Southeast Asia is a hotspot of riverine export of terrigenous organic carbon to the ocean, accounting for ~10% of the global land-to-ocean riverine flux of terrigenous dissolved organic carbon (tDOC). While anthropogenic disturbance is thought to have increased the tDOC loss from peatlands in Southeast Asia, the fate of this tDOC in the marine environment and the potential impacts of its remineralization on coastal ecosystems remain poorly understood. We collected a multi-year biogeochemical time series in the central Sunda Shelf (Singapore Strait), where the seasonal reversal of ocean currents delivers water masses from the South China Sea first before (during Northeast Monsoon) and then after (during Southwest Monsoon) they have mixed with run-off from peatlands on Sumatra. The concentration and stable isotope composition of dissolved organic carbon, and colored dissolved organic matter spectra, reveal a large input of tDOC to our site during Southwest Monsoon. Using isotope mass balance calculations, we show that 60–70% of the original tDOC input is remineralized in the coastal waters of the Sunda Shelf, causing seasonal acidification by up to 0.10 pH units. The persistent CO2 oversaturation drives a CO2 efflux of 4.1 – 8.2 mol C m-2 yr-1 from the Singapore Strait, suggesting that a large proportion of the remineralized peatland tDOC is ultimately emitted to the atmosphere. However, incubation experiments show that the remaining 30–40% tDOC exhibits surprisingly low lability to microbial and photochemical degradation, suggesting that up to 20–30% of peatland tDOC might be relatively refractory and exported to the open ocean.
The structure(s), distribution and dynamics of CDOM have been investigated over the last several decades largely through optical spectroscopy (including both absorption and fluorescence) due to the fairly inexpensive instrumentation and the easy-to-gather data (over thousands published papers from 1990-2016). Yet, the chemical structure(s) of the light absorbing and emitting species or constituents within CDOM has only recently being proposed and tested through chemical manipulation of selected functional groups (such as carbonyl and carboxylic/phenolic containing molecules) naturally occurring within the organic matter pool. Similarly, fitting models (among which the PArallel FACtor analysis, PARAFAC) have been developed to better understand the nature of a subset of DOM, the CDOM fluorescent matter (FDOM). Fluorescence spectroscopy coupled with chemical tests and PARAFAC analyses could potentially provide valuable insights on CDOM sources and chemical nature of the FDOM pool. However, despite that applications (and publications) of PARAFAC model to FDOM have grown exponentially since its first application/publication (2003), a large fraction of such publications has misinterpreted the chemical meaning of the delivered PARAFAC ‘components’ leading to more confusion than clarification on the nature, distribution and dynamics of the FDOM pool. In this context, we employed chemical manipulation of selected functional groups to gain further insights on the chemical structure of the FDOM and we tested to what extent the PARAFAC ‘components’ represent true fluorophores through a controlled chemical approach with the ultimate goal to provide insights on the chemical nature of such ‘components’ (as well as on the chemical nature of the FDOM) along with the advantages and limitations of the PARAFAC application.
While liquid environments with high salt content are of broad interest to the Earth and Planetary Science communities, instruments face challenges in detecting organics in hypersaline samples due to the effects of salts. Therefore, technology to desalt samples before analysis by these instruments would be enabling for liquid sampling on missions to Mars or ocean worlds. Electrodialysis (ED) removes salt from aqueous solutions by applying an electric potential across a series of ion-selective membranes, and is demonstrated to retain a significant percentage of dissolved organic molecules (DOM) in marine samples. However, current electrodialysis systems used for DOM recovery are too large for deployment on missions or for use in terrestrial fieldwork. Here we present the design and evaluation of the Minature Robotic Electrodialysis (MR ED) system, which is approximately 1/20th the size of heritage instruments and processes as little as 50 mL of sample at a time. We present tests of the instrument efficiency and DOM recovery using lab-created solutions as well as natural samples taken from an estuary of the Skidaway River (Savannah, GA) and from South Bay Saltworks (San Diego, CA). Our results show that the MR ED system removed 97-99% of the salts in most samples, with an average DOC recovery range from 53 to 77%, achieving similar capability to tabletop instruments. This work both demonstrates MR ED as a possible field instrument and increases the technology readiness level of miniaturized electrodialysis systems for future missions.
