Yongchen Wang

Decrease in the CO2 Uptake Capacity in an Ice-Free Arctic Ocean Basin

Sinking in Slowly As the Arctic warms and its sea ice continues to melt, more of the ocean surface will be exposed, creating the potential for greater uptake of carbon dioxide from the atmosphere. Cai et al. (p. 556, published online 22 July) present results from a series of Arctic Ocean transects that show that the amount of CO2 in the surface waters has increased greatly recently. This will act as a barrier to future CO2 uptake and suggests that the Arctic Ocean will not become the large CO2 sink that some have predicted. The current carbon dioxide levels in the Arctic Ocean basin will limit further uptake under ice-free conditions. It has been predicted that the Arctic Ocean will sequester much greater amounts of carbon dioxide (CO2) from the atmosphere as a result of sea ice melt and increasing primary productivity. However, this prediction was made on the basis of observations from either highly productive ocean margins or ice-covered basins before the recent major ice retreat. We report here a high-resolution survey of sea-surface CO2 concentration across the Canada Basin, showing a great increase relative to earlier observations. Rapid CO2 invasion from the atmosphere and low biological CO2 drawdown are the main causes for the higher CO2, which also acts as a barrier to further CO2 invasion. Contrary to the current view, we predict that the Arctic Ocean basin will not become a large atmospheric CO2 sink under ice-free conditions.

The geochemistry of dissolved inorganic carbon in a surficial groundwater aquifer in North Inlet, South Carolina, and the carbon fluxes to the coastal ocean

Abstract We report measurements of pH, total dissolved inorganic carbon (DIC), total or titration alkalinity (TAlk), Ca2+, Mg2+, sulfate, and sulfide data at the seawater-freshwater interface in a shallow groundwater aquifer in North Inlet, South Carolina. These measurements and a diagenetic modeling analysis indicate that the groundwaters at North Inlet are mixtures of seawater and freshwater end-members and are seriously modified by carbon dioxide inputs from organic carbon degradation via SO42− reduction across the entire salinity range and fermentation and CaCO3 dissolution in the low-salinity region. DIC and TAlk are several times higher than the theoretical dilution line, whereas Ca2+ is slightly higher and SO42− is somewhat lower than the dilution line. Partial pressure of CO2 in the groundwater is extremely high (0.05 to 0.12 atm). These deviations are consistent with theoretical predictions from known diagenetic reactions. Estimated groundwater DIC fluxes to the South Atlantic Bight from either the surficial aquifer (via salt marshes) or the Upper Floridan Aquifer (direct input) are significant when compared to riverine flux in this area.

Acid-Base Properties of Dissolved Organic Matter in the Estuarine Waters of Georgia, USA

The contribution of dissolved organic matter (DOM) to alkalinity in estuarine waters and its relationship to CO2 chemistry has not received much previous attention. In this paper, we present some of the first organic alkalinity measurements in the context of estuarine mixing, focusing on three rivers in the Southeastern US. The simple model presented here illustrates that the organic contribution to alkalinity in estuarine waters is largely controlled by the dramatic pH change in the early stage of the mixing. As mixing progresses, organic alkalinity becomes nearly conservative with respect to salinity change. Although no DOM removal during estuarine mixing was detected here, this work provides an alternative approach to evaluate the issues of colloid and small particle formation and coagulation during mixing. This paper demonstrates that a large fraction of the proton binding sites of humic substances is either completely protonated or deprotonated during the estuarine mixing processes and, therefore, that these sites contribute neither to alkalinity nor to proton transfer reactions. One class of chargeable sites may be adequate to model the acid-base properties of humic substances during the mixing process.

