Emission of CO2 and its related carbonate system dynamics in a hotspot area during winter and summer: The Changjiang River estuary

The carbonate chemistry in river-dominated marginal seas is highly heterogeneous, and there is ongoing debate regarding the definition of atmospheric CO2 source or sink. On this basis, we investigated the carbonate chemistry and air-sea CO2 fluxes in a hotspot estuarine area: the Changjiang Estuary during winter and summer. The spatial characteristics of the carbonate system were influenced by water mixing of three end-members in winter, including the Changjiang freshwater with low total alkalinity (TA) concentration, the less saline Yellow Sea Surface Water with high TA, and the saline East China Sea (ECS) offshore water with moderate TA. While in summer with increased river discharge, the carbonate system was regulated by simplified two end-member mixing between the Changjiang freshwater and the ECS offshore water. By performing the end-member mixing model on DIC variations in the river plume region, significant biological addition of DIC was found in winter with an estimation of -120 ± 113 μmol kg-1 caused by wintertime organic matter remineralization from terrestrial source. While this biological addition of DIC shifted to DIC removal due to biological production in summer supported by the increased nutrient loading from Changjiang River. The pCO2 dynamics in the river plume and the ECS offshore were both subjected to physical mixing of freshwater and seawater, whether in winter and summer. In the inner estuary without horizontal mixing, the pCO2 dynamics were mainly influenced by biological uptake in winter and temperature in summer. The inner estuary, the river plume, and the ECS offshore were sources of atmospheric CO2, with their contributions varying seasonally. The Changjiang runoff enhanced the inner estuary's role as a CO2 source in summer, while intensive biological uptake reduced the river plume's contribution.

Effects of HCO3- and CO2 conversion rates on carbon assimilation strategies in marine microalgae: Implication by stable carbon isotope analysis of fatty acids

Marine microalgae are an essential component of marine plankton and critical primary producers, playing a vital role in marine ecosystems. The seawater carbonate system is a dynamic equilibrium system, and changes in any component can alter the carbonate balance. In CO2-concentrating mechanisms (CCMs), carbonic anhydrase (CA) regulates CO2 concentration by catalyzing the interconversion between CO2 and HCO3-. Therefore, limiting the activity of extracellular carbonic anhydrase (exCA) alters the rate at which carbonate equilibrium is reached and further affects the carbon assimilation process in microalgae. In this study, two different microalgae, Phaeodactylum tricornutum and Nannochloropsis oceanica, were selected to investigate the effects of changes in the carbonate system on photosynthetic carbon assimilation in microalgae by inhibiting exCA activity with acetazolamide (AZ). Inhibition of exCA activity reduces specific growth rates and photosynthetic efficiency of microalgae. The total alkalinity, HCO3- concentration, and CO2 concentration of the cultures increased with the decrease of pH, but the changes of the ribulose 1,5- bisphosphate carboxylase/oxygenase (Rubisco) activities of the two microalgae were different. In addition, the two microalgae possessed different lipid and carbohydrate synthesis strategies, but both restricted triacylglycerol (TAG) synthesis. Meanwhile, the microalgal cells had to utilize more 13CO2 when HCO3- and CO2 conversion rates were limited and restricted. This led to the continuous accumulation of 13C in fatty acids and the elevation of δ13CFAs. In conclusion, our study provides a new perspective on the role of microalgae in correlating carbonate changes with photosynthetic carbon assimilation strategies under mechanistic constraints on inorganic carbon utilization.

Daily variability of pH and temperature in seawater from a near-pristine oceanic atoll, Southwest Atlantic

The study of pH and temperature variability in reef environments, and the underlying processes that control this variability, is of great importance for ocean acidification research. Therefore, in the reef environment of Rocas Atoll, we conducted continuous monitoring of pH and temperature and periodic sampling of carbonate chemistry, and we hypothesize that seawater temperature is not the determining factor in the daily variability of pH at this atoll. Our results showed that the seawater of the atoll presented a high daily variability in pH, [H+], and temperature. The cycles of variations occurred primarily with a periodicity of ∼24 h, related to the daily light cycle, and secondarily with a periodicity of ∼12 h, associated with the semi-diurnal tidal cycles of the atoll. The results indicate that the relative balance of net organic carbon metabolism is the main process modulating carbonate chemistry on the atoll throughout the day.