Although iron and light are understood to regulate the Southern Ocean biological carbon pump, observations have also indicated a possible role for manganese. Low concentrations in Southern Ocean surface waters suggest manganese limitation is possible, but its spatial extent remains poorly constrained and direct manganese limitation of the marine carbon cycle has been neglected by ocean models. Here, using available observations, we develop a new global biogeochemical model and find that phytoplankton in over half of the Southern Ocean cannot attain maximal growth rates because of manganese deficiency. Manganese limitation is most extensive in austral spring and depends on phytoplankton traits related to the size of photosynthetic antennae and the inhibition of manganese uptake by high zinc in Antarctic waters. Importantly, manganese limitation expands under the increased iron supply of past glacial periods, reducing the response of the biological carbon pump. Overall, these model experiments describe a mosaic of controls on Southern Ocean productivity that emerge from the interplay of light, iron, manganese and zinc, shaping the evolution of Antarctic phytoplankton since the opening of the Drake Passage.
Rising temperatures and an acceleration of the hydrological cycle due to climate change are increasing river discharge, causing permafrost thaw, glacial melt, and a shift to a groundwater-dominated system in the Arctic. These changes are funnelled to coastal regions of the Arctic Ocean where the implications for the distributions of nutrients and biogeochemical constituents are unclear. In this study, we investigate the impact of terrestrial runoff on marine biogeochemistry in Inuit Nunangat (the Canadian Arctic Archipelago) --- a key pathway for transport and modification of waters from the Arctic Ocean to the North Atlantic --- using sensitivity experiments from 2002-2020 with an ocean model of manganese (Mn). The micronutrient Mn traces terrestrial runoff and the modification of geochemical constituents of runoff during transit. The heterogeneity in Arctic runoff composition creates distinct terrestrial fingerprints of influence in the ocean: continental runoff influences Mn in the southwestern Archipelago, glacial runoff dominates the northeast, and their influence co-occurs in central Parry Channel. Glacial runoff carries micronutrients southward from Nares Strait in the late summer and may help support longer phytoplankton blooms in the Pikialasorsuaq polynya. Enhanced glacial runoff may increase micronutrients delivered downstream to Baffin Bay, accounting for up to 18% of dissolved Mn fluxes seasonally and 6% annually. These findings highlight how climate induced changes to terrestrial runoff may impact the geochemical composition of the marine environment, and will help to predict the extent of these impacts from ongoing alterations of the Arctic hydrological cycle.
During Marine Isotope Stage 3 (MIS-3; 57–29 ka) Antarctic ice cores reveal a glacial climate state punctuated by millennial-scale warming events and pulses of CO2. Changes in iron-fertilised export production and ocean circulation/upwelling, interpreted from South Atlantic sediment cores, suggest that the Southern Ocean is a conduit for the storage and release of CO2 from the deep ocean. However, it is unclear whether this occurs throughout the Southern Ocean as these processes have not previously been investigated in the southwest Pacific . Here we describe localised iron limitation linked to glaciation changes in New Zealand, which reduced export production during early MIS-3 (60–48 ka) and caused decreases/increases in export production during late MIS-3 (48–29 ka) millennial-scale warming/cooling. Consistent decreases in foraminifera-bound δ15N during all MIS-3 warming events may reflect changes in the supply of nitrate to the subantarctic Pacific, possibly from increased wind-driven upwelling in the Antarctic and northward eddy-driven transport and/or shifting SO fronts. Concomitant decreases in bottom water oxygen and increases in the 14C age of deep waters suggest that old, nutrient-rich waters influenced upper circumpolar deep water in the southwest Pacific during warming events. This signature may reflect an expansion of Pacific Deep Water into the Southern Ocean as Southern Ocean overturning strengthens. Iron-limitation of export production, the expansion of Pacific Deep water, and increased wind-driven upwelling would all work to contribute to increasing atmospheric CO2 through reduced drawdown, and increased outgassing from the Pacific carbon reservoir during the millennial-scale warming events of MIS-3.
The Kuroshio current separates from the Japanese coast to become the Kuroshio Extension (KE) characterized by a strong latitudinal density front, high levels of mesoscale (eddy) energy, and high chlorophyll (CHL). Recent work has also shown that the KE carries subsurface nutrients into the region horizontally. While satellite measurements of CHL show evidence of the impact of eddies on the standing stock of phytoplankton, there have been very limited in situ estimates of productivity over synoptic scales in this region. Here, we present highly spatially resolved estimates of net community production (NCP) for the KE region derived from underway O2/Ar measurements made in spring, summer, and early autumn. We find large seasonal differences in the relationships between NCP, CHL, and sea level anomaly (SLA, a proxy for local thermocline depth deviations driven by mesoscale eddies). The KE front is a pronounced hotspot of NCP in spring when NCP is almost completely decorrelated with CHL. Conversely, we find that NCP and CHL are strongly correlated in summer away from the front. We explore the mechanistic underpinnings of the relationship between NCP and CHL and suggest that the KE nutrient stream as well as vertical motions associated with mesoscale eddies might be a key factor in supporting an NCP hotspot that is seasonally decoupled from CHL at the KE front. Our observations also highlight seasonal and regional (de)coupling between NCP and CHL which may impact the accuracy of CHL-based estimates of productivity.
