Air Water CO2 Research Articles

Distribution and control mechanism of pCO2 and water-air CO2 efflux in the Pearl River Estuary.

Estuaries are generally considered to be important sources of atmospheric CO2. However, the differences between estuaries, and inadequate observations of partial pressure of CO2 in estuarine water (pCO2water) hamper global estuarine CO2 budgeting. In this study, the longitudinal distribution of CO2 in the waters of Modaomen (MSE) and Lingdingyang (LSE), two sub-estuaries of the Pearl River Estuary (PRE), and its influencing mechanism are studied. The change in the distribution of pCO2water along the distance from the upstream estuary to the ocean between LSE and MSE was significantly different. pCO2water at the LSE ranges from 238 to 7267 µatm, whereas the MSE ranges from 406 to 3078 µatm. Stronger microbial respiration and relatively long water retention times were the main influences that led to higher pCO2water at LSE than at MSE. Seasonally, the increase of soil CO2 into the water in the upstream basin caused by precipitation is the potential influencing factor that the water pCO2water in the flood season is higher than in the dry season. PRE was a net source of atmospheric CO2 with an average annual water-air flux of 41.2 ± 33.3mmolm-2day-1. Our results suggest that the differences in longitudinal gradients of pCO2water between estuaries in the same region and the effects of different gas transport velocity models on CO2 emission estimates need to be considered in estuarine CO2 emission budgeting.

High stability of carbonate weathering relevant carbon sink under biological pump effect in inland waters: Insights from Shawan Karst Experimental Site, Southwest China

The dissolved inorganic carbon (DIC) produced by carbonate weathering may be lost by degassing or be biologically fixed as it transports from inland water to the ocean. Knowledge of DIC stability under these two processes in surface water systems is crucial for evaluating the role of carbonate weathering in the global carbon cycle. Here, we evaluated the net ecosystem productivity of aquatic ecosystems (NEPs), water‒air CO2 exchange fluxes (FCO2), and Carbon budgets (Carbonbudget=(TOCoutput+0.5DICoutput)−(TOCinput+0.5DICinput)) in five simulated spring-pond systems controlled by carbonate weathering and different land uses. Using these parameters and the unique structure of our simulation site, the stability of the carbonate weathering-driven carbon sink (DIC) in surface water was discussed. The results showed that the NEP and FCO2 of the study site presented significant negative relationships and were mainly controlled by aquatic ecosystem metabolisms than abiotic processes. Most aquatic ecosystems in our study site were autotrophic (NEP>0) due to the strong biological carbon pump effect (BCP) induced by carbonate weathering. Moreover, the DIC input enhanced the FCO2 in the warm season. The intensive BCP enhanced both CO2 release and atmospheric CO2 invasion. Moreover, we found that the stability of DIC was positively related to DIC input, which can be influenced by BCP and human land use. After considering the net changes in Carbon budget through the groundwater-surface water systems, a high average carbon stabilization rate (CSR) of approximately 103.7% of carbonate weathering carbon sink was found in our study site. This indicated that BCP could retain most of the DIC that was driven by carbonate weathering. Therefore, due to the high stability of the carbonate weathering carbon sink under the BCP, we stress that it should be considered a significant carbon sink in the global carbon cycle.

High carbon dioxide emissions from Australian estuaries driven by geomorphology and climate

Estuaries play an important role in connecting the global carbon cycle across the land-to-ocean continuum, but little is known about Australia’s contribution to global CO2 emissions. Here we present an Australia-wide assessment, based on CO2 concentrations for 47 estuaries upscaled to 971 assessed Australian estuaries. We estimate total mean (±SE) estuary CO2 emissions of 8.67 ± 0.54 Tg CO2-C yr−1, with tidal systems, lagoons, and small deltas contributing 94.4%, 3.1%, and 2.5%, respectively. Although higher disturbance increased water-air CO2 fluxes, its effect on total Australian estuarine CO2 emissions was small due to the large surface areas of low and moderately disturbed tidal systems. Mean water-air CO2 fluxes from Australian small deltas and tidal systems were higher than from global estuaries because of the dominance of macrotidal subtropical and tropical systems in Australia, which have higher emissions due to lateral inputs. We suggest that global estuarine CO2 emissions should be upscaled based on geomorphology, but should also consider land-use disturbance, and climate.

