Surface seawater partial pressure of CO2 (pCO2) is a critical parameter in the quantification of air-sea CO2 flux, which further plays an important role in quantifying the global carbon budget and ...understanding ocean acidification. Yet, the remote estimation of pCO2 in coastal waters (under influences of multiple processes) has been difficult due to complex relationships between environmental variables and surface pCO2. To date there is no unified model to remotely estimate surface pCO2 in oceanic regions that are dominated by different oceanic processes. In our study area, the Gulf of Mexico (GOM), this challenge is addressed through the evaluation of different approaches, including multi-linear regression (MLR), multi-nonlinear regression (MNR), principle component regression (PCR), decision tree, supporting vector machines (SVMs), multilayer perceptron neural network (MPNN), and random forest based regression ensemble (RFRE). After modeling, validation, and extensive tests using independent cruise datasets, the RFRE model proved to be the best approach. The RFRE model was trained using data comprised of extensive pCO2 datasets (collected over 16 years by many groups) and MODIS (Moderate Resolution Imaging Spectroradiometer) estimated sea surface temperature (SST), sea surface salinity (SSS), surface chlorophyll concentration (Chl), and diffuse attenuation of downwelling irradiance (Kd). This RFRE-based pCO2 model allows for the estimation of surface pCO2 from satellites with a spatial resolution of ~1 km. It showed an overall performance of a root mean square difference (RMSD) of 9.1 μatm, with a coefficient of determination (R2) of 0.95, a mean bias (MB) of −0.03 μatm, a mean ratio (MR) of 1.00, an unbiased percentage difference (UPD) of 0.07%, and a mean ratio difference (MRD) of 0.12% for pCO2 ranging between 145 and 550 μatm. The model, with its original parameterization, has been tested with independent datasets collected over the entire GOM, with satisfactory performance in each case (RMSD of ≤~10 μatm for open GOM waters and RMSD of ≤~25 μatm for coastal and river-dominated waters). The sensitivity of the RFRE-based pCO2 model to uncertainties of each input environmental variable was also thoroughly examined. The results showed that all induced uncertainties were close to, or within, the uncertainty of the model itself with higher sensitivity to uncertainties in SST and SSS than to uncertainties in Chl and Kd. The extensive validation, evaluation, and sensitivity analysis indicate the robustness of the RFRE model in estimating surface pCO2 for the range of 145–550 μatm in most GOM waters. The RFRE model approach was applied to the Gulf of Maine (a contrasting oceanic region to GOM), with local model training. The results showed significant improvement over other models suggesting that the RFRE may serve as a robust approach for other regions once sufficient field-measured pCO2 data are available for model training.
•Remote sensing of ocean surface pCO2 in complex regions has large uncertainties.•A machine learning approach is developed for the Gulf of Mexico and Gulf Maine.•The approach shows significantly improved performance over other approaches.•Uncertainties in the estimated surface pCO2 are within 10 μatm for a large range.
Interactions between riverine inputs, internal cycling, and oceanic exchange result in dynamic variations in the partial pressure of carbon dioxide (pCO₂) in large estuaries. Here, we report the ...first bay-wide, annual-scale observations of surface pCO₂ and air–water CO₂ flux along the main stem of the Chesapeake Bay, revealing large annual variations in pCO₂ (43–3408 μatm) and a spatial-dependence of pCO₂ on internal and external drivers. The low salinity upper bay was a net source of CO₂ to the atmosphere (31.2 mmol m−2 d−1) supported by inputs of CO₂-rich Susquehanna River water and the respiration of allochthonous organic matter, but part of this region was also characterized by low pCO₂ during spring and fall phytoplankton blooms. pCO₂ decreased downstream due to CO₂ ventilation supported by long water residence times, stratification, mixing with low pCO₂ water masses, and carbon removal by biological uptake. The mesohaline middle bay was a net CO₂ sink (−5.8 mmol m−2 d−1) and the polyhaline lower bay was nearly in equilibrium with the atmosphere (1.0 mmol m−2 d−1). Although the main stem of the bay was a weak CO₂ source (3.7 ± 3.3 × 10⁹ mol C) during the dry hydrologic (calendar) year 2016, our observations showed higher river discharge could decrease CO₂ efflux. In contrast to many other estuaries worldwide that are strong sources of CO₂ to the atmosphere, the Chesapeake Bay and potentially other large estuaries are very weak CO₂ sources in dry years, and could even turn into a CO₂ sink in wet years.
