6H2O crystals in natural sea ice provides the necessary evidence for the evaluation of previous assumptions and lays the foundation for further studies to help elucidate the role of ikaite in the carbon cycle of the seasonally sea ice-covered regions

During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.

Blooms of the coccolithophorid Emiliania huxleyi can be infected by viruses, which can lead to bloom-termination. This pilot study used an expressed sequence tag (EST) approach to get a first view of gene-expression changes that occur during viral infection of E. huxleyi. cDNA libraries were constructed from uninfected cultures and 6, 12, and 24 h after infection with E. huxleyi-specific virus 86 (EhV-86). From each library 60-90 ESTs were randomly selected and annotated manually with PhyloGena. Viral genes were identified using BLAST-Search of the known viral genome. The data of this study show, that 6 h after viral infection the algal transcriptome changed significantly although few viral transcripts were present. At this point, changes mainly concerned transcripts related to photosynthesis and protein metabolism. However, after 24 h viral transcripts were most abundant. Viral transcripts found at this stage of viral infection encode proteins involved in protein degradation, nucleic acid degradation, transcription and replication.

By means of numerical modeling, we analyze the cycling of iron between its various physical (dissolved, colloidal, particulate) and chemical (redox state and organic complexation) forms in the upper mixed layer. With our proposed model it is possible to obtain a first quantitative assessment of how this cycling influences iron uptake by phytoplankton and its loss via particle export. The model is forced with observed dust deposition rates, mixed layer depths, and solar radiation at the site of the Bermuda Atlantic Time-series Study (BATS). It contains an objectively optimized ecosystem model which yields results close to the observational data from BATS that has been used for the data-assimilation procedure. It is shown that the mixed layer cycle strongly influences the cycling of iron between its various forms. This is mainly due to the light dependency of photoreductive processes, and to the seasonality of primary production. The daily photochemical cycle is driven mainly by the production of superoxide, and its amplitude depends on the concentration and speciation of dissolved copper. Model results are almost insensitive to the dominant form of dissolved iron within dust deposition, and also to the form of iron that is taken up directly during algal growth. In our model solutions, the role of the colloidal pumping mechanism depends strongly on assumptions on the colloid aggregation and photoreduction rate.

Ecosystem processes are important determinants of the biogeochemistry of the ocean, and they can be profoundly affected by changes in climate. Ocean models currently express ecosystem processes through empirically derived parameterizations that tightly link key geochemical tracers to ocean physics. The explicit inclusion of ecosystem processes in models will permit ecological changes to be taken into account, and will allow us to address several important questions, including the causes of observed glacial-interglacial changes in atmospheric trace gases and aerosols, and how the oceanic uptake of CO2 is likely to change in the future. There is an urgent need to assess our mechanistic understanding of the environmental factors that exert control over marine ecosystems, and to represent their natural complexity based on theoretical understanding. We present a prototype design for a Dynamic Green Ocean Model (DGOM) based on the identification of (a) key plankton functional types that need to be simulated explicitly to capture important biogeochemical processes in the ocean; (b) key processes controlling the growth and mortality of these functional types and hence their interactions; and (c) sources of information necessary to parameterize each of these processes within a modeling framework. We also develop a strategy for model evaluation, based on simulation of both past and present mean state and variability, and identify potential sources of validation data for each. Finally, we present a DGOM-based strategy for addressing key questions in ocean biogeochemistry. This paper thus presents ongoing work in ocean biogeochemical modeling, which, it is hoped will motivate international collaborations to improve our understanding of the role of the ocean in the climate system.

International audience; Marine diatoms take up silicic acid for the buildup of their opaline shells and discriminate against the heavier silicon isotope. For the first time, the overall oceanic distribution of silicon isotopes has been estimated by integration of the Hamburg Model of the Ocean Carbon Cycle, version 4 (HAMOCC4). It is shown that the relationship between the silicic acid concentration and its silicon isotope composition is not a simple Rayleigh distillation curve. Only the Southern Ocean and the equatorial Pacific show a clear functional dependency similar to the Rayleigh distillation curve. Model results can be used to predict opal silicon isotope compositions in the sediment and constrain the use of silicon isotopes as a proxy for silicic acid utilization. Owing to the structure of the Pacific current system, it might be valid to apply a relationship between surface silicic acid concentrations and the silicon isotope signal in the equatorial Pacific sediments.

