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Biogeochemical-Argo (BGC-Argo) is a network of profiling floats carrying sensors that enable observation of as many as six essential biogeochemical and bio-optical variables: oxygen, nitrate, pH, chlorophyll a, suspended particles, and downwelling irradiance. This sensor network represents today's most promising strategy for collecting temporally and vertically resolved observations of biogeochemical properties throughout the ocean. All data are freely available within 24 hours of transmission. These data fill large gaps in ocean-observing systems and support three ambitions: gaining a better understanding of biogeochemical processes (e.g., the biological carbon pump and air–sea CO2 exchanges) and evaluating ongoing changes resulting from increasing anthropogenic pressure (e.g., acidification and deoxygenation); managing the ocean (e.g., improving the global carbon budget and developing sustainable fisheries); and carrying out exploration for potential discoveries. The BGC-Argo network has already delivered extensive high-quality global data sets that have resulted in unique scientific outcomes from regional to global scales. With the proposed expansion of BGC-Argo in the near future, this network has the potential to become a pivotal observation system that links satellite and ship-based observations in a transformative manner.

Continuous monitoring of dissolved oxygen, pH and carbon dioxide are of great importance in oceanography. Sensors are the optimal tools for in situ measurements from mobile platforms, like A utonomous U nderwater V ehicles (AUVs) or Argo profiling floats, and for shipboard deployments. A validation study of small, versatile, easy-to-use, stand-alone optodes is presented. Each analyte can be read out with the identical optoelectronics which greatly minimizes the costs of the hardware needed. Several deployments were performed to evaluate the applicability of the sensors. The deployments varied in terms of duration (profiling, long-term monitoring 5 days to 8 weeks) and environmental conditions (salinity: 6–33 PSS; temperature: 9–25 °C). A set of sensors was successfully deployed at a mooring buoy, in an aquaculture facility and in the Monterey Bay Aquarium Open Sea Exhibition. They were also integrated in an AUV and a profiling float. The performance of the optodes was evaluated in comparison with commercially available sensors for dissolved oxygen (Aanderaa Data Instruments AS, Sea-Bird Scientific, OxyGuard®), pH (Hach, Satlantic) and carbon dioxide (Turner design). The data collected by our optodes and the commercially available sensors is generally in good agreement showing that the new, compact sensor device in combination with sensor foils (pO 2 , pH, pCO 2 ) can be a valuable tool for many applications in oceanography. The data also revealed the importance of the calibration strategy since inappropriate calibration resulted in an offset in the measured parameter. The efficiency of simple biofouling protection strategy (copper guard) for prolong measurements in highly dynamic environments was also demonstrated.

The most accurate calculation of nitrate concentration from the ultraviolet (UV) absorption spectrum of seawater requires that the absorption signal due to bromide in seawater be removed before nitrate concentrations are computed. Recent work suggests that the UV absorption spectrum of bromide in seawater has a pressure dependence. Neglect of this signal could add a bias when nitrate concentrations at high pressure are computed from UV measurements. Laboratory tests were conducted to determine the pressure dependence of the bromide absorption in seawater. Our results confirm the existence of a pressure coefficient in the bromide spectrum. The percentage change in bromide molar absorptivity is wavelength and temperature independent. The effect of pressure on the absorptivity of sea salt (ESW), which is dominated by bromide ion, can therefore be calculated as ESW pressure=ESW 1 dbar∗ (1 – 0.0 26 ∗ Pressure (dbar)/1000) The correction amounts to an error of around 0.95 μM nitrate at 1000 dbar. The pressure correction should be used in the calculations of nitrate concentrations from UV absorption spectra at high pressures.

Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (Δ V ) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The Δ V of Tris in seawater media of salinity 35 (Δ V Tris ⁎ ) was determined between 10 and 30 °C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode directly exposed to solution. The effects of pressure on the potentiometric cell were quantified in aqueous HCl solution prior to measurements in Tris buffer. The experimentally determined Δ V Tris ⁎ were fitted to the equation Δ V Tris ⁎ = 4.528 + 0.04912 t where t is temperature in Celsius; the resultant fit agreed to experimental data within uncertainty of the measurements, which was estimated to be 0.9 cm − 3 mol − 1 . Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200 bar, and can be expressed as: ∆ pH Tris = − 4.528 + 0.04912 t P ln 10 RT . where T is temperature in Kelvin, R is the universal gas constant (83.145 cm 3 bar K − 1 mol − 1 ), and P is gauge pressure in bar. On average, pH of Tris buffer changes by approximately − 0.02 at 200 bar.

A recent analysis of full water column hydrographic data revealed a pH-dependent discrepancy between spectrophotometrically measured pH using purified meta-cresol purple and pH calculated from dissolved inorganic carbon (DIC) and total alkalinity (TA). The discrepancy (pHspec – pHTA,DIC) is approximately −0.018 and 0.014 at pH 7.4 and 8.2, respectively. This discrepancy has a wide range of implications for marine inorganic carbon measurements, such as establishing robust calibration protocols for pH sensors operating on profiling floats. Here, we conducted a series of lab based experiments to assess the magnitude of pH-dependent errors for spectrophotometric pH measurements in seawater by directly comparing its performance to pH measured by an Ion Sensitive Field Effect Transistor (ISFET) pH sensor known to have Nernstian behavior. Natural seawater was titrated with high CO2 seawater while simultaneously measuring pH using spectrophotometry and an ISFET sensor over a large range in pH (7–8.5) and temperature (5–30 °C). The two pH measurements were consistent to better than ±0.003 (range) at all temperatures except at 5 and 10 °C and very low and high pH, where discrepancies were as large as ±0.005. These results demonstrate that pH-dependent errors in spectrophotometric pH measurements can be rejected as the cause of the pH-dependent discrepancy between pHspec and pHTA,DIC. The cause of this discrepancy is thus likely due to our incomplete understanding of the marine inorganic carbon model that could include errors in thermodynamic constants, concentrations of major ions in seawater, systematic biases in measurements of TA or DIC, or contributions of organic compounds that are not accounted for in the definition of total alkalinity. This should be a research priority for the inorganic carbon community.

