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International audience; Picoplankton populations dominate the planktonic community in the surface oligotrophic ocean. Yet, their strategies in the acquisition and the partitioning of organic and inorganic sources of nitrogen (N) and carbon (C) are poorly described. Here, we measured at the single-cell level the uptake of dissolved inorganic C (C-fixation), C-leucine, N-leucine, nitrate (NO3−), ammonium (NH4+), and N-urea in pigmented and nonpigmented picoplankton groups at six low-N stations in the northwestern Atlantic Ocean. Our study highlights important differences in trophic strategies between Prochlorococcus, Synechococcus, photosynthetic pico-eukaryotes, and nonpigmented prokaryotes. Nonpigmented prokaryotes were characterized by high leucine uptake rates, nonsignificant C-fixation and relatively low NH4+, N-urea, and NO3− uptake rates. Nonpigmented prokaryotes contributed to 7% ± 3%, 2% ± 2%, and 9% ± 5% of the NH4+, NO3−, and N-urea community uptake, respectively. In contrast, pigmented groups displayed relatively high C-fixation rates, NH4+ and N-urea uptake rates, but lower leucine uptake rates than nonpigmented prokaryotes. Synechococcus and photosynthetic pico-eukaryotes NO3− uptake rates were higher than Prochlorococcus ones. Pico-sized pigmented groups accounted for a significant fraction of the community C-fixation (63% ± 27%), NH4+ uptake (47% ± 27%), NO3− uptake (62% ± 49%), and N-urea uptake (81% ± 35%). Interestingly, Prochlorococcus and photosynthetic pico-eukaryotes showed a greater reliance on C- and N-leucine than Synechococcus on average, suggesting a greater reliance on organic C and N sources. Taken together, our single-cell results decipher the wide diversity of C and N trophic strategies between and within marine picoplankton groups, but a clear partitioning between pigmented and nonpigmented groups still remains.

At depth, O2 depleted because of organic matter remineralization is generally estimated based on apparent oxygen utilization (AOU). However, AOU is an imperfect measure of oxygen utilization because of O2 air-sea disequilibrium at the site of deep-water formation. Recent methodological and instrumental advances have paved the way to further deconvolve processes driving the O2 signature. Using numerical model simulations of the global ocean, we show that measurements of the dissolved O2/Ar ratio, which so far have been confined to the ocean surface, can provide improved estimates of oxygen utilization, especially in regions where the disequilibrium at the site of deep-water formation is associated with physical processes. We discuss applications of this new approach and implications for current tracers relying on O2 such as remineralization ratios, respiratory quotients and preformed nutrients. Finally, we propose a new composite geochemical tracer, [O2]*bio combining dissolved O2/Ar and phosphate concentration. Being insensitive to photosynthesis and respiration, the change in this new tracer reflects gas exchange at the air-sea interface at the sites of deep-water formation. Plain Language Summary Oxygen utilization at depth offers insight into organic matter remineralization and the strength of the biological carbon pump. However, the oxygen concentration in the ocean interior is also impacted by additional biotic and abiotic processes occurring at the sites of deep-water formation and during transit in the ocean interior. In this study, we summarize, formalize, and model these processes to explore how decomposing the O2 signal at depth with new tools can provide new insight and more refined budgets of the broad-scale oceanic biogeochemical cycling of oxygen and nutrients. This is particularly important in light of the recent evidence that the role of physical processes in regions of convective deep-water formation may currently be underestimated.

