Sea CO2 Research Articles

A seasonal carbon budget for the sub-Antarctic Ocean, South of Australia

Changes from winter (July) to summer (February) in mixed layer carbon tracers and nutrients measured in the sub-Antarctic zone (SAZ), south of Australia, were used to derive a seasonal carbon budget. The region showed a strong winter to summer decrease in dissolved inorganic carbon (DIC; ∼ 45 µmol/kg) and fugacity of carbon dioxide ( fCO 2; ∼ 25 µatm), and an increase in stable carbon isotopic composition of DIC ( δ 13C DIC; ∼ 0.5‰), based on data collected between November 1997 and July 1999. The observed mixed layer changes are due to a combination of ocean mixing, air–sea exchange of CO 2, and biological carbon production and export. After correction for mixing, we find that DIC decreases by up to 42 ± 3 µmol/kg from winter (July) to summer (February), with δ 13C DIC enriched by up to 0.45 ± 0.05‰ for the same period. The enrichment of δ 13C DIC between winter and summer is due to the preferential uptake of 12CO 2 by marine phytoplankton during photosynthesis. Biological processes dominate the seasonal carbon budget (≈ 80%), while air–sea exchange of CO 2 (≈ 10%) and mixing (≈ 10%) have smaller effects. We found the seasonal amplitude of fCO 2 to be about half that of a study undertaken during 1991–1995 [Metzl, N., Tilbrook, B. and Poisson, A., 1999. The annual fCO 2 cycle and the air–sea CO 2 flux in the sub-Antarctic Ocean. Tellus Series B—Chemical and Physical Meteorology, 51(4): 849–861.] for the same region, indicating that SAZ may undergo significant inter-annual variations in surface fCO 2. The seasonal DIC depletion implies a minimum biological carbon export of 3400 mmol C/ m 2 from July to February. A comparison with nutrient changes indicates that organic carbon export occurs close to Redfield values (ΔP:ΔN:ΔC = 1:16:119). Extrapolating our estimates to the circumpolar sub-Antarctic Ocean implies a minimum organic carbon export of 0.65 GtC from the July to February period, about 5–7% of estimates of global export flux. Our estimate for biological carbon export is an order of magnitude greater than anthropogenic CO 2 uptake in the same region and suggests that changes in biological export in the region may have large implications for future CO 2 uptake by the ocean.

Mechanisms controlling the air–sea [formula omitted] flux in the North Sea

The mechanisms driving the air–sea exchange of carbon dioxide ( CO 2 ) in the North Sea are investigated using the three-dimensional coupled physical–biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001–2002 and identify the controls of the air–sea CO 2 flux for two locations representative for the North Sea's biogeochemical provinces. In the seasonally stratified northern region, net CO 2 uptake is high ( 2.06 mol m - 2 a - 1 ) due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in CO 2 partial pressure ( pCO 2 ) during the productive season. In contrast, the permanently mixed southern region is a weak net CO 2 source ( 0.78 mol m - 2 a - 1 ). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO 2 appears to be controlled by temperature.

Partial pressure and air–sea CO 2 flux in the Aegean Sea during February 2006

Data on the distribution of fCO 2 were obtained during a cruise in the Aegean Sea during February 2006. The fCO 2 of surface water ( fCO 2 sw) was lower than the atmospheric fCO 2 ( fCO 2 atm) throughout the area surveyed and Δ fCO 2 values varied from −34 to −61 μatm. The observed under-saturation suggests that surface waters in the Aegean represent a sink for atmospheric CO 2 during the winter of 2006. Higher fCO 2 sw values were recorded in the ‘less warm’ and ‘less saline’ shallow northernmost part of the Aegean Sea implying that the lower seawater temperature and salinity in this area play a crucial role in the spatial distribution of fCO 2 sw. A first estimate of the magnitude of the air–sea CO 2 exchange and the potential role of the Aegean Sea in the transfer of atmospheric CO 2 was also obtained. The air–sea CO 2 fluxes calculated using different gas transfer formulations showed that during February 2006, the Aegean Sea absorbs atmospheric CO 2 at a rate ranging from −6.2 to −11.8 mmol m −2 d −1 with the shipboard recorded wind speeds and at almost half rate (−3.5 to −5.5 mmol m −2 d −1) with the monthly mean model-derived wind speed. Compared to recent observations from other temperate continental shelves during winter period, the Aegean Sea acts as a moderate to rather strong sink for atmospheric CO 2. Further investigations, including intensive spatial and temporal high-resolution observations, are necessary to elucidate the role of the Aegean Sea in the process of transfer of atmospheric CO 2 into the deep horizons of the Eastern Mediterranean.