Characterization of several potentiometric cells without a liquid junction has been carried out in universal buffer, aqueous HCl, and artificial seawater media. The electrodes studied include Ion Sensitive Field Effect Transistor (ISFET) pH electrodes, and Chloride-Ion Selective Electrodes (Cl-ISE) directly exposed to the solution. These electrodes were compared directly to the conventional hydrogen electrode and silver-silver chloride electrode in order to report the degree to which they obey ideal Nernstian laws. These data provide a foundation for operating the ISFET|Cl-ISE pair in seawater as a pH sensor. In order to obtain the highest quality pH measurements from this sensor, its response to changes in pH and salinity must be properly characterized. Our results indicate near-ideal Nernstian response for both electrodes over a wide range of pH (2-12) and Cl(-) molality (0.01-1). We conclude that the error due to sub-Nernstian response of the cell ISFET|seawater|Cl-ISE over the range of seawater pH and salinity is negligible (0.0001 pH). The cross sensitivity of the Cl-ISE to Br(-) does not seem to be a significant source of error (0.003 pH) in seawater media in the salinity range 20-35.

Performance of autonomous pH sensors is evaluated by comparing in situ data to independent bench-top measurements of pH and to co-located pH, O2, and p CO 2 sensors. While the best practice is always to deploy a properly calibrated sensor, the lengthy time period required for sensor conditioning and calibration often results in sensor deployment without comprehensive calibration. Quality control (QC) procedures are examined to determine the errors associated with different in situ calibration approaches and lay a framework for best practices. Sensor packages employing the Honeywell Durafet remained stable across multiple deployments for over nine months. However, sensor performance was often limited by biofouling. Regional empirical relationships for estimating carbonate system parameters are shown to enable identification of otherwise indistinguishable sensor offset and drift when multiple sensor types are co-located. Uncertainty is determined by calibration approach and must be quantified on a case-by-case basis. Our results indicate that the Durafet is capable of accuracy, relative to a chosen reference, of better than 0.03 pH units over multiple months. Accuracy is improved when a robust shore-side calibration is performed, an independent means of QC is available throughout a deployment, and effective biofouling prevention measures are taken.

Long-term in situ pH measurements in the extreme conditions of the deep ocean remain a challenge for many oceanographers. Until recently pH in the deep ocean was measured by traveling to a specific location on the ocean surface, obtaining a seawater sample at a specific depth and analyzing the sample using pH indicator dye spectrophotometry. This process is laborious, time-consuming, expensive, and is incapable of providing the spatiotemporal pH data chemical oceanographers need to understand the processes that are contributing to ocean acidification. As such, the need for a robust and reliable in-situ deep ocean pH sensor is urgent. The Honeywell Durafet, an Ion Sensitive Field-Effect Transistor (ISFET), offers many advantages over current pH monitoring technologies and has been recently adapted for deep ocean monitoring on carousels, profiling floats and moorings. The chief concern in the development of the deep ocean capable Durafet is to maintain the accuracy of the sensor over the temperature and pressure range found in the deep ocean. The calibration of the Durafet will be described and field data will be presented.

Increasing atmospheric carbon dioxide is driving a long-term decrease in ocean pH which is superimposed on daily to seasonal variability. These changes impact ecosystem processes, and they serve as a record of ecosystem metabolism. However, the temporal variability in pH is observed at only a few locations in the ocean because a ship is required to support pH observations of sufficient precision and accuracy. This paper describes a pressure tolerant Ion Sensitive Field Effect Transistor pH sensor that is based on the Honeywell Durafet ISFET die. When combined with a AgCl pseudoreference sensor that is immersed directly in seawater, the system is capable of operating for years at a time on platforms that cycle from depths of several km to the surface. The paper also describes the calibration scheme developed to allow calibrated pH measurements to be derived from the activity of HCl reported by the sensor system over the range of ocean pressure and temperature. Deployments on vertical profiling platforms enable self-calibration in deep waters where pH values are stable. Measurements with the sensor indicate that it is capable of reporting pH with an accuracy of 0.01 or better on the total proton scale and a precision over multiyear periods of 0.005. This system enables a global ocean observing system for ocean pH.

This study examines the utility of combining pH measurements with other inorganic carbon parameters for autonomous mooring-based carbon cycle research. Determination of the full suite of inorganic carbon species in the oceans has previously been restricted to ship-based studies. Now with the availability of autonomous sensors for pH and the partial pressure of CO 2 ( p CO 2 ), it is possible to characterize the inorganic carbon system on moorings and other unmanned platforms. The indicator-based pH instrument, SAMI-pH, was deployed with an autonomous equilibrator-infrared p CO 2 system in Monterey Bay, California USA from June to August 2007. The two-month time-series show a high degree of short-term variability, with pH and p CO 2 changing by as much as 0.32 pH units and 240 μatm, respectively, during upwelling periods. The pH and salinity-derived alkalinity (A Tsalin ) were used to calculate the other inorganic carbon parameters, including p CO 2 , total dissolved inorganic carbon (DIC) and CaCO 3 saturation states. The calculated p CO 2 was within 2 μatm of the measured p CO 2 during the first day of the deployment and within 8 μatm over the first month. The DIC calculated from pH–A Tsalin and p CO 2 –A Tsalin were within 5 μmol kg −1 of each other during the first month. However, DIC calculated from pH– p CO 2 differed by ~ 50 μmol kg −1 from the other estimates over the same period, reflecting the sensitivity of the pH– p CO 2 calculation to measurement error. The data continued to diverge during the final month and this difference was likely driven by extensive biofouling. Because of the relative insensitivity of CO 3 2− concentration to these errors, aragonite saturation calculated from the pH– p CO 2 pair was within 0.15 of the pH–A Tsalin values over the entire deployment. These results show that in situ pH, when combined with other CO 2 parameters, can provide valuable insights into both data quality and inorganic carbon cycling.