In the traditional view, riverine organic matter typically has a higher C:N ratio than marine phytoplankton 6.7:1 and has therefore been thought to be a carbon source in estuaries and coastal waters. ...Thus, a decrease in the riverine organic C:N ratio to <6.7:1 would potentially switch riverine organic matter from a coastal carbon source to sink. However, few studies have paid an attention to such a change. Our field investigation showed that organic C:N ratio was 11.8:1 in the pristine upstream section of a natural reserve, but decreased after the river passed through several urban cities, reaching 5.0:1 in near the Pearl River estuary. Along the river, dissolved inorganic nitrogen, total organic carbon and nitrogen all increased and they were highly negatively correlated with organic C:N ratios. The observation has a great implication that organic matter with a decreased C:N ratio from the Pearl River would potentially switch from a coastal carbon source of 2.8 × 1011 g C/year to a sink of 2.2 × 1011 g C/year. This carbon sink (2.2 × 1011 g C/year) contributes to 56% of the previous estimate of the Pearl River estuarine-coastal net carbon sink. Such a decrease in organic C:N ratio also occurs in some other large rivers, which should be considered in the assessment of global coastal carbon budgets and metabolic balance.
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•Organic C:N decreases downstream from 11.8:1 to 5.0:1 along the Pearl River.•The Pearl River organic matter switches from a coastal carbon source to sink.•River organic C:N decrease affects global coastal metabolic balance and carbon budgets.
Every year a large quantity of wastewater is generated worldwide, but its influence on the carbon dioxide (CO2) uptake by coastal oceans is not well understood. Here, sea surface CO2 partial pressure ...(pCO2) and air-sea CO2 flux were examined in the Jiaozhou Bay (JZB), a temperate coastal bay strongly disturbed by wastewater inputs. Monthly surveys from April 2014 through March 2015 showed that surface pCO2 in the JZB substantially varied both temporally and spatially between 163 μatm and 1222 μatm, with an annual average of 573 μatm. During April–December, surface pCO2 was oversaturated with respect to the atmosphere, with high values exceeding 1000 μatm in the northeastern part of the bay, where seawater salinity was low mainly due to the inputs of wastewater with salinity close to zero. During January–March, surface pCO2 was undersaturated, with the lowest value of <200 μatm also mainly in the northeastern part because of low water temperature and strong biological production. Over an annual cycle, apparently sea surface temperature dominated the monthly variation of surface pCO2 in this shallow bay, while wastewater inputs and related biological production/respiration dominated its spatial variability. Overall, the JZB was a net CO2 source to the atmosphere, emitting 9.6 ± 10.8 mmol C m−2 d−1, unlike its adjacent western part of the Yellow Sea and most of the temperate coastal oceans which are a net CO2 sink. This was possibly associated with wastewater inputs that cause high sea surface pCO2 via direct inputs of CO2 and degradation of organic matter. Thus, from this viewpoint reducing wastewater discharge or lowering CO2 levels in discharged wastewater may be important paths to enhancing the CO2 uptake by coastal oceans in the future.
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•This temperate bay was a net CO2 source, unlike most of temperate coastal oceans.•Wastewater inputs and biological processes dominated pCO2 spatial variability.•Even treated wastewater inputs reduce the CO2 uptake by coastal oceans.