The stable boron isotope composition (δ11B) of fossil foraminiferal shells is used as a paleo-pH recorder and is therefore one of the most promising paleocarbonate chemistry proxies (“paleoacidimetry”). One crucial question regarding this proxy is whether foraminifera record the pH of the bulk seawater or the pH of the microenvironment (diffusive boundary layer, ∼500 μm), which is strongly influenced by life processes. Here we present a novel theoretical approach to address this question by using a diffusion-reaction model. Model results indicate that the δ11B in planktonic foraminifera is primarily controlled by the pH of the microenvironment. We therefore predict that the δ11B of different species (e.g., symbiont-bearing versus symbiont-barren) or of foraminifera grown in the dark and in the light should be offset from the δ11B of inorganic calcite. This theoretical prediction was experimentally confirmed while this paper was written [Honisch et al., 2003]. Most importantly, the model predicts that this offset is constant over a wide pH range. Thus the use of δ11B as a paleo-pH indicator is not compromised through vital effects as modeled here.

Biotic and abiotic processes, climate change, regional and global environmental issues, greenhouse gas emission, stratospheric ozone depletion, loss of biodiversity, desertification, freshwater availability, and air quality are all inter-linked. Understanding the driving forces and feedback mechanisms of these processes is essential for understanding the evolution and future of the earth, the environment, and society. It is important to realize that the ocean's carbon cycle plays an important role in the global element fluxes. In the following, we look at two major processes representing a sink for carbon in the ocean: 1) sedimentation of biogenic carbonate from productive surface waters, and 2) carbon sequestration by methane oxidation above gashydrate and other sites of methane seepage. We point out the necessity to monitor and understand the submarine environments at the interface between the geo- and biosphere.

THE supply of dissolved inorganic carbon (DIC) is not considered to limit oceanic primary productivity1, as its concentration in sea water exceeds that of other plant macronutrients such as nitrate and phosphate by two and three orders of magnitude, respectively. But the bulk of oceanic new production2 and a major fraction of vertical carbon flux is mediated by a few diatom genera whose ability to use DIG components other than CO2, which comprises < 1% of total DIC3, is unknown4. Here we show that under optimal light and nutrient conditions, diatom growth rate can in fact be limited by the supply of CO2. The doubling in surface water pCO2 levels since the last glaciation from 180 to 355 p.p.m.5,6 could therefore have stimulated marine productivity, thereby increasing oceanic carbon sequestration by the biological pump.

3.1 The HPP lattice-gas cellular automata 3.1.1 Model description 3.1.2 Implementation of the HPP model: How to code lattice-gas cellular automata? 3.1.3 Initialization 3.1.4 Coarse graining 3.2 The FHP lattice-gas cellular automata 3.2.1 The lattice and the collision rules 3.2.2 Microdynamics of the FHP model 3.2.3 The Liouville equation 3.2.4 Mass and momentum density 3.2.5 Equilibrium mean occupation numbers 3.2.6 Derivation of the macroscopic equations: multi-scale analysis 3.2.7 Boundary conditions 3.2.8 Inclusion of body forces 3.2.9 Numerical experiments with FHP 3.2.10 The 8-bit FHP model 3.3 Lattice tensors and isotropy in the macroscopic limit 3.3.1 Isotropic tensors 3.3.2 Lattice tensors: single-speed models 3.3.3 Generalized lattice tensors for multi-speed models 3.3.4 Thermal LBMs: D2Q13-FHP (multi-speed FHP model) 3.3.5 Exercises 3.4 Desperately seeking a lattice for simulations in three dimensions 3.4.1 Three dimensions 3.4.2 Five and higher dimensions 3.4.3 Four dimensions 3.5 FCHC 3.5.1 Isometric collision rules for FCHC by Henon 3.5.2 FCHC, computers and modified collision rules 3.5.3 Isometric rules for HPP and FHP 3.5.4 What else? 3.6 The pair interaction (PI) lattice-gas cellular automata 3.6.1 Lattice, cells, and interaction in 2D 3.6.2 Macroscopic equations 3.6.3 Comparison of PI with FHP and FCHC 3.6.4 The collision operator and propagation in C and FORTRAN 3.7 Multi-speed and thermal lattice-gas cellular automata 3.7.1 The D3Q19 model 3.7.2 The D2Q9 model 3.7.3 The D2Q21 model 3.7.4 Transsonic and supersonic flows: D2Q25, D2Q57, D2Q129 3.8 Zanetti (‘staggered’) invariants 3.8.1 FHP 3.8.2 Significance of the Zanetti invariants 3.9 Lattice-gas cellular automata: What else?