While it has been known that wave breaking and bubble generation at high wind speeds enhance air‐sea CO2 exchange rates (F), quantification of their contribution at the global scale remains a formidable challenge. There is urgency to make progress on this issue as a significant uptick in both magnitude and frequency of high wind events (HW) has been documented over the last three decades. Using a wind‐wave dependent expression for gas transfer velocity (k) that explicitly considers bubbles and a widely used wind‐only parameterization, the spatial pattern of k at high winds can be explained by sea surface temperature distribution. The HW, which represent some 3% of wind conditions, contribute disproportionally to the global F (18%) with an increasing trend. Approximately 50% of the global F at high winds is attributed to bubble contribution. The findings are of significance to quantifying CO2 transfer to the ocean interior. Plain Language Summary Studies on air‐sea carbon dioxide (CO2) exchange seek to determine how rapidly CO2 molecules traverse the air‐water interface. This exchange impacts a plethora of processes related to ocean biogeochemistry, oceanic carbon cycling, and CO2 buildup in the atmosphere. At high wind speeds, both conventional aerodynamic transfer processes and bubbles generated by wave breaking are expected to enhance air‐sea CO2 gas exchange. Yet the impact of bubbles in isolation on global and regional CO2 exchange remains a subject of inquiry and speculation. There is some urgency to make progress on this topic because the magnitude and frequency of high wind events (HW) have been steadily increasing over the last three decades. The work here demonstrates that bubbles contribute as much as 50% of CO2 gas exchange under high wind speeds conditions, which rarely occur over the ocean (less than 3% of the time). Yet, HW contribute disproportionally to the global air‐sea CO2 gas exchange (about 18%).

International audience; Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N-2) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N-2 fixation have been hypothesized to be generally controlled by temperature, phosphorus, and iron availability in the global ocean. However, even in the North Atlantic where most research on diazotrophs and N-2 fixation has taken place, environmental controls remain contentious. Here we measure diazotroph composition, abundance, and activity at high resolution using newly developed underway sampling and sensing techniques. We capture a diazotrophic community shift from Trichodesmium to UCYN-A between the oligotrophic, warm (25-29 degrees C) Sargasso Sea and relatively nutrient-enriched, cold (13-24 degrees C) subpolar and eastern American coastal waters. Meanwhile, N-2 fixation rates measured in this study are among the highest ever recorded globally and show significant increase with phosphorus availability across the transition from the Gulf Stream into subpolar and coastal waters despite colder temperatures and higher nitrate concentrations. Transcriptional patterns in both Trichodesmium and UCYN-A indicate phosphorus stress in the subtropical gyre. Over this iron-replete transect spanning the western North Atlantic, our results suggest that temperature is the major factor controlling the diazotrophic community structure while phosphorous drives N-2 fixation rates. Overall, the occurrence of record-high UCYN-A abundance and peak N-2 fixation rates in the cold coastal region where nitrate concentrations are highest (similar to 200 nM) challenges current paradigms on what drives the distribution of diazotrophs and N-2 fixation.

Export production reflects the amount of organic matter transferred from the surface ocean to depth through biological processes. This export is in great part controlled by nutrient and light availability, which are conditioned by mixed layer depth (MLD). In this study, building on Sverdrup’s critical depth hypothesis, we derive a mechanistic model of an upper bound on carbon export based on the metabolic balance between photosynthesis and respiration as a function of MLD and temperature. We find that the upper bound is a positively skewed bell-shaped function of MLD. Specifically, the upper bound increases with deepening mixed layers down to a critical depth, beyond which a long tail of decreasing carbon export is associated with increasing heterotrophic activity and decreasing light availability. We also show that in cold regions the upper bound on carbon export decreases with increasing temperature when mixed layers are deep, but increases with temperature when mixed layers are shallow. A metaanalysis shows that our model envelopes field estimates of carbon export from the mixed layer. When compared to satellite export production estimates, our model indicates that export production in some regions of the Southern Ocean, most particularly the Subantarctic Zone, is likely limited by light for a significant portion of the growing season.

Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.

Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles. The geographical distribution and controlling factors of marine N2 fixation are understudied. Here the authors find increasing rates of N2 fixation from the Sargasso Sea to the coastal waters of North America, driven primarily by cyanobacterial diazotrophs and best correlated with phosphorus availability and chlorophyll-a concentrations.

