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(a) Schematic of major currents in the East China Sea shelf and (b) sampling locations in the Changjiang River plume.
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A monthlong, high-resolution buoy time series from the surface ocean of the Changjiang River plume in early autumn 2013 (30-min sampling frequency) shows great variability in the partial pressure of carbon dioxide (pCO2) and other physical and biogeochemical parameters. Early in the deployment, surface pCO2 decreased by ~117 μatm in a single day (1...
Contexts in source publication
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... midlatitude East China Sea (ECS) continental shelf ( Figure 1) is regarded as a typical river-dominated wes- tern boundary current shelf system. The Changjiang River and intruding Kuroshio water deliver tremendous amounts of nutrients and carbon to the shelf, fueling both primary production and microbial respiration (Chen & Borges, 2009;Chen & Wang, 1999;Jiao et al., 1998;Liu et al., 2010). ...
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... moored oceanographic buoy was deployed at an ECS inner-shelf site (water depth ~40 m; Figure 1b) on 9 September 2013, in the salinity-front area of the Changjiang River plume (122.79°E, 30.68°N; ...
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... cross-shelf transect survey (Figure 1b) was conducted near the buoy site on 27 August 2013, about two weeks before buoy deployment. A Multi Water Sampler (MWS6, Hydro-Bios) was used to measure hydro- graphic conditions and collect water samples. ...
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... correlation of pCO 2 -SST than npCO 2 -SSS during periods I and II was possibly because the surface water during mixing was a time-varying mixture of two water types (Changjiang plume water and offshore water) characterized with contrasting salinity but similar temperature (B. . Previous study have revealed that seawaters along Subei coast have very high pCO 2 ( Guo et al., 2015;; however, the remote-sensing SST around our buoy position was much lower than that in the northern Changjiang Estuary and southern Yellow Sea ( Figure S1). Thus, we suggest that the pCO 2 increase was not caused by advection of water from the southern Yellow Sea. ...
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... noted in previous studies (Chen et al., 2006; Chou, Gong, Sheu, Jan, et al., 2009), the manner in which sea surface pCO 2 responds to vertical mixing is important in determining when the ECS outgases versus absorbs CO 2 . Predeployment DIC differences between 2-and 10-m depth near the buoy site (stations 7, 8, and 9; Figure 1) ranged from 44 to123 μmol/kg ( Figure 7d); vertical gradients of pCO 2 were also quite steep (Figure 7g). A previous study reported that the vertical increase between surface pCO 2 and 10 m pCO 2 in this area can be as high as 200 μatm during summer ( Chou et al., 2013). ...
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... end-member values (T, S, and DIC; Table 2) were calculated from the transect data obtained near the buoy site (Figure 1) and from the in situ buoy data. For CDW during period I, the T, S, and DIC characteristics were obtained by averaging over SST, SSS, and DIC at survey stations 3 through 8. To characterize the CDW end-member during period II, we used the in situ buoy data of 23-24 September (specifically, the average DIC value of samples with SSS <31) because the CDW DIC of 22-24 September was greatly reduced from that of 10 September by biological production. ...
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... ΔpCO 2(sea À atm) and wind speed exhibited great daily and weekly variations (Figures 10a and 10b). Daily average ΔpCO 2(sea À atm) ranged from À24 to +147 μatm, and daily average 2-m wind speed ranged from 1.35 to 11.03 m/s. ...
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... assess potential bias associated with sampling frequency, we simulated air-sea CO 2 flux samplings at dif- ferent frequencies and calculated the corresponding probability distributions ( Leinweber et al., 2009; Figures 10d-10f). The commercial software Crystal Ball (built on the Monte Carlo method) was used to predict the probability of CO 2 fluxes. ...
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... simulations indicate that flux error increases sharply with decreasing sampling frequency (Figures 10d-10f), consistent with the findings of Leinweber et al., 2009. Sampling only once per month (Figure 10f)-for example, as on a monthly ship visit-results in an apparent air-sea CO 2 flux that differs from the monthly average flux by up to +15 mmol · m À2 · day À1 ; in other words, the apparent flux is twice the monthly average flux. ...