Temporal variability of air-water gas exchange of carbon dioxide in clam and fish aquaculture ponds

The growing trend of land-based aquaculture has heightened the significance of comprehensively assessing air-water carbon dioxide (CO2) gas exchange in these inland waters, given their potential impact on carbon neutral strategies. However, temporal variations of partial pressure of CO2 (pCO2) and CO2 flux in clam and fish aquaculture ponds were barely investigated. We assessed the water surface pCO2 in one to five months intervals by deploying a lab-made buoy in three clam ponds and three fishponds located in tropical and subtropical climates. Measurements were conducted over a 24 h period each time, spanning from April 2021 to June 2022, covering the stocking, middle, and harvesting stages of the culture cycle. Diurnal pCO2 variations were dominantly controlled by biologically driven changes in dissolved inorganic carbon and total alkalinity (~97 %), while temperature and salinity effects were minor (~3 %). Clam ponds acted as a sink of atmospheric CO2 during stocking stages and transitioned to a source during middle to harvesting stages. In contrast, fishponds acted as a source of atmospheric CO2 throughout culture cycles and CO2 flux strengthened when reaching harvesting stages. Overall, clam ponds acted as a weak sink for atmospheric CO2 (−2.8 ± 17.3 mmol m−2 d−1), whereas fishponds acted as a source (16.8 ± 21.7 mmol m−2 d−1). CO2 emission was stronger during daytime coinciding with higher windspeeds compared to nighttime in fishponds. We suggest incorporating high temporal resolution measurements to account for diurnal and culture-stage variations, enabling more accurate estimates of air-water CO2 flux in aquaculture ponds. Moreover, the findings of this study highlight the importance of feeding, aeration, and biological activities (photosynthesis, remineralization, and calcification) in controlling the air-water CO2 flux in aquaculture ponds and such information can be used in implementing better strategies to achieve carbon neutral goals.

Recent inorganic carbon increase in a temperate estuary driven by water quality improvement and enhanced by droughts

Abstract. Estuaries are an important contributor to the global carbon budget, facilitating carbon removal, transfer, and transformation between land and the coastal ocean. Estuaries are susceptible to global climate change and anthropogenic perturbations. We find that a long-term significant mid-estuary increase in dissolved inorganic carbon (DIC) of 6–21 µmol kg−1 yr−1 (1997–2020) in a temperate estuary in Germany (Elbe Estuary) was driven by an increase in upper-estuary particulate organic carbon (POC) content of 8–14 µmol kg−1 yr−1. The temporal POC increase was due to an overall improvement in water quality observed in the form of high rates of primary production and a significant drop in biological oxygen demand. The magnitude of mid-estuary DIC gain was equivalent to the increased POC production in the upper estuary, suggesting that POC is effectively remineralized and retained as DIC in the mid-estuary, with the estuary acting as an efficient natural filter for POC. In the context of this significant long-term DIC increase, a recent extended drought period (2014–2020) significantly lowered the annual mean river discharge (468 ± 234 m3 s−1) compared to the long-term mean (690 ± 441 m3 s−1, 1960–2020), while the late spring internal DIC load in the estuary doubled. The drought induced a longer dry season, starting in May (earlier than normal), increased the residence time in the estuary and allowed for a more complete remineralization period of POC. Annually, 77 %–94 % of the total DIC export was laterally transported to the coastal waters, reaching 89 ± 4.8 Gmol C yr−1, and thus, between 1997 and 2020, only an estimated maximum of 23 % (10 Gmol C yr−1) was released via carbon dioxide (CO2) evasion. Export of DIC to coastal waters decreased significantly during the drought, on average by 24 % (2014–2020: 38 ± 5.4 Gmol C yr−1), compared to the non-drought period. In contrast, there was no change in the water–air CO2 flux during the drought. We have identified that seasonal changes in DIC processing in an estuary require consideration when estimating both the long-term and future changes in water–air CO2 flux and DIC export to coastal waters. Regional and global carbon budgets should therefore take into account carbon cycling estimates in estuaries, as well as their changes over time in relation to impacts of water quality changes and extreme hydrological events.