WOS:000497952500008; International audience; Despite generally low primary productivity and diatom abundances in oligotrophic subtropical gyres, the North Atlantic Subtropical Gyre (NASG) exhibits significant diatom-driven carbon export on an annual basis. Subsurface pulses of nutrients likely fuel brief episodes of diatom growth, but the exact mechanisms utilized by diatoms in response to these nutrient injections remain understudied within near-natural settings. Here we simulated delivery of subsurface nutrients and compare the response among eukaryotic phytoplankton using a combination of physiological techniques and metatranscriptomics. We show that eukaryotic phytoplankton groups exhibit differing levels of transcriptional responsiveness and expression of orthologous genes in response to release from nutrient limitation. In particular, strategies for use of newly delivered nutrients are distinct among phytoplankton groups. Diatoms channel new nitrate to growth-related strategies while physiological measurements and gene expression patterns of other groups suggest alternative strategies. The gene expression patterns displayed here provide insights into the cellular mechanisms that underlie diatom subsistence during chronic nitrogen-depleted conditions and growth upon nutrient delivery that can enhance carbon export from the surface ocean.

Nitrogen (N) is a limiting nutrient in vast regions of the world's oceans, yet the sources of N available to various phytoplankton groups remain poorly understood. In this study, we investigated inorganic carbon (C) fixation rates and nitrate (NO

We present estimates of mixed-layer net community oxygen production (N) and gross oxygen production (G) of the Bellingshausen Sea in March and April 2007. N was derived from oxygen-to-argon (O2/Ar) ratios; G was derived using the dual-delta method from triple oxygen isotope measurements. In addition, O2 profiles were collected at 253 CTD stations. N is often approximated by the biological oxygen air–sea exchange flux (Fbio based on the O2/Ar supersaturation, assuming that significant horizontal or vertical fluxes are absent. Here we show that the effect of vertical fluxes alone can account for Fbio values < 0 in large parts of the Bellingshausen Sea towards the end of the productive season, which could otherwise be mistaken to represent net heterotrophy. Thus, improved estimates of mixed-layer N can be derived from the sum of Fbio, Fe (entrainment from the upper thermocline during mixed-layer deepening) and Fv (diapycnal eddy diffusion across the base of the mixed layer). In the winter sea ice zone (WSIZ), the corresponding correction results in a small change of Fbio = (30 ± 17) mmol m−2 d−1 to N = (34 ± 17) mmol m−2 d−1. However, in the permanent open ocean zone (POOZ), the original Fbio value of (−17 ± 10) mmol m−2 d−1 gives a corrected value for N of (−2 ± 18) mmol m−2 d−1. We hypothesize that in the WSIZ, enhanced water column stability due to the release of freshwater and nutrients from sea ice melt may account for the higher N value. These results stress the importance of accounting for physical biases when estimating mixed-layer marine productivity from in situ O2/Ar ratios.

WOS:000426143100062; International audience; Because of the difficulty in resolving the large variability of N-2 fixation with current methods which rely on discrete sampling, the development of new methods for high resolution measurements is highly desirable. We present a new method for high-frequency measurements of aquatic N-2 fixation by continuous flow-through incubations and spectral monitoring of the acetylene (C2H2, a substrate analog for N-2) reduction to ethylene (C2H4). In this method, named Flow-through Incubation Acetylene Reduction Assays by Cavity Ring-Down Laser Absorption Spectroscopy (FARACAS), dissolved C2H2 is continuously admixed with seawater upstream of a continuous flow stirred-tank reactor (CFSR) in which C2H2 reduction takes place. Downstream of the flow-through incubator, the C2H4 gas is stripped using a bubble column contactor and circulated with a diaphragm pump into a wavelength-scanned cavity ring down laser absorption spectrometer (CRDS). Our method provides high-resolution and precise mapping of aquatic N-2 fixation, its diel cycle, and its response to environmental gradients, and can be adapted to measure other biological processes. The short-duration of the flow-through incubations without preconcentration of cells minimizes potential artifacts such as bottle containment effects while providing near real-time estimates for adaptive sampling. We expect that our new method will improve the characterization of the biogeography and kinetics of aquatic N-2 fixation rates.

