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Seagrass meadows are some of the most productive ecosystems in the world and could help to mitigate the increase of atmospheric CO2 from human activities. However, understanding the role of seagrasses in the global carbon cycle requires knowledge of air–sea CO2 fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocities were determined using the 3He and SF6 dual tracer technique in a seagrass ecosystem in south Florida, Florida Bay, near Bob Allen Keys (25.02663∘ N, 80.68137∘ W) between 1 and 8 April 2015. The observed gas transfer velocity, normalized for CO2 in freshwater at 20 ∘C, k(600), was 4.8 ± 1.8 cm h−1, which was lower than that calculated from published wind speed/gas exchange parameterizations. The deviation in k(600) from other coastal and offshore regions was only weakly correlated with tidal motion and air–sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in the study area. A new wind speed/gas exchange parameterization is proposed (k600=0.143u102), which might be applicable to other seagrass ecosystems and wind-fetch-limited environments.

The Earth’s climate is strongly affected by the partitioning of carbon between its mobile reservoirs, primarily between the atmosphere and the ocean. The distribution between the reservoirs is being massively perturbed by human activities, primarily due to fossil fuel emissions, with a range of consequences, including ocean warming and acidification, sea-level rise and coastal erosion, and changes in ocean productivity. These changes directly impact valuable habitats in many coastal regions and threaten the important services the habitats provide to mankind. Among the most productive and diverse systems are coral reefs and vegetated habitats, including saltmarshes, seagrass meadows, and mangroves. Coral reefs are particularly vulnerable to ocean warming and acidification. Vegetated habitats are receiving heightened attention for their ability to sequester carbon, but they are being impacted by land-use change, sea-level rise, and climate change. Overall, coasts play an important, but poorly quantified, role in the global cycling of carbon. Carbon reservoirs on land and in the ocean are connected through the so-called land–ocean aquatic continuum, which includes rivers, estuaries, and the coastal ocean. Terrestrial carbon from soils and rocks enters this continuum via inland water networks and is subject to transformations and exchanges with the atmosphere and sediments during its journey along the aquatic continuum. The expansive permafrost regions, comprised of ground on land and in the seabed that has been frozen for many years, are of increasing concern because they store vast amounts of carbon that is being mobilized due to warming. Quantitative estimates of these transformations and exchanges are relatively uncertain, in large part because the systems are diverse and the fluxes are highly variable in space and time, making observation at the necessary spatial and temporal coverage challenging. But despite their uncertainty, existing estimates point to an important role of these systems in global carbon cycling.

Seagrass meadows are one of the most productive ecosystems in the world and could play a role in mitigating the increase of atmospheric CO2 from human activities. Understanding their role in the global carbon cycle requires knowledge of air-sea CO2 fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocity and its controlling processes were examined in a seagrass ecosystem in south Florida. Gas transfer velocity was determined using the 3He and SF6 dual tracer technique in Florida Bay near Bob Allen Keys (25.02663° N, 80.68137° W) between 3 and 8 April 2015. The observed gas transfer velocity normalized for CO2 in freshwater at 20 °C, k(600), was 4.8 ± 1.8 cm h-1. The result gas transfer velocities were lower than previous experiments in the coastal and open oceans at the same wind speeds. Therefore, using published wind speed/gas exchange parameterizations would overpredict gas transfer velocities and CO2 fluxes in this area. The deviation in k(600) from other settings was weakly correlated to tidal motion and air-sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in this area. A new wind speed/gas exchange parameterization is proposed (k(600)=0.125u102), which might be applicable to other seagrass ecosystems and wind fetch limited environments.

Blue carbon ecosystems, including mangroves, saltmarshes, and seagrasses, mitigate climatechange by storing atmospheric carbon. Previous blue carbon research has focused on organic carbonstocks. However, recent studies suggest that lateral inorganic carbon export might be equally important.Lateral export is a long-term carbon sink if carbon is exported as alkalinity (TAlk) produced via sulfatereduction coupled to pyrite formation. This study evaluates drivers of pyrite formation in blue carbonecosystems, compares pyrite production to TAlk outwelling rates, and estimates global pyrite stocks inmangroves. We quantified pyrite stocks in mangroves, saltmarshes, and seagrasses along a latitudinalgradient on the Australian East Coast, including a mangrove dieback area, and in the Everglades(Florida, USA). Our results indicate that pyrite stocks were driven by a combination of biomass, tidalamplitude, sediment organic carbon, sediment accumulation rates, rainfall, latitude, temperature, andiron availability. Pyrite stocks were three-times higher in mangroves (103 ± 61 Mg/ha) than in saltmarshes(30 ± 30 Mg/ha) and seagrasses (32 ± 1 Mg/ha). Mangrove pyrite stocks were linearly correlated toTAlk export at sites where sulfate reduction was the dominant TAlk producing process. However, pyritegeneration could not explain all TAlk outwelling. We present the first global model estimating pyritestocks in mangroves, giving a first-order estimate of 197 Mg/ha (RMSE = 24 Mg/ha). In mangroves,estimated global TAlk production coupled to pyrite formation (∼3 mol/m2/y) is equal to ∼24% of theirglobal carbon burial rate, highlighting the importance of including TAlk export in future blue carbonbudgets.