We report on the first seawater tests at 1 atm of the Honeywell Durafet® pH sensor, a commercially available ion sensitive field effect transistor (ISFET). Performance of this sensor was evaluated in a number of different situations including a temperature-controlled calibration vessel, the MBARI test tank, shipboard underway mapping, and a surface mooring. Many of these tests included a secondary reference electrode in addition to the internal reference supplied with the stock Durafet sensor. We present a theoretical overview of sensor response using both types of reference electrode. The Durafet sensor operates with a short term precision of ± 0.0005 pH over periods of several hours and exhibits stability of better than 0.005 pH over periods of weeks to months. Our tests indicate that the Durafet pH sensor operates at a level of performance satisfactory for many types of biogeochemical studies at low pressure.

We describe a novel method for determination of carbonate ion concentration in seawater by acidimetric titration with UV detection. Because CO 3 2− absorbs light at wavelengths of less than ~ 250 nm, it is feasible to titrate most carbonate-containing natural waters with acid and observe an increase in %Transmittance. The observed signal is proportional to the concentration of carbonate ion in the original sample. Present technology permits a theoretical precision in the determination of [CO 3 2− ] in natural seawater background of ~ 0.5% (at 10 cm pathlength, 200 μM CO 3 2− , ± 0.0001 AU). The procedure has been tested at 1 and 10 cm pathlengths using single and multipoint titration methods, respectively. Results using natural seawater test solutions indicate a resolution in [CO 3 2− ] of 3.6% in a standard 1 cm cuvette using a very simple manual method, and 0.7% using a custom-built 10 cm closed titration cell. Estimates of the relative distribution of CO 3 2− between inner and outer sphere complexes with Mg 2+ and Na + were also determined and the equilibrium constants agree with published values. This method provides a new tool for studies of several fundamental aspects CO 2 chemistry, including the second dissociation constant of carbonic acid, CO 3 2− ion pairing, and can be used to directly measure the distribution of carbonate ion in seawater and many other types of natural waters.

Improvements in the data processing algorithm and calibration procedures have greatly increased the accuracy of nitrate measurements using an in situ ultraviolet spectrophotometer (ISUS). Two major changes in the algorithm involve application of a temperature-dependent correction to the bromide spectrum and then using the observed temperature and salinity to subtract the bromide component before fitting nitrate. By reducing the degrees of freedom in calculating nitrate concentrations, the accuracy of the ISUS instrument is substantially improved. The new algorithm was tested in environments ranging from the Southern Ocean to oligotrophic eastern Pacific seawater and found to be applicable at all temperatures and depths. The standard error of the estimate for regression between ISUS nitrate values and discrete samples measured by standard wet chemistry methods for the combined data set is reduced by greater than 2-fold (1.4 down to 0.65 µM) using the new algorithm. This corresponds to a 5-fold reduction in variance (2.0 down to 0.4 µM2). Although biofouling and calibration drift remain issues for any instrument deployed in situ for long periods of time, using the measured salinity and temperature to correct the ultraviolet spectra before the nitrate calculations will reduce the impacts of these confounding processes.

We developed an autonomous, in situ instrument to detect dissolved copper in seawater, suitable for deployment on time scales from weeks to a month. A commercially available in situ nitrate analyzer (YSI 9600) was adapted to measure copper (II) by chemiluminescence. Modifications included construction of a photomultiplier (PMT)–based detector and flow-cell, the use of more chemically resistant plastics for parts in contact with the reagents, addition of an online acidification step, and optimization of the method and flow parameters. Microsolenoid pumps take in sample and dispense reagent, standard, and blank solutions, which are stored in 1 L plastic bags. Filtration (0.45 im) and acidification (pH ~1.7) permit a measurement of total dissolved Cu(II). Calibration is achieved by periodically analyzing ligand-stabilized seawater standard and blank solutions stored at pH 8 and acidified online. Averaged over all in situ deployments, the instrument had a detection limit of 0.8 ± 0.3 nM, a sample precision of 11% (n = 612), and an accuracy of 17% (compared to discrete samples n = 27). The instrument is capable of functioning autonomously for 25 d when sampling every hour and calibrating every 6 h. Reagent consumption is the limiting factor.

In July 2002, a combination of underway mapping and discrete profiles revealed significant along-shore variability in the concentrations of manganese and iron in the vicinity of Monterey Bay, California. Both metals had lower concentrations in surface waters south of Monterey Bay, where the shelf is about 2.5 km wide, than north of Monterey Bay, where the shelf is about 10 km wide. During non-upwelling conditions over the northern broad shelf, dissolvable iron concentrations measured underway in surface waters reached 3.5 nmol L−1 and dissolved manganese reached 25 nmol L−1. In contrast, during non-upwelling conditions over the southern narrow shelf, dissolvable iron concentrations in surface waters were less than 1 nmol L−1 and dissolved manganese concentrations were less than 5 nmol L−1. A pair of vertical profiles at 1000 m water depth collected during an upwelling event showed dissolved manganese concentrations of 10 decreasing to 2 nmol L−1, and dissolvable iron concentrations of 12–20 nmol L−1 in the upper 100 m in the north, compared to 3.5–2 nmol L−1 Mn and ∼0.6 nmol L−1 Fe in the upper 100 m in the south, suggesting the effect of shelf width influences the chemistry of waters beyond the shelf. These observations are consistent with current understanding of the mechanism of iron supply to coastal upwelling systems: Iron from shelf sediments, predominantly associated with particles greater than 20 μm, is brought to the surface during upwelling conditions. We hypothesize that manganese oxides are brought to the surface with upwelling and are then reduced to dissolved manganese, perhaps by photoreduction, following a lag after upwelling. Greater phytoplankton biomass, primary productivity, and nutrient drawdown were observed over the broad shelf, consistent with the greater supply of iron. Incubation experiments conducted 20 km offshore in both regions, during a period of wind relaxation, confirm the potential of these sites to become limited by iron. There was no additional growth response when copper, manganese or cobalt was added in addition to iron. The growth response of surface water incubated with bottom sediment (4 nmol L−1 dissolvable Fe) was slightly greater than in control incubations, but less than in the presence of 4 nmol L−1 dissolved iron. This may indicate that dissolvable iron is not as bioavailable as dissolved iron, although the influence of additional inhibitory elements in the sediment cannot be ruled out.

