Highlights

- A new gas tension device (GTD) improves on previous designs of GTDs.

- The new GTD can operate up to 1000 m depth with no pressure dependency.

- Two GTDs successfully measured total dissolved gases in an Oxygen Minimum Zone.

- The GTDs were accurate to ±0.6% compared with independent dissolved gas data.

Oxygen minimum zones (OMZs) play important roles in regulating the ocean's global carbon and nitrogen cycles. In these functionally anoxic waters, denitrifying and anammox (short for anaerobic ammonium oxidation) microbes remove nitrogenous nutrients from the biosphere by transformation to biologically unavailable nitrogen gas (N₂). A newly developed sensor can detect this "excess" N₂ in OMZ regions in order to quantify these nitrogen-loss processes. The near-term goal is to explore OMZs and collect high-quality excess N₂ data to document their baseline inventories. The long-term objective is to determine if excess N₂ inventories in OMZs are increasing as a result of ocean deoxygenation.

Oceanic bubbles play an important role in the air-sea exchange of weakly soluble gases at moderate to high wind speeds. A Lagrangian bubble model embedded in a large eddy simulation model is developed to study bubbles and their influence on dissolved gases in the upper ocean. The transient evolution of mixed-layer dissolved oxygen and nitrogen gases at Ocean Station Papa (50°N, 145°W) during a winter storm is reproduced with the model. Among different physical processes, gas bubbles are the most important in elevating dissolved gas concentrations during the storm, while atmospheric pressure governs the variability of gas saturation anomaly (the relative departure of dissolved gas concentration from the saturation concentration). For the same wind speed, bubble-mediated gas fluxes are larger during rising wind with smaller wave age than during falling wind with larger wave age. Wave conditions are the primary cause for the bubble gas flux difference: when wind strengthens, waves are less-developed with respect to wind, resulting in more frequent large breaking waves. Bubble generation in large breaking waves is favorable for a large bubble-mediated gas flux. The wave-age dependence is not included in any existing bubble-mediated gas flux parameterizations.

We propose a novel method for constructing a pseudo-synoptic view of estuarine features from non-synoptic observations captured by mobile platforms. The model-aided Lagrangian interpretation (MALI) method is based on relocating observations to a common reference moment in time along three-dimensional Lagrangian trajectories derived from a numerical model of estuarine circulation. The method relies on the model skill to capture large-scale circulation features, and on high-resolution in situ observations to characterize small-scale hydrographic structure. We demonstrate our technique by applying MALI to autonomous underwater vehicle observations in the Columbia River estuary, with the aid of a validated unstructured-grid finite-element numerical simulation. The method can be readily adapted to a broader range of environments, observational platforms, and model-data combinations.

To meet societal needs, modern estuarine science needs to be interdisciplinary and collaborative, combine discovery with hypotheses testing, and be responsive to issues facing both regional and global stakeholders. Such an approach is best conducted with the benefit of data-rich environments, where information from sensors and models is openly accessible within convenient timeframes. Here, we introduce the operational infrastructure of one such data-rich environment, a collaboratory created to support (a) interdisciplinary research in the Columbia River estuary by the multi-institutional team of investigators of the Science and Technology Center for Coastal Margin Observation & Prediction and (b) the integration of scientific knowledge into regional decision making. Core components of the operational infrastructure are an observation network, a modeling system and a cyber-infrastructure, each of which is described. The observation network is anchored on an extensive array of long-term stations, many of them interdisciplinary, and is complemented by on-demand deployment of temporary stations and mobile platforms, often in coordinated field campaigns. The modeling system is based on finiteelement unstructured-grid codes and includes operational and process-oriented simulations of circulation, sediments and ecosystem processes. The flow of information is managed through a dedicated cyber-infrastructure, conversant with regional and national observing systems.