Carbon dioxide removal (CDR) has become part of the portfolio of solutions to mitigate climate change. In combination with emission reductions, CDR may be critical to achieving the goal of limiting global warming to below 2°C, as outlined in the Paris Agreement. Due to its potential high productivity and environmental co-benefits, macroalgae cultivation has recently become a prominent ocean-based CDR strategy. However, estimates of the CDR potential of large-scale deployment are highly limited. Here we simulate idealized global deployment of macroalgae-based CDR using the NEMO-PISCESv2 ocean biogeochemical model at high spatial resolution (0.25° nominal horizontal resolution). Macroalgae growth is confined to the upper 100m of the water column in Exclusive Economic Zones (EEZ) free of sea ice and with an appropriate nitrate/phosphate regime. Although the loss of dissolved inorganic carbon (DIC) through macroalgal growth enhances the flux of atmospheric carbon into the ocean, this increase in carbon uptake is less than the rate of macroalgal production. In the absence of any nutrient limitation on growth, the enhancement in ocean carbon uptake is only 73-77% of the carbon lost from the water column due to macroalgal production. However, when macroalgae nutrient limitation/uptake is additionally accounted for, the increase in ocean carbon uptake accounts for only 41-42% of the potential carbon lost through macroalgae production. These inefficiencies are due to ocean transport replacing part of the DIC lost in the upper water column with DIC from depth, the influence of local nutrient concentrations on the vertical profile of macroalgal production, and feedbacks on the nutrient resources available for phytoplankton net primary production. CDR efficiency is shown to scale near-linearly between scenarios assuming 1% to 10% of the global EEZ area is cultivated for macroalgae. The efficiency of macroalgal CDR shows significant regional variability, with much of the enhancement in ocean carbon uptake (43%-46%) occurring outside EEZs, posing potential difficulties to national scale accounting.

Predicting the responses of animals to environmental changes is a fundamental goal of ecology and is necessary for conservation and management of species. While most studies focus on relatively gradual changes, extreme events may have lasting impacts on populations. Animals respond to major disturbances such as hurricanes by seeking shelter, migrating, or they may fail to respond appropriately. We assessed the effects of Hurricane Irma in 2017 on the behavior and survival of juvenile bull sharks (Carcharhinus leucas) within a nursery of the Florida coastal Everglades using long-term acoustic telemetry monitoring. Most of our tagged sharks (n = 14) attempted to leave the shallow waters of the Shark River Estuary before the hurricane strike, but individuals varied in the timing and success of their movements. Eight bull sharks left within hours or days before the hurricane, but three left more than a week in advance. Nine of 11 bull sharks (~ 82%) eventually returned to the array within weeks or months of the storm. Six of these returning individuals were detected in a different coastal array in nearshore waters ca. 80 km away from the mouth of the estuary during their absence. The remaining three bull sharks moved downstream relatively late (after the hurricane) and may have died. We used binomial generalized linear mixed models to estimate the probability of presence within the array as a function of several environmental variables. Departure from the array was predicted by declining barometric pressure, increasing rate of change in pressure, and potentially fluctuations in river stage. Juvenile bull sharks may weigh multiple environmental cues, perceived predation risk, their own physical size, and shifting prey resources when making decisions during and after hurricanes.

Coastal vegetated habitats, including mangroves, saltmarshes and seagrasses, mitigate climate change by storing atmospheric carbon. Previous blue carbon research has mainly focused on organic carbo...

Most research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial. However, the few studies that have quantified lateral exchange of carbon and alkalinity, indicate that the dissolved carbon and alkalinity export may be several-fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon and greenhouse gas fluxes of the mangrove-dominated Shark River estuary located in the Everglades National Park in Florida, USA. Time series and spatial surveys were conducted to asses total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Dominant metabolic processes driving dissolved carbon and greenhouse gas dynamics varied along the estuary salinity gradient. Dissolved carbon and greenhouse gas concentrations were strongly coupled to porewater input, which was examined using Rn-222. Shark River was a source of CO2 (92 mmol/m­2/d), CH4 (60 µmol/m­2/d) and N2O (2 µmol/m­2/d) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m­2/d, DOC = 39 mmol/m­2/d) was several-fold higher than burial (~28 mmol/m2/d) and represents an additional carbon sink. Furthermore, the estuary was a source of TAlk (97 mmol/m­2/d) to the coastal ocean, potentially buffering coastal acidification. Despite accounting for only a small share of TAlk, OAlk had a large effect on the estuarine pH. By integrating our results with previous studies, we argue that alkalinity, dissolved carbon and greenhouse gas fluxes should be considered in future blue carbon budgets.

We diagnosed the effect of various physical processes on the distribution of mixed-layer colored dissolved organic matter (CDOM) and a sulfur hexauoride (SF6) tracer during the Southern Ocean Gas Exchange Experiment (SO GasEx). The biochemical upper ocean state estimate uses in situ and satellite biochemical and physical data in the study region, including CDOM (absorption coefcient and spectral slope), SF6, hydrography, and sea level anomaly. Modules for photobleaching of CDOM and surface transport of SF6 were coupled with an ocean circulation model for this purpose. The observed spatial and temporal variations in CDOM were captured by the state estimate without including any new biological source term for CDOM, assuming it to be negligible over the 26 days of the state estimate. Thermocline entrainment and photobleaching acted to diminish the mixed-layer CDOM with time scales of 18 and 16 days, respectively. Lateral advection of CDOM played a dominant role and increased the mixed-layer CDOM with a time scale of 12 days, whereas lateral diffusion of CDOM was negligible. A Lagrangian view on the CDOM variability was demonstrated by using the SF6 as a weighting function to integrate the CDOM elds. This and similar data assimilation methods can be used to provide reasonable estimates of optical properties, and other physical parameters over the short-term duration of a research cruise, and help in the tracking of tracer releases in large-scale oceanographic experiments, and in oceanographic process studies.