We present a novel method for continuous and automated shipboard measurements of dissolved inorganic carbon concentration ([DIC]) in surface water. The method is based on dual isotope dilution and cavity ring-down spectroscopy (DID-CRDS). In this method, seawater is continuously sampled and mixed with a flow of NaH(13)CO3 solution that is also enriched in deuterated water (the spike). The isotopic composition of CO2 (δ(13)C(spiked_sample)) derived from the DIC in the mixture, and the D/H ratio of the mixed water (δD(spiked_sample)), are measured by CRDS analyzers. The D/H of the water in the mixture allows accurate estimates of the mixing ratio of the sample and the spike. [DIC] of the sample is then calculated from the mixing ratio, [DI(13)C] of the spike, and δ(13)C(spiked_sample). In the laboratory, the precision of the method is0.02% (±0.4 μmol kg(-1) when [DIC] = 2000 μmol kg(-1)). A shipboard test was conducted in the Delaware Bay and Estuary. For 2 min average [DIC], a precision of0.03% was achieved. Measurements from the DID-CRDS showed good agreement with independent measurements of discrete samples using the well-established coulometric method (mean difference = -1.14 ± 1.68 μmol kg(-1)), and the nondispersive infrared(NDIR)-based methods (mean difference = -0.9 ± 4.73 μmol kg(-1)).

Continuous measurements of CO 2 partial pressure, dissolved oxygen, and the oxygen argon ratio (O 2 /Ar) in surface waters, complemented by discrete observations of inorganic carbon, along a late-summer south-to-north transect in the Australian Sector of the Southern Ocean (115°E, WOCE I9S) are presented. The largest net community production (NCP, based on O 2 /Ar), and air–sea CO 2 disequilibrium (ΔDpCO 2 ) were found in three regions: (1) between the southern boundary of the Antarctic Circumpolar Current (63.6°S) and southern extent of the Polar Front (58.6°S, NCP = 30 mmol C m − 2 d − 1 , ΔpCO 2 = 30 to 50 μatm); (2) between the northern extent of the Sub-Antarctic Front (45.5°S) and the Sub-Tropical Front (39.8°S, NCP = 50 mmol C m − 2 d − 1 , ΔpCO 2 = 25 μatm); and (3) near 35°S (NCP = 50 mmol C m − 2 d − 1 , ΔpCO 2 = 20 μatm). Areas of enhanced NCP were correlated with shallow mixed layer depths, suggesting that production is controlled in part by light limitation. We found good agreement between estimates of NCP based on O 2 /Ar measurements and those derived from seasonal (mixed layer) inorganic carbon deficits. In ice covered areas, and in areas where surface temperature changes act on shorter timescales than the equilibration of oxygen in the mixed layer, we find that physical controls on surface O 2 are of equal magnitude to biological controls, which has important implications for the use of oxygen sensors on autonomous platforms to infer NCP.

Large-scale patterns of net community production (NCP) were estimated during the late summer cruise ARK-XXVI/3 (TransArc, August/September 2011) to the central Arctic Ocean. Several approaches were used based on the following: (i) continuous measurements of surface water oxygen to argon ratios (O2/Ar), (ii) underway measurements of surface partial pressure of carbon dioxide (pCO2), (iii) discrete samples of dissolved inorganic carbon, and (iv) dissolved inorganic nitrogen and phosphate. The NCP estimates agreed well within the uncertainties associated with each approach. The highest late summer NCP (up to 6 mol C m−2) was observed in the marginal sea ice zone region. Low values (

We observe surface and subsurface fluorescence-derived chlorophyll maxima in southern Drake Passage during austral summer. Backscatter measurements indicate that the deep chlorophyll maxima (DCMs) are also deep biomass maxima, and euphotic depth estimates show that they lie below the euphotic layer. Subsurface, offshore and near-surface, onshore features lie along the same isopycnal, suggesting advective generation of DCMs. Temperature measurements indicate a warming of surface waters throughout austral summer, capping the winter water (WW) layer and increasing off-shelf stratification in this isopycnal layer. The outcrop position of the WW isopycnal layer shifts onshore, into a surface phytoplankton bloom. A lateral potential vorticity (PV) gradient develops, such that a down-gradient PV flux is consistent with offshore, along-isopycnal tracer transport. Model results are consistent with this mechanism. Subduction of chlorophyll and biomass along isopycnals represents a biological term not observed by surface satellite measurements which may contribute significantly to the strength of the biological pump in this region.