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... simulations indicate that flux error increases sharply with decreasing sampling frequency (Figures 10d-10f), consistent with the findings of Leinweber et al., 2009. Sampling only once per month (Figure 10f)-for example, as on a monthly ship visit-results in an apparent air-sea CO 2 flux that differs from the monthly average flux by up to +15 mmol · m À2 · day À1 ; in other words, the apparent flux is twice the monthly average flux. For weekly sampling, the maximum uncertainty (Figure 10e) can be as large as +4.7 mmol · m À2 · day À1 (63% error, with the 90% confidence interval based on a Gaussian fit to the prob- ability distribution). ...
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... only once per month (Figure 10f)-for example, as on a monthly ship visit-results in an apparent air-sea CO 2 flux that differs from the monthly average flux by up to +15 mmol · m À2 · day À1 ; in other words, the apparent flux is twice the monthly average flux. For weekly sampling, the maximum uncertainty (Figure 10e) can be as large as +4.7 mmol · m À2 · day À1 (63% error, with the 90% confidence interval based on a Gaussian fit to the prob- ability distribution). The probability of a flux bias within À2 to +2 mmol · m À2 · day À1 is 56%. ...
Citations
... These areas display high temporal and spatial heterogeneity due to the delivery of carbon and nutrients from rivers and the ocean (Chen and Borges, 2009;Dai et al., 2013;Gruber, 2015). High resolution and high quality TA monitoring is hence crucial for the observation of daily and seasonal variations in such complex and dynamic environments (Bates et al., 2014;Xue et al., 2016;Li et al., 2018). ...
... The Changjiang River plume (CRP) is the most spatiotemporally heterogeneous area in the East China Sea (ECS) continental shelf, where CO 2 dynamics is significantly affected by the Changjiang River discharge and ECS's current system (Chen and Borges, 2009;Li et al., 2018;Wang et al., 2014). The ECS continental shelf is usually considered a sink for atmospheric CO 2 in early autumn, attributed to its high biological production (Chou et al., 2009;Dai et al., 2013). ...
... K. Wang et al. (2017) and Wu et al. (2020) respectively demonstrated the significant impact of episodic nutrient transport and typhoon events on surface carbonate systems and air-sea CO 2 exchange in the CRP and ECS, which would be impossible to capture by shipboard observation. Furthermore, the controlling factors of surface seawater pCO 2 have been quantified in different regions of the ECS through buoy observations, with a strong emphasis on the key roles of mixing and biological processes (Kao et al., 2023;Li et al., 2018). However, most of these studies focus on the CO 2 exchange at the sea air interface, with less attention paid to the coupling effect of ocean boxes and the CO 2 sourcesink dynamics at the sea air interface. ...
... In short, the SAMI-CO 2 was placed in a custom-made chamber infused with CO 2 gas standards ranging from 300 to 920 μatm, which were measured using a nondispersive infrared CO 2 analyzer (LI-7000, LiCor, Inc.). The precision of SAMI-pCO 2 was ±2 μatm (Li et al., 2018). The buoy was also equipped with sensors to measure sea surface temperature (SST), sea surface salinity (SSS), dissolved oxygen (DO), surface turbidity (nephelometric turbidity units, NTU) and chlorophyll a (Chl a) fluorescence, all integrated by Seabird sensors in WET labs® WQM (water quality meters). ...
Estuaries are crucial components of the global ocean carbon cycle due to their high productivity. However, our understanding of the carbon source-sink dynamics at the air-sea interface of estuaries is incomplete, largely due to the rapidly changing environmental conditions. To address this, we conducted a study in early autumn 2016 using high-resolution biogeochemical data collected through buoy observations in the Changjiang River plume (CRP). Using a mass balance approach, we examined the factors driving changes in the sea surface partial pressure of carbon dioxide (pCO2) and quantified the net community production (NCP) in the mixed layer. We also explored the relationship between NCP and the carbon source-sink dynamics at the air-sea interface. Our results revealed that biological activities (64.0 %) and seawater mixing (19.7 %, including lateral transport and vertical mixing) were the dominant factors controlling changes in sea surface pCO2 during the study period. Moreover, NCP in the mixed layer was affected by factors such as light availability and the presence of respired organic carbon associated with vertical mixing of seawater. Notably, we observed a strong correlation between NCP and the difference in pCO2 between air and sea (δpCO2), with a threshold NCP value of 308.4 mmol m-2 d-1 identified as an indicator of the transition from a CO2 source to a sink in the CRP. Hence, we suggest that the NCP in a specific ocean box has a threshold, beyond which the air-sea interface in estuaries will change from a carbon source to a carbon sink, and vice versa.