Development of a two-dimensional model to assess carbon dynamics and anthropogenic effects on CO2 emissions in the Tan river, southern China

Tidal rivers are key biochemical reaction channels along the land-ocean aquatic continuum, receiving carbon from wastewater and agricultural drains, which can considerably affect CO2 emissions. We developed a two-dimensional hydrodynamic and ecological model coupled with an inorganic carbon module along the Tan River in southern China. The simulations of and observations regarding discharge, temperature, total organic carbon (TOC), total inorganic carbon (TIC), and other common water quality variables were generally in good agreement. Based on the validated model, we employed statistical and scenario analyses to evaluate the carbon distribution, TOC and TIC budgets, and the imbalances induced by climatic and anthropogenic changes, providing insights into their potential greenhouse effect. The Tan River was consistently supersaturated with CO2 with an annual mean air-water CO2 emission flux (FCO2) of 226.1 ± 84.9 mmol m−2 d−1, and significant temporal and spatial variations of FCO2, TOC, and TIC were observed. Urban small streams tended to emit additional CO2 during wet seasons, and rural tributaries usually had an increase in TOC concentrations during the dry season. FCO2 was significantly positively correlated with air temperature and negatively correlated with total nitrogen, total phosphorus, and TOC. The annual riverine input of carbon to the urban river network was 17.37 Gg C yr−1, with 59.82% of TOC, and carbon output was 15.31 Gg C yr−1, with 66.87% of TOC. The retention rates for TOC and total carbon were 50.7% and 11.8% in the urban branch, respectively. Furthermore, warming and wastewater treatment could prevent urban river networks and downstream rivers from becoming carbon sources. Therefore, our findings suggest that riverine management strategies change the global CO2 release from tidal rivers and estuarine systems under climate change.

Multi-annual variability of pCO2(aq) and air-water CO2 flux in the mangrove-dominated Dhamra Estuary draining into the Bay of Bengal (India).

Small estuaries often remain neglected while characterizing air-water CO2 flux dynamics. This study reports the seasonal, spatial, and multi-annual variability of carbon biogeochemistry, emphasizing air-water CO2 flux from a small tropical mangrove-dominated estuary (Dhamra Estuary) of the Bay of Bengal, based on the 9-year-long sampling survey (2013 to 2021). The sampling covered twelve pre-fixed locations of this estuary. A suite of biogeochemical parameters was kept within the purview of this study to deliniate the interrelationship between CO2 fluxes and potential factors that can regulate/govern pCO2(aq) dynamics. Air water CO2 exchange rates were calculated using five globally accepted empirical gas transfer velocity equations and varied in a range of - 832.5 to 7904 μmol m-2 h-1. The estuary was a sink for CO2 in monsoon season, having the highest average flux rates of - 380.9 ± 125.5 μmol m-2 h-1, whereas a source in pre-monsoon (38.29 ± 913.1 μmol m-2 h-1) and post-monsoon (91.81 ± 1009.8 μmol m-2 h-1). The significant factors governing pCO2 were pH, salinity, total alkalinity and dissolved inorganic carbon (DIC). This long-term seasonal study emphasizes the need to include small regional estuaries for more accurate estimates of global CO2 flux to upscale the global carbon budget and its controlling mechanism.

Influence of typology and management practices on water pCO[formula omitted] and atmospheric CO2 fluxes over two temperate shelf–estuary–marsh water continuums

Within the coastal zone, salt marshes often behave as atmospheric CO2 sinks, allowing for blue carbon (C) sequestration associated with intense autotrophic metabolism. However, C dynamics over salt marshes are complex since various biogeochemical processes and fluxes occur at different terrestrial–aquatic–atmospheric exchange interfaces and spatiotemporal scales. This study focuses on seasonal, tidal and diurnal variations of water pCO2, estimated water–air CO2 fluxes and controlling factors along two temperate shelf–estuary–marsh continuums. The latter include typical coastal systems with artificial salt marshes that have contrasting water management practices and primary producer types. Our high-frequency biogeochemical measurements (seasonal 24-hour cycles) highlighted a strong control of ecosystem typology on inorganic C dynamics with lower water pCO2 values in the artificial salt marshes, due to stronger biological activity and longer water residence times, than in the tidal estuary. In the marine-dominated estuary, water pCO2 variations (267 - 569 ppmv) were strongly controlled by tidal effects and phytoplankton activity particularly in spring/summer. On the contrary, the greatest amplitudes in water pCO2 were recorded in the artificial salt marshes (6 - 721 ppmv) due to intense macrophyte activity. In the rewilded marsh, eutrophication favoured spring/summer fast-growing macroalgae produced, in turn, strong fall atmospheric CO2 outgassing from degraded algae waters and thus a net annual source of CO2 to the atmosphere (17.5 g C m−2 yr−1). Conversely, specific management practices at the working marsh for salt-farming activity favoured rather slow-growing macrophytes (i.e. seagrasses) which greatly contribute to the yearly observed atmospheric CO2 sink (-97.7 g C m−2 yr−1). In this work, we suggest that salt marsh management can be used to control the contribution of primary producers to marsh C budget as atmospheric CO2 (sink and/or source).