We present a new physically based calibration equation for Aanderaa Inc. oxygen sensing optodes. We use the two site fluorescence quenching model of Demas et al. (1995) to describe the nonlinear Stern-Volmer response of the optode foil to oxygen partial pressure. Seven (minimally six) coefficients quantify foil response to oxygen and temperature; another quantifies response to hydrostatic pressure. These eight coefficients are related, theoretically, to basic physical properties of the foil. The equation provides a framework to study causes of variability and drift in optodes and to develop better quality control and handling procedures. We tested the equation using factory calibrations of 24 optode foils. When accurate multi-point calibration data are unavailable, two additional coefficients empirically correct the usually large differences observed between factory foil calibrations and post-factory laboratory/field calibrations; we cannot eliminate this major cause of uncertainty in optode calibrations.

Excluding two potentially anomalous foils, the calibration equation fits 13 similarly calibrated foils, totaling 455 calibration points over 3 – 40°C to –0.57 ± 1.48 mbar. Analysis of the resulting best fit coefficients reveals an underlying variability in optodes associated with variability in site 2 and site 1 Stern-Volmer coefficients of 32% and 20%, respectively. The fraction of unquenched fluorophores responding with the more accessible site 1 quenching characteristics varies by only 3%. The equation fits multi-point data for two optodes within manufacturer's specifications, the greater of ± 2.5 µmol kg^-1 and ± 1.5%. Detailed measurements of calibration changes over time will be required to understand the causes of optode drift.

The calibration accuracy and stability of three Aanderaa 3835 optodes and three Seabird SBE-43 oxygen sensors were evaluated over four years using in situ and laboratory calibrations. The sensors were mostly in storage, being in the ocean for typically only a few weeks per year and operated for only a few days per year. Both sensors measure partial pressure of oxygen, or equivalently saturation at standard pressure; results are expressed in this variable. It is assumed that sensor drift occurs in the oxygen sensitivity of the sensors, not the temperature compensation; this is well justified for the SBE-43 based on multiple calibrations.

Neither sensor had significant long-term drift in output when sampling anoxic water. Sensor output at 100% saturation differed from the factory calibrations by up to ±6% (averaging –2.3%±3%) for the SBE-43 and up to –12.6% for the optodes. The optode output at 100% saturation is well described by a single decaying exponential with a decay constant of ~2 yr and an amplitude of 28%. The mechanism of this drift is unknown, but is not primarily due to sensor operation. It may be different from that experienced by sensors continuously deployed in the ocean. Thus, although the optodes in this study did not have a stable calibration, their drift was stable and, once calibrated, allowed optode and SBE-43 pairs mounted on the same autonomous floats to be calibrated to an accuracy of ±0.4% over a 4-yr period.

The carbon system of the western Arctic Ocean is undergoing a rapid transition as sea ice extent and thickness decline. These processes are dynamically forcing the region, with unknown consequences for CO₂ fluxes and carbonate mineral saturation states, particularly in the coastal regions where sensitive ecosystems are already under threat from multiple stressors. In October 2011, persistent wind-driven upwelling occurred in open water along the continental shelf of the Beaufort Sea in the western Arctic Ocean. During this time, cold (<–1.2°C), salty (>32.4) halocline water — supersaturated with respect to atmospheric CO₂ (pCO₂ > 550 µatm) and undersaturated in aragonite (< 1.0) was transported onto the Beaufort shelf. A single 10-day event led to the outgassing of 0.18–0.54 Tg-C and caused aragonite undersaturations throughout the water column over the shelf. If we assume a conservative estimate of four such upwelling events each year, then the annual flux to the atmosphere would be 0.72–2.16 Tg-C, which is approximately the total annual sink of CO₂ in the Beaufort Sea from primary production. Although a natural process, these upwelling events have likely been exacerbated in recent years by declining sea ice cover and changing atmospheric conditions in the region, and could have significant impacts on regional carbon budgets. As sea ice retreat continues and storms increase in frequency and intensity, further outgassing events and the expansion of waters that are undersaturated in carbonate minerals over the shelf are probable.