... b The unit is Gt C year −1 . (Li et al., 2018). Bates et al. (1998) found that short-term pCO 2sea variability has a minor impact on annual F CO2 values in the Sargasso Sea. ...
... The large diurnal variations observed in the pCO 2sea and wind velocity measurements substantially influenced the F CO2 estimations (Table 1). Inadequate sampling of these parameters may cause the estimated F CO2 values to be seriously biased in coastal oceans (Honkanen et al., 2021;Leinweber et al., 2009;Li et al., 2018;Xue et al., 2016) and slightly biased in the open sea (Bates et al., 1998). A contradictory CO 2 flux would be obtained when underway measurements were obtained during the transition periods or during periods with episodic biological production events (Xue et al., 2016). ...
... The deviation probability between annual F CO2 at different sampling frequencies and "true" flux was calculated ( Figure 12). These trials indicated that the sampling error decreased suddenly as the sampling frequency increased (Figure 12), which was consistent with previous findings in coastal waters (Leinweber et al., 2009;Li et al., 2018). ...
Diurnal variability of the CO2 partial pressure (pCO2) at the sea surface (pCO2sea) and its impact on air‐sea CO2 flux estimations are currently poorly understood. In this work, diurnal variations in pCO2sea were examined in the Bay of Bengal (BoB) based on 3‐hourly observations collected at the BoB Ocean Acidification moored buoy between November 2013 and November 2018 by a moored autonomous pCO2 system. Significant mean diurnal pCO2sea cycles and a mean diurnal amplitude of 11 ± 9 μatm were observed based on 1215 complete diurnal cycles. One‐dimensional mass budget model calculations suggest that temperature was responsible for these mean diurnal variations in pCO2sea and that these variations were intensified by biological activity. The effects of air‐sea gas fluxes and mixing on the mean diurnal pCO2sea variability were very low. A weak annual net source of 12.1 mmol m⁻² year⁻¹ was estimated at this site. The sampling frequency greatly impacted the annual air‐sea CO2 flux estimations. Compared to the 3‐hr sampling frequency, random daily sampling resulted in a bias of ±7.7 mmol m⁻² year⁻¹ or 64%, and regular daily sampling resulted in a maximum bias of 400%. Factorial experiments showed that wind speed had the largest impact on the air‐sea CO2 flux estimations, followed by pCO2sea and atmospheric pCO2 (pCO2air). These findings indicate the importance of obtaining continuous pCO2 measurements to accurately estimate air‐sea CO2 flux; additionally, pCO2 and wind speed measurements should be obtained at frequencies higher than 6 hr.
... The changes in pCO 2 due to dissolution/formation of CaCO 3 and salinity are omitted. The daily changes in the pCO 2 were estimated as previously carried out by various authors (Chierici et al., 2006;Xue et al., 2016;Fransson et al., 2017;Li et al., 2018;Gac et al., 2021). Changes in pCO 2 (DpCO 2 ) are driven by changes in temperature (DpCO 2tem ), air-sea CO 2 exchange (DpCO 2gas ), mixing (DpCO 2mix ), and biological activity (DpCO 2bio ). ...
The biogeochemical dynamics of fjords in the southeastern Pacific Ocean are strongly influenced by hydrological and oceanographic processes occurring at a seasonal scale. In this study, we describe the role of hydrographic forcing on the seasonal variability of the carbonate system of the Sub-Antarctic glacial fjord, Seno Ballena, in the Strait of Magellan (53°S). Biogeochemical variables were measured in 2018 during three seasonal hydrographic cruises (fall, winter and spring) and from a high-frequency pCO2-pH mooring for 10 months at 10 ± 1 m depth in the fjord. The hydrographic data showed that freshwater input from the glacier influenced the adjacent surface layer of the fjord and forced the development of undersaturated CO2 (< 400 μatm) and low aragonite saturation state (ΩAr < 1) water. During spring, the surface water had relatively low pCO2 (mean = 365, range: 167 - 471 μatm), high pH (mean = 8.1 on the total proton concentration scale, range: 8.0 - 8.3), and high ΩAr (mean = 1.6, range: 1.3 - 4.0). Concurrent measurements of phytoplankton biomass and nutrient conditions during spring indicated that the periods of lower pCO2 values corresponded to higher phytoplankton photosynthesis rates, resulting from autochthonous nutrient input and vertical mixing. In contrast, higher values of pCO2 (range: 365 – 433 μatm) and relatively lower values of pHT (range: 8.0 – 8.1) and ΩAr (range: 0.9 – 2.0) were recorded in cold surface waters during winter and fall. The naturally low freshwater carbonate ion concentrations diluted the carbonate ion concentrations in seawater and decreased the calcium carbonate saturation of the fjord. In spring, at 10 m depth, higher primary productivity caused a relative increase in ΩAr and pHT. Assuming global climate change will bring further glacier retreat and ocean acidification, this study represents important advances in our understanding of glacier meltwater processes on CO2 dynamics in glacier–fjord systems.