An autonomous system using isotope dilution as its core method has been developed to obtain high-frequency measurements of dissolved inorganic carbon (DIC) concentrations in the surface ocean. This system accurately mixes a seawater sample and a 13C-labeled sodium bicarbonate solution (spike). The mixed solution is then acidified and sent through a gas permeable membrane contactor. CO2 derived from DIC in the mixture is extracted by a CO2-free gas stream, and is sent to a cavity ring-down spectrometer to analyze its 13C/12C ratio. [DIC] of the seawater can then be derived from the measured 13C/12C, the known mixing ratio and the [DI13C] of the spike. The method has been tested under a wide [DIC] range (1800–2300 µmol/kg) in the laboratory. It has also been deployed on a cruise that surveyed ocean waters to the south of Florida. At a sampling resolution of 4 min (15 samples per hour), the relative standard deviation of DIC determined from the laboratory tests and the field deployment is ± 0.07% and ± 0.09%, respectively. The accuracy of the method is better than 0.1% except where [DIC] varies faster than 5 µmol/kg per minute. Based on the laboratory and field evaluations, we conclude that this method can provide accurate underway [DIC] measurements at high resolution in most oceanic regions.

The most common currency for estimating N2 fixation is acetylene reduction to ethylene. Real-time esti- mates of nitrogen fixation are needed to close the global nitrogen budget and these remain a critical gap in both laboratory and field experiments. We present a new method for continuous real-time measurements of ethylene pro- duction: Acetylene Reduction Assays by Cavity ring-down laser Absorption Spectroscopy (ARACAS). In ARACAS, air in the headspace of an incubation chamber is circulated with a diaphragm pump through a cavity ring-down eth- ylene spectrometer and back to the incubation chamber. This paper describes the new approach and its benefits compared to the conventional detection of ethylene by flame ionization detector gas chromatography. First, the detection of acetylene reduction to ethylene is non-intru- sive and chemically non-destructive, allowing for real-time measurements of nitrogenase activity. Second, the mea- surements are made instantaneously and continuously at ppb levels, allowing for observation of real-time kinetics on time intervals as short as a few seconds. Third, the instrument can be automated for long time periods of measurement. Finally, the technique will be widely accessible by the research community as it can be readily adapted to most existing acetylene reduction protocols and is based on a modestly priced, commercially available instrument. We illustrate its use for measuring N2 fixation using two species, the diazotrophic bacterium Azotobacter vinelandii and the lichen Peltigera praetextata. We also discuss potential limitations of the approach, primarily the implications of leaks in the analyzer, as well as future improvements.

A three dimensional, time-evolving field of atmospheric potential oxygen (APO ~O2/N2+CO2) was estimated using surface O2, N2 and CO2 fluxes from the WHOI ocean ecosystem model to force the MATCH atmospheric transport model. Land and fossil carbon fluxes were also run in MATCH and translated into O2 tracers using assumed O2:CO2 stoichiometries. The modeled seasonal cycles in APO agree well with the observed cycles at 13 global monitoring stations, with agreement helped by including oceanic CO2 in the APO calculation. The modeled latitudinal gradient in APO is strongly influenced by seasonal rectifier effects in atmospheric transport. An analysis of the APO-vs.-CO2 mass-balance method for partitioning land and ocean carbon sinks was performed in the controlled context of the MATCH simulation, in which the true surface carbon and oxygen fluxes were known exactly. This analysis suggests uncertainty of up to ±0.2 PgC in the inferred sinks due to variability associated with sparse atmospheric sampling. It also shows that interannual variability in oceanic O2 fluxes can cause large errors in the sink partitioning when the method is applied over short timescales. However, when decadal or longer averages are used, the variability in the oceanic O2 flux is relatively small, allowing carbon sinks to be partitioned to within a standard deviation of 0.1 Pg C/yr of the true values, provided one has an accurate estimate of long-term mean O2 outgassing.