Primary production in the Strait of Gibraltar: Carbon fixation rates in relation to hydrodynamic and phytoplankton dynamics

Primary production was studied at nine sites distributed within the Strait of Gibraltar (Southern Spain) and North-Western (NW) Alboran Sea by analyzing photosynthesis-irradiance (P–I) relationships and integrated primary production rates in relation to the different types of Deep Chlorophyll Maxima (DCM) detected in the area. The characteristics of the DCM were examined by several methods, including flow cytometry, quantification of transparent expolymer particles and fluorimetric measurements that were applied in order to assess the photo-physiological state of the phytoplankton assemblages with respect to their species composition and water column structure (hydrology). The photosynthetic parameters (derived from P–I relationships) and integrated primary production (range 6–644 mg m −2 d −1) responded greatly to the diverse DCM identified and thereby the spatial variability of the primary production observed in the region was found to depend upon the occurrence of the different types of phytoplankton accumulations, which were themselves indicative of the previous history of the water column. The net contribution of the primary production to the air–sea CO 2 exchange process was also evaluated in the area. Results indicated that this region behaved as a net sink for the atmospheric CO 2, with the intensity of the flux being strongly modulated by the wind intensity.

On the seasonal variation of air – sea CO 2 fluxes in the outer Changjiang (Yangtze River) Estuary, East China Sea

Based upon seven field surveys conducted during April 2005 – April 2008, we examined the surface partial pressure of CO 2 ( pCO 2) and dissolved oxygen (DO) in the outer Changjiang (Yangtze River) Estuary, on the inner shelf of the East China Sea (ECS). This area represents a most dynamic zone of the ECS where high pCO 2 riverine water meets with highly productive shelf waters, covering a 2° × 3° area, ~ 10% of the surface area of the entire ECS. Surface pCO 2 ranged 320 – 380 µatm (average ~ 345 µatm) in winter, 180 – 450 µatm (average ~ 330 µatm) in spring, 150 – 620 µatm (average ~ 310 µatm) in summer and 120 – 540 µatm (average ~ 375 µatm) in autumn. The seasonal variation pattern of surface DO generally mirrored that of pCO 2, ranging 95% – 105% in winter, 96% – 142% (average 110%) in spring, 73% – 192% (average 118%) in summer and 81% – 178% (average 102%) in autumn. The dynamics of pCO 2 drawdown and DO enhancement in the warm seasons (from April to October) appeared to be controlled by primary productivity and air – sea exchange, while mixing dominated the aqueous pCO 2 in the cold seasons (from November to March of the following year). This study showed that the outer Changjiang Estuary served as a moderate or significant sink of atmospheric CO 2 in winter, spring and summer, while it turned to a net source in autumn. The integrated sea – air CO 2 flux in the outer Changjiang Estuary was estimated as − 1.9 ± 1.3 mol m − 2 year − 1, which is double the recent sea–air CO 2 flux estimation for the northern ECS.

Satellite-derived surface water pCO 2 and air–sea CO 2 fluxes in the northern South China Sea in summer

An empirical approach is presented for the estimation of the partial pressure of carbon dioxide ( pCO 2) and air–sea CO 2 fluxes in the northern South China Sea in summer using satellite-derived sea surface temperatures (SSTs), chlorophyll-a (Chl a) concentrations, and wind fields. Two algorithms were tested. The first used an SST-dependent equation, and the other involved the introduction of Chl a. Regression equations were developed for summer based on in situ data obtained in July, 2004. Using the monthly average SST and Chl a fields derived from the advanced very high resolution radiometer (AVHRR) and the SeaWiFS (sea-viewing wide field of view sensor), respectively, the monthly pCO 2 fields were computed. The derived pCO 2 was compared with the shipboard pCO 2 observations conducted in July, 2000. This resulted in a root-mean-square error of 4.6 μatm, suggesting that the satellite-derived pCO 2 was in general agreement with the in situ observations. The air–sea CO 2 flux was further computed with the aid of the monthly mean QuikSCAT wind speed. We contend that more shipboard data are necessary for refining the empirical algorithms and reducing the uncertainty in the results.

Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

AbstractThe RIVERSTRAHLER model, an idealized biogeochemical model of the river system, has been coupled to MIRO‐CO2, a complex biogeochemical model describing diatom and Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the dual role of changing nutrient loads and increasing atmospheric CO2 as drivers of air–sea CO2 exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt), is characterized by variable diatom and Phaeocystis colonies blooms which impact on the trophic status and air–sea CO2 fluxes of the coastal ecosystem. For this application, the MIRO‐CO2 model is implemented in a 0D multibox frame covering the eutrophied Eastern English Channel and Southern North Sea and receiving loads from the rivers Seine and Scheldt. Model simulations are performed for the period between 1951 and 1998 using real forcing fields for sea surface temperature, wind speed and atmospheric CO2 and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model results suggest that the BCZ shifted from a source of CO2 before 1970 (low eutrophication) towards a sink during the 1970–1990 period when anthropogenic DIN and P loads increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is characterized by a progressive decrease of P loads concomitant with a decrease of primary production and of the CO2 sink in the BCZ. At the end of the simulation period, the BCZ ecosystem is again net heterotroph and acts as a source of CO2 to the atmosphere. R‐MIRO‐CO2 scenarios testing the relative impact of temperature, wind speed, atmospheric CO2 and river loads variability on the simulated air–sea CO2 fluxes suggest that the trend in air–sea CO2 fluxes simulated between 1951 and 1998 in the BCZ was mainly controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative nutrient changes control the level of primary production while qualitative changes modulate the relative contribution of diatoms and Phaeocystis to this flux and hence the sequestration of atmospheric CO2.

Mechanisms governing interannual variability in upper-ocean inorganic carbon system and air–sea CO2 fluxes: Physical climate and atmospheric dust

Abstract We quantify the mechanisms governing interannual variability in the global, upper-ocean inorganic carbon system using a hindcast simulation (1979–2004) of an ecosystem-biogeochemistry model forced with time-evolving atmospheric physics and dust deposition. We analyze the variability of three key, interrelated metrics—air–sea CO 2 flux, surface-water carbon dioxide partial pressure p CO 2 , and upper-ocean dissolved inorganic carbon (DIC) inventory—presenting for each metric global spatial maps of the root mean square (rms) of anomalies from a model monthly climatology. The contribution of specific driving factors is diagnosed using Taylor expansions and linear regression analysis. The major regions of variability occur in the Southern Ocean, tropical Indo-Pacific, and Northern Hemisphere temperate and subpolar latitudes. Ocean circulation is the dominant factor driving variability over most of the ocean, modulating surface dissolved inorganic carbon that in turn alters surface-water p CO 2 and air–sea CO 2 flux variability (global integrated anomaly rms of 0.34 Pg C yr −1 ). Biological export and thermal solubility effects partially damp circulation-driven p CO 2 variability in the tropics, while in the subtropics, thermal solubility contributes positively to surface-water p CO 2 and air–sea CO 2 flux variability. Gas transfer and net freshwater inputs induce variability in the air–sea CO 2 flux in some specific regions. A component of air–sea CO 2 flux variability (global integrated anomaly rms of 0.14 Pg C yr −1 ) arises from variations in biological export production induced by variations in atmospheric iron deposition downwind of dust source regions. Beginning in the mid-1990s, reduced global dust deposition generates increased air–sea CO 2 outgassing in the Southern Ocean, consistent with trends derived from atmospheric CO 2 inversions.