Deckboard enrichment experiments were conducted during the US JGOFS Antarctic Environment Southern Ocean Process Study to determine the community response to added iron and zinc and their effect on phytoplankton growth. Seawater was collected into acid-cleaned 20-l polycarbonate carboys and incubated with varying additions of iron and zinc. Cells representing individual taxa were counted before and after incubation. Chlorophyll a, particulate organic carbon, and particulate organic nitrogen were measured periodically throughout the experiments. Zinc is not a limiting factor on phytoplankton growth in both the Ross Sea and the Antarctic Circumpolar Current (ACC). The Ross Sea is less iron-stressed than the ACC, in particular at the southern site. However, iron addition largely increased phytoplankton growth and chlorophyll a in both regions. Community growth showed a stronger response to iron addition in high-silicate water than in low-silicate water. Dinoflagellates had the lowest abundance in both natural and iron-enriched seawaters. Prymnesiophytes and pennate diatoms were most sensitive to iron enrichment and were responsible for the bulk of the growth signal. In the high-silicate water of the ACC, Phaeocystis showed the strongest response to iron addition, increasing its abundance from

A submersible osmotically pumped analyzer (Fe-OsmoAnalyzer) has been adapted for the long-term continuous high resolution monitoring of iron(II) and (III) in deep-sea hydrothermal vents. Ferrozine is used as the colorimetric reagent. The detection limit for our analyzer is 0.1 μM for a 0.7 cm pathlength with a linear response up to 50 μM. The Fe-OsmoAnalyzer uses periodic injections of standards and blanks to self calibrate and can operate unattended for over a year collecting data every 15 min. The Fe-OsmoAnalyzer was field tested during a 6 week test deployment at 900 m in Monterey Bay and a year-long deployment at 1100 m off the coast of Hawaii. A second year-long deployment in a low temperature hydrothermal vent on the Juan de Fuca Ridge provided high resolution continuous monitoring of Fe and was able to distinguish tidally influenced fluctuations in Fe concentrations.

Fluxes of dissolved cadmium were measured in situ using benthic flux chambers at stations in Los Angeles/Long Beach harbour and at sites on the California continental margin. Cadmium fluxes ranged from −0·212 to 0·118 μmol m−2 d−1indicating that Cd flux may either be into or out of sediments. Correlations between Cd flux and carbon oxidation rate and between carbon oxidation rate and sulfate reduction indicate that anaerobic microbial degradation was the major process controlling both the sign and the magnitude of cadmium fluxes at stations in Los Angeles and Long Beach harbours. A simple box model based on sediment fluxes and water column concentrations indicates cadmium has a residence time of 47 days within the Los Angeles/Long Beach harbour system, similar to the hydraulic residence time. Sedimentary flux is, therefore, sufficient to account for water column cadmium concentration in inner harbour areas, suggesting that the sediments are the dominant source and sink of cadmium in these areas of the harbour. Comparison of the cadmium: phosphate ratio for the Los Angeles/Long Beach harbour water column with those reported for the northeast Pacific suggests that distributions of Cd in the study area were elevated over expected values but appear to be dominated by natural physical and biogeochemical processes.

Oceanic Nitrogen Fixation Nitrogen fixation in the oceans is important in sustaining global marine productivity and balances carbon dioxide export to the deep ocean. It was previously believed that marine nitrogen fixation was due to a single genus of filamentous cyanobacteria, Trichodesmium . The recent discovery of unicellular open-ocean cyanobacteria has raised the question of how they contribute to global ocean nitrogen fixation and how they compare in distribution and activity to Trichodesmium . Using data collected from the southwest Pacific Ocean, Moisander et al. (p. 1512 , published online 25 February) show that the unicellular nitrogen-fixing cyanobacteria (UCYN-A and Crocosphaera watsonii ) have distinct ecophysiologies and distinct oceanic distributions from each other, and from Trichodesmium . These data can be incorporated into models to retune estimates of the global rates of oceanic nitrogen fixation and carbon sequestration.

The question of whether the plankton communities in low-nutrient regions of the ocean, comprising 80% of the global ocean surface area, are net producers or consumers of oxygen and fixed carbon is a key uncertainty in the global carbon cycle. Direct measurements in bottle experiments indicate net oxygen consumption in the sunlit zone, whereas geochemical evidence suggests that the upper ocean is a net source of oxygen. One possible resolution to this conflict is that primary production in the gyres is episodic and thus difficult to observe: in this model, oligotrophic regions would be net consumers of oxygen during most of the year, but strong, brief events with high primary production rates might produce enough fixed carbon and dissolved oxygen to yield net production as an average over the annual cycle. Here we examine the balance of oxygen production over three years at sites in the North and South Pacific subtropical gyres using the new technique of oxygen sensors deployed on profiling floats. We find that mixing events during early winter homogenize the upper water column and cause low oxygen concentrations. Oxygen then increases below the mixed layer at a nearly constant rate that is similar to independent measures of net community production. This continuous oxygen increase is consistent with an ecosystem that is a net producer of fixed carbon (net autotrophic) throughout the year, with episodic events not required to sustain positive oxygen production.