Multiple isotope systematics incorporating paired carbon isotopes (δ13C and ∆14C), strontium isotopes (87Sr/86Sr) and water isotopes (δ2H and δ18O) are used to investigate the coherent relationships ...among flow paths, chemical weathering regimes, and Sr export fluxes from the Lower Mississippi River. Monthly water samples were collected at a site near Baton Rouge, Louisiana, during 2006–2008 for measurements of water isotopic composition, the concentration and isotopic composition of dissolved inorganic carbon (DIC), and the concentration and isotopic ratio of Sr along with other selected major elements. Both δ2H and δ18O followed a similar seasonal pattern with a steady increase from a minimum in March to a maximum in July, indicating a shift of water sources from the snowmelt-dominant uppermost Upper Mississippi River during spring freshet to rainfall-induced midcontinent surface runoff and groundwater during other seasons. Values of δ13C-DIC ranged from −8.67‰ to −5.96‰ while Δ14C-DIC varied from −56.8‰ to 27.9‰, corresponding to a 14C age from contemporary to 415 yr BP. Generally, Δ14C-DIC increased with increasing δ13C-DIC, suggesting variations in bicarbonate sources in response to the shifts of flow paths and chemical weathering regimes. Depleted Δ14C-DIC and δ13C-DIC values during the wet seasons are likely contributed by carbonate mineral dissolution involving soil-derived CO2, while the higher Δ14C and relatively enriched δ13C-DIC values during the dry seasons mirror the atmospheric CO2 signatures, implying the supply by silicate weathering and a seasonal CO2 exchange between riverwater and the atmosphere which is enhanced by high primary production. Sr concentrations and 87Sr/86Sr ratios averaged 1.80 ± 0.26 μmol L−1 and 0.709866 ± 0.000248, respectively. Both Sr concentrations and 87Sr/86Sr ratios show a significant correlation with δ18O values, supporting a hydrologic control of the Sr provenance in the Mississippi River basin. Indeed, the radiogenic 87Sr from the Archean and early Proterozoic terrain in the uppermost Upper Mississippi River, the Sr released from carbonate-mineral dissolution, and the radiogenic 87Sr from silicate weathering are manifested in the Lower Mississippi River with modifications from the snowmelt flow, rainfall-induced surface runoffs, and subsurface/groundwater, respectively. Overall, our results suggest that different hydrological flow regimes play unique roles in regulating the chemical weathering processes and therefore the seasonal variations in isotope systematics and in the concentrations and export fluxes of both DIC and Sr from the Mississippi River.
•The weathering regimes in the MRB were examined with multiple isotope systematics•Both Sr concentrations and 87Sr/86Sr are correlated with water isotopes•Variations in water sources regulate the Sr and bicarbonate provenance•Flow regimes control chemical weathering and isotope systematics
The spatiotemporal variabilities and drivers of ocean acidification (OA) metrics, H+, pH, and aragonite saturation state (Ωarag) across environmental gradients remain poorly constrained. We use a ...novel high‐precision measurement of underway pH to investigate the hemispheric‐scale distributions of OA metrics from East Asia to the Arctic Ocean. While temperature and its induced air‐sea gas exchange fundamentally control the OA metrics distributions, we show that biological activity exerts the most prominent but different modifications on pH and Ωarag patterns. Strong photosynthesis counteracts the temperature‐driven pH pattern but reinforces that of Ωarag. Ice melt‐induced dilution in the Arctic Ocean additionally strengthens the Ωarag‐temperature relationship but insignificantly affects H+ and pH. This study provides the first coherent assessment of comprehensive processes on OA metrics across large spatial regions, and highlights the potential of sea‐ice melt in changing Ωarag distribution, which should be included by Earth system models projecting future climate change.