Strategies for high-latitude northern hemisphere CO2 sampling now and in the future

Abstract A sampling strategy to determine the annual mean air–sea CO 2 fluxes in the high-latitude North Pacific and North Atlantic was developed using a combination of signal-to-noise ratios and 2D Fourier transforms in conjunction with a coupled climate carbon model. To account for unresolved mesoscale variability in our simulations, we used summer and winter cruises to determine the magnitude of this variability and estimate its impact on the air–sea fluxes. From our analysis, we propose a regular sampling strategy of every 6° in latitude and 10° in longitude every 3 months for the North Pacific and North Atlantic. Applying this sampling strategy to the simulation returned an annual mean air–sea flux value within ±15% of the total simulated value, when applied now and into the future. Our study highlights several key points for measuring annual mean basin-scale CO 2 fluxes: (1) the combination of temporal and spatial sampling dramatically reduces the noise in the observations and provides a good representation of the fluxes with far fewer measurements than required for resolving the spatial or temporal signals independently; (2) that sampling at higher than recommended frequencies in time and space provides little improvement in the estimated annual mean flux; (3) the uncertainty in the decadal annual mean uptake is limited by interannual variability and not by sampling error or unresolved mesoscale variability; and (4) our high-latitude sampling strategies remain valid until at least the end of this century.

Estimation of air–sea CO 2 fluxes in the Bay of Biscay based on empirical relationships and remotely sensed observations

An empirical algorithm has been developed to compute the sea surface CO 2 fugacity (fCO 2 sw) in the Bay of Biscay from remotely sensed sea surface temperature (SST RS) and chlorophyll a (chl a RS) retrieved from AVHRR and SeaWiFS sensors, respectively. Underway fCO 2 sw measurements recorded during 2003 were correlated with SST RS and chl a RS data yielding a regression error of 0.1 ± 7.5 µatm (mean ± standard deviation). The spatial and temporal variability of air–sea fCO 2 gradient (ΔfCO 2) and air–sea CO 2 flux (FCO 2) was analyzed using remotely sensed images from September 1997 to December 2004. An average FCO 2 of − 1.9 ± 0.1 mol m − 2 yr − 1 characterized the Bay of Biscay as a CO 2 sink that is suffering a significant long-term decrease of 0.08 ± 0.05 mol m − 2 yr − 2 in its capacity to store atmospheric CO 2. The main parameter controlling the long-term variability of the CO 2 uptake from the atmosphere was the air–sea CO 2 transfer velocity (57%), followed by the SST RS (10%) and the chl a RS (2%).

Air–sea carbon dioxide fluxes in the coastal southeastern tropical Pacific

Comprehensive sea surface surveys of the partial pressure of carbon dioxide (pCO 2) have been made in the upwelling system of the coastal (0–200 km from shore) southeastern tropical Pacific since 2004. The shipboard data have been supplemented by mooring and drifter based observations. Air–sea flux estimates were made by combining satellite derived wind fields with the direct sea surface pCO 2 measurements. While there was considerable spatial heterogeneity, there was a significant flux of CO 2 from the ocean to the atmosphere during all survey periods in the region between 4° and 20° south latitude. During periods of strong upwelling the average flux out of the ocean exceeded 10 moles of CO 2 per square meter per year. During periods of weaker upwelling and high productivity the CO 2 evasion rate was near 2.5 mol/m 2/yr. The average annual fluxes exceed 5 mol/m 2/yr. These findings are in sharp contrast to results obtained in mid-latitude upwelling systems along the west coast of North America where the average air–sea CO 2 flux is low and can often be from the atmosphere into the ocean. In the Peruvian upwelling system there are several likely factors that contribute to sea surface pCO 2 levels that are well above those of the atmosphere in spite of elevated primary productivity: (1) the upwelling source waters contain little pre-formed nitrate and are affected by denitrification, (2) iron limitation of primary production enhanced by offshore upwelling driven by the curl of the wind stress and (3) rapid sea surface warming. The combined carbon, nutrient and oxygen dynamics of this region make it a candidate site for studies of global change.

Application of satellite remote sensing techniques for estimating air–sea CO 2 fluxes in Hudson Bay, Canada during the ice-free season