Coastal nitrogen (N) enrichment is a global environmental problem that can influence acidification, deoxygenation, and subsequent habitat loss in ways that can be synergistic with global climate change impacts. In the Southern California Bight, an eastern boundary upwelling system, modeling of wastewater discharged through ocean outfalls has shown that it effectively doubles N loading to urban coastal waters. However, effects of wastewater outfalls on biogeochemical rates of primary production and respiration, key processes through which coastal acidification and deoxygenation are manifested, have not been directly linked to observed trends in ambient chlorophyll a, oxygen and pH. In this paper, we compare observations of nutrient concentrations and forms, as well as rates of nitrification, primary production, and respiration, in areas within treated wastewater effluent plumes compared to areas spatially distant from ocean outfalls where we expected minimum influence of the plume. We document that wastewater nutrient inputs have an immediate, local effect on nutrient stoichiometry, elevating ammonium and nitrite concentrations and increasing dissolved nitrogen: phosphorus ratios, as well as increasing rates of nitrification within the plume. We did not observe a near plume effect on nitrate assimilation into the biomass, primary production, chlorophyll a, respiration, or dissolved oxygen concentration, suggesting any potential impact from wastewater on these processes is moderated by offshore factors, notably mixing of water masses. These results indicate that a “reference-area” approach, wherein stations within or near the zone of initial dilution (ZID) from the wastewater outfall are compared to stations farther afield (reference areas) to assess contaminant impacts, is insufficient to document regional scale impacts of nutrients. Understanding of the complex interactions between local, regional, and global drivers on coastal eutrophication requires coupled observational-numerical modeling approaches where numerical models are carefully validated with observed state and rate data to develop effective, evidence-based solutions to coastal eutrophication.
The abundance and size distribution of marine organic particles are two major factors controlling biological carbon sequestration in the ocean. These quantities are the result of complex physical-biological interactions that are difficult to observe, and their spatial and temporal patterns remain uncertain. Here, we present a novel analysis of particle size distributions (PSD) from a global compilation of in situ Underwater Vision Profiler 5 (UVP5) optical measurements. Using a machine learning algorithm, we extrapolate sparse UVP5 observations to the global ocean from well-sampled oceanographic variables. We reconstruct global maps of PSD parameters (biovolume and slope) for particles at the base of the euphotic zone. These reconstructions reveal consistent global patterns, with high chlorophyll regions generally characterized by high particle biovolume and flatter PSD slope, i.e., a high relative abundance of large vs. small particles. The resulting negative correlations between particle biovolume and slope further suggests amplified effects on sinking particle fluxes. Our approach and estimates provide a baseline for an improved understanding of particle cycles in the ocean, and pave the way to global, three-dimensional reconstructions of sinking particle fluxes from UVP5 observations.
The Southern Ocean (SO) provides the largest oceanic sink of carbon. Observational datasets highlight decadal-scale changes in SO CO2 uptake, but the processes leading to this decadal-scale variability remain debated. Here, using an eddy-permitting ocean, sea-ice, carbon cycle model, we explore the impact of changes in Southern Hemisphere (SH) westerlies on contemporary (i.e. total), anthropogenic and natural CO2 fluxes using idealised sensitivity experiments as well as an interannually varying forced (IAF) experiment covering the years 1948 to 2007. We find that a strengthening of the SH westerlies reduces the contemporary CO2 uptake by leading to a high southern latitude natural CO2 outgassing. The enhanced SO upwelling and associated increase in Antarctic Bottom Water decrease the carbon content at depth in the SO, and increase the transport of carbon-rich waters to the surface. A poleward shift of the westerlies particularly enhances the CO2 outgassing south of 60S, while inducing an asymmetrical DIC response between high and mid southern latitudes. Changes in the SH westerlies in the 20th century in the IAF experiment lead to decadal-scale variability in both natural and contemporary CO2 fluxes. The ~10% strengthening of the SH westerlies since the 1980s led to a 0.016 GtC/yr^2 decrease in natural CO2 uptake, while the anthropogenic CO2 uptake increased at a similar rate, thus leading to a stagnation of the total SO CO2 uptake. The projected poleward strengthening of the SH westerlies over the coming century will thus reduce the capability of the SO to mitigate the increase in atmospheric CO2.