Gas transfer velocities (k) of CO2 and CH4 were determined from 209 deployments of a newly designed floating chamber in six mangrove dominated estuaries in Australia and the United States to estimate mangrove system specific k. k600-CO2 and k600-CH4 (k normalized to the Schmidt number of 600) varied greatly within and between mangrove creeks, ranging from 0.9 cm h−1 to 28.3 cm h−1. The gas transfer velocity correlated well with current velocity at all study sites suggesting current generated turbulence was the main driver controlling k. An empirical relationship that accounts for current velocity and a linearly additive contribution of wind speed and water depth was a good predictor of k600-CO2 (R2 = 0.67) and k600-CH4 (R2 = 0.57) in the mangrove creeks in Australia. In a side-by-side study, good agreement was found between k determined from this new floating chamber and a 3He/SF6 dual tracer release experiment (∼5% discrepancy). k600-CH4 correlated well with k600-CO2 (R2 = 0.81), however, k600-CH4 was on average 1.2 times higher than k600-CO2, most likely reflecting a microbubble flux contribution. The microbubble flux contributed up to 73% of the total CH4 flux and was best predicted by a model that included CH4 supersaturation, temperature, and current velocity. A large overestimation was found for both CO2 and CH4 fluxes when calculated using empirically derived k models from previous studies in estuaries. The high temporal and spatial variabilities of kCO2 and kCH4 highlights the importance of site specific transfer velocity measurements in dynamic ecosystems such as mangrove estuaries.

Nasal tip mechanical stability is important for functional and cosmetic nasal airway surgery. Palpation of the nasal tip provides information on tip strength to the surgeon, though it is a purely subjective assessment. Providing a means to simulate nasal tip deformation with a validated model can offer a more objective approach in understanding the mechanics and nuances of the nasal tip support and eventual nasal mechanics as a whole. Herein we present validation of a finite element (FE) model of the nose using physical measurements recorded using an ABS plastic-silicone nasal phantom. Three-dimensional photogrammetry was used to capture the geometry of the phantom at rest and while under steady state load. The silicone used to make the phantom was mechanically tested and characterized using a linear elastic constitutive model. Surface point clouds of the silicone and FE model were compared for both the loaded and unloaded state. The average Hausdorff distance between actual measurements and FE simulations across the nose were 0.39mm ± 1.04 mm and deviated up to 2mm at the outermost boundaries of the model. FE simulation and measurements were in near complete agreement in the immediate vicinity of the nasal tip with millimeter accuracy. We have demonstrated validation of a two-component nasal FE model, which could be used to model more complex modes of deformation where direct measurement may be challenging. This is the first step in developing a nasal model to simulate nasal mechanics and ultimately the interaction between geometry and airflow.

Stable carbon isotopic compositions of water-soluble organic carbon (WSOC) and organic molecular markers were measured to investigate the relative contributions of the sea surface sources to the water-soluble fraction of submicron organic aerosols collected over the eastern equatorial Pacific during the Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOCs (TORERO)/KA-12-01 cruise. On average, the water-soluble organic fraction of the total carbon (TC) mass in submicron aerosols was ∼ 30–35 % in the oceans with the low chlorophyll a (Chl a) concentrations, whereas it was ∼ 60 % in the high-Chl a regions. The average stable carbon isotope ratio of WSOC (δ13CWSOC) was −19.8 ± 2.0 ‰, which was systematically higher than that of TC (δ13CTC) (−21.8 ± 1.4 ‰). We found that in the oceans with both high and low Chl a concentrations the δ13CWSOC was close to the typical values of δ13C for dissolved organic carbon (DOC), ranging from −22 to −20 ‰ in surface seawater of the tropical Pacific Ocean. This suggests an enrichment of marine biological products in WSOC aerosols in the study region regardless of the oceanic area. In particular, enhanced levels of WSOC and biogenic organic marker compounds together with high values of WSOC / TC ( ∼ 60 %) and δ13CWSOC were observed over upwelling areas and phytoplankton blooms, which was attributed to planktonic tissues being more enriched in δ13C. The δ13C analysis estimated that, on average, marine sources contribute ∼ 90 ± 25 % of the aerosol carbon, indicating the predominance of marine-derived carbon in the submicron WSOC. This conclusion is supported by Lagrangian trajectory analysis, which suggests that the majority of the sampling points on the ship had been exposed to marine boundary layer (MBL) air for more than 80 % of the time during the previous 7 days. The combined analysis of the δ13C and monosaccharides, such as glucose and fructose, demonstrated that DOC concentration was closely correlated with the concentration levels of submicron WSOC across the study region regardless of the oceanic area. The result implies that DOC may characterize background organic aerosols in the MBL over the study region.