An on-line preconcentration method for the determination of cadmium in seawater was developed for graphite furnace atomic absorption spectrometry (GFAAS) using deuterium lamp background correction. The technique was adapted from a previously reported technique and was fully automated. Automated preconcentration of cadmium reduced the concentrations of matrix elements in samples to levels that did not interfere with the determination of cadmium at ultratrace levels. The soluble ligand, 1-pyrrolidinecarbodithioic acid (APDC), was mixed with the sample in a continuous flow system to form a non-polar metal complex which was absorbed onto a C18 column. The metal complex was then eluted from the column directly into the graphite furnace. The addition of an in-line nitric acid rinse step between sample load cycles allowed for the determination of cadmium with one elute cycle. Other modifications included laboratory purification of the ammonium acetate rinse solution, a non-buffered APDC ligand solution, and a computer-activated load cycle to reduce sample consumption. The average column recovery was 73% with a sample loading flow rate of 2.2 ml/min and 55 s preconcentration time. Sample throughput was approximately 15 samples/h with 2 ml of sample preconcentrated and analyzed every 4 min. The small sample volume required made this method useful where sample volumes are limited. The 3σ limit of detection for replicate blanks was 42 pM Cd (n=5) for a 2 ml sample. The effectiveness of the method was demonstrated for the determination of Cd in certified open ocean reference materials NASS-3 and NASS-4. Analysis of Cd in equatorial Pacific seawater demonstrated that the system produces oceanographically consistent results.

Flow injection analysis with chemiluminescence detection (FIA–CL) via the reaction of copper with 1,10-phenanthroline was used for the determination of copper complexation in seawater. A detection limit of 0.1 nM Cu in undiluted seawater without sample preconcentration was obtained. Copper speciation was assessed by titrating natural seawater ligands with added copper. This method was used to measure copper complexation throughout the water column in Monterey Bay, California. Two ligand classes were identified: a strong ligand (L1), ranging in concentration from 1.3 to 3.1 nM, and a weaker ligand (L2), ranging from 9.4 to 25.6 nM. The average conditional stability constant for the strong ligand was log K 1 ′ 13.1±0.4 and for the weaker ligand log K 2 ′ 9.4±0.6. These measurements are in good agreement with complexation studies previously performed using established methods. This technique may eventually be adapted for in situ complexation measurements that can be used to monitor natural spatial and temporal variations in copper complexation, as well as coastal ecosystem responses to urban and industrial change.

Dissolved ( 5 μm, leachable and refractory) trace elements were measured during the IronEx I and PlumEx experiments in October and November 1993 near the Galapagos Islands. Iron was measured in the enriched patch and at control stations over a 9 day period following fertilization. The dissolved iron was initially depleted at a rate that gave an iron half-life of 28–40 h. The loss rate gradually decreased, and dissolved iron concentrations did not decrease below about 0.25 nmol kg-1 throughout the experiment. These results were most consistent with a kinetic model that was second order in iron concentration for the scavenging removal of each iron fraction. Other trace elements measured did not change significantly either in concentration or partitioning during the IronEx I experiment. Biological production tracked iron concentrations over time, which suggests that productivity within the fertilized patch was regulated by the availability of iron. The PlumEx study consisted primarily of two transects, a meridional course to the east of the Galapagos Islands and a zonal section to the west. Surface dissolved iron was very low at all stations except those near the Galapagos Islands. Sections of trace metal and other properties distinctly confirm the upwelling of the Equatorial Undercurrent as it intersects the islands. This upwelling has a great effect on the surface distributions of the trace metals on the west side of the islands. Productivity within the mixed layer of this region is highly correlated with iron and nitrate. West of the islands highest productivity is found at stations with the highest iron. North of the Equatorial front, in nitrate-depleted waters, elevated iron concentrations do not enhance productivity.

Manganese oxidation rates were determined at low (∼ 20 nM) concentrations in seawater by measuring dissolved manganese (Mn(II)) using flow injection analysis with chemiluminescence detection. Mn(II) was measured in samples that had been filtered (0.2 μm) and kept in the dark under controlled temperature and pH conditions for time periods up to 6 months. Eight 9 L carboys with mean pH values ranging from 8 to 8.7 were held at 25°C, another carboy (pH = 9.32) was kept at 5°C. Oxidation followed the Morgan (1967) homogeneous rate equation (d[Mn(II)]/dt = k1 [O2][OH−]2[Mn(II)]). The mean rate constant k1 = 1.7 ± 0.7 × 1012 M−3 d−1 (95% CI), determined using hydroxide ion activities determined with pH measurements on the NBS scale, was in agreement with work by Morgan (1967; k1 = 4 × 1012 M−3 d−1) and Davies and Morgan (1989; k1 = 1.1 × 1012 M−3 d−1) in dilute solutions. The rate constant at 5°C was 1.3 ± 0.3 × 1012 M−3 d−1. If free hydroxide concentrations (based on the free proton pH scale) are used, then the rate constant at 25°C was k 1 ∗ = 0.34 ± 0.14 × 10 12 M−3d−1. Autocatalytic increases in Mn(II) oxidation rates, as predicted by a heterogeneous reaction mechanism (Morgan, 1967) (d[Mn(II)]/dt= k2′[Mn(II)][MnO2]) were not observed, indicating that the homogeneous reaction dominates Mn(II) oxidation at low nM concentrations in seawater. Bacteria were enumerated by 4′,6-diamidino-2-phenylindole (DAPI) staining during the experiments. No significant correlation between bacterial concentrations and Mn(II) oxidation rates was found.