The role of coastal seas as either a sink or a source of CO 2 is subject to a great deal of uncertainty. This uncertainty largely arises from a lack of observations in the coastal zones. Remote sensing offers an avenue for expanding these observations by allowing for the extrapolation of relatively limited data sets of dissolved CO 2 ( pCO 2sw). In this paper, predictive algorithms for pCO 2sw that could be applied to remote sensing products were created from a field data set collected from September–October, 2005 in Hudson Bay, Canada. The field data showed that an effective pCO 2sw interpolation algorithm could be created using sea surface temperature (SST) as a predictor, and that a slight improvement of the algorithm could be achieved if measurements of absorption due to coloured dissolved organic material ( a CDOM) were included. Unfortunately, satellite retrievals of a CDOM did not match well with in situ observations, and so only SST (obtained from the MODIS Aqua sensor) was used to create monthly maps of pCO 2sw for the period of August–October. To estimate fluxes of CO 2, constructed surfaces of pCO 2sw were combined with estimates of gas transfer velocity derived from QuikSCAT wind retrievals, and pCO 2air based on field observations. The results of these calculations revealed that Hudson Bay acts as a source of CO 2 during August and September, but reverts to a sink of CO 2 in October as the water temperature decreases. Overall, a positive flux of 1.60 TgC was estimated for the region during the ice-free season. This result is in contrast to most Arctic or sub-Arctic continental shelf seas, where usually strong absorptions of CO 2 are observed.

Skill metrics for confronting global upper ocean ecosystem-biogeochemistry models against field and remote sensing data

We present a generalized framework for assessing the skill of global upper ocean ecosystem-biogeochemical models against in-situ field data and satellite observations. We illustrate the approach utilizing a multi-decade (1979–2004) hindcast experiment conducted with the Community Climate System Model (CCSM- 3) ocean carbon model. The CCSM-3 ocean carbon model incorporates a multi-nutrient, multi-phytoplankton functional group ecosystem module coupled with a carbon, oxygen, nitrogen, phosphorus, silicon, and iron biogeochemistry module embedded in a global, three-dimensional ocean general circulation model. The model is forced with physical climate forcing from atmospheric reanalysis and satellite data products and time-varying atmospheric dust deposition. Data-based skill metrics are used to evaluate the simulated time-mean spatial patterns, seasonal cycle amplitude and phase, and subannual to interannual variability. Evaluation data include: sea surface temperature and mixed layer depth; satellite-derived surface ocean chlorophyll, primary productivity, phytoplankton growth rate and carbon biomass; large-scale climatologies of surface nutrients, pCO 2, and air–sea CO 2 and O 2 flux; and time-series data from the Joint Global Ocean Flux Study (JGOFS). Where the data is sufficient, we construct quantitative skill metrics using: model–data residuals, time–space correlation, root mean square error, and Taylor diagrams.

Daily variability of dissolved inorganic radiocarbon at three sites in the surface ocean

We report radiocarbon measurements of dissolved inorganic carbon (DIC) in surface water samples collected daily during cruises to the central North Pacific, the Sargasso Sea and the Southern Ocean. The ranges of Δ 14C measurements for each cruise (11–30‰) were larger than the total uncertainty (7.8‰, 2-sigma) of the measurements. The variability is attributed to changes in the upper water mass that took place at each site over a two to four week period. These results indicate that variability of surface Δ 14C values is larger than the analytical precision, because of patchiness that exists in the DIC Δ 14C signature of the surface ocean. This additional variability can affect estimates of geochemical parameters such as the air–sea CO 2 exchange rate using radiocarbon.

Riverine input and air–sea CO 2 exchanges near the Changjiang (Yangtze River) Estuary: Status quo and implication on possible future changes in metabolic status

Due to anthropogenic activities, the nutrient loadings of the Changjiang (Yangtze River) are strickly on the rise. The high nutrient concentrations notwithstanding, river water was pCO 2 supersaturated in the inner estuary during summer 2003 but decreased quickly in the mid-estuary due to mixing with low pCO 2 waters from offshore. In addition, settling of particles in the estuary resulted in better light conditions so that phytoplankton bloomed, driving down pCO 2 to ∼200 μatm. In the outer estuary and outside of the bloom area, pCO 2 increased again to near or just below saturation. Literature data also reveal that the mainstream of the Changjiang is always supersaturated with respect to CO 2 probably because the decomposition of terrestrial organic matter overwhelms the consumption of CO 2 due to biological production. Because the Changjiang outflow accounts for 90% of the total river flow to the East China Sea (ECS), any variation in the Changjiang could have significant implications for the ECS. For instance, completion of the Three Gorge's Dam could change the metabolic status of the estuary by cutting off 70% of the downstream transport of organic carbon-containing particles. This would reduce the extent of organic carbon decomposition, producing better light conditions and enhancing autotrophy. As a result, the estuary could become a smaller source of CO 2 to the atmosphere. On the other hand, if the Three Gorge's Dam reduced freshwater output, especially in summer, upwelling of nutrient-rich offshore waters would be reduced resulting in a reduction in autotrophy in the much wider ECS shelves. This effect could outweigh the reduced heterotrophy in the estuary and the ECS as a whole could become a smaller CO 2 sink.