Geographical and seasonal patterns in the carbonate chemistry of Narragansett Bay, RI

This study examined geographical and seasonal patterns in carbonate chemistry and will facilitate assessment of acidification conditions and the current state of the seawater carbonate chemistry system in Narragansett Bay. Direct measurements of total alkalinity, dissolved inorganic carbon, dissolved oxygen percent saturation, water temperature, salinity and pressure were performed during monthly sampling cruises carried out over three years. These measurements were used to calculate the following biologically relevant carbonate system parameters: total pH ( p H T ) , the partial pressure of carbon dioxide in the gas phase p C O 2 , and the aragonite saturation state Ω A . The information provided by carbonate chemistry analysis allowed for the characterization of acidification events which have the potential to disrupt the species composition and ecological functioning of coastal biological communities and threaten commercially important aquatic life. We found very robust relationships between salinity and total alkalinity R adjusted 2 = 0.82 and between salinity and dissolved inorganic carbon R adjusted 2 = 0.81 that persisted through all regions, seasons, and depth-layers with mixing of coastal waters with freshwater entering in the upper bay being an important driver on alkalinity and dissolved inorganic carbon distributions. We compared the metabolically linked calculated carbonate system parameters with dissolved oxygen (DO) saturation and found high correlation, with DO percent saturation exhibiting robust correlation with the calculated carbonate system parameters total pH ( r = 0.70 ) and with partial pressure of carbon dioxide in the gas phase ( r = - 0.71 ) . Using a statistical model to correct for the confounded effects of time and space that are a common challenge in marine survey design, we found that acidification events occurred in the Northern Region of the bay, primarily during the Summer and Fall, and likely due to a combination of microbial respiration and stratification. These acidification events, especially in the Northern Region, have the potential to adversely impact aquatic life.

The carbon sink of the Coral Sea, the world's second largest marginal sea, weakened during 2006-2018

The latest reports show that the ocean absorbs approximately 26 % of anthropogenic CO₂ and that the carbon sink of the global ocean (air-sea CO₂ flux) is continually increasing, while variations in different marginal seas are complicated. The Coral Sea, the second largest marginal sea in the world, is characterized by a generally oligotrophic basin and borders the biodiversity hotspot of Great Barrier Reef. In this study, we proposed a semianalytical method and reconstructed the first high-resolution satellite-based pCO₂ and air-sea CO₂ flux dataset from 2006 to 2018 for the Coral Sea. This dataset performed well in the basin (RMSE<10 μatm, R² > 0.72) and coral reef areas (RMSE<12 μatm, R² > 0.8) based on validation by a massive independent dataset. We found that sea surface pCO₂ is increasing (1.8 to 2.7 μatm/year) under the forcing of increasing atmospheric CO₂, and the pCO₂ growth rate in water is faster than that in the atmosphere. The combination of increasing sea surface pCO₂, high pCO₂ seawater from coral reef areas, and the low depletion capacity of the oligotrophic basin led to a gradual weakening of the carbon sink in the Coral Sea, with the 2016 carbon sink being 52 % of that in 2006. This weakening was more pronounced after strong El Niño events (e.g., 2007, 2010, and 2016), with the corresponding high SST and low wind speed further weakening the carbon sink. This understanding of the long-term change in the Coral Sea provides new insight on the carbonate system variation and carbon sink capacity evolution in seawater under increasing atmospheric CO₂.

Haemolymph pH of two important mollusc species is susceptible to seawater buffering capacity instead of pH or pCO2

The acid-base status and balance of molluscs are considered to be susceptible to environmental changes, especially in the context of ocean acidification (OA). Here, we studied the effects of manipulated seawater carbonate chemistry on the acid-base status of scallop Chlamys farreri and abalone Haliotis discus hannai. The haemolymph pH of the tested individuals showed a fast response to acidified seawater incubation, and the pH level was restored to a normal value within 1 h of recovery in control seawater. However, no significant correlation (P > 0.05) was found between haemolymph pH and seawater pCO₂ or pH, while the squared Pearson correlation coefficient (R²) ranged from close to zero to 0.41. In addition, although the pCO₂ level of total alkalinity (TA)-lowered seawater was lower than half of that in the control, molluscs eliminated less CO₂ (less than 80%) to TA lowered waters than to the control waters. These findings seem to disagree with the crucial role of seawater pCO₂ in influencing the acid-base balance of molluscs. CO₂ elimination occurs in the microenvironment, and CO₂ first diffuses to limited amounts of seawater that tightly surround the gills, causing dissolved inorganic carbon (DIC) accumulation in the ventilation sites, which leads to a sharp increase in the pCO₂ of the surrounding seawater. Moreover, in this microenvironment, the pCO₂ level increases much faster and more greatly if the environmental seawater is acidified or contains a lower level of TA. Therefore, mollusc acid-base status is influenced by rapidly varying pCO₂ levels at the ventilation site, which is largely independent of that of the rest of the incubating seawater. In summary, CO₂ elimination by molluscs relies heavily on the carbonate chemistry of environmental seawater, and seawater buffering capacity should be taken into consideration instead of considering only pCO₂ or pH in studying the acid-base balance of marine molluscs.