A continuous flow analyzer powered by osmotic pumps has been developed to monitor nitrate concentrations while submerged in freshwater or seawater. Osmotic pumps driven by a NaCl gradient propel both sample and reagents through a miniature continuous flow manifold. The analyzer operates with sample and reagent flow rates of about 12 and 1 μL h -1 , respectively (≃8 and 0.7 mL month -1 ), shows a linear response from 0 to 20 μM nitrate with a detection limit of 0.1 μM, and has a 90% response time of approximately 30 min. The analyzer can automatically standardize itself by correcting for baseline and sensitivity drift by periodically injecting and analyzing known standard solutions. The system has operated continuously for month-long periods in freshwater and seawater and can be easily adapted to a variety of colorimetric analyses

A sensitive technique for the shipboard determination of zinc in seawater has been developed. The technique couples flow injection analysis with fluorometric detection (FIA-FL). A cation exchange column was used to separate zinc from interering alkali and alkaline earth ions and to concentrate Zn from seawarer. The organic indicator ligan, p-tosyl-8-aminoquinoline, was used to form a complex with zinc, the fluorescence of which was determined with a flow-through fluorometer. The deterction limit (defined as three times the standard deviation of the blank, n=4) was 0.1 nM for a 4.4-mL sample

In situ measurements of dissolved manganese and total dissolved iron were conducted in hydrothermal plumes over the Juan de Fuca Ridge using a submersible chemical analyzer (Scanner). The Scanner was deployed as part of a CTD/transmissometer/rosette instrument package on both tow-yos and vertical casts during the VENTS Leg I cruise in 1989. Dissolved manganese and total dissolved iron concentrations, along with temperature and light attenuation anomalies, were determined over the ridge crest every 5 s. Discrete samples for laboratory analyses of dissolved iron II, total dissolved iron II+III and manganese were also collected. Metal to heat ratios (Me:Q) measured in situ were extremely variable in one steady state plume, while an event plume had constant Me:Q. Uniform values of Mn:Q in the event plume demonstrate that Mn behaves conservatively in the near-field plume. Variability in the Mn: Q ratios in a steady state plume indicated the presence of at least two hydrothermal sources with distinct Me:Q values. A simple mixing model shows that the contribution of Mn from high Me:Q sources, with a composition characteristic of black smoker vents, varies between 1% and 99% within the core of the steady state plume with an average value of 55%. On average, over 50% of the excess heat within the plume originates from low Me:Q ratio sources, with a composition characteristic of low-temperature, diffuse flow vent fluids. Less than 4% of the volume of hydrothermal fluids in the plume originates from black smokers. The Fe II concentrations were used to provide an estimate of plume age on a transect across the ridge axis. Plume ages were about 2.5 days on axis and >12 days off axis. These plume ages were modeled to provide estimates of plume transport and horizontal diffusion and show excellent agreement with ages determined using 222Rn.

Dissolved copper can he rapidly determined in sea water by using a modification of a chemiluminescence method. The determination is based on the formation of a complex between copper and 1,10-phenanthroline and the subsequent emission of light during the oxidation of the complex by hydrogen peroxide. A flow-injection manifold was constructed which provides for the separation of copper from the sea water matrix with a column of immobilized 8-hydroxyquinoline. The column eluant was injected into a carrier stream, mixed with the reagents and delivered to a flow cell where emitted light was detected and used to quantify the amount of copper in the sample. The technique is noteworthy for its low detection limit (s 0.4 nM), the rapidly of sample processing (approximately 8 min sample−1), the small sample volumes required (approximately 4 ml), and the minimal precautions necessary for the prevention of sample contamination during shipboard processing.

A spectrophotometric method using the complexing reagent 1-(2-pyridylazo)-2-naphthol (PAN) was developed for the determination of dissolved manganese in seawater. A detection limit of approximately 22 nM was obtained when the analyses were performed in situ with a submersible chemical analyzer (Scanner). Color formation was rapid (less than 1 s), which permitted the technique to be used in flow injection and continuous flow systems. An investigation of possible interferences revealed that the only significant interferent to manganese determinations in seawater (hydrothermal plumes) was iron. The iron-specific chelating agent desferrioxamine B was used to mask this interference. Temperature and pressure did not adversely affect the sensitivity of the technique, which made it amenable to applications in situ to depths of 3000 m. This method was tested by measuring dissolved manganese concentrations in situ in submarine hydrothermal plumes using a Scanner. Scanner deployments over the Juan de Fuca Ridge produced the first near real time analyses of manganese from a hydrothermal plume environment. Estuarine samples have also been successfully analyzed in the laboratory and the results confirmed by an independent method.

Seasonal (spring and summer) depletion of dissolved inorganic carbon occurs in the surface mixed layers of most of the unproductive or oligotrophic regions of the world's oceans. The missing dissolved inorganic carbon is thought to be converted to particulate carbon by photosynthesis, yet there are seemingly insufficient dissolved nutrients present to support the required amount of plant growth. A possible nutrient supply mechanism has now been identified in a study in the North Pacific. Short-lived transport events — lasting less than 10 days — connect the nutrient-poor surface waters with deep-water stocks of dissolved nitrates. When the water column from the surface to a depth of 250 metres is considered as a whole, there is near equivalence in nutrient supply and demand. Concentrations of dissolved inorganic carbon (DIC) decrease in the surface mixed layers during spring and summer in most of the oligotrophic ocean. The missing DIC is thought to be converted to particulate carbon by photosynthesis, but known mechanisms do not seem to supply enough nutrients for the photosynthesis. Here it is shown that short-lived transport events connect deep-water nitrate stocks with nutrient-poor surface waters. Concentrations of dissolved inorganic carbon (DIC) decrease in the surface mixed layers during spring and summer in most of the oligotrophic ocean. Mass balance calculations require that the missing DIC is converted into particulate carbon by photosynthesis1,2,3. This DIC uptake represents one of the largest components of net community production in the world ocean2,4. However, mixed-layer waters in these regions of the ocean typically contain negligible concentrations of plant nutrients such as nitrate and phosphate3,5. Combined nutrient supply mechanisms including nitrogen fixation, diffusive transport and vertical entrainment are believed to be insufficient to supply the required nutrients for photosynthesis6,7. The basin-scale potential for episodic nutrient transport by eddy events is unresolved8,9. As a result, it is not understood how biologically mediated DIC uptake can be supported in the absence of nutrients. Here we report on high-resolution measurements of nitrate (NO3−) and oxygen (O2) concentration made over 21 months using a profiling float deployed near the Hawaii Ocean Time-series station in the North Pacific subtropical gyre. Our measurements demonstrate that as O2 was produced and DIC was consumed over two annual cycles, a corresponding seasonal deficit in dissolved NO3− appeared in water at depths from 100 to 250 m. The deep-water deficit in NO3− was in near-stoichiometric balance with the fixed nitrogen exported to depth. Thus, when the water column from the surface to 250 m is considered as a whole, there is near equivalence between nutrient supply and demand. Short-lived transport events (