Knowledge of air-water gas transfer velocities and water residence times is necessary to study the fate of mangrove derived carbon exported into surrounding estuaries and ultimately to determine carbon balances in mangrove ecosystems. For the first time, the He/SF6 dual tracer technique, which has been proven to be a powerful tool to determine gas transfer velocities in the ocean, is applied to Shark River, an estuary situated in the largest contiguous mangrove forest in North America. The mean gas transfer velocity was 3.3 ± 0.2 cmh 1 during the experiment, with a water residence time of 16.5 ± 2.0 days. We propose a gas exchange parameterization that takes into account the major sources of turbulence in the estuary (i.e., bottom generated shear and wind stress).

The Shark and Harney Rivers, located on the southwest coast of Florida, USA, originate in the freshwater, karstic marshes of the Everglades and flow through the largest contiguous mangrove forest in North America. In November 2010 and 2011, dissolved carbon source-sink dynamics were examined in these rivers during SF6 tracer release experiments. Approximately 80 % of the total dissolved carbon flux from all sources (i.e., freshwater wetlands, mangrove, carbonate dissolution, and marine input) out of the Shark and Harney Rivers during these experiments was as inorganic carbon, either via air-water CO2 exchange or longitudinal flux of inorganic carbon to the coastal ocean. Of the total mangrove-derived dissolved inorganic carbon (DIC) exported from the forests into these rivers, between 42 and 48 % was emitted to the atmosphere, with the remaining discharged to the coastal ocean. Dissolved organic carbon (DOC) represented ca. 10 % of the total mangrove-derived dissolved carbon export from the forests. The sum of mangrove-derived DIC and DOC export to these rivers was estimated to be at least 18.9 to 24.5 mmol m−2 d−1, a rate lower than other independent estimates from Shark River and from other mangrove forests. Results from these experiments also suggest that in this region, mangrove contribution to the estuarine flux of dissolved carbon to the ocean is less than 10 %.

High-resolution time series measurements of temperature, salinity, pH and pCO2 were made during the period October 2014-September 2015 at the midpoint of Shark River, a 15km tidal river that originates in the freshwater Everglades of south Florida (USA) and discharges into the Gulf of Mexico. Dissolved inorganic carbon dynamics in this system vary over time, and during this study could be classified into three distinct regimes corresponding to October 2014-February 2015 (a wet to dry season transition period), March-May 2015 (dry period) and July-September 2015 (wet period). Average net longitudinal dissolved inorganic carbon (DIC) fluxes and air-water CO2 fluxes from the Shark River estuary were determined for the three periods. Net DIC fluxes to the coast were estimated to vary between 23.2 and 25.4×105mold-1 with an average daily DIC flux of 24.3×105mold-1 during the year of study. CO2 emissions ranged between 5.5 and 7.8×105mold-1 with an average daily value of 6.4×105mold-1 during the year. The differences in estuarine carbon fluxes during the study period are attributed to differences in the relative importance of hydro-climatological drivers. Results suggest that, during months characterized by reduced rainfall, carbon fluxes are affected by water management via control structures in the upstream Everglades marshes. During months with high rainfall, when culverts are closed and rainfall events are more frequent, carbon fluxes depend more on other forcings, such as rainfall and groundwater discharge.

[1] Mangrove forests are highly productive ecosystems, but the fate of mangrove-derived carbon remains uncertain. Part of that uncertainty stems from the fact that gas transfer velocities in mangrove-surrounded waters are not well determined, leading to uncertainty in air-water CO2 fluxes. Two SF6 tracer release experiments were conducted to determine gas transfer velocities (k(600) = 8.3 ± 0.4 and 8.1 ± 0.6 cm h−1), along with simultaneous measurements of pCO2 to determine the air-water CO2 fluxes from Shark River, Florida (232.11 ± 23.69 and 171.13 ± 20.28 mmol C m−2 d−1), an estuary within the largest contiguous mangrove forest in North America. The gas transfer velocity results are consistent with turbulent kinetic energy dissipation measurements, indicating a higher rate of turbulence and gas exchange than predicted by commonly used wind speed/gas exchange parameterizations. The results have important implications for carbon fluxes in mangrove ecosystems.

[1] Concentrations of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are highly variable in time and space. What is driving the variability in DMS(P), and can those variability be explained by physical processes and changes in the biological community? During the Southern Ocean Gas Exchange Experiment (SO GasEx) in the austral fall of 2008, two 3He/SF6 labeled patches were created in the surface water. SF6 and DMS were surveyed continuously in a Lagrangian framework, while direct measurements of air-sea exchange further constrained the gas budgets. Turbulent diffusivity at the base of the mixed layer was estimated from SF6 profiles and used to calculate the vertical fluxes of DMS and nutrients. Increasing mixed layer nutrient concentrations due to mixing were associated with a shift in the phytoplankton community structure, which in turned likely affected the sulfur dynamics on timescales of days. DMS concentration as well as air-sea DMS flux appeared to be decoupled from the DMSP concentration, possibly due to grazing and bacterial DMS production. Contrary to expectations, in an environment with high winds and modest productivity, physical processes (air-sea exchange, photochemistry, vertical mixing) only accounted for a small fraction of DMS loss from the surface water. Among the DMS sinks, inferred biological consumption most likely dominated during SO GasEx.