Coral Energy Reserves and Calcification in a High-CO2 World at Two Temperatures

Rising atmospheric CO2 concentrations threaten coral reefs globally by causing ocean acidification (OA) and warming. Yet, the combined effects of elevated pCO2 and temperature on coral physiology and resilience remain poorly understood. While coral calcification and energy reserves are important health indicators, no studies to date have measured energy reserve pools (i.e., lipid, protein, and carbohydrate) together with calcification under OA conditions under different temperature scenarios. Four coral species, Acropora millepora, Montipora monasteriata, Pocillopora damicornis, Turbinaria reniformis, were reared under a total of six conditions for 3.5 weeks, representing three pCO2 levels (382, 607, 741 µatm), and two temperature regimes (26.5, 29.0°C) within each pCO2 level. After one month under experimental conditions, only A. millepora decreased calcification (−53%) in response to seawater pCO2 expected by the end of this century, whereas the other three species maintained calcification rates even when both pCO2 and temperature were elevated. Coral energy reserves showed mixed responses to elevated pCO2 and temperature, and were either unaffected or displayed nonlinear responses with both the lowest and highest concentrations often observed at the mid-pCO2 level of 607 µatm. Biweekly feeding may have helped corals maintain calcification rates and energy reserves under these conditions. Temperature often modulated the response of many aspects of coral physiology to OA, and both mitigated and worsened pCO2 effects. This demonstrates for the first time that coral energy reserves are generally not metabolized to sustain calcification under OA, which has important implications for coral health and bleaching resilience in a high-CO2 world. Overall, these findings suggest that some corals could be more resistant to simultaneously warming and acidifying oceans than previously expected.

Microelectrode characterization of coral daytime interior pH and carbonate chemistry

Reliably predicting how coral calcification may respond to ocean acidification and warming depends on our understanding of coral calcification mechanisms. However, the concentration and speciation of dissolved inorganic carbon (DIC) inside corals remain unclear, as only pH has been measured while a necessary second parameter to constrain carbonate chemistry has been missing. Here we report the first carbonate ion concentration ([CO32−]) measurements together with pH inside corals during the light period. We observe sharp increases in [CO32−] and pH from the gastric cavity to the calcifying fluid, confirming the existence of a proton (H+) pumping mechanism. We also show that corals can achieve a high aragonite saturation state (Ωarag) in the calcifying fluid by elevating pH while at the same time keeping [DIC] low. Such a mechanism may require less H+-pumping and energy for upregulating pH compared with the high [DIC] scenario and thus may allow corals to be more resistant to climate change related stressors.

pH and pCO2 microelectrode measurements and the diffusive behavior of carbon dioxide species in coastal marine sediments

The performance of a polymeric membrane-based pH microelectrode and a new pCO2 microelectrode in marine sediments is evaluated for the purpose of collecting fine-scale and undisturbed porewater profiles of carbon dioxide parameters. Both microelectrodes have a precision of 0.01–0.02 logarithmic units. A fine-scale porewater profile of total dissolved inorganic carbon (TCO2) is calculated from the pH and pCO2 data and is in good agreement with coarse-scale TCO2 profiles measured in squeezed porewater. The measuring uncertainty for TCO2 is within 5%. The measurements of the pH microelectrode also agree in general with those of a glass mini-electrode. The mini-electrode, however, measures smaller pH changes as a result of mixing the sediments. Very low pH values (<6.0) are measured in this salt marsh-influenced sediment and are correlated to the oxidation of Mn2+, Fe2+, and possibly, solid sulfides. Concentration profiles and fluxes of TCO2, CO2, HCO3− and CO32− are calculated from the pH and pCO2 data. These millimeter-scale profiles demonstrate that the calculation of benthic flux of TCO2 based on porewater TCO2 profiles and on a HCO3− (or a concentration “weighted”) diffusion coefficient can be erroneous. When porewater pH is very low (<7.0) in the near-surface sediment, as is the case in many organic-rich coastal sediments, TCO2 transport is accomplished significantly through the diffusion of CO2 and can be greatly enhanced due to a sharp concentration gradient and a large diffusion coefficient of CO2.

A long pathlength liquid-core waveguide sensor for real-time pCO2 measurements at sea

An improved spectrophotometric pCO2 sensor based on a long pathlength liquid-core waveguide is described for pCO2 underway measurements. A low refractive index (RI) amorphous fluoropolymer (Teflon AF 2400) tubing, the heart of the sensor, served as both a CO2-permeable membrane equilibrator and a long pathlength liquid-core waveguide spectrophotometric cell. By using absorbance ratios at three wavelengths and carefully preparing and storing indicator solution, good reproducibility and long-term stability were achieved. The sensor behaved closely to theoretical prediction. Two pronounced features of the sensor were fast response (approximately 2 min to reach 99% of full response) due to high permeability of the Teflon AF, and high precision (about ±2–3 μatm in the pCO2 range of 200–500 μatm) due to the long pathlength. The temperature dependence of the sensor is also discussed. The sensors measurements agreed well with the “showerhead equilibrator plus infrared detector” method during an underway survey of sea surface pCO2 along a transect off the Georgia coast in December 2000. Besides underway mapping, the sensor shows broad applicability for measuring pCO2 in different environments.