Variations in aragonite saturation state and its controlling factors in the South Yellow Sea in spring and autumn

To assess the progression of ocean acidification in the South Yellow Sea (SYS), the aragonite saturation state (Ω_arag) was determined from dissolved inorganic carbon (DIC) and total alkalinity (TA) in the surface and bottom waters of the SYS in spring and autumn. The Ω_arag exhibited large spatiotemporal variations in the SYS; DIC was a major factor controlling the Ω_arag variations, whereas temperature, salinity, and TA were minor factors. Surface DIC concentrations were mainly influenced by the lateral transport of the DIC-enriched Yellow River waters and DIC-depleted East China Sea Surface Water; bottom DIC concentrations were affected by aerobic remineralization in spring and autumn. Ocean acidification is now seriously progressing in the SYS, particularly in the Yellow Sea Bottom Cold Water (YSBCW) where the mean value of Ω_arag substantially decreased from 1.55 in spring to 1.22 in autumn. All Ω_arag values measured in the YSBCW in autumn were lower than the critical threshold value of 1.5 necessary for the survival of calcareous organisms.

Assessment on seasonal acidification and its controls in the Muping Marine Ranch, Yantai, China

Ocean acidification has emerged as a major challenge affecting the development of the marine aquaculture. Seasonal variations of seawater pH and aragonite saturation (Ω_arag) were investigated in the Muping Marine Ranch, Yantai. The results showed that the seasonal variations of pH and Ω_arag were distinct. The temperature exerted opposite effects on pH and Ω_arag and played a dominant role in pH variation, while limited role in Ω_arag. The air-sea exchange had a syntropic effect on pH and Ω_arag but less impact on their seasonal variations. Biological activities affected seasonal variations of surface seawater pH and Ω_arag, but they largely canceled each other out with other non-temperature effects; while bottom seawater Ω_arag was mainly controlled by biological respiration in summer. This study demonstrates that pH is primarily controlled by seasonal temperature changes, whereas Ω_arag would be a better indicator for ocean acidification caused by non-temperature processes.

Responses of the marine carbonate system to a green tide: A case study of an Ulva prolifera bloom in Qingdao coastal waters

As an environmental nuisance, Ulva prolifera green tides have occurred annually in the southern Yellow Sea since 2007. While it is expected that high levels of biological activity during these blooms can alter seawater carbonate chemistry, there has been little research on the responses of marine carbonate system to green tides. Here, the effects of the bloom on the carbonate system were examined on three cruises in June, July, and September, corresponding to the early-, late-, and after-bloom periods of the U. prolifera bloom in Qingdao coastal waters in 2018. Among these three stages, the pH (National Bureau of Standards scale), dissolved inorganic carbon (DIC), total alkalinity (TA), and partial pressure of CO₂ (pCO₂) were all affected by bloom, with the highest pH and lowest DIC and TA concentrations of the surface seawater occurring at the late-bloom stage. While pCO₂ continuously increased from the beginning to the end of the bloom. TA increased by ∼40 μmol kg^-1 between the early- and after-bloom periods likely due to the shifts in the carbonate system equilibrium caused by increased CO₃^2- concentrations and the organic matter released by U. prolifera during decomposition. Compared to nearby areas with no U. prolifera bloom, the green tide, along with increasing temperature, reduced the pH and DIC but increased the TA and pCO₂. This large-scale bloom also turned the coastal waters from being an atmospheric CO₂ sink to a strong source, with the estimation of air-sea CO₂ fluxes about 1.69 ± 1.70, 2.28 ± 1.16, and 7.44 ± 5.84 mmol m^-2 d^-1 during the early-, late-, and after-bloom periods, respectively. This bloom event also promoted the formation of CaCO₃ and was an important source of low molecular weight organic acids. These new findings provide nuances for the current conversations on the role of biological processes in modulating marine carbonate system and the contribution of organic matter to alkalinity.