The SOLAS air–sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive subpolar zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment was conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX's), SAGE was designed as a Lagrangian study, quantifying key biological and physical drivers influencing the air–sea gas exchange processes of CO₂, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom.

A dual tracer SF₆/³He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air–sea tracer exchange at tenths of kilometer scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describe air–sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties and windspeed were quantified to further assist the development of gas exchange models for high-wind environments.

Simultaneous observations of upper-ocean bubble clouds, and dissolved gaseous nitrogen (N₂) and oxygen (O₂) from three winter storms are presented and analyzed. The data were collected on the Canadian Surface Ocean Lower Atmosphere Study (C-SOLAS) mooring located near Ocean Station Papa (OSP) at 50N, 145W in the NE Pacific during winter of 2003/2004. The bubble field was measured using an upward looking 200 kHz echosounder. Direct estimates of bubble mediated gas fluxes were made using assumed bubble size spectra and the upward looking echosounder data.

A one-dimensional biogeochemical model was used to help compare data and various existing models of bubble mediated air-sea gas exchange. The direct bubble flux calculations show an approximate quadratic/cubic dependence on mean bubble penetration depth. After scaling from N₂/O₂ to carbon dioxide, near surface, nonsupersaturating, air-sea transfer rates, K_T, for U₁₀ > 12 m s^-1 fall between quadratic and cubic relationships. Estimates of the subsurface bubble induced air injection flux, V_T, show an approximate quadratic/cubic dependence on mean bubble penetration depth. Both K_T and V_T are much higher than those measured during Hurricane Frances over the wind speed range 12 < U₁₀ < 23 m s^-1. This result implies that over the open ocean and this wind speed range, older and more developed seas which occur during winter storms are more effective in exchanging gases between the atmosphere and ocean than younger less developed seas which occur during the rapid passage of a hurricane.

Measurements of the air–sea fluxes of N₂ and O<sub>2 were made in winds of 15–57 m s^-1 beneath Hurricane Frances using two types of air-deployed neutrally buoyant and profiling underwater floats. Two "Lagrangian floats" measured O2 and total gas tension (GT) in pre-storm and post-storm profiles and in the actively turbulent mixed layer during the storm. A single "EM-APEX float" profiled continuously from 30 to 200 m before, during and after the storm. All floats measured temperature and salinity. N2 concentrations were computed from GT and O2 after correcting for instrumental effects. Gas fluxes were computed by three methods. First, a one-dimensional mixed layer budget diagnosed the changes in mixed layer concentrations given the pre-storm profile and a time varying mixed layer depth. This model was calibrated using temperature and salinity data. The difference between the predicted mixed layer concentrations of O2 and N2 and those measured was attributed to air–sea gas fluxes FBO and FBN. Second, the covariance flux FCO(z) = < w>O2%u2032%u3009(z) was computed, where w is the vertical motion of the water-following Lagrangian floats, O2' is a high-pass filtered O2 concentration and <>(z) is an average over covariance pairs as a function of depth. The profile FCO(z) was extrapolated to the surface to yield the surface O2 flux FCO(0). Third, a deficit of O2 was found in the upper few meters of the ocean at the height of the storm. A flux FSO, moving O2 out of the ocean, was calculated by dividing this deficit by the residence time of the water in this layer, inferred from the Lagrangian floats. The three methods gave generally consistent results. At the highest winds, gas transfer is dominated by bubbles created by surface wave breaking, injected into the ocean by large-scale turbulent eddies and dissolving near 10-m depth. This conclusion is supported by observations of fluxes into the ocean despite its supersaturation; by the molar flux ratio FBO/FBN, which is closer to that of air rather than that appropriate for Schmidt number scaling; by O2 increases at about 10-m depth along the water trajectories accompanied by a reduction in void fraction as measured by conductivity; and from the profile of FCO(z), which peaks near 10 m instead of at the surface.