... The study demonstrated that, despite estuaries representing < 10 % of the domain area, interactions with shelf seas must be considered as the estuarine waters gave off more CO 2 than the shelves drew down. Eulerian studies in the estuarine zone have identified large tidal signals in pCO 2 with data collected from research vessels (Borges and Frankignoulle, 1999;Jeffrey et al., 2018a) and from moorings or fixed sites (Dai et al., 2009;Jeffrey et al., 2018a;Li et al., 2018;Bakker et al., 1996;Call et al., 2015;Ferrón et al., 2007;Santos et al., 2012). pCO 2 levels at the mouth of different rivers have been observed to co-vary with changes in river flow rate and with the tidal cycle (Ribas-Ribas et al., 2013;Canning et al., 2021;Najjar et al., 2018;Frankignoulle et al., 1996). ...
Surface ocean carbon dioxide (CO2) measurements are used to compute the oceanic air–sea CO2 flux. The CO2 flux component
from rivers and estuaries is uncertain due to the high spatial and seasonal heterogeneity of CO2 in coastal waters. Existing high-quality
CO2 instrumentation predominantly utilises showerhead and percolating style equilibrators optimised for open-ocean observations. The intervals between measurements made with such instrumentation make it difficult to resolve the fine-scale spatial variability of surface water
CO2 at timescales relevant to the high frequency variability in estuarine and coastal environments. Here we present a novel dataset with
unprecedented frequency and spatial resolution transects made at the Western Channel Observatory in the south-west of the UK from June to September
2016, using a fast-response seawater CO2 system. Novel observations were made along the estuarine–coastal continuum at different stages of
the tide and reveal distinct spatial patterns in the surface water CO2 fugacity (fCO2) at different stages of the tidal
cycle. Changes in salinity and fCO2 were closely correlated at all stages of the tidal cycle and suggest that the mixing of oceanic
and riverine endmembers partially determines the variations in fCO2. The correlation between salinity and fCO2
was different in Cawsand Bay, which could be due to enhanced gas exchange or to enhanced biological activity in the region. The observations
demonstrate the complex dynamics determining spatial and temporal patterns of salinity and fCO2 in the region. Spatial variations
in observed surface salinity were used to validate the output of a regional high-resolution hydrodynamic model. The model enables a novel estimate
of the air–sea CO2 flux in the estuarine–coastal zone. Air–sea CO2 flux variability in the estuarine–coastal boundary region is
influenced by the state of the tide because of strong CO2 outgassing from the river plume. The observations and model output demonstrate
that undersampling the complex tidal and mixing processes characteristic of estuarine and coastal environment biases quantification of air–sea
CO2 fluxes in coastal waters. The results provide a mechanism to support critical national and regional policy implementation by reducing
uncertainty in carbon budgets.
... Similar to the Mississippi River plume, the Changjiang plume is a strong carbon sink in summer due to intense biological production. The biological uptake of CO 2 and the air-sea CO 2 flux in the Changjiang plume have large temporal variations associated with episodic wind events (Li et al., 2018(Li et al., , 2019Wu et al., 2020). Typhoon winds cause Changjiang plume waters to become a strong carbon source through the upward transport of high-CO 2 bottom waters (Li et al., 2019) or cause the waters to become carbon sinks by in situ biological production or the advection of undersaturated-CO 2 waters (Zhang et al., 2018;Wu et al., 2020). ...