2.1 What are cellular automata? 2.2 A short history of cellular automata 2.3 One-dimensional cellular automata 2.3.1 Qualitative characterization of one-dimensional cellular automata 2.4 Two-dimensional cellular automata 2.4.1 Neighborhoods in 2D 2.4.2 Fredkin’s game 2.4.3 ‘Life’ 2.4.4 CA: what else? Further reading 2.4.5 From CA to LGCA

5.1 From lattice-gas cellular automata to lattice Boltzmann models 5.1.1 Lattice Boltzmann equation and Boltzmann equation 5.1.2 Lattice Boltzmann models of the first generation 5.2 BGK lattice Boltzmann model in 2D 5.2.1 Derivation of the W i 5.2.2 Entropy and equilibrium distributions 5.2.3 Derivation of the Navier-Stokes equations by multi-scale analysis 5.2.4 Storage demand 5.2.5 Simulation of two-dimensional decaying turbulence 5.2.6 Boundary conditions for LBM 5.3 Hydrodynamic lattice Boltzmann models in 3D 5.3.1 3D-LBM with 19 velocities 5.3.2 3D-LBM with 15 velocities and Koelman distribution 5.3.3 3D-LBM with 15 velocities proposed by Chen et al. (D3Q15) 5.4 Equilibrium distributions: the ansatz method 5.4.1 Multi-scale analysis 5.4.2 Negative distribution functions at high speed of sound 5.5 Hydrodynamic LBM with energy equation 5.6 Stability of lattice Boltzmann models 5.6.1 Nonlinear stability analysis of uniform flows 5.6.2 The method of linear stability analysis (von Neumann) 5.6.3 Linear stability analysis of BGK lattice Boltzmann models 5.6.4 Summary 5.7 Simulating ocean circulation with LBM 5.7.1 Introduction 5.7.2 The model of Munk (1950) 5.7.3 The lattice Boltzmann model 5.8 A lattice Boltzmann equation for diffusion 5.8.1 Finite differences approximation 5.8.2 The lattice Boltzmann model for diffusion 5.8.3 Multi-scale expansion 5.8.4 The special case \(\omega = 1\) 5.8.5 The general case 5.8.6 Numerical experiments 5.8.7 Summary and conclusion 5.8.8 Diffusion equation with a diffusion coefficient depending on concentration 5.8.9 Further reading 5.9 Lattice Boltzmann model: What else? 5.10 Summary and outlook

4.1 The Boltzmann equation 4.1.1 Five collision invariants and Maxwell’s distribution 4.1.2 Boltzmann’s H-theorem 4.1.3 The BGK approximation 4.2 Chapman-Enskog: From Boltzmann to Navier-Stokes 4.2.1 The conservation laws 4.2.2 The Euler equation 4.2.3 Chapman-Enskog expansion 4.3 The maximum entropy principle