The industrial-scale production of biofuels from cultivated microalgae has gained considerable interest in the last several decades. While the climate benefits of microalgae cultivation that result from the capture of atmospheric CO(2) are known, the counteracting effect from the potential emission of other greenhouse gases has not been well quantified. Here, we report the results of a study conducted at an industrial pilot facility in Hawaii to determine the air-water fluxes of N(2)O and CH(4) from open raceway ponds used to grow the marine diatom Staurosira sp. as a feedstock for biofuel. Dissolved O(2), CH(4), and N(2)O concentrations were measured over a 24 h cycle. During this time, four SF(6) tracer release experiments were conducted to quantify gas transfer velocities in the ponds, and these were then used to calculate air-water fluxes. Our results show that pond waters were consistently supersaturated with CH(4) (up to 725%) resulting in an average emission of 19.9 ± 5.6 μmol CH(4) m(-2) d(-1). Upon NO(3)(-) depletion, the pond shifted from being a source to being a sink of N(2)O, with an overall net uptake during the experimental period of 3.4 ± 3.5 μmol N(2)O m(-2) d(-1). The air-water fluxes of N(2)O and CH(4) expressed as CO(2) equivalents of global warming potential were 2 orders of magnitude smaller than the overall CO(2) uptake by the microalgae.

Elevated mixing ratios of chlorofluorocarbons (CFC-11 and CFC-12), and sulfur hexafluoride (SF6) have been observed at Lamont-Doherty Earth Observatory (LDEO), located approximately 25 km north of New York City (NYC). Emissions and transport of these gases are of interest because of their global warming potential, the role of CFCs in depletion of stratospheric ozone and information they provide on the transport of atmospheric pollutants. Comparison of trace gas time series with meteorological data indicates that both NYC and the region to the southwest (New Jersey and the Philadelphia –Washington DC area) are significant sources of CFCs, and confirms that NYC is an unusually large source of SF6. From 1996 to 2005 the elevation of CFC-12 mixing ratio above that of the remote (well mixed) atmosphere has decreased on average by 5.2 ± 0.6 ppt y−1, whereas that of CFC-11 has not changed significantly (0.0 ± 2.0 ppt y−1). From 1998 to 2006, the elevation of SF6 mixing ratios above that of the remote atmosphere declined by 0.4 ± 0.1 ppt y−1. Time series of the same gases measured at Harvard Forest, 205 km northeast of LDEO, demonstrate transport of air masses with elevated levels of these gases from their source region to central Massachusetts. Emissions in the local area around LDEO were quantified through analysis of diurnal cycles. Local CFC-12 emissions decreased ca. 95% between 1996 and 2005 while CFC-11 emission decreased ca. 51% during the same period. Local SF6 emissions decreased by 47% between 1998 and 2005.

Air-water gas exchange is an important process in aquatic systems, including tidal rivers and estuaries. While there are now reliable and routine methods for determining gas exchange over a range of temporal and spatial scales in the ocean and these measurements have resulted in widely used wind speed parameterizations to calculate air-sea gas exchange, the same has not been true for coastal inland waterways. Some studies have suggested that this difference is methodological, while others point to the existence of additional drivers for gas exchange besides wind in rivers and estuaries. Here, we present gas transfer velocities measured in the tidal Hudson River with a method widely used in oceanic studies, the 3 He/SF6 dual tracer technique. Airside and waterside forcings were determined with an anemometer and an acoustic Doppler current profiler, respectively. The results confirm that wind is the dominant driver of gas exchange in the tidal Hudson River, with negligible contribution from bottom-generated turbulence. Furthermore, a parameterization between wind speed and gas exchange developed for the ocean is able to predict gas exchange in this environment with high accuracy. It is hoped that by transferring methodology used in oceanic studies to rivers and estuaries, robust data can be obtained that will eventually allow development of widely applicable relationships between easily measured environ- mental variables and gas exchange in tidal inland waters.

A dual tracer experiment was carried out during the SAGE experiment using the inert tracers SF6 and 3He, in order to determine the gas transfer velocity, k, at high wind speeds in the Southern Ocean. Wind speed/gas exchange parameterization is characterised by significant variability and we examine the major measurement uncertainties that contribute to that scatter. Correction for the airflow distortion over the research vessel, as determined by computational fluid dynamics (CFD) modelling, had the effect of increasing the calculated value of k by 30%. On the short time scales of such experiments, the spatial variability of the wind field resulted in differences between ship and satellite QuikSCAT winds, which produced significant differences in transfer velocity. With such variability between wind estimates, the comparison between gas exchange parameterizations from diverse experiments should clearly be made on the basis of the same wind product. Uncertainty in mixed layer depth of ∼10% arose from mixed layer deepening at high wind speed and limited resolution of vertical sampling. However the assumption of equal mixing of the two tracers is borne out by the experiment. Two dual tracer releases were carried out during SAGE, and showed no significant difference in transfer velocities using QuikSCAT winds, despite the differences in wind history. In the SAGE experiment, duration limitation on the development of waves was shown to be an important factor for Southern Ocean waves, despite the presence of long fetches.