... The buoy was deployed at a Changjiang plume site (water depth ∼45 m, 122.8 • E, 30.6 • N, Figure 1), which is located at the track of typhoon "Chan-Hom" (July 11, 2015). The details of the buoy observations were explained by Li et al. (2018). In brief, a mounted SAMI-CO 2 FIGURE 1 | Advanced Scatterometer winds over the East China Sea on July 24. ...
... In addition, the temperature increase observed during period I should accompany elevated pCO 2 . The significant relationship between npCO 2 and DO observed during period I (Figure 4B) demonstrated that the pCO 2 decrease was probably dominated by biological production (Li et al., 2018). ...
The partial pressure of CO2 (pCO2) in the sea and the air-sea CO2 flux in plume waters are subject to interactions among biological production, horizontal advection, and upwelling under wind events. In this study, time series of pCO2 and other biogeochemical parameters in the dynamic Changjiang plume were presented to illuminate the controlling factors of pCO2 and the air-sea CO2 flux after a strong south wind event (July 23–24, maximum of 11.2 ms–1). The surface pCO2 decreased by 310 μatm (to 184 μatm) from July 24 to 26. Low-pCO2 waters (<200 μatm) were observed in the following 2 days. Corresponding chlorophyll a and dissolved oxygen (DO) increase, and a significant relationship between DO and npCO2 indicated that biological uptake drove the pCO2 decrease. The salinity of undersaturated-CO2 waters decreased by 3.57 (from 25.03 to 21.46) within 2 days (July 27–28), suggesting the offshore advection of plume waters in which CO2 had been biologically reduced. Four days after the wind event, the upwelling of high-CO2 waters was observed, which increased the pCO2 by 428 μatm (up to 584 μatm) within 6 days. Eight days after the onset of upwelling, the surface pCO2 started to decrease (from 661 to 346 μatm within 3 days), which was probably associated with biological production. Regarding the air-sea CO2 flux, the carbon sink of the plume was enhanced as the low-pCO2 plume waters were pushed offshore under the south winds. In its initial stage, the subsequent upwelling made the surface waters act as a carbon source to the atmosphere. However, the surface waters became a carbon sink again after a week of upwelling. Such short-term air-sea carbon fluxes driven by wind have likely occurred in other dynamic coastal waters and have probably induced significant uncertainties in flux estimations.
... Estuaries are especially challenging to fully understand because of the heterogeneity between and within estuaries that is driven by diverse processes operating on different timescales such as river discharge, nutrient and or-ganic matter loading, stratification, and coastal upwelling (Jiang et al., 2013;Mathis et al., 2012). The traditional sampling method for carbonate system characterization involving discrete water sample collection and laboratory analysis is known to lead to biases in average pCO 2 and CO 2 flux calculations due to daytime sampling that neglects to capture diel variability (Li et al., 2018). Mean diel ranges in pH can exceed 0.1 unit in many coastal environments, and especially high diel ranges (even exceeding 1 pH unit) have been reported in biologically productive areas or areas with higher mean pCO 2 (Challener et al., 2016;Cyronak et al., 2018;Schulz and Riebesell, 2013;Semesi et al., 2009;Yates et al., 2007). ...
The coastal ocean is affected by an array of co-occurring biogeochemical and
anthropogenic processes, resulting in substantial heterogeneity in water
chemistry, including carbonate chemistry parameters such as pH and partial
pressure of CO2 (pCO2). To better understand coastal and estuarine
acidification and air-sea CO2 fluxes, it is important to study baseline
variability and driving factors of carbonate chemistry. Using both discrete
bottle sample collection (2014–2020) and hourly sensor measurements
(2016–2017), we explored temporal variability, from diel to interannual
scales, in the carbonate system (specifically pH and pCO2) at the
Aransas Ship Channel located in the northwestern Gulf of Mexico. Using other
co-located environmental sensors, we also explored the driving factors of
that variability. Both sampling methods demonstrated significant seasonal
variability at the location, with highest pH (lowest pCO2) in the winter
and lowest pH (highest pCO2) in the summer. Significant diel variability
was also evident from sensor data, but the time of day with elevated
pCO2 and depressed pH was not consistent across the entire monitoring
period, sometimes reversing from what would be expected from a biological
signal. Though seasonal and diel fluctuations were smaller than many other
areas previously studied, carbonate chemistry parameters were among the most
important environmental parameters for distinguishing between time of day and
between seasons. It is evident that temperature, biological activity,
freshwater inflow, and tide level (despite the small tidal range) are all
important controls on the system, with different controls dominating at
different timescales. The results suggest that the controlling factors of
the carbonate system may not be exerted equally on both pH and pCO2 on
diel timescales, causing separation of their diel or tidal relationships
during certain seasons. Despite known temporal variability on shorter
timescales, discrete sampling was generally representative of the average
carbonate system and average air-sea CO2 flux on a seasonal and annual
basis when compared with sensor data.