Fossil foraminiferal shells are utilized in paleoceanography to extract information about environmental conditions of the past ocean. Based on several assumptions, the ratio of 13 C and 12 C preserved in their shells is used to reconstruct, for example, the paleoproductivity or the oceanic pCO2. Metabolism of the living organism and the sea water chemistry, however, can influence the incorporation of carbon isotopes during calcification such that the signal of the shells differ from the signal of the sea water. These effects occur because the chemical microenvironment of the foraminifer (boundary layer thickness ∼500 μm) differs from the bulk sea water. Here, we present a numerical model that calculates the δ 13 C of the foraminiferal shell as a function of the sea water chemistry and the magnitude of vital effects. Concentration profiles of the chemical species of the carbonate system within the microenvironment of foraminifera are obtained by solving diffusion-reaction equations. The compounds of dissolved forms of carbon dioxide containing either the stable carbon isotope 13 C or 12 C are considered separately. Spherical symmetry of the foraminifer is assumed. The model outcome is compared to results from culture experiments with the planktonic foraminifer Orbulina universa. Model results indicate that the interaction between vital effects of the foraminifer and the sea water chemistry can account for changes in the δ 13 C of foraminiferal calcite of 0.3–0.4‰ when glacial and interglacial sea water conditions are compared. These effects occur even though the δ 13 C of the total dissolved inorganic carbon is kept constant. Thus, changes in sea water chemistry should be distinguished from events which changed the δ 13 C of the inorganic carbon of the sea water.

Diatoms have evolved a multitude of morphologies, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protection or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. Ar scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller scale of diatom cells, however, turbulence creates a roughly linear shear and nutrients can locally become strongly depleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent environments, Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The model results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects of sinking.

Ongoing global warming induced by anthropogenic emissions has opened the debate as to whether geoengineering is a 'quick fix' option. Here we analyse the intended and unintended effects of one specific geoengineering approach, which is enhanced weathering via the open ocean dissolution of the silicate-containing mineral olivine. This approach would not only reduce atmospheric CO2 and oppose surface ocean acidification, but would also impact on marine biology. If dissolved in the surface ocean, olivine sequesters 0.28 g carbon per g of olivine dissolved, similar to land-based enhanced weathering. Silicic acid input, a byproduct of the olivine dissolution, alters marine biology because silicate is in certain areas the limiting nutrient for diatoms. As a consequence, our model predicts a shift in phytoplankton species composition towards diatoms, altering the biological carbon pumps. Enhanced olivine dissolution, both on land and in the ocean, therefore needs to be considered as ocean fertilization. From dissolution kinetics we calculate that only olivine particles with a grain size of the order of 1 μm sink slowly enough to enable a nearly complete dissolution. The energy consumption for grinding to this small size might reduce the carbon sequestration efficiency by ~30%.

[1] The influence of the overturning circulation on the anthropogenic carbon sink in the North Atlantic is investigated with a simple box model. The net air-sea flux of anthropogenic carbon in the North Atlantic is the result of two opposing fluxes: The first is the uptake caused by the disequilibrium between the rapidly rising atmospheric pCO2 and the dissolved carbon content in the ocean, depending mainly on the water exchange rate between mixed layer and interior North Atlantic ocean. Superimposed is a second flux, related to the northward transport of heat within the Atlantic basin, that is directed out of the ocean, contrary to conventional wisdom. It is caused by a latitudinal gradient in the ratio of seawater alkalinity to total dissolved inorganic carbon that in turn is related to the cooling and freshening of surface water on its way north. This flux depends strongly on the vertical structure of the upper branch of the overturning circulation and on the distribution of undersaturation and supersaturation of CO2 in Atlantic surface waters. A data-based estimate of anthropogenic carbon inventory in the North Atlantic is consistent with a dominance of the disequilibrium flux over the heat-flux-related outgassing at the present time, but, in our model, does not place a strong constraint on the net anthropogenic air-sea flux. Stabilization of the atmospheric pCO2 on a higher level will change the relative role of the two opposing fluxes, making the North Atlantic a source of anthropogenic carbon to the atmosphere. We discuss implications for the interpretation of numerical carbon cycle models.