In vitro activities of phosphoenolpyruvate carboxylase (PEPC) and phosphoenolpyruvate carboxykinase (PEPCK) were measured in chemostat cultures of the marine diatom Phaeodactylum tricornutum grown under either phosphate-or nitrate-limited conditions and at low (1 μmol kg−1) and high (70–100 μmol kg−1) CO2 concentrations. A comparison of in vitro methods for measuring PEPC activity underscored the importance of including glycerol in the reaction mixture because of its ability to stabilize the quaternary structure of PEPC. Assays for total β-carboxylase activity (PEPC + PEPCK) identified the importance of substituting Mn2+ for Mg2+ as an activator for PEPC since Mg2+ is inhibitory to PEPCK activity. Results indicated that PEPC was constitutive, that is, present in a constant ratio relative to the limiting nutrient over a wide range of growth conditions. PEPCK acclimation was greater, and its intracellular concentration was positively correlated with CO2 concentrations. In vitro β-carboxylase activi...

A theoretical model was developed to explain the characteristics of carbon isotopic fractionation (eP) by the marine diatom Phaeodactylum tricornutum under nutrient- and light-limited growth conditions. The model takes into consideration active transport and diffusion of inorganic carbon through the cell membrane and chloroplast membrane and the energetic tradeoff between production of Rubisco and operation of a carbon-concentrating mechanism to achieve a given growth rate. The model is able to explain 88% of the variance in experimental ep data reported in this study and in previous work and is able to account for the observed pattern of Rubisco activity in nitrate-limited chemostats. Two important implications of the model include the fact that ep is not a unique function of the ratio of growth rate to external CO2 concentration (as opposed to the predictions of several previous models) and that changes in light-limited and nutrient-limited growth rates have opposite effects on the fraction of CO2 taken up by the chloroplast that is lost to diffusion and hence on certain patterns of carbon isotopic fractionation.

Models describing the evolution of the partial pressure of atmospheric oxygen over Phanerozoic time are constrained by the mass balances required between the inputs and outputs of carbon and sulfur to the oceans. This constraint has limited the applicability of proposed negative feedback mechanisms for maintaining levels of atmospheric O 2 at biologically permissable levels. Here we describe a modeling approach that incorporates O 2 -dependent carbon and sulfur isotope fractionation using data obtained from laboratory experiments on carbon-13 discrimination by vascular land plants and marine plankton. The model allows us to calculate a Phanerozoic O 2 history that agrees with independent models and with biological and physical constraints and supports the hypothesis of a high atmospheric O 2 content during the Carboniferous (300 million years ago), a time when insect gigantism was widespread.

A closed eddy core in the Atlantic Subantarctic Southern Ocean was fertilized twice with two tons of iron (as FeSO4) to test whether iron addition enhances downward particle flux into the deep ocean. The ~300 km2 fertilized patch was occupied for 39 d. Chlorophyll-a and primary productivity doubled after fertilization, and photosynthetic quantum yield (FV/FM) increased from 0.33 to ≥0.40. Silicic acid was at limiting concentrations (