Massive shellfish farming might accelerate coastal acidification: A case study on carbonate system dynamics in a bay scallop (Argopecten irradians) farming area, North Yellow Sea

Seven cruises were carried out in a bay scallop (Argopecten irradians) farming area and its surrounding waters, North Yellow Sea, from March to November 2017 to study the dynamics of the carbonate system and its controlling factors. Results indicated that the studied parameters were highly variability over a range of spatiotemporal scales, comprehensively forced by various physical and biological processes. Mixing effect and scallop calcification played the most important role in the seasonal variation of total alkalinity (TAlk). For dissolved inorganic carbon (DIC), in addition to mixing, air-sea exchange and microbial activity, e.g. photosynthesis and microbial respiration processes, had more important effects on its dynamics. Different from the former, the changes of water pH_T, partial pressure of CO₂ (pCO₂) and aragonite saturation state (Ω_A) were mainly controlled by the combining of the temperature, air-sea exchange, microbial activity and scallop metabolic activities. In addition, the results indicated that massive scallop farming can significantly increase the DIC/TAlk ratio by reducing the TAlk concentration in seawater, thereby reducing the buffering capacity of the carbonate system in seawater especially for Ω_A. Preliminary calculated, ~75.7 and ~45.5 μmol kg^-1 of TAlk were removed from the surface and bottom waters respectively in one scallop cultivating cycle. If these carbonates cannot be replenished in time, it is likely to accelerate the acidification process of coastal waters. This study highlighted the control mechanism of the carbonate system under the influence of bay scallop farming, and provided useful information for revealing the potential link between human activities (shelled-mollusc mariculture) and coastal acidification.

Carbonate chemistry variability in the southern Yellow Sea and East China Sea during spring of 2017 and summer of 2018

Marginal seas are highly productive and disproportionately large contributors to global air-sea CO₂ fluxes. Due to complex physical and biogeochemical conditions, the southern Yellow-East China Sea is an ideal site for studying carbonate chemistry variability. The carbonate system was investigated in the area in spring of 2017 and summer of 2018. Dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations were higher in the SYS than the ECS due to material from carbonate weathering and erosion carried by the Yellow River. High pH and low DIC and TA were observed in the Zhe-Min Coastal Current in spring due to high primary productivity caused by Changjiang River input and the Taiwan Warm Current. Temperature and biological activity were the primary drivers controlling the partial pressure of CO₂ (pCO₂) in the SYS, pCO₂ was controlled by primary productivity related to nutrients carried by the Changjiang River and physical mixing in the Changjiang River plume and inner/middle shelves of the ECS, whereas temperature was the dominant factor determining pCO₂ distributions in the ECS outer shelf waters influenced by the Kuroshio Current. Overall, the entire study area shifted from a CO₂ sink (-4.18 ± 5.60 mmol m^-2 d^-1) to a weak source (1.02 ± 4.87 mmol m^-2 d^-1) from spring to summer. Specifically, the SYS and ECS offshore waters changed from CO₂ sinks in spring to sources in summer, while the Changjiang River plume was always a CO₂ sink.

Seasonal variation in aragonite saturation states and the controlling factors in the southeastern Yellow Sea

The aragonite saturation state (Ω_arag) was determined to assess its seasonal variations and the major controlling factors in the southeastern Yellow Sea (YS) over four seasons. Ω_arag showed large seasonal variation in the surface waters, with dissolved inorganic carbon (DIC) as a major factor controlling the seasonal variation. In the bottom waters, Ω_arag exhibited only small seasonal variation compared with the surface waters; DIC and total alkalinity were the main factors contributing to the variation. The bottom water of the southeastern YS was undersaturated with aragonite during the fall, even though the southeastern YS was not typically associated with upwelling, freshwater discharge, or eutrophication processes. Aragonite undersaturation was most likely due to ocean dumping of organic materials. Therefore, ocean pumping should be prohibited in shallow marginal seas to prevent aragonite undersaturation.

Formation and characteristics of a ternary pH buffer system for in-situ biogas upgrading in two-phase anaerobic membrane bioreactor treating starch wastewater

Biochemical biogas upgrading retaining more CO₂ from biogas to form biomethane opens new avenues for sustainable biofuel production. For developing this technology, maintaining sustain pH for CO₂·H₂O is fundamental. This study proposes an innovative control strategy for in-situ biogas upgrading retaining and converting the CO₂ in the biogas into CH₄, via hydrogenotrophic methanogenesis without external agent. The Biogas-pH strategy limited pH drop over 7.4 by stop feeding and maintained the methanogenesis activity by biogas flow rate over 98 ml·h^-1. Low pH (7.37-7.80) decrease CO₂·H₂O as a substrate in stage-I, higher pH (7.40-8.41) enhances CO₂ to CO₂·H₂O transfer by 6.29 ± 2.20% in stage-II. Because of that 95% CO₂·H₂O converts to HCO₃^- and CO₃^2- when pH > 7.9, higher pH > 7.9 did not further upgrading the biogas. The carbonate buffer system shown open and close system characteristics in gas and liquid phase. The biogas CH₄ was upgraded from 67.27 ± 5.21% to 73.56 ± 5.01%.