[1] The production, transport, and dissolution of carbonate shells plays an important role in the global carbon cycle. For instance, these processes affect the ocean's CO32− concentration, which, on glacial-interglacial timescales, is involved in the regulation of atmospheric pCO2. The saturation state of the ocean with respect to calcium carbonate suggests that any dissolution of calcium carbonate takes place not in the water column but at the seafloor. On the other hand, several authors suggest that a significant part of the dissolution takes place in the upper part of the water column. In the present paper, a simple model is presented to investigate under which circumstances dissolution of calcium carbonate in the water column may be plausible. Our analysis also includes dissolution facilitated by CO2 release from organic carbon remineralization, which may generate a microenvironment that is undersaturated with respect to CaCO3. In marine snow aggregates, however, respiration-driven dissolution is shown to be insignificant due to the size and settling velocities of the aggregates. With respect to aragonite, dissolution in the water column attributable to the bulk saturation state alone is significant and may well contribute to the alkalinity maximum in the undersaturated intermediate waters of the North Pacific Ocean. The suggested large loss of calcium carbonate dissolved in the water column (60–80% of CaCO3 production), however, cannot be explained by the mechanisms investigated here.

The results of an existing one-dimensional diagnostic model that calculates the fugacity of CO 2 ( f CO 2 ) in the surface layer of the southern ocean were compared with in situ observations from diVerent ocean sectors and seasons. Our model is based on the translation of monthly variations of constraints fields into surface water f CO 2 variations, and was used to assess the CO 2 uptake of the southern ocean. In situ observations are useful to verify the model results and were here applied to improve the estimation of the CO 2 uptake of the southern ocean south of 50°S. The model reproduces the f CO 2 distribution in both Pacific and Indian sectors of the southern ocean satisfactorily, the mean deviation being only 5 μatm. This discrepancy requires only a minor modification of the CO 2 uptake calculated by the model for that area. By contrast, the model strongly underestimates the f CO 2 levels in early spring and early winter in the Weddell gyre. This indicates that the CO 2 uptake by the Atlantic sector of the southern ocean as calculated by the model, amounting to 0.47 GtC yr −1 , should be reduced, possibly by about half of this value. The reason for this mismatch lies in the use of climatological physical constraints by the model, that do not sufficiently well describe reality. Partly, the mismatch is also caused by a difference of seasonal stage between the model which reflects climatological conditions and the real ocean which is affected by interannual variability. Based on this study it is concluded that the CO 2 uptake of the southern ocean south of 50°S is likely to lie somewhere between 0.6 and 0.7 GtC yr −1 for the 1990s, which is a high value compared to estimates from other investigations. DOI: 10.1034/j.1600-0889.1999.00029.x

The assessment of direct effects of anthropogenic CO 2 increase on the marine biota has received relatively little attention compared to the intense research on CO 2 -related responses of the terrestrial biosphere. Yet, due to the rapid air–sea gas exchange, the observed past and predicted future rise in atmospheric CO 2 causes a corresponding increase in seawater CO 2 concentrations, [CO 2 ], in upper ocean waters. Increasing [CO 2 ] leads to considerable changes in the surface ocean carbonate system, resulting in decreases in pH and the carbonate concentration, [CO 2 −3 ]. These changes can be shown to have strong impacts on the marine biota. Here we will distinguish between CO 2 -related responses of the marine biota which (a) potentially affect the ocean's biological carbon pumps and (b) are relevant to the interpretation of diagnostic tools (proxies) used to assess climate change on geological times scales. With regard to the former, three direct effects of increasing [CO 2 ] on marine plankton have been recognized: enhanced phytoplankton growth rate, changing elemental composition of primary produced organic matter, and reduced biogenic calcification. Although quantitative estimates of their impacts on the oceanic carbon cycle are not yet feasible, all three effects increase the ocean's capacity to take up and store atmospheric CO 2 and hence, can serve as negative feedbacks to anthropogenic CO 2 increase. With respect to proxies used in palaeo-reconstructions, CO 2 -sensitivity is found in carbon isotope fractionation by phytoplankton and foraminifera. While CO 2 - dependent isotope fractionation by phytoplankton may be of potential use in reconstructing surface ocean p CO 2 at ancient times, CO 2 -related effects on the isotopic composition of foraminiferal shells confounds the use of the difference in isotopic signals between planktonic and benthic shells as a measure for the strength of marine primary production. The latter effect also offers an alternative explanation for the large negative swings in δ 13 C of foraminiferal calcite between glacial and interglacial periods. Changes in [CO 2 −3 ] affect the δ 18 O in foraminiferal shells. Taking this into account brings sea surface temperature estimates for the glacial tropics closer to those obtained from other geochemical proxies. DOI: 10.1034/j.1600-0889.1999.00023.x