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. There was a significant increase in the photosynthetic competence (Fv/Fm) of resident phytoplankton within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary production and column-integrated chlorophyll a concentrations had only doubled relative to the unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions totalling 1.1 ton Fe2+, this was a very modest response compared to other mesoscale iron enrichment experiments. An investigation of the factors limiting bloom development considered co-limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst incident light levels and the initial Si:N ratio were the lowest recorded in all FeAXs to date, there was only a small seed-stock of diatoms (less than 1% of biomass) and the main response to iron addition was by the picophytoplankton. A high rate of dilution of the fertilised patch relative to phytoplankton growth rate, the greater than expected depth of the surface mixed layer and microzooplankton grazing were all considered as factors that prevented significant biomass accumulation. In line with the limited response, the enhanced biological draw-down of pCO2 was small and masked by a general increase in pCO2 due to mixing with higher pCO2 waters. The DMS precursor DMSP was kept in check through grazing activity and in contrast to most FeAX’s dissolved dimethylsulfide (DMS) concentration declined through the experiment. SAGE is an important low-end member in the range of responses to iron addition in FeAX’s. In the context of iron fertilisation as a geoengineering tool for atmospheric CO2 removal, SAGE has clearly demonstrated that a significant proportion of the low iron ocean may not produce a phytoplankton bloom in response to iron addition.

Summary SF 6 is a promising transient tracer for groundwater dating, but elevated levels of atmospheric SF 6 may limit application of this dating method in urban areas. To determine the magnitude of this limitation within the United States (US), this study derives average atmospheric SF 6 mixing ratios from soil air measurements from five major urban areas. All surveyed sites showed SF 6 elevated above Northern Hemisphere clean air levels. In many cases, SF 6 mixing ratios were highest within large cities, but significantly elevated levels persisted hundreds of kilometers beyond the urban centers. Estimated SF 6 emissions from New York city were anomalously large, resulting in average levels over 200% above remote atmosphere values. Emissions from other urban areas included in this study appear to be typical for large cities in the US and maximum observed elevations of SF 6, in these areas, were in the range of 50–100%. These findings imply that caution is needed in choosing an atmospheric input function when using SF 6 as a transient tracer within or near urbanized regions. Within the Northeast US, the assumption of remote atmospheric SF 6 may lead to significant errors in estimation of groundwater ages, especially in the case of an expected future flattening of the atmospheric mixing ratio curve.

A series of experiments were conducted at University of Delaware's Air–Sea Interaction Laboratory to examine the combined effects of rain and wind on air–water gas exchange. During this study, ASIL WRX I, a combination of 3 rain rates and 4 wind speeds were used, for a total of 12 different environmental conditions. The SF6 evasion method was used to determine the bulk gas transfer velocities, and airside profiles of wind and CO2 were used to estimate flux–profiles of momentum and carbon dioxide. In addition to measurements of fluxes with and without rain in a wind–wave boundary layer, measurements of wave properties were also obtained. Rain is shown to alter the wind profile in the flume, and dampen surface waves. Also, SF6 evasion indicates that with the present experimental setup, for most of the experimental conditions, rain and wind combine linearly to influence air–water gas exchange. Flux–profile relationships for marine atmospheric boundary layers, which were performed to scale up to field measurements, were explored by a comparison between SF6-derived bulk fluxes and airside CO2 profile measurements.

We present the first large-scale comparison of a fluorescent dye [fluorescein ( C20 H10 O5 Na2 ) ] and a gas [sulfur hexafluoride ( SF6 ) ] as tracers of advection and longitudinal dispersion from a dual tracer release experiment conducted in the tidal Hudson River. At the beginning of the experiment, 36 kg of fluorescein and ∼4.3 mol of SF6 were injected into the Hudson River at an averaged depth of 9.5 m , ∼1 m above the bottom, near Hyde Park, N.Y. After injection, fluorescein distributions were surveyed for 4 days (until it became undetectable) and SF6 distributions were surveyed for 10 days. The dye resolves initial vertical mixing on the day of injection, and then net advection and longitudinal dispersion, whereas SF6 provides information on net advection and longitudinal mixing on larger spatial scales and longer time scales. Quantitative estimates of transport processes (net advection and longitudinal dispersion) calculated from the two methods are consistent for the first three days, and start to...

In October 2001, ∼0.2mol of SF6 was injected into the upper Hudson River, a modified natural channel with multiple dams, at Ft. Edward, N.Y. The tracer was monitored for 7 days as it moved ∼50km downriver. The longitudinal evolution of the tracer distribution was used to estimate one-dimensional advection (9.0±0.2kmd-1) and dispersion (17.3±4.0m2s-1) along the river axis. Comparison of these results to tracer studies on channels without dams suggests that dams reduce longitudinal dispersion below the value expected in a natural channel with the same discharge. SF6 loss through air–water gas exchange along the river and at two dams (10.7m combined height) was estimated by observing decay in peak concentration. Losses at dams (approximately 50% per dam) were dominant. The estimated gas exchange at dams was compared to a simple model adapted from those available in literature. Small amounts of tracer were trapped in a canal segment (∼5km long) that parallels the river, where advection and dispersion were sha...