[1] Sea-air fluxes of gases are commonly calculated from the product of the gas transfer velocity (k) and the departure of the seawater concentration from atmospheric equilibrium. Gas transfer velocities, generally parameterized in terms of wind speed, continue to have considerable uncertainties, partly because of limited field data. Here we evaluate commonly used gas transfer parameterizations using a historical data set of 222Rn measurements at 105 stations occupied on Eltanin cruises and the Geosecs program. We make this evaluation with wind speed estimates from meteorological reanalysis products (from National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasting) that were not available when the 22Rn data were originally published. We calculate gas transfer velocities from the parameterizations by taking into account winds in the period prior to the date that 222Rn profiles were sampled. Invoking prior wind speed histories leads to much better agreement than simply calculating parameterized gas transfer velocities from wind speeds on the day of sample collection. For individual samples from the Atlantic Ocean, where reanalyzed winds agree best with observations, three similar recent parameterizations give k values for individual stations with an rms difference of ∼40% from values calculated using 222Rn data. Agreement of basin averages is much better. For the global data set, the average difference between k constrained by 222Rn and calculated from the various parameterizations ranges from −0.2 to +0.9 m/d (average, 2.9 m/d). Averaging over large domains, and working with gas data collected in recent years when reanalyzed winds are more accurate, will further decrease the uncertainties in sea-air fluxes.

The roles of iron and light in controlling biomass and primary productivity are clearly established in the Southern Ocean. However, their influence on net community production (NCP) and carbon export remains to be quantified. To improve our understanding of NCP and carbon export production in the Subantarctic Zone (SAZ) and the northern reaches of the Polar Frontal Zone (PFZ), we conducted continuous onboard determinations of NCP as part of the Sub-Antarctic Sensitivity to Environmental Change (SAZ-Sense) study, which occurred in January–February 2007. Biological O2 supersaturation was derived from measuring O2/Ar ratios by equilibrator inlet mass spectrometry. Based on these continuous measurements, NCP during the austral summer 2007 in the Australian SAZ was approximately 43 mmol O2 m−2 d−1. NCP showed significant spatial variability, with larger values near the Subtropical front, and a general southward decrease. For shallower mixed layers (

The oxygen (O(2)) concentration in the surface ocean is influenced by biological and physical processes. With concurrent measurements of argon (Ar), which has similar solubility properties as oxygen, we can remove the physical contribution to O(2) supersaturation and determine the biological oxygen supersaturation. Biological O(2) supersaturation in the surface ocean reflects the net metabolic balance between photosynthesis and respiration, i.e., the net community productivity (NCP). We present a new method for continuous shipboard measurements of O(2)/Ar by equilibrator inlet mass spectrometry (EIMS). From these measurements and an appropriate gas exchange parametrization, NCP can be estimated at high spatial and temporal resolution. In the EIMS configuration, seawater from the ship's continuous intake flows through a cartridge enclosing a gas-permeable microporous membrane contactor. Gases in the headspace of the cartridge equilibrate with dissolved gases in the flowing seawater. A fused-silica capillary continuously samples headspace gases, and the O(2)/Ar ratio is measured by mass spectrometry. The ion current measurements on the mass spectrometer reflect the partial pressures of dissolved gases in the water flowing through the equilibrator. Calibration of the O(2)/Ar ion current ratio (32/40) is performed automatically every 2 h by sampling ambient air through a second capillary. A conceptual model demonstrates that the ratio of gases reaching the mass spectrometer is dependent on several parameters, such as the differences in molecular diffusivities and solubilities of the gases. Laboratory experiments and field observations performed by EIMS are discussed. We also present preliminary evidence that other gas measurements, such as N(2)/Ar and pCO(2) measurements, may potentially be performed with EIMS. Finally, we compare the characteristics of the EIMS with the previously described membrane inlet mass spectrometry (MIMS) approach.