Open ocean deep convection is examined with a nonhydrostatic model based on primitive equations. Strong cooling on the surface of the ocean enforces vertical motion which takes place in the narrow regions of convective cells. Different equations of state are considered, and it is shown that a linear equation of state with a constant thermal expansion coefficient cannot represent the buoyancy field and fluxes properly. The inclusion of thermobaric effects leads to additional vertical acceleration in the whole water column. The parametrization of the convective fluxes by a convective adjustment algorithm represents the horizontal mean temperature quite well but suppresses the entire vertical mass transport within the convective cells. This mass transport may be important for the transport of other tracers. Investigation of an energy cycle of the convective motion sorts out sources and sinks and reveals the conversion between different forms of energy during convective events. Both the vertical and the horizontal component of the Earth rotation vector contribute to the time mean energy balance with the same order of magnitude, but the instantaneous amplitudes of the distinct terms may differ substantially. A sensitivity study with respect to eddy and thermal diffusion coefficients distinguishes two regions; at high values of the diffusivities the velocities and tracer distributions show that a strong dependence on this values occurs, while for sufficiently small coefficients, only a weak dependence is observed.

A numerical model is proposed for investigatingthe potential of calcium carbonatedissolution in copepod guts. A sensitivityanalysis is performed to reveal criticalparameters. Gut clearance rate is dependent ontemperature and grazing rate and determines thetime scale on which ingested calcite is subjectto dissolution. Highest dissolution isobtained when the individual zooplankton isalternating between grazing and non-grazing and feedingis restricted to the night-time period. Model resultsshow that up to 70% of the ingested carbonate may bedissolved in the guts, considering reingestion of faecalpellets in the absence of a phytoplanktonbloom, while approximately15% dissolution is to be expectedin a bloom situation. An estimate is made for the contributionof calcite dissolution in copepod guts to the proposed globalcalcite loss in the water column.

Constraints on the hydrography and geostrophic velocity shear of the Southern Ocean implicit in its potential vorticity field are discussed and illustrated by diagnostic study of observed and modeled potential vorticity fields. A stress-driven, thermodynamically inactive, eddy-resolving quasigeostrophic model of the Southern Ocean suggests that, through the systematic erosion of potential vorticity gradients by geostrophic eddies, the large-scale flow equilibrates toward a state in which interior potential vorticity gradients are small. Observations of the large-scale isopycnal distribution of potential vorticity (IPV), deduced from climatological hydrographic data, reveal a much richer structure. The most striking feature of the IPV field is the presence of large, near-surface gradients of IPV (a vortocline) coinciding with the axis of the Antarctic Circumpolar Current (ACC). To the south of this front potential vorticity (PV) is large; to its north, PV has low values, and relative to those foun...

Diatoms, one of the most productive phytoplankton groups in the ocean, have an essential requirement for silicic acid for the construction of their opaline shells. In seawater silicic acid is present in three chemical species (H4SiO4, H3SiO4-, H2SiO42-). The silicic acid molecule actually taken up by diatoms is still under discussion. Algal cells are surrounded by a diffusive boundary layer (DBL) which has an effective thickness of the order of the cell radius. The nutrient transport through this layer is by diffusion only and thus may limit the supply of silicic acid to the cell. Due to uptake of one of the species of silicic acid by the cell, the chemical system in the DBL is out of equilibrium. We have developed a diffusion-reaction model for the components H4SiO4, H3SiO4-, H2SiO42-, OH-, and H+ in the DBL which allows us to calculate maximum silicic acid supply rates as a function of the total concentration of dissolved silicon, pH, algal radius, and silicic acid species taken up by the cell. In addition, analytical solutions for the simplified diffusion-reaction system are presented. Model calculations of the silicic acid maximum uptake rates are compared with uptake data for Thalassiosira weissflogii from recent laboratory experiments. Results indicate that the supply of H4SiO4 to T. weissflogii by diffusion-reaction processes is suffcient to sustain the observed growth rates. Thus, the H4SiO4 uptake of T. weissffogii is not diffusion-limited. This does not hold true for H3SiO4-.