Interannual and seasonal variability and future forecasting of pCO2(water) using the ARIMA model and CO2 fluxes in a tropical estuary.

The seasonal and interannual variation in the partial pressure of carbon dioxide in water [pCO2(water)] and air-water CO2 exchange in the Mahanadi estuary situated on the east coast of India was studied between March 2013 and March 2021. The principal aim of the study was to analyze the spatiotemporal variability and future trend of pCO2 and air-water CO2 fluxes along with the related carbonate chemistry parameters like water temperature, pH, salinity, nutrients, and totalalkalinity, over 9 years. The seasonal CO2 flux over nine years was also calculated using five worldwide accepted equations. The seasonal map of pCO2(water) followed a general trend of being high in monsoon (2628 ± 3484 μatm) associated with high river inflow and low during pre-monsoon (445.6 ± 270.0 μatm). High pCO2 in water compared to the atmosphere (average 407.6-409.4 μatm) was observed in the estuary throughout the sampling period. The CO2 efflux computed using different gas transfer velocity formulas was also consistent with pCO2 water acquiring the peak during monsoon in the Mahanadi estuary (6033 ± 9478 μmol m-2 h-1) and trough during pre-monsoon (21.66± 187.2 μmol m-2 h-1). The estuary acted as a net source of CO2 throughout the study period, with significant seasonality in the flux magnitudes. However, CO2 sequestration via photosynthesis by phytoplankton resulted in lower emission rates toward the atmosphere in summer. This study uses the autoregressive integrated moving average (ARIMA) model to forecast pCO2(water) for the future. Using measured and predicted values, our work demonstrated that pCO2(water) has an upward trend in the Mahanadi estuary. Our results demonstrate that long-term observations from estuaries should be prioritized to upscale the global carbon budget.

Interannual variability of air-water CO2 flux in a large eutrophic estuary

Air-water CO2 fluxes in estuarine environments are characterized by high interannual variability, in part due to hydrological variability that alters estuarine carbonate chemistry through multiple physical and biogeochemical processes. To understand the relative contributions of these varied controls on interannual air-water CO2 fluxes in the mainstem Chesapeake Bay, we implemented both hindcast and scenario simulations using a coupled physical-biogeochemical model. Significant spatiotemporal variability in bay-wide fluxes was found over a 10-year period (1996-2005), where the mainstem Bay was primarily a net CO2 sink, except in drought periods. Sensitivity scenario results suggested substantial effects of riverine nutrient and organic matter (OM) inputs to CO2 flux variations. The high correlations between riverine inputs and upper-Bay fluxes were due to elevated respiration under increased OM inputs. The interannual flux variations in the lower Bay was mostly regulated by the nutrient inputs. Both nutrient and OM inputs contributed to the flux variability in the mid Bay. It is found that the interannual CO2 flux of the mainstem was most sensitive to riverine nutrient inputs associated with the hydrological changes. For each hindcast simulation we computed the ratio of organic carbon turnover time to water residence time (λ), a proxy for CO2 efflux potential, and found that the wetter periods had a relatively lower λ. The variability of mainstem CO2 fluxes can be well represented using a generic function of λ. The model results showed that higher river flows would lead to enhanced CO2 sinks into a large eutrophic estuary by promoting net autotrophy.