The light is emitted by the reaction of brillant sulfoflavin with hydrogen peroxide and Fe(II) in a neutral medium. A cation-exchange column is used to preconcentrate the iron from seawater. The detection limit was 0.45 nmol/L when 4.4 mL of sample was passed through the column

Flow-injection analysis (FIA) with chemiluminescence detection was used to determine manganese in sea water. The oxidation of 7,7,8,8-tetracyanoquinodimethane in an alkaline solution produces light. Manganese(II) catalyzes this reaction and its concentration can be determined by measuring the rate of photon emission. A column containing 8-hydroxyquinoline immobilized on a solid support was used in-line in the FIA system to concentrate manganese. The analysis time was 6 min per sea-water sample with a corresponding detection limit of 0.1 nM (3σ). Lower detection limits can be obtained by increasing the amount of manganese loaded onto the column. The relative standard deviation of analyses of sea water containing 1–10 nM manganese was typically 3%. Analyses of North Atlantic Surface Sea Water (NASS-1) and Coastal Atlantic Surface Sea Water (CASS-1) standards gave manganese concentrations within the accepted ranges, confirming the accuracy of the technique. Shipboard determinations of manganese produced excellent agreement with previous reliable data. This technique is widely applicable to a variety of natural water and process-oriented studies.

[1] Here we report measurements of iron and aluminum in surface and subsurface waters during late March and late May of 2001 on transects between central California and Hawaii. A large cloud of Asian dust was detected during April 2001, and there was a clear signal in surface water iron due to aerosol deposition on the May transect. Iron and aluminum concentrations increased synchronously by 0.5 and 2 nM along the southern portion of the transect, which includes the Hawaii Ocean Time series (HOT) station, from background values in March (0.1 to 0.2 nM Fe). These changes occurred in a ratio that is close to the crustal abundance ratio of the metals, which indicates a soil aerosol source. A vertical profile of dissolved iron was also measured at the HOT station in late April and this profile also shows a large increase near the surface. Direct observations of aerosol iron concentration at Mauna Loa Observatory on Hawaii indicate that aerosol concentrations were significantly lower than climatological values during this period. Soil aerosol concentrations along the transect were estimated using the real-time Navy Aerosol Analysis and Prediction System (NAAPS). The NAAPS results show a large meridional gradient with maximum concentrations in the boundary layer north of 30°N. However, the deposition of iron and aluminum to surface waters was highest south of 25°N, near Hawaii. There were only weak signals in the ecosystem response to the aerosol deposition.

The effect of Ocean Acidification (OA) on marine biota is quasi-predictable at best. While perturbation studies, in the form of incubations under elevated pCO 2, reveal sensitivities and responses of individual species, one missing link in the OA story results from a chronic lack of pH data specific to a given species' natural habitat. Here, we present a compilation of continuous, high-resolution time series of upper ocean pH, collected using autonomous sensors, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. These observations reveal a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The nature of the observed variability was also highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO 2, often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100. Our data provide a first step toward crystallizing the biophysical link between environmental history of pH exposure and physiological resilience of marine organisms to fluctuations in seawater CO 2. Knowledge of this spatial and temporal variation in seawater chemistry allows us to improve the design of OA experiments: we can test organisms with a priori expectations of their tolerance guardrails, based on their natural range of exposure. Such hypothesis-testing will provide a deeper understanding of the effects of OA. Both intuitively simple to understand and powerfully informative, these and similar comparative time series can help guide management efforts to identify areas of marine habitat that can serve as refugia to acidification as well as areas that are particularly vulnerable to future ocean change. © 2011 Hofmann et al.

Stoichiometric association constants, which have been measured for the ion pairs of Cl − and SO 4 −2 with Na + , K + , Mg 2+ , and Ca 2+ , were used to determine the speciation in an artificial sea water containing only these ions. The resulting distribution is quite different to that found in earlier models in which chloride ion association was ignored. The concentrations of chloride ion pairs with the cations are 4 to 5 times larger than the concentrations of the sulphate ion pairs with the same cations. The total activity coefficients of the neutral salts in sea water calculated from the model are in good agreement with the experimentally measured values. The concentration of MgSO 4 0 calculated to be present agrees with the amount determined from ultrasonic absorption data. The calculated solubility of gypsum is also in good agreement with the measured values.

The mineral rhodochrosite (MnCO3) causes rapid precipitation of calcium carbonate when it is added to seawater. The calcium carbonate forms a solid solution of the type Mn1−x Cax CO3 on the rhodochrosite surface. As a result, seawater becomes undersaturated with respect to pure calcite. The degree of saturation ranged from 0.42 to 0.10, and it is a function of the solid to liquid ratio. Seawater that has been equilibrated with rhodochrosite can, therefore, be concentrated substantially before it again becomes supersaturated with respect to calcite. Potential calcium carbonate scale is also reduced by equilibrating seawater with rhodochrosite.

A reagent-injection flow technique, which allows automatic control of the volume of reagent to be injected, is described. The sensitivity of the measurements can be adjusted over a wide range by changing the injection volume. This technique also eliminates problems in photometric determinations arising from refractive index effects and from light scattering by suspended particles in the sample. Consequently, absorbances as small as 0.00004 can be measured. The capabilities of this reagent-injection flow technique were tested by using it to determine hydrogen peroxide in seawater at nanomolar levels with photometric detection. The concentration of hydrogen peroxide was determined as the colored condensation product of N-ethyl-N-(sulfopropyl)aniline and 4-aminoantipyrene; the detection limit was 12 nM.