Two common strategies for acquisition of iron by terrestrial plants, fungi and bacteria are the excretion of siderophores and the reduction of organic iron‐complexes at the cell surface. We investigate the feasibility of these strategies for single phytoplankton cells in a marine environment, where diffusion plays a much larger role. Using numerical models we determine the efficiency of both mechanisms, where efficiency is defined as the ratio between the increase in iron uptake rates and the excretion rate of either siderophores or electrons required to obtain this increase. This allows an estimation of the physiological cost of either mechanism. Based on our results we discuss how the maximum possible efficiency depends on the chemical kinetics of iron coordination reactions, on the biological kinetics of iron uptake, and on other parameters.

The interaction between Io and the plasma torus is simulated by a three-dimensional numerical model which allows the calculation of electric fields, current density distributions, and magnetic fields in Io's vicinity and in the Alfven wings. The model is self-consistent in the sense that the influence of magnetic field disturbances on the electric field is taken into account and vice versa. A two-dimensional elliptic differential equation for the electric potential is derived by using Euler potentials. The current density in Io's vicinity is decomposed into poloidal and toroidal fields. This representation allows an effective integration to get the magnetic field disturbances. Pedersen, Hall, inertial (or polarization) and field-aligned currents are calculated. We show results for different upstream plasma conditions and atmosphere/ionosphere models and discuss the consequences of variations in neutral density, Alfvenic Mach number, and other parameters. A series of simple spherically symmetric atmosphere/ionosphere models with neutral gas densities between 1016 m−3 and 1017 m−3 are investigated. By comparison with Voyager magnetic field measurement we get agreement for a column density of 7.5 × 1021 m−2 (surface density N0 = 5×1016 m−3, scale height H = 150 km).

Lake eutrophication is a pervasive problem globally, particularly serious in agricultural and densely populated areas. Whenever nutrients nitrogen and phosphorus do not limit phytoplankton growth directly, high growth rates will rapidly lead to biomass increases causing self-shading and light-limitation, and eventually CO2 depletion. The paradigm of phytoplankton limitation by nutrients and light is so pervasively established, that the lack of nutrient limitation is ordinarily interpreted as sufficient evidence for the condition of light limitation, without considering the possibility of limitation by inorganic carbon. Here, we firstly evaluated how frequently CO2 undersaturation occurs in a set of eutrophic lakes in the Pampa plains. Our results confirm that conditions of CO2 undersaturation develop much more frequently (yearly 34%, summer 44%) in these agriculturally impacted lakes than in deep, temperate lakes in forested watersheds. Secondly, we used Generalized Additive Models to fit trends in CO2 concentration considering three drivers: total incident irradiance, chlorophyll a concentration, and lake depth; in eight multi-year datasets from eutrophic lakes from Europe, North and South America, Asia and New Zealand. CO2 depletion was more often observed at high irradiance levels, and shallow water. CO2 depletion also occurred at high chlorophyll concentration. Finally, we identified occurrences of light- and carbon-limitation at the whole-lake scale. The different responses of chlorophyll a and CO2 allowed us to develop criteria for detecting conditions of CO2 limitation. For the first time, we provided whole-lake evidence of carbon limitation of phytoplankton biomass. CO2 increases and eutrophication represent two major and converging environmental problems that have additive and contrasting effects, promoting phytoplankton, and also leading to carbon depletion. Their interactions deserve further exploration and imaginative approaches to deal with their effects.