Marine estuaries act as better sink for greenhouse gases during winter in undisturbed mangrove than degraded ones in Sundarban, India

The estuaries provide the key pathway for travelling carbon across the land-ocean interfaces and behave as both source and sink of greenhouse gases (GHGs) in water-atmosphere systems. The sink-source characteristics of estuaries for GHGs vary spatially. The primary driving factors are adjacent ecologies (agriculture, aquaculture, etc.) and proximities to the sea. To study the sink-source characteristics of estuaries for GHGs (methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2)), the water samples were collected from 53 different locations in the estuaries for estimation of dissolved GHGs concentration and air-water GHGs exchanges. The locations represent five zones (Zone I, II, III, IV and V) based on the type and degradation status of mangroves (degraded and undisturbed), anthropogenic activities, and distance from the sea. Zone I, III, V represents to the degraded mangroves far from sea, whereas, Zone II, IV surrounded by undisturbed mangroves and nearer to sea. The average dissolved CH4 concentrations were higher in the estuaries which were adjacent to degraded mangroves (154.4 nmol L−1) than undisturbed mangroves (81.7 nmol L−1). Further, the average dissolved N2O concentrations were 48% higher in the estuaries nearer to degraded mangroves than that of undisturbed ones. Among the degraded mangrove sites, the dissolved CO2 concentrations were higher at Zone I (30.1 μmol L−1) followed by Zone III and IV, whereas in undisturbed sites, it was higher in Zone IV (22.3 μmol L−1) than Zone II (17.6 μmol L−1). Among the 53 locations, 36, 51 and 33 locations acted as a sink (negative value of exchanges) for CH4, N2O and CO2, respectively. The higher sink potential for CH4 was recorded to those estuaries adjacent to undisturbed mangroves (−791.69 μmol m−2 d−1) than the degraded ones (−23.18 μmol m−2 d−1). Similarly, the average air-water N2O and CO2 exchanges were more negative in the estuaries which were nearer to undisturbed mangroves indicating higher sink potential. The pH, and salinity of the estuary water were negatively correlated with air-water CH4 and N2O exchanges, whereas those were positively correlated with CO2 exchanges. Significantly lower dissolved GHGs and air-water GHGs exchange was observed in the estuaries adjacent to the undisturbed mangrove as compared to the degraded mangrove. The reason behind higher sink behaviours of estuaries nearer to undisturbed mangroves are higher intrusion of seawater, less nutrient availability, higher salinity, low carbon contents and alkaline pH compared to estuaries adjacent to degraded mangroves and far from sea.

Seasonal variability in water‒air CO2 exchanges and carbon origin in a subtropical estuary

The seasonal and interannual variabilities in the water‒air net carbon dioxide (CO2) flux were investigated in the lower zone of the Patos Lagoon Estuary (PLE), the largest choked lagoon worldwide. The studied region experienced periods of CO2 ingassing (outgassing) during ocean-dominated (riverine-dominated) conditions. During ocean-dominated conditions (summer/autumn), CO2 uptake was an average of −15 mmol m–2 d–1, whereas under riverine-dominated conditions (winter/spring), CO2 emissions prevailed, reaching an average of 22 mmol m–2 d–1. The magnitude of CO2 uptake in summer/autumn was comparable to the CO2 released to the atmosphere in winter/spring in the mesomixohaline zone. However, the CO2 uptake in summer/autumn was 4-fold higher than the CO2 released in winter/spring in the euhaline zone of the PLE. Unlike most estuarine systems, the PLE acted as a net CO2 sink, with a net CO2 uptake of −2 mmol m–2 d–1 during the period investigated (May 2017 to June 2021). The water‒air CO2 exchanges are modulated mainly by the balance between continental freshwater discharge and the inflow/outflow of seawater. However, the combined effect of wind speed, biological activity, and water residence time adds complexity to understanding the carbon exchanges in this region. The higher concentration of organic matter entails the highest estuarine CO2 concentration during autochthonous production, which indicates heterotrophy in estuarine waters. Part of this carbon produced in the estuary is likely exported to the coast, as suggested by the high CO2 concentration in the estuary mouth. The CO2 export from the PLE to the coastal zone appears to make the lagoon resilient to regional anthropogenic CO2 build-up. Owing to the water‒air net CO2 flux range associated with complex PLE biogeochemistry dynamics, further investigations as a long-term time-series program will improve knowledge of regional CO2 exchanges in the context of climate and elucidate the role of large estuaries and coastal bays in the global carbon budget.

Carbon Biogeochemistry of the Estuaries Adjoining the Indian Sundarbans Mangrove Ecosystem: A Review.