The solubility product of rhodochrosite (MnCO 3 ) was measured in seawater, deionized water and dilute NaCl solutions. The solubility product extrapolated to infinite dilution at 25.0 C was (2.60 ± 0.07)× 10 −11 . The stoichiometric solubility product measured in seawater of 34.27%. salinity was (3.24 ± 0.23) × 10 −9 at 25.0 C and (2.28 ± 0.24) × 10 −9 at 3.3 C. The stoichiometric solubility product is in good agreement with the value calculated from an ion association model. The enthalpy of the reaction is in fair agreement with the estimated value.

The concentrations of a suite of redox reactive chemicals were measured in the Rose Garden hydrothermal vent field of the Galapagos spreading center. Sulfide, silicate, oxygen and temperature distributions were measured in situ with a submersible chemical analyser. In addition, 15 chemical species were measured in discrete samples. Variability in the slope of the temperature-silicate plots indicates that heat is lost from these relatively low temperatures (

Dosage du silicate dans l'eau de mer par la methode au bleu de molybdene. Analyse de 80 echantillons a l'heure

The trace metals cobalt and copper are removed from the oceans interior by scavenging on to particle surfaces1, but the mechanisms for removal of these two metals are probably quite different. Cobalt appears to be scavenged by manganese oxide particles1,2, whereas organic compounds are the main carrier phase for copper3,4. Remobilization of these metals in marine sediments therefore proceeds by different pathways. The differences in the pathways of remobilization are accentuated in oxygen-deficient environments: manganese oxide reduction is accelerated at low oxygen levels5 and organic carbon is preserved6. Cobalt fluxes from sediments underlying oxygen-deficient waters should be enhanced and copper fluxes reduced. We report here measurements of the cobalt and copper distributions in the waters of an oxygen-deficient marine basin in the Southern California Bight. Cobalt concentrations near the bottom are raised four times above the background level, whereas copper concentrations show no increase. These measurements confirm features of existing models for the oceanic cycles of these metals.

A reliable and sensitive technique for the rapid determination of ammonia in seawater is described. The procedure is based on the conversion of NH 4 + in seawater to NH3 and the subsequent diffusion of NH3 across a hydrophobic membrane using flow-injection analysis. Sixty determinations can be made per hour on a flowing stream of seawater or discrete samples. The lower limit of detection is 0.05 μM. Results of two applications that demonstrate the potential of this technique are presented, a laboratory excretion experiment employing the crabPachygrapsus crassipes, and small-scale vertical mapping of ammonia in the ocean.

Flow injection analysis (FIA) was used to automate the determination of cobalt in seawater by the Co-enhanced chemiluminescent oxidation of gallic acid in alkaline hydrogen peroxide. A preconcentration/separation step in the FIA manifold with an in-line column of immobilized 8-hydroxyquinoline was included to separate the Co from alkaline-earth ions. One sample analysis takes 8 min, including the 4-min sample load period. The detection limit is approximately 8 pM. The average standard deviation of replicate analyses at sea of 80 samples was +/- 5%. The method was tested and intercalibrated on samples collected off the California coast.

The precision of pH measurements of seawater relative to the pH 7.413 NBS buffer with various glass electrode—reference electrode pairs was found to be ±0.010 pH at the 95% confidence level. The differences between the residual liquid junction potentials of reference electrodes having ceramic frit, asbestos fibre, or ground glass sleeve liquid junctions was at the maximum 0.006 pH while the total difference between measurements with various electrode pairs was 0.016 pH. A change to a pH scale designed to minimize residual liquid junction potentials would, therefore, result in little improvement in the reproducibility of pH measurements relative to those made against the NBS buffer scale. The carbonate alkalinity obtained by the method of Culberson, Pytkowicz and Hawley (Journal of Marine Research, 1970, 28, 15–21) and that by a Gran (Analyst, 1952, 77, 661–671) titration of seawater showed both methods have similar accuracy and precision. The total CO2 measured and that calculated from the pH and carbonate alkalinity agreed well.

Rate constants for the hydration and dehydration reactions of carbon dioxide with water and with hydroxyl ion were measured in seawater by a pH-stat method at salinities (X lO:l) from 3.4 to 37.06 at 25°C and from 5” to 35°C at a salinity of 33.77. Transition state theory was used to extrapolate the rate constants to infinite dilution, with the following results at 25°C: kcoz = 0.037 ? 0.002. s-l, /COII-&~ = (7.1 + 0.7) x lo-l1 mol.dm-‘. s-‘, k,, = (7.6 + 1.1) x lo4 dm”* mol-’ * s-’ , and kIIcoB- = (1.8 f 0.3) x 10m4* s-l. The corresponding values in scawatcl of 33.77 salinity and 25°C calculated from equations fit to the experimental data are 0.036, 13.5 x 10 I’, 3.5 x 10d, and 1.40 x 10-4.

The activity of NaCl in artificial seawater was measured potentiometrically with Na+- and Cl− -sensitive electrodes. The salinity of the solutions, examined at 25°C, ranged from 10–40‰ salinity. The change in the activity from 5–25°C was measured at 35‰ salinity. The molal mean activity coefficient of NaCl in 35‰ seawater at 25°C is 0.667. The relative partial molal enthalpy of NaCl in 35‰ seawater is −130 ±50 cal mol−1. This value is in good agreement with the value measured in pure 0.72 M NaCl. The results were compared with activity coefficients predicted by a specific interaction model and by an ion association model. Good agreement was found in both cases.

Flow injection sample processing is used with fluorescence detection for the determination of total primary amines in seawater and nectar. The effects of carrifer stream flow rate and dispersion tube length on sensitivity and sampling rates were studied. Relative responses of several amino acids and other primary amines were determined using two dispersion tube lengths. Linear calibration curves were obtained over the ranges 0–10 -6 M and O–1O -5 M glycine. Precisions of better than 2% at 10 -6 M and a detection limit of 1 × lO -8 M glycine were obtained. Applications to the analysis of seawater from the vicinity of a baited lobster trap and diluted nectar samples from Erythrina sp. are described.