A long pathlength spectrophotometric pCO2 sensor using a gas-permeable liquid-core waveguide

The first long pathlength fiber optic-based sensor system to measure pCO(2) in natural waters and the atmosphere is described. The sensor is based on a liquid-core (an indicator-HCO(3)(-)/CO(3)(2-) buffer solution) waveguide made of a low refractive index amorphous fluoropolymer tubing, the wall of which serves as a gas-permeable membrane to sense pCO(2) changes. The system detects the indicator absorbance changes when the liquid-core reaches CO(2) equilibrium with the surrounding sample. Theoretical calculations demonstrate that due to indicator buffer effects, increasing the optical pathlength is a more efficient way to obtain higher sensitivity than increasing the indicator concentration. Using an 18-cm cell with low indicator concentrations (10 muM), this system achieves a precision and an accuracy of +/-2-3 muatm in the pCO(2) range of 200-500 muatm. The sensor also features a response time (99%) of only 2 min for low-level (<1000 muatm) pCO(2) measurements as a result of high CO(2) permeability of the amorphous fluoropolymer membrane. Field tests indicate that this new sensor is capable of handling both atmospheric and aquatic pCO(2) monitoring.

Physiological response to elevated temperature and pCO2 varies across four Pacific coral species: Understanding the unique host + symbiont response

The physiological response to individual and combined stressors of elevated temperature and pCO2 were measured over a 24-day period in four Pacific corals and their respective symbionts (Acropora millepora/Symbiodinium C21a, Pocillopora damicornis/Symbiodinium C1c-d-t, Montipora monasteriata/Symbiodinium C15, and Turbinaria reniformis/Symbiodinium trenchii). Multivariate analyses indicated that elevated temperature played a greater role in altering physiological response, with the greatest degree of change occurring within M. monasteriata and T. reniformis. Algal cellular volume, protein, and lipid content all increased for M. monasteriata. Likewise, S. trenchii volume and protein content in T. reniformis also increased with temperature. Despite decreases in maximal photochemical efficiency, few changes in biochemical composition (i.e. lipids, proteins, and carbohydrates) or cellular volume occurred at high temperature in the two thermally sensitive symbionts C21a and C1c-d-t. Intracellular carbonic anhydrase transcript abundance increased with temperature in A. millepora but not in P. damicornis, possibly reflecting differences in host mitigated carbon supply during thermal stress. Importantly, our results show that the host and symbiont response to climate change differs considerably across species and that greater physiological plasticity in response to elevated temperature may be an important strategy distinguishing thermally tolerant vs. thermally sensitive species.

Coral calcification under environmental change: a direct comparison of the alkalinity anomaly and buoyant weight techniques

Two primary methods—the buoyant weight (BW) and alkalinity anomaly (AA) techniques—are currently used to quantify net calcification rates (G) in scleractinian corals. However, it remains unclear whether they are directly comparable since the few method comparisons conducted to date have produced inconsistent results. Further, such a comparison has not been made for tropical corals. We directly compared GBW and GAA in four tropical and one temperate coral species cultured under various pCO2, temperature, and nutrient conditions. A range of protocols for conducting alkalinity depletion incubations was assessed. For the tropical corals, open-top incubations with manual stirring produced GAA that were highly correlated with and not significantly different from GBW. Similarly, GAA of the temperate coral was not significantly different from GBW when incubations provided water motion using a pump, but were significantly lower than GBW by 16% when water motion was primarily created by aeration. This shows that the two techniques can produce comparable calcification rates in corals but only when alkalinity depletion incubations are conducted under specific conditions. General recommendations for incubation protocols are made, especially regarding adequate water motion and incubation times. Further, the re-analysis of published data highlights the importance of using appropriate regression statistics when both variables are random and measured with error. Overall, we recommend the AA technique for investigations of community and short-term day versus night calcification, and the BW technique to measure organism calcification rates integrated over longer timescales due to practical limitations of both methods. Our findings will facilitate the direct comparison of studies measuring coral calcification using either method and thus have important implications for the fields of ocean acidification research and coral biology in general.