The present study reviewed the carbon-biogeochemistry-related observations concerning CO2 and CH4 dynamics in the estuaries adjoining the Indian Sundarbans mangrove ecosystem. The review focused on the partial pressure of CO2 and CH4 [pCO2(water) and pCH4(water)] and air-water CO2 and CH4 fluxes and their physical, biogeochemical, and hydrological drivers. The riverine-freshwater-rich Hooghly estuary has always exhibited higher CO2 emissions than the marine-water-dominated Sundarbans estuaries. The mangrove sediment porewater and recirculated groundwater were rich in pCO2(water) and pCH4(water), enhancing their load in the adjacent estuaries. Freshwater-seawater admixing, photosynthetically active radiation, primary productivity, and porewater/groundwater input were the principal factors that regulated pCO2(water) and pCH4(water) and their fluxes. Higher chlorophyll-a concentrations, indicating higher primary production, led to the furnishing of more organic substrates that underwent anaerobic degradation to produce CH4 in the water column. The northern Bay of Bengal seawater had a high carbonate buffering capacity that reduced the pCO2(water) and water-to-air CO2 fluxes in the Sundarbans estuaries. Several authors traced the degradation of organic matter to DIC, mainly following the denitrification pathway (and pathways between aerobic respiration and carbonate dissolution). Overall, this review collated the significant findings on the carbon biogeochemistry of Sundarbans estuaries and discussed the areas that require attention in the future.

Carbon stocks and fluxes in Asia-Pacific mangroves: current knowledge and gaps

Mangrove forest plays a key role in regulating climate change, earth carbon cycling and other biogeochemical processes within blue carbon ecosystems. Therefore, mangrove forests should be incorporated into Earth system climate models with the aim of understanding future climate change. Despite multiple carbon stock and flux assessments taking place over the past couple of decades, concrete knowledge of carbon source/sink patterns is largely lacking, particularly in the biodiversity-rich Asia-Pacific (AP) region with its 68 493 km2 of mangrove area. Thus, to understand the gaps in mangrove blue carbon research in the AP region, we summarize a recent decade-long inventory of carbon stock pools (aboveground, belowground and soil) and biogeochemical flux components (burial, export/import, soil-air and water-air CO2 flux) across 25 AP countries to understand the current knowledge and gaps. While carbon stock assessments of individual components are available for all 25 countries, whole ecosystem carbon stocks—including live and standing dead aboveground and belowground, downed woody debris and soil carbon stocks—are often lacking, even in highly researched countries like Indonesia. There is restricted knowledge around biogeochemical carbon fluxes in 55% of the countries, suggesting poor carbon flux research across the region. Focusing on flux components, reports on sediment-to-sea carbon exports are extremely limited (coming from just nine countries in the AP region). There is notable scarcity of data on carbon export fluxes in Indonesian mangroves. Given the key role AP mangroves play in climate change mitigation worldwide, more detailed and methodologically comparable investigation of biogeochemical source/sink processes is required to better understand the role of this large carbon source in global carbon stocks and fluxes, and hence, global climate.

Legacy Effects of Late Macroalgal Blooms on Dissolved Inorganic Carbon Pool through Alkalinity Enhancement in Coastal Ocean.

Taking the world's largest green tide caused by the macroalga Ulva prolifera in the South Yellow Sea as a natural case, it is studied here if macroalgae can perform inorganic carbon sequestration in the ocean. Massive macroalgae released large amounts of organic carbon, most of which were transformed by microorganisms into dissolved inorganic carbon (DIC). Nearshore field investigations showed that, along with seawater deoxygenation and acidification, both DIC and total alkalinity (TAlk) increased significantly (both >50%) in the areas covered by dense U. prolifera at the late-bloom stage. Offshore mapping cruises revealed that DIC and TAlk were relatively higher at the late-bloom stage than at the before-bloom stage. Laboratory cultivation of U. prolifera at the late-bloom stage further manifested a significant enhancement effect on DIC and TAlk in seawater. Sulfate reduction and/or denitrification likely dominated the production of TAlk. Notably, half of the generated DIC and almost all the TAlk could persist in seawater under varying conditions, from hypoxia to normoxia and from air-water CO2 disequilibrium to re-equilibrium. The enhancement of TAlk allowed more DIC to remain in the seawater rather than escape into the atmosphere, thus having the long-term legacy effect of increasing DIC pool in the ocean.