Improvements made to an established mass spectrometric method for measuring changes in atmospheric O 2 /N 2 are described. With the improvements in sample handling and analysis, sample throughput and analytical precision have both increased. Aliquots from duplicate flasks are repeatedly measured over a period of 2 weeks, with an overall standard error in each flask of 3-4 per meg, corresponding to 0.6-0.8 ppm O 2 in air. Records of changes in O 2 /N 2 from six global sampling stations (Barrow, American Samoa, Cape Grim, Amsterdam Island, Macquarie Island, and Syowa Station) are presented. Combined with measurements of CO 2 from the same sample flasks, land and ocean carbon uptake were calculated from the three sampling stations with the longest records (Barrow, Samoa, and Cape Grim). From 1994-2002, We find the average CO 2 uptake by the ocean and the land biosphere was 1.7 ± 0.5 and 1.0 ± 0.6 GtC yr -1 respectively; these numbers include a correction of 0.3 Gt C yr -1 due to secular outgassing of ocean O 2 . Interannual variability calculated from these data shows a strong land carbon source associated with the 1997-1998 El Nifio event, supporting many previous studies indicating that high atmospheric growth rates observed during most El Nino events reflect diminished land uptake. Calculations of interannual variability in land and ocean uptake are probably confounded by non-zero annual air sea fluxes of O 2 . The origin of these fluxes is not yet understood.

uptake observed was attributable to direct HCO3 uptake, the other half being direct CO2 uptake mediated either by passive diffusion or active uptake mechanisms. The increase in growth rates and decrease in CO2 concentration associated with the iron fertilization did not trigger any noticeable changes in the mode of DIC acquisition, indicating that under most environmental conditions the carbon concentrating mechanism (CCM) is constitutive. A low-CO2 treatment induced an increase in uptake of CO2, which we attributed to increased extracellular carbonic anhydrase activity, at the expense of direct HCO3 transport across the plasmalemma. Isotopic disequilibrium experimental results are consistent with Southern Ocean carbon stable isotope fractionation data from this and other studies. Although iron fertilization has been shown to significantly enhance phytoplankton growth and may potentially increase carbon flux to the deep ocean, an important source of the inorganic carbon taken up by phytoplankton in this study was HCO3 � , whose concentration is negligibly affected by the anthropogenic rise in CO2. We conclude that biological productivity in this region of the world’s ocean is unlikely to be directly regulated by natural or anthropogenic variations in atmospheric CO2 concentrations because of the presence of a constitutive CCM. INDEX TERMS: 4207 Oceanography: General: Arctic and Antarctic oceanography; 4806 Oceanography: Biological and Chemical: Carbon cycling; 4853 Oceanography: Biological and Chemical: Photosynthesis; 4870 Oceanography: Biological and Chemical: Stable isotopes; KEYWORDS: bicarbonate, phytoplankton, Southern Ocean

Diatoms are an important functional group of marine phytoplankton because of their role in the fixation of atmospheric carbon dioxide (CO 2 ) and transfer of organic carbon to deep waters. Carbon-concentrating-mechanisms, such as active CO 2 and bicarbonate (HCO - 3 ) uptake and carbonic anhydrase activity, are believed to be essential to marine photosynthesis, because the main carbon-fixing enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase, is less than half saturated at normal seawater CO 2 concentrations. On the basis of short-term inorganic 14 C uptake experiments, Tortell et al. (1997; Nature 390: 243-244) recently argued that marine diatoms are capable of HCO - 3 uptake. However, as discussed herein, the extent of HCO - 3 uptake cannot be assessed on the basis of these experiments. Using short-term 14 CO 2 -disequilibrium experiments, we show that a clone of the marine diatom Phaeodactylum tricornutum takes up little or no HCO - 3 even under conditions of severe CO 2 limitation. Predicting the response of the oceans to increased CO 2 concentrations will require, among other things, a careful assessment of the extent to which marine algae take up HCO - 3 or CO 2 . Because the plasmalemma of microalgae is gas permeable, all phytoplankton exchange CO 2 with the growth medium. Experimental results that are merely consistent with HCO - 3 uptake are insufficient to prove that HCO - 3 uptake is occurring. Our results are in accord with predictions based on stable carbon isotopic fractionation data. Combining isotopic disequilibrium experiments with continuous growth cultures and stable isotope fractionation experiments is a powerful tool for understanding the response of oceanic primary producers to anthropogenic CO 2 emissions as well as for interpreting paleoceanographic carbon isotope data.