Plain Language Summary
The ocean uptake of anthropogenic carbon dioxide (CO2) is causing increase in hydrogen ion concentration (H+) and reductions in pH and carbonate mineral aragonite saturation state (Ωarag), together of which are commonly referred to as ocean acidification (OA). The coupled behavior of these affected OA metrics responding to physical and biogeochemical processes across environmental gradients has barely been examined in a comparative manner. To address this issue, we conduct a survey measuring high‐precision underway pH from East Asia to the Arctic Ocean. We find that, besides the temperature effects, which ultimately control the distributions of OA metrics, biological activity induces the strongest interruptions. Photosynthesis weakens the temperature‐driven pH pattern but reinforces that of Ωarag. In addition, ice melt‐induced dilution in the polar region strengthens Ωarag‐temperature relationship but makes less difference to H+ and pH. These findings are important as they are based on the first large‐scale direct measurements of underway pH, and have implications for future studies on ocean acidification in the context of climate change.
Key Points
High‐frequency underway pH across large spatial regions is measured, which is very rare in the oceanographic community
Biological activity counteracts temperature‐driven pattern in pH, but reinforces that in Ωarag
Sea‐ice melt driven dilution in the polar regions contributes significantly to lowering Ωarag
Inorganic carbon chemistry data from the surface and subsurface waters of the West Coast of North America have been compared with similar data from the northern Gulf of Mexico to demonstrate how ...future changes in CO2 emissions will affect chemical changes in coastal waters affected by respiration-induced hypoxia (O2 ≤ ~ 60µmolkg−1). In surface waters, the percentage change in the carbon parameters due to increasing CO2 emissions are very similar for both regions even though the absolute decrease in aragonite saturation is much higher in the warmer waters of the Gulf of Mexico. However, in subsurface waters the changes are enhanced due to differences in the initial oxygen concentration and the changes in the buffer capacity (i.e., increasing Revelle Factor) with increasing respiration from the oxidation of organic matter, with the largest impacts on pH and CO2 partial pressure (pCO2) occurring in the colder West Coast waters. As anthropogenic CO2 concentrations begin to build up in subsurface waters, increased atmospheric CO2 will expose organisms to hypercapnic conditions (pCO2 >1000 µatm) within subsurface depths. Since the maintenance of the extracellular pH appears as the first line of defense against external stresses, many biological response studies have been focused on pCO2-induced hypercapnia. The extent of subsurface exposure will occur sooner and be more widespread in colder waters due to their capacity to hold more dissolved oxygen and the accompanying weaker acid-base buffer capacity. Under present conditions, organisms in the West Coast are exposed to hypercapnic conditions when oxygen concentrations are near 100µmolkg−1 but will experience hypercapnia at oxygen concentrations of 260µmolkg−1 by year 2100 under the highest elevated-CO2 conditions. Hypercapnia does not occur at present in the Gulf of Mexico but will occur at oxygen concentrations of 170µmolkg−1 by the end of the century under similar conditions. The aragonite saturation horizon is currently above the hypoxic zone in the West Coast. With increasing atmospheric CO2, it is expected to shoal up close to surface waters under the IPCC Representative Concentration Pathway (RCP) 8.5 in West Coast waters, while aragonite saturation state will exhibit steeper gradients in the Gulf of Mexico. This study demonstrates how different biological thresholds (e.g., hypoxia, CaCO3 undersaturation, hypercapnia) will vary asymmetrically because of local initial conditions that are affected differently with increasing atmospheric CO2. The direction of change in amplitude of hypercapnia will be similar in both ecosystems, exposing both biological communities from the West Coast and Gulf of Mexico to intensification of stressful conditions. However, the region of lower Revelle factors (i.e., the Gulf of Mexico), currently provides an adequate refuge habitat that might no longer be the case under the most severe RCP scenarios.
•In surface waters the percentage change in the carbon parameters due to increasing CO2 emissions are similar.•In subsurface waters the changes are enhanced due to changes in the buffer capacity.•Increased anthropogenic CO2 concentrations will expose organisms to hypercapnic conditions.