... The study demonstrated that, despite estuaries representing <10 % of the domain area, interactions with shelf seas must be considered as the estuarine waters gave off more CO 2 than the shelves drew down. Eulerian studies in the estuarine zone have identified large tidal signals in pCO 2 with data collected from research vessels (Borges and 60 Frankignoulle, 1999;Jeffrey et al., 2018a) and from moorings or fixed sites (Dai et al., 2009;Jeffrey et al., 2018a;Li et al., 2018;Bakker et al., 1996;Call et al., 2015;Ferrón et al., 2007;Santos et al., 2012). pCO 2 levels at the mouth of different rivers have been observed to co-vary with changes in river flow rate and with the tidal cycle (Ribas-Ribas et al., 2013;Canning et https://doi.org/10.5194/bg-2021-166 ...
Surface ocean CO2 measurements are used to compute the oceanic air–sea CO2 flux. The CO2 flux component from rivers and estuaries is uncertain. Estuarine and coastal water carbon dioxide (CO2) observations are relatively few compared to observations in the open ocean. The contribution of these regions to the global air–sea CO2 flux remains uncertain due to systematic under-sampling. Existing high-quality CO2 instrumentation predominantly utilise showerhead and percolating style equilibrators optimised for open ocean observations. The intervals between measurements made with such instrumentation make it difficult to resolve the fine-scale spatial variability of surface water CO2 at timescales relevant to the high frequency variability in estuarine and coastal environments. Here we present a novel dataset with unprecedented frequency and spatial resolution transects made at the Western Channel Observatory in the south west of the UK from June to September 2016, using a fast response seawater CO2 system. Novel observations were made along the estuarine–coastal continuum at different stages of the tide and reveal distinct spatial patterns in the surface water CO2 fugacity (fCO2) at different stages of the tidal cycle. Changes in salinity and fCO2 were closely correlated at all stages of the tidal cycle and suggest that the mixing of oceanic and riverine end members determines the variations in fCO2. The observations demonstrate the complex dynamics determining spatial and temporal patterns of salinity and fCO2 in the region. Spatial variations in observed surface salinity were used to validate the output of a regional high resolution hydrodynamic model. The model enables a novel estimate of the air–sea CO2 flux in the estuarine–coastal zone. Air–sea CO2 flux variability in the estuarine–coastal boundary region is dominated by the state of the tide because of strong CO2 outgassing from the river plume. The observations and model output demonstrate that undersampling the complex tidal and mixing processes characteristic of estuarine and coastal environment bias quantification of air-sea CO2 fluxes in coastal waters. The results provide a mechanism to support critical national and regional policy implementation by reducing uncertainty in carbon budgets.
... High riverine pCO 2 is dramatically reduced, however, when river water mixes with oceanic waters with a high buffering capacity and lower pCO 2 . The location and lagrangian timescale associated with the transition from CO 2 -supersaturated river water to CO 2 -undersaturated plume water is a key topic of interest for coastal carbon cycle studies involving large river plumes (Huang et al., 2013;Li et al., 2018). ...