In June 2003, two injections of approximately 3.9 mol of sulfur hexafluoride (SF6) were made 8 days apart in the East River, a 25 km tidal strait, to observe solute mixing and dissipation. The first injection occurred at slack before flood, and the second at slack before ebb (flood = northward flow). Tidally synchronized surveys of the SF6 tracer patch, supplemented by vertical profiles, were conducted by boat for 6 and 4 days following the flood and ebb injections, respectively. Residence times for the tracer-tagged water mass in the East River were estimated to be 3.3 +/- 0.7 days and 1.7 +/- 0.5 days for the flood and ebb injections, respectively, after correcting SF6 inventories for losses of SF6 from the water column by air-water gas exchange. The data indicate that the majority of East River solutes are transported to New York Harbor and that tidal mixing dominates subtidal circulation with respectto solute transport. Surveys of the adjacent lower Hudson River revealed a northward-moving, intermediate layer of East River water. Our results suggest that tidal phasing of contaminant discharges in the East River could reduce environmental impacts, by increasing flushing rates and directing a greater fraction of material away from Long Island Sound.

The effects of estuarine circulation and tidal trapping on transport in the Hudson estuary were investi- gated by a large-scale, high-resolution numerical model simulation of a tracer release. The modeled and measured longitudinal profiles of surface tracer concentrations (plumes) differ from the ideal Gaussian shape in two ways: on a large scale the plume is asymmetric with the downstream end stretching out farther, and small-scale (1-2 km) peaks are present at the upstream and downstream ends of the plume. A number of diagnostic model simulations (e.g., remove freshwater flow) were performed to understand the processes responsible for these features. These simulations show that the large-scale asymmetry is related to salinity. The salt causes an estuarine circulation that decreases vertical mixing (vertical density gradient), increases longitudinal dispersion (increased vertical and lateral gradients in longitudinal ve- locities), and increases net downstream velocities in the surface layer. Since salinity intrusion is confined to the down- stream end of the tracer plume, only that part of the plume is effected by those processes, which leads to the large- scale asymmetry. The small-scale peaks are due to tidal trapping. Small embayments along the estuary trap water and tracer as the plume passes by in the main channel. When the plume in the main channel has passed, the tracer is released back to the main channel, caiising a secondary peak in the longitudinal profile.

In July 2002, ∼0.9 mol of sulfur hexafluoride (SF6) was injected into Newark Bay, NJ, a 14 km2 estuary that forms part of New York Harbor, to investigate circulation, mixing, and the transport and fate of solutes. The SF6 tracer was observed over 11 consecutive days using a high-resolution measurement system. Total tracer mass in the sheltered waters declined quasiexponentially at a rate of 0.29 ± 0.03 d-1. Air−water gas exchange was estimated to account for 56% of tracer mass loss, upon the basis of wind speed/gas exchange parametrizations. Large-scale tidal transfer of solutes through the Kill van Kull strait (7 km long) caused net seaward flushing contrary to the apparent residual circulation. Seaward transport via the Arthur Kill strait (20 km long) appeared to depend on longitudinal dispersion, residual circulation, and freshwater discharge and was ∼1 order of magnitude lower. The loss rate due to flushing alone was 0.13 ± 0.02 d-1, indicating a mean residence time for water and solutes in Newark Bay...

Specific docking interactions between MAPKs and their activating MAPK kinases (MKKs or MEKs) are crucial for efficient and accurate signal transmission. Here, we report the identification of a MAPK-docking site, or "D-site," in the N terminus of human MKK4/JNKK1. This docking site conforms to the consensus sequence for known D-sites in other MKKs and contains the first of the two cleavage sites for anthrax lethal factor protease that have been found in the N terminus of MKK4. This docking site was both necessary and sufficient for the high affinity binding of the MAPKs JNK1, JNK2, JNK3, p38 alpha, and p38 beta to MKK4. Mutations that altered conserved residues in this docking site reduced JNK/p38 binding. In addition, a peptide version of this docking site, as well as a peptide version of the JNK-binding site of the JIP-1 scaffold protein, inhibited both MKK4/JNK binding and MKK4-mediated phosphorylation of JNK1. These same peptides also inhibited JNK2-mediated phosphorylation of c-Jun and ATF2, suggesting that transcription factors, MKK4, and the JIP scaffold compete for docking to JNK. Finally, the selectivity of the MKK4, MEK1, and MEK2 D-sites for JNK versus ERK was quantified. The MEK1 and MEK2 D-sites displayed a strong selectivity for their cognate MAPK (ERK2) versus a non-cognate MAPK (JNK). In contrast, the MKK4 D-site exhibited only limited selectivity for JNK versus ERK.

Local emissions of SF6 are of interest for studying their impact on the use of SF6 as a groundwater-dating tool near source regions as well as for investigating the spatial distributions of (inert) gaseous compounds spreading from urban or industrial centers. A precondition for the use of SF6 in such studies is the capability to document the temporal and spatial evolution of SF6 in and around source regions with sufficient resolution. Here we present a time series of SF6 measurements in soil air at a site (Sparkill, NY) about 25 km north of New York City carried out between May 2000 and January 2002. The data show that, below about 2 m depth, the vadose zone integrates atmospheric SF6 mixing ratios over time scales greater than 1 month. SF6 mixing ratios in soil air at these depths match averaged high-resolution atmospheric measurements performed at Lamont-Doherty Earth Observatory in Palisades, NY, located about 3 km south of Sparkill. To a first-order approximation, a simple one-dimensional diffusion model reproduces the measured SF6 profiles in the vadose zone, suggesting that the soil indeed acts as a low-pass filter for inert atmospheric gases. These findings indicate that measurements of soil air can be used to determine the spatial pattern of SF6 excess relative to the remote atmosphere for a given region. A transect of soil profiles from Manhattan to the tip of Long Island indicates that emissions from sites close to New York City lead to significant SF6 excesses (ca. 25% or more) above the clean air mixing ratios over distances of the order of 80 km.