Effects of calcification on air-water CO2 fluxes in tropical seagrass meadows: A mesocosm experiment

Seagrass meadows deliver a range of ecosystem services, where one of the more important is the capacity to store carbon and serve as sinks for atmospheric carbon dioxide. The capacity of seagrass meadows for carbon storage might, however, be modified and complicated by several factors; one important factor is the possible effects of calcification within the meadows. In tropical areas, seagrass meadows can contain high proportions of calcareous organisms, which through their calcification may cause release of CO2. To study this aspect of the CO2 balance within tropical seagrass systems, we investigated the air-water CO2 flux in seagrass mesocosms with different plant community compositions, i.e. mixtures of seagrass and calcifying macroalgae, having similar overall photosynthetic oxygen evolution rates. The measured CO2 fluxes changed both in rate and direction over the day and were significantly related to plant community composition. Downward fluxes of CO2 were found only over vegetation with high proportion of seagrass and in the afternoon, whereas occurrence of calcifying algae appeared to reverse the flow. A partial least squares (PLS) regression model indicated that pH, pCO2 and dissolved inorganic carbon (DIC) were the primary environmental variables predicting the CO2 fluxes. Our findings show that algal calcification might partly counteract the carbon sequestration in seagrass meadows.

Dynamics and internal links of dissolved carbon in a karst river system: Implications for composition, origin and fate

Dissolved carbon (DC) deciphers biotic and abiotic processes in aquatic ecosystems, representing a critical component of global carbon cycling. However, underlying drivers of riverine DC dynamics and internal links have yet to be studied. Here, we investigated fluvial physicochemical characteristics, dissolved inorganic carbon (DIC) species, carbon dioxide (CO2) exchange, dissolved organic carbon (DOC) compositions and properties in a karst river system Qijiang, Southwest China. Carbonate dissolution combined with photosynthetic uptake could explain dynamics of DIC species. Carbon sequestration caused low-magnitude of partial pressure of aqueous CO2 (pCO2, 620.3 ± 1028.7 μatm) and water-air CO2 flux (F, 154.3 ± 772.6 mmol/m2/d), yielding an annual CO2 emission of 0.079 Tg CO2/y. Relatively high biological index (BIX, 0.77–0.96 on average) but low humification index (HIX, 0.67–0.78 on average) indicated notable autochthonous processes. Humic-like component was the predominant DOC, accounting for 39.0%-75.2% with a mean of 57.2% ± 6.17%. Meanwhile, tryptophan-like component (5.84% ± 2.31%) was also identified as collective DOC by parallel factor analysis (PARAFAC) across samples. Biological metabolism established internal linkages between DIC and DOC in the karst river system. Our findings highlighted biological process as a determinant for DC cycling in karst aquatic ecosystems.

Air-water CO2 and water-sediment O2 exchanges over a tidal flat in Tokyo Bay

Despite the potential for carbon storage in tidal flats, little is known about the details of relevant processes because of the complexity of intertidal physical and chemical environments and the uniqueness of the biota. We measured air-water carbon dioxide (CO2) fluxes and water-sediment oxygen (O2) fluxes over a tidal flat in Tokyo Bay by the eddy covariance method, which has the potential to facilitate long-term, broad-scale, continuous monitoring of carbon flows in tidal flats. The results indicated that throughout the tidal flat in Tokyo Bay, CO2 was taken up from the atmosphere at a rate of 6.05 ± 7.14 (mean ± SD) mmol m−2 hour−1, and O2 was taken up from the water into the sediment at a rate of 0.62 ± 1.14 (mean ± SD) mmol m−2 hour−1. The fact that the CO2 uptake rate was about 18 times faster than the previously reported average uptake rate in the whole area of Tokyo Bay was attributable to physical turbulence in the water column caused by bottom friction. Statistical analysis suggested that light intensity and water temperature were the major factors responsible for variations of CO2 and O2 exchange, respectively. Other factors such as freshwater inputs, atmospheric stability, and wind speed also affected CO2 and O2 exchange. High rates of O2 uptake from the water into the sediment surface and high rates of atmospheric CO2 uptake into the water column occurred simultaneously (R2 = 0.44 and 0.47 during day and night, respectively). The explanation could be that photosynthetic consumption of CO2 and production of O2 in the water column increased the downward CO2 (air to water) and O2 (water to sediment) fluxes by increasing the concentration gradients of those gases. Resuspension of sediment in the low-O2 layer by physical disturbance would also increase the O2 concentration gradient and the O2 flux in the water.