At the highest winds O2 and N2 are injected into the ocean by bubbles dissolving at depth. This, plus entrainment of gas-rich water from below, supersaturates the mixed layer causing gas to flux out of the near-surface ocean. A net influx of gas results from the balance of these two competing processes. At lower speeds, the total gas fluxes, FBO, FBN and FCO(0), are out of the ocean and downgradient.</sub>

Air–sea flux measurements of O₂ and N₂ obtained during Hurricane Frances in September 2004 using air-deployed neutrally buoyant floats reveal the first evidence of a new regime of air–sea gas transfer occurring at wind speeds in excess of 35 m s^-1. In this regime, plumes of bubbles 1 mm and smaller in size are transported down from near the surface of the ocean to greater depths by vertical turbulent currents with speeds up to 20–30 cm s^-1. These bubble plumes mostly dissolve before reaching a depth of approximately 20 m as a result of hydrostatic compression. Injection of air into the ocean by this mechanism results in the invasion of gases in proportion to their tropospheric molar gas ratios, and further supersaturation of less soluble gases. A new formulation for air–sea fluxes of weakly soluble gases as a function of wind speed is proposed to extend existing formulations to span the entire natural range of wind speeds over the open ocean, which includes hurricanes.

The new formulation has separate contributions to air–sea gas flux from: 1) non-supersaturating near-surface equilibration processes, which include direct transfer associated with the air–sea interface and ventilation associated with surface wave breaking; 2) partial dissolution of bubbles smaller than 1 mm that mix into the ocean via turbulence; and 3) complete dissolution of bubbles of up to 1 mm in size via subduction of bubble plumes. The model can be simplified by combining "surface equilibration" terms that allow exchange of gases into and out of the ocean, and "gas injection" terms that only allow gas to enter the ocean. The model was tested against the Hurricane Frances data set. Although all the model parameters cannot be determined uniquely, some features are clear. The fluxes due to the surface equilibration terms, estimated both from data and from model inversions, increase rapidly at high wind speed but are still far below those predicted using the cubic parameterization of Wanninkhof and McGillis at high wind speed. The fluxes due to gas injection terms increase with wind speed even more rapidly, causing bubble injection to dominate at the highest wind speeds.

The development and testing of a new, fast response, profiling gas tension device (GTD) that measures total dissolved air pressure is presented. The new GTD equilibrates a sample volume of air using a newly developed (patent pending) tubular silicone polydimethylsiloxane (PDMS) membrane interface. The membrane interface is long, flexible, tubular, and is contained within a seawater-flushed hose. The membrane interface communicates pressure to a precise pressure gauge using low dead-volume stainless steel tubing. The pressure sensor and associated electronics are located remotely from the membrane interface. The new GTD has an operating depth in seawater of 0–300 m. The sensor was integrated onto an upper-ocean mixed layer, neutrally buoyant float, and used in air–sea gas exchange studies. Results of laboratory and pressure tank tests are presented to show response characteristics of the device. A significant hydrostatic response of the instrument was observed over the depth range of 0–9 m, and explained in terms of expulsion (or absorption) of dissolved air from the membrane after it is compressed (or decompressed). This undesirable feature of the device is unavoidable since a large exposed surface area of membrane is required to provide a rapid response. The minimum isothermal response time varies from (2 ± 1) min near the sea surface to (8 ± 2) min at 60-m depth. Results of field tests, performed in Puget Sound, Washington, during the summer of 2004, are reported, and include preliminary comparisons with mass-spectrometric analysis of in situ water samples analyzed for dissolved N₂ and Ar. These tests served as preparations for deployment of two floats by aircraft into the advancing path of Hurricane Frances during September 2004 in the northwest Atlantic. The sensors performed remarkably well in the field. A model of the dynamical response of the GTD to changing hydrostatic pressure that accounts for membrane compressibility effects is presented. The model is used to correct the transient response of the GTD to enable a more precise measurement of gas tension when the float was profiling in the upper-ocean mixed layer beneath the hurricane.