Three′s a crowd: The title reaction provides a tetracylic core bearing three chiral centers (see scheme; Ts=4‐toluenesulfonyl) with excellent enantioselectivity and good diastereoselectivity. The ...reaction was used to efficiently construct the (+)‐kreysiginine skeleton.
The Arctic Ocean has experienced tremendous changes in recent years. To evaluate temporal and spatial variations of the marine carbonate system, a rigorous evaluation of the quality and internal ...consistency of Arctic field data and the applicability of existing thermodynamic constants to Arctic conditions is needed. Low Arctic temperatures fall outside the range of conditions used to experimentally determine these constants. Using data collected during the Chinese National Arctic Research Expedition (CHINARE) cruise of summer 2010, we compared underway measurements of the partial pressure of carbon dioxide (pCO2) to pCO2 values calculated using measurements of dissolved inorganic carbon (DIC) and total alkalinity (TAlk) and seven sets of dissociation constants. For waters sampled outside areas of active sea ice melt, calculated and measured pCO2 values agreed best when the calculations incorporated the carbonic acid dissociation constants of Mehrbach et al. (1973) (as refit by Dickson and Millero (1987)) (mean difference of 1.5μatm±standard deviation of 5.7μatm) or Lueker et al. (2000) (2.3±5.4μatm). Differences between calculated and measured pCO2 values were related to temperature and salinity and were, for all sets of constants, increasing with increasing T and S over a temperature range of −1.5 to 10.5°C and a salinity range of 25.8 to 33.1. In the relatively warm Bering Sea, calculated pCO2 was higher than measured pCO2, but in the colder Canada Basin, calculated values were lower than measured values. This pattern indicates that calculations of pCO2 in very cold waters may underestimate pCO2, with an uncertainty of ~5μatm in accuracy. In areas of active sea ice melt (ice cover <35%), large differences between calculated and measured pCO2 values occurred. We explored possible explanations for these large differences and concluded that dissolution of CaCO3 precipitates from sea ice in samples is the most likely cause. Further research including a comparison of filtered and unfiltered samples is needed to resolve this issue. Many processes influence the marine carbonate system of the Arctic Ocean, and further assessment of their relative roles is needed.
•We evaluate the internal consistency of the CO2 system through pCO2, DIC, and TAlk in the Arctic Ocean•Mehrbach et al. (1973) as refit by Dickson and Millero (1987), and Lueker et al. (2000) are recommended in the Arctic•Calculated pCO2 values from DIC and TAlk are about 5µatm underestimated in the cold Arctic waters•Potential sampling artifacts are discussed to explore the cause of disagreement between calculated and measured pCO2 values
The diamondback moth Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae) is one of the most destructive insect pests of cruciferous plants worldwide. Biological, ecological and genetic studies ...have indicated that this moth is migratory in many regions around the world. Although outbreaks of this pest occur annually in China and cause heavy damage, little is known concerning its migration. To better understand its migration pattern, we investigated the population genetic structure and demographic history of the diamondback moth by analyzing 27 geographical populations across China using four mitochondrial genes and nine microsatellite loci. The results showed that high haplotype diversity and low nucleotide diversity occurred in the diamondback moth populations, a finding that is typical for migratory species. No genetic differentiation among all populations and no correlation between genetic and geographical distance were found. However, pairwise analysis of the mitochondrial genes has indicated that populations from the southern region were more differentiated than those from the northern region. Gene flow analysis revealed that the effective number of migrants per generation into populations of the northern region is very high, whereas that into populations of the southern region is quite low. Neutrality testing, mismatch distribution and Bayesian Skyline Plot analyses based on mitochondrial genes all revealed that deviation from Hardy-Weinberg equilibrium and sudden expansion of the effective population size were present in populations from the northern region but not in those from the southern region. In conclusion, all our analyses strongly demonstrated that the diamondback moth migrates within China from the southern to northern regions with rare effective migration in the reverse direction. Our research provides a successful example of using population genetic approaches to resolve the seasonal migration of insects.
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