Physical processes such as advection, dispersion, and air−water gas exchange play important roles in determining the movement and change in concentration of contaminants discharged into rivers. In the following, we report results from a large-scale SF6 tracer release experiment conducted in the tidal Hudson River to examine longitudinal dispersion and net advection. SF6 was injected into the Hudson River near Newburgh, NY, and surveyed for 13 days using a new, fully automated, high-resolution SF6 sampling and analysis system. Net down river advection of the water body originally tagged with SF6 was slow, averaging mean displacement rates of about 0.5 ± 0.2 km d-1. In contrast, spreading of the tracer was driven by tidal movement, causing rapid mixing of the water up and down river. By examining the change in the second moment of the tracer distribution with time, we determined the mean longitudinal dispersion coefficient to be 70.1 ± 4.3 m2 s-1. Temporal evolution of the SF6 inventory indicates an average...

Sulfur hexafluoride (SF6) has the potential to be a valuable transient tracer for dating of young ground water. However, near urban areas, there are typically numerous point sources of SF6 which have a significant impact on the temporal evolution of its atmospheric mixing ratio, thereby complicating its use as an age-dating tool. Here, we present and discuss a 12-month record of atmospheric SF6 from a location near New York City. The data were obtained by gas chromatographic analyses performed at intervals of approximately 10 minutes yielding about 40,000 data points for the time series. Nearly all measured SF6 values are in excess of remote Northern Hemisphere (NH) atmospheric mixing ratios. Temporal trends in the baseline data from LDEO indicate that atmospheric SF6 at LDEO decreased at a rate of 0.4 ppt yr−1 over the 12-month period. The SF6 data are compared to records of CFCs obtained during the same period. Whereas the CFCs show daily, weekly, and seasonal patterns of variability near New York City, the SF6 data exhibit only a daily cycle. The observed SF6 excesses are far greater than those found for CFCs during the same time period. This indicates that in order to use SF6 as an age dating tool of groundwater near source regions, its input function, i.e., its concentration in soil air above the groundwater table, needs to be defined explicitly.

Chlorofluorocarbons (CFCs) are frequently used as tracers for age dating of young groundwaters. However, in urban environments with many CFC point sources, uncertainties in the delivery of CFCs to groundwater (input function) complicate quantitative interpretation of observed CFC distributions. To assess the potential impact of elevated atmospheric CFC mixing ratios on CFC-based groundwater ages near a large coastal urban center, we measured atmospheric mixing ratios of CFCs in Palisades, NY, 25 km north of New York City. We present and discuss a 16-month record of atmospheric CCl3F (CFC-11) and CCl2F2 (CFC-12) obtained from gas chromatographic analyses taken at intervals of approximately 10 min. Nearly all measured values are in excess of remote Northern Hemisphere (NH) atmospheric mixing ratios. The mean mixing ratios of CFC-11 and CFC-12 are 6 and 13% higher, respectively, than those measured at a remote NH location during the same time period. The temporal trends of CFC-11 and CFC-12 differ from those...

The efficiency of transfer of gases and particles across the air-sea interface is controlled by several physical, biological and chemical processes in the atmosphere and water which are described here (including waves, large- and small-scale turbulence, bubbles, sea spray, rain and surface films). For a deeper understanding of relevant transport mechanisms, several models have been developed, ranging from conceptual models to numerical models. Most frequently the transfer is described by various functional dependencies of the wind speed, but more detailed descriptions need additional information. The study of gas transfer mechanisms uses a variety of experimental methods ranging from laboratory studies to carbon budgets, mass balance methods, micrometeorological techniques and thermographic techniques. Different methods resolve the transfer at different scales of time and space; this is important to take into account when comparing different results. Air-sea transfer is relevant in a wide range of applications, for example, local and regional fluxes, global models, remote sensing and computations of global inventories. The sensitivity of global models to the description of transfer velocity is limited; it is however likely that the formulations are more important when the resolution increases and other processes in models are improved. For global flux estimates using inventories or remote sensing products the accuracy of the transfer formulation as well as the accuracy of the wind field is crucial.

GasEx-98 was an air-sea exchange process cruise conducted aboard the NOAA ship Ronald H. Brown in the North Atlantic during May and June of 1998. During the cruise, air-sea gas transfer velocities for carbon dioxide were measured using the direct-covariance method. Because the sampling times for the covariance method are on the same order as the timescales of changes in meteorological forcing, the GasEx-98 results provide a unique data set for investigating whether changes in different forcing mechanisms correlate with changes in gas transfer. In particular, fractional area whitecap coverage, W C , was measured during daylight hours using a dual-camera video system mounted on a bow tower. Several high wind speed events occurred during the cruise, and the resulting correlation between wind speed and W C is consistent with previous oceanic measurements. The whitecap coverage data were combined with the wind speed records and these data were used in a parameterization of whitecap-mediated gas transfer to predict transfer velocities. These predicted transfer velocities are in good agreement with the transfer velocities derived from the direct-covariance data.