The annual cycle of carbon dioxide and parameters influencing the air–sea carbon exchange in the Baltic Proper

A land-based field station, two moored buoys and data from the Finnpartner ship were used to investigate the variability of the air–sea CO 2-flux and parameters controlling the flux during one year in the Baltic Sea region. The agreement between the sea surface partial pressure of CO 2 measured near the tower and from the ship in the central parts of the Baltic Proper was relatively good during most of the period. But, during periods with intense biological activity or strong upwelling there were significant differences. The flux of CO 2 was measured with the eddy-correlation method. The transfer velocity was calculated from the flux measurements and the instrumental uncertainty in calculations of the hourly values of transfer velocity was of the order of 20%. The calculated value of the transfer velocity increased with increasing the wind speed. The relation showed, however, great scatter and no clear wind-dependent relation could be determined. It was shown that for the measured flux and for transfer velocities estimated from measurements it is important to know the variability of pCO 2 w in the footprint area. This is of particular importance when investigating the processes influencing the flux. When calculating the air–sea flux of CO 2 the greatest uncertainty is in the determination of the transfer velocity, but it was shown that also the partial pressure of CO 2 in the surface water is crucial to determine with good accuracy.

Satellite-driven modeling of the upper ocean mixed layer and air–sea CO 2 flux in the Mediterranean Sea

The air–sea CO 2 flux, the carbon export to the deep layers, and more generally the carbon budgets are presently poorly characterized in the Mediterranean Sea. An approach to the simulation of these fluxes at regional scale is proposed, based on an array of unconnected one-dimensional (1D) physical–biological–chemical coupled models. The rationale is to massively assimilate satellite information, on one hand to derive accurate surface heat fluxes, and, on the other hand, to implicitly account for the horizontal processes not explicitly represented in the 1D scheme. This method is applied here to simulate the upper ocean physical and biogeochemical dynamics of the entire Mediterranean Sea over the years 1998–2004, and at a 0.5° spatial resolution. The mixed-layer annual cycles are validated against the values determined from a database of in situ temperature profiles, demonstrating the validity of the approach in various physical regimes. A validation of the simulated annual cycles of the total inorganic carbon (TCO 2) and of the carbon dioxide partial pressure ( pCO 2) is presented at a measurement site in the northwestern Mediterranean where both properties were determined in 1998–1999 and in 2003–2004. An additional validation of the pCO 2 fields is presented using along-track data collected during the “Productivité des Systèmes Océaniques Pélagiques” (PROSOPE) cruise. The basin-scale air–sea carbon budget derived from the model outputs shows that the Mediterranean Sea, over the study period, is close to equilibrium with the atmosphere, with a slight sink for the atmospheric CO 2. This is in agreement with the results previously obtained via indirect methods and is supported here by a series of sensitivity studies. A detailed analysis of the pCO 2 and TCO 2 seasonal distributions in the Mediterranean Sea is provided for the first time. It shows that the biological processes play a major role in shaping the pCO 2 seasonal evolution in the eastern and western basins.

The response to phase-dependent wind stress and cloud fraction of the diurnal cycle of SST and air–sea CO 2 exchange

We present the results of a 1D modelling study using a two equation k– ϵ turbulence closure model to generate realistic upper-ocean temperatures. The model is forced hourly with a variety of idealized diurnal wind and cloud profiles to investigate the sensitivity of the diurnal response and its effect on air–sea CO 2 transfer at low to moderate wind speeds. In many cases a diurnal warm layer is formed and this affects gas fluxes. The results show that phase-dependent wind speed and cloud fraction with respect to solar insolation are important for determining the magnitude and shape of the diurnal variability and the depth of the diurnal mixed layer. Phasing the wind forcing could double the diurnal temperature difference (e.g. for solar insolation 12 h out of phase with maximum wind stress), and in the same case the net CO 2 flux was increased by approximately 30%. Exact calculations of air–sea CO 2 flux are compared to simplified calculations based on night-time temperatures, with or without a ‘cool-skin’ correction and average wind speed. These simplified calculations underestimate the net CO 2 flux in a weak source region. When Δ T was at it largest the net daily air–sea CO 2 flux was reduced by up to 61.7% if daily average values for wind speed and SST were used, compared to the flux obtained from the 1D model. Short-term wind speed variability leads to large discrepancies as a result of the non-linear relationship between wind speed and CO 2 transfer, which is even further enhanced by diurnal processes and subsequent convective enhancement. Irrespective of varying winds or clouds, the formation of a diurnal warm layer has a significant effect on air–sea CO 2 transfer resulting in increased ocean degassing.