Seasonal, Diurnal, and Tidal Variations of Dissolved Inorganic Carbon and pCO2 in Surface Waters of a Temperate Coastal Lagoon (Arcachon, SW France)

We report on diurnal, tidal, and seasonal variations of dissolved inorganic carbon (DIC), water partial pressure of CO2 (pCO2), and associated water–air CO2 fluxes in a tidal creek of a temperate coastal lagoon with 70% of intertidal flats, during eight tidal/diurnal cycles and two consecutive years covering all seasons. Surface waters of the lagoon were always slightly oversaturated in CO2 with respect to the atmosphere with an average pCO2 value of 496 ± 36 ppmv. Seasonally, subsurface water pCO2 values were controlled by both temperature and biological/tidal advection effects that compensated each other and resulted in weak annual variations. High-resolution temporal pCO2 records reveal that the highest fluctuations (192 ppmv) occurred at the tidal/diurnal scale as a result of biological activity, advection from the tidal flat, and porewater pumping that all contributed to water pCO2 and carbonate chemistry variations. Total alkalinity (TA) versus salinity plots suggest a net production of alkalinity in the lagoon attributed to benthic carbonate dissolution and/or anaerobic degradation of organic matter. We specifically highlighted that for the same salinity range, during flooding, daytime pCO2 were generally lower than nighttime pCO2 values because of photosynthesis, whereas during ebbing, daytime pCO2 were higher than nighttime pCO2 values because of heating. Waters in the lagoon were a relatively weak CO2 source to the atmosphere over the year compared to other estuarine and lagoon waters elsewhere, and to sediment-air fluxes measured simultaneously by atmospheric Eddy Covariance (EC) in the Arcachon lagoon. Because of low values and small variations of the air-sea pCO2 gradient, the variability of fluxes calculated using the piston velocity parameterization was greatly controlled by the wind speed at the diurnal and, to a lesser extent, seasonal time scales. During the emersion, the comparison of these pCO2 data in the tidal creek with EC fluxes measured 1.8 km away on the tidal flat suggests high heterogeneity in air-sea CO2 fluxes, both spatially and at short time scales according to the inundation cycle and the wind speed. In addition to tidal pumping when the flat becomes emerged, our data suggest that lateral water movement during the emersion of the flat generates strong spatial heterogeneity in water–air CO2 flux.

Distribution and Dynamics of Radiatively Active Gas (RAG) Emissions From Major Estuaries of the Sundarbans Mangrove, India

The world’s largest mangroves ecosystem, the Sundarbans, being highly productive and a place for extensive organic matter cycling, is considered to be the hotspot for biogeochemical studies in the tropical estuarine environment. Hence, the spatial and temporal dynamics of the biogenic gases (CO2, CH4, and N2O), also known as radiatively active gases, were measured in mangrove-dominated estuaries of the system. In addition to spatial and seasonal observation, three full tidal cycles were observed at one site. Results showed that the air/water gas saturations were widely distributed and highly variable along the stretch. The gas saturations showed varying responses to salinity and tidal fluctuations. This indicated that localized biogeochemical processes may be more influential than simple mixing and dilution processes in controlling the variability of these gases. The surface waters were always supersaturated with CH4 (Up to 13,133%) relative to the atmosphere. However, N2O ranged from 8 to 1,286% and CO2 from 30 to 2075%. N2O fluxes were ∼4.8 times higher in the pre-monsoon than the post-monsoon. CH4 fluxes were ∼3.6 times higher in the pre-monsoon than both the monsoon and the post-monsoon. CO2 fluxes were ∼10 times higher in the monsoon than both the pre-monsoon and the post-monsoon. The seasonality in the gas saturation could be linked more to the availability of substrates than physicochemical parameters. Overall, air/water CH4 fluxes varied maximally (0.4–18.4 μmol m−2 d−1), followed by CO2 fluxes (−0.6–10.9 mmol m−2 d−1), and N2O fluxes varied the least of all (−0.6–5.4 μmol m−2 d−1). Interestingly, CH4 and N2O fluxes were positively correlated to each other (p < 0.05), suggesting organic matter decomposition as the key factor in the production of these two gases. Finally, these water–air CO2, CH4, and N2O flux estimates show that the estuaries are a modest source of CH4 but fluctuate between sources and sinks for CO2 and N2O gases.