The partial pressure of carbon dioxide (pCO2) was surveyed across the Amazon River plume and the surrounding western tropical North Atlantic Ocean (15–0°N, 43–60°W) during three oceanic expeditions (May–June 2010, September–October 2011, and July 2012). The survey timing was chosen according to previously described temporal variability in plume behavior due to changing river discharge and winds. In situ sea surface pCO2 and air‐sea CO2 flux exhibited robust linear relationships with sea surface salinity (SSS; 15 < SSS < 35), although the relationships differed among the surveys. Regional distributions of pCO2 and CO2 flux were estimated using SSS maps from high‐resolution ocean color satellite‐derived (MODIS‐Aqua) diffuse attenuation coefficient at 490 nm (Kd490) during the periods of study. Results confirmed that the plume is a net CO2 sink with distinctive temporal variability: the strongest drawdown occurred during the spring flood (−2.39 ± 1.29 mmol m⁻² d⁻¹ in June 2010), while moderate drawdown with relatively greater spatial variability was observed during the transitional stages of declining river discharge (−0.42 ± 0.76 mmol m⁻² d⁻¹ in September–October 2011). The region turned into a weak source in July 2012 (0.26 ± 0.62 mmol m⁻² d⁻¹) when strong CO2 uptake in the mid‐plume was overwhelmed by weak CO2 outgassing over a larger area in the outer plume. Outgassing near the mouth of the river was observed in July 2012. Our observations draw attention to the importance of assessing the variable impacts of biological activity, export, and air‐sea gas exchange before estimating regional CO2 fluxes from salinity distributions alone.
... Long-term time-series observations (e.g., Bates et al. 2014; Sutton et al. 2014a,b) are critically needed to decipher the dynamics of the oceanic environment, and to detect changes in the global ocean carbon cycle, due to both natural processes and anthropogenic perturbations, in the broad context of global warming and ocean acidification. Given the highly temporal and spatial heterogeneity of coastal oceans (Chen and Borges 2009;Dai et al. 2013;Gruber 2015), as a consequence of large, variable riverine inputs of carbon and nutrients, and dynamic exchange with the open ocean, it is also of critical importance to carry out long-term high-frequency observations to better understand the coastal ocean CO 2 system (Bates et al. 2014;Xue et al. 2016;Li et al. 2018). ...
... inputs of nutrients (Chen et al. 2012;Wang et al. 2017a,b;Li et al. 2018), and those which increase pCO 2 , such as vertical mixing driven by episodic strong winds (Bates et al. 1998;Nemoto et al. 2009) or the seasonal collapse of surface stratification ) resulting in the supply of subsurface high-CO 2 waters into the surface mixed layer. In addition, temperature also exerts a dominant control on sea surface pCO 2 variability by affecting CO 2 solubility (Weiss 1974). ...
... However, these studies are limited in terms of temporal resolution, leading to large uncertainties in estimations of air-sea CO 2 fluxes and an incomplete understanding of their underlying processes (Chou et al. 2009b;Nemoto et al. 2009). For instance, Li et al. (2018) investigated the dependence of air-sea CO 2 fluxes on sampling frequency (e.g., every 3 d, weekly, and monthly) on the inner shelf of the East China Sea, showing that the potential error in estimated fluxes increases sharply with decreasing sampling frequency. Even weekly sampling is capable of introducing flux biases of up to ± 63% (Li et al. 2018). ...
Carbon dioxide partial pressure (pCO2) in surface water was continuously measured every 3 h from July 2012 to June 2013 using a moored autonomous pCO2 system (MAPCO2) deployed on a moored buoy on the East China Sea shelf (31°N 124.5°E). Sea surface pCO2 and pH had the largest variations in summer, ranging from 215 to 470 µatm, and 7.941 to 8.263 (averagely 8.084±0.080), respectively. They varied little in winter, ranging from 328 to 395 µatm, and 8.003 to 8.074 (averagely 8.052±0.010), respectively. The seasonal average sea surface pCO2 was respectively 335±70, 422±43, 362±11 and 311±59 µatm in summer, autumn, winter and spring, and was overall undersaturated with respect to atmosphere on a yearly basis. Although the average sea surface pCO2 in summer was below the atmospheric level, the net CO2 flux has suggested a CO2 source status due to the influence of typhoon. Our observation thus demonstrated the significant, even dominant impact of episodic typhoon events on surface ocean CO2 chemistry and air-sea CO2 gas exchange, which would be impossible to capture by shipboard observation. The high wind stress and curl associated with the northward movement of typhoon induced complex sea surface water movement, vertical mixing, and subsequent biological drawdown, which differ in pre-, onset and post-typhoon stages. Based on our estimates, the degassing fluxes during typhoon reached as high as 82 and 242 mmol m-2 CO2 in summer and autumn, respectively, accounting for twice as large as the summer CO2 sink during non-typhoon period, and 28% of the total CO2 source in autumn.