Air–sea CO 2 exchange in a subtropical estuarine-coral reef system, Kaneohe Bay, Oahu, Hawaii

The present study represents the first time-series evaluation of air–sea CO 2 exchange for a coastal marine system of a subtropical high island of the Pacific. From September 2003 through September 2004, surface water total alkalinity (TA) and dissolved inorganic carbon (DIC) data were collected bimonthly throughout the Kaneohe Bay estuarine-coral reef system. The partial pressure of carbon dioxide ( pCO 2) and the air–sea CO 2 exchange flux were calculated using TA, DIC, atmospheric CO 2 at sea level, and five wind speed relationships for gas transfer velocity. pCO 2 values were above the atmospheric level (average: 378 μatm) throughout Kaneohe Bay (400–500 μatm) and Kaneohe Stream waters (600–1300 μatm) during baseline conditions. pCO 2 levels above atmospheric in bay water were driven mainly by calcification while pCO 2 levels above atmospheric in stream water were driven mainly by remineralization of terrestrial organic matter. pCO 2 values above the atmospheric level were observed in surface water that extended almost 4 km beyond the boundary of the bay indicating that processes inside the bay such as calcification affect the inorganic carbon properties of the surrounding near shore open ocean. Precipitation from storm events in the Kaneohe watershed increases stream and land runoff that brings excess nutrients to bay waters. These excess nutrients stimulate phytoplankton blooms which draw down CO 2 in Kaneohe Bay water through photosynthesis. Despite the significant effects of the storms, Kaneohe Bay was a net annual source of CO 2 to the atmosphere of − 0.046 Gmol C year − 1 (− 0.55 kton C year − 1 ) with an area-specific flux of − 1.45 mol C m − 2 year − 1 (− 17.4 g C m − 2 year − 1 ). The source of CO 2 in Kaneohe Bay could be stronger on a long-term basis since dry years may be stronger net annual sources than wet years such as this study period. As shown by Kaneohe Bay data, the dynamic nature of estuarine systems makes closely spaced time-series data imperative for characterizing accurately the inorganic carbon system and the net annual air–sea exchange flux of CO 2.

Inorganic carbon and nutrient fluxes on the Arctic Shelf

Historic data from the Russian-American Hydrochemical Atlas of Arctic Ocean together with data from the TRANSDRIFT II 1994 and TUNDRA 1994 cruises have been used to assess the spatial and inter-annual variability of carbon and nutrient fluxes, as well as air–sea CO 2 exchange in the Laptev and western East Siberian Seas during the summer season. Budget computations using summer data of dissolved inorganic phosphate (DIP), dissolved inorganic nitrogen (DIN) and dissolved inorganic carbon (DIC) gives that the Laptev Sea shelf is a net sink of DIP and DIN of 2.5×10 6, 23.2×10 6 mol d −1, respectively, while it is a net source of DIC (excluding air–sea exchange) of 1249×10 6 mol d −1. In the East Siberian Seas the budget computations give 0.5×10 6, −11.4×10 6 and −173×10 6 mol d −1 (minus being a sink) for DIP, DIN, and DIC, respectively. In summers, the Laptev Sea Shelf is net autotrophic while the East-Siberian Sea Shelf is net heterotrophic, and both systems are weak net denitrifying. The Laptev Sea Shelf takes up 2.1 mmol CO 2 m −2 d −1 from atmosphere, whereas the western part of the East-Siberian Sea Shelf loose 0.3 mmol CO 2 m −2 d −1 to the atmosphere. The variability of DIP, DIN and DIC fluxes during summer in the different regions of the Laptev and East Siberian Seas depends on bottom topography, river runoff, exchange with surrounding seas and wind field.