The carbonate chemistry of Arctic Ocean seafloor and its vulnerability to ocean acidification remains poorly explored. This limits our ability to quantify how biogeochemical processes and bottom water conditions shape sedimentary carbonate chemistry, and to predict how climate change may affect such biogeochemical processes at the Arctic Ocean seafloor. Here, we employ an integrated model assessment that explicitly resolves benthic pH and carbonate chemistry along a S—N transect in the Barents Sea. We identify the main drivers of observed carbonate dynamics and estimate benthic fluxes of dissolved inorganic carbon and alkalinity to the Arctic Ocean. We explore how bottom water conditions and in-situ organic matter degradation shape these processes and show that organic matter transformation strongly impacts pH and carbonate saturation (Ω) variations. Aerobic organic matter degradation drives a negative pH shift (pH < 7.6) in the upper 2—5 cm, producing Ω < 1. This causes shallow carbonate dissolution, buffering porewater pH to around 8.0. Organic matter degradation via metal oxide (Mn/Fe) reduction pathways further increases pH and carbonate saturation state. At the northern stations, where Ω > 5 at around 10–25 cm, model simulations result in authigenic carbonate precipitation. Furthermore, benthic fluxes of dissolved inorganic carbon (12.5—59.5 µmol cm−2 yr−1) and alkalinity (11.3—63.2 µmol cm−2 yr−1) are 2—3-fold greater in the northern sites due to greater carbonate dissolution. Our assessment is of significant relevance to predict how changes in the Arctic Ocean may shift carbon burial and pH buffering into the next century.
The deep ocean naturally releases large amounts of old, pre-industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, closing the global ocean carbon cycle. This Southern Ocean CO2 release is relevant to the global climate, because its changes could alter atmospheric CO2 levels on long time scales and the present-day potential of the Southern Ocean to take up anthropogenic CO2. Here, based on observational data, we show that this CO2 release arises from a zonal band of subsurface waters between the Subantarctic Front and wintertime sea-ice edge with a potential partial pressure of CO2 exceeding current atmospheric CO2 levels (∆PCO2) by 175 µatm. This band of high ∆PCO2 subsurface water conincides with the outcropping of the 27.8 kg m−3 isoneutral density surface that marks the upwelling of Indo-Pacific Deep Water (IPDW). Vertically, the IPDW layer exhibits a distinct ∆PCO2 maximum in the deep ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO2 decreases. Most of this vertical ∆PCO2 decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2 outgassing from the high-carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2 fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles.
What controls the distribution of barium (Ba) in the oceans? Answers to this question have been sought since early studies revealed relationships between particulate Ba (pBa) and POC and dissolved Ba (dBa) and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. Herein, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ~50% of the dBa inventory (upper 500 m of the Arctic water column) is not supplied by conservatively advected inputs. Calculated areal dBa fluxes are up to 10 µmol m-2 d-1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. Surprisingly, the Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin-bay derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a terrigenous source (e.g., submarine groundwater discharge or fluvial particles) is an important contributor
The Indian Ocean (IO) is witnessing acidification as a direct consequence of the continuous rising of atmospheric CO2 concentration and indirectly due to the rapid ocean warming, which disrupts the pH of the surface waters. This study investigates the pH seasonality and trends over various bio-provinces of the IO and regionally assesses the contribution of each of its controlling factors. Simulations from a global and a regional ocean model coupled with biogeochemical modules were validated with pH measurements over the basin, and used to discern the regional response of pH seasonality (1990-2010) and trend (1961-2010) in response to changes in Sea Surface Temperature (SST), Dissolved Inorganic Carbon (DIC), Total Alkalinity (ALK) and Salinity (S). DIC and SST are significant contributors to the seasonal variability of pH in almost all bio-provinces. Total acidification in the IO basin was 0.0675 units from 1961 to 2010, with 69.3% contribution from DIC followed by 13.8% contribution from SST. For most of the bio-provinces, DIC remains a dominant contributor to changing trends in pH except for the Northern Bay of Bengal and Around India (NBoB-AI) region, wherein the pH trend is dominated by ALK (55.6%) and SST (16.8%). Interdependence of SST and S over ALK is significant in modifying the carbonate chemistry and biogeochemical dynamics of NBoB-AI and a part of tropical, subtropical IO bio-provinces. A strong correlation between SST and pH trends infers an increasing risk of acidification in the bio-provinces with rising SST and points out the need for sustained monitoring of IO pH in such hotspots.
Climate change is expected to increase the strength of ocean Oxygen Deficient Zones (ODZs), but we lack detailed understanding of the temporal or spatial variability of these ODZs. A fifty-year time series in the Eastern Tropical North Pacific (ETNP) ODZ revealed that it strengthened by 30% from 1994 to 2019. We subdivided the ODZ into a core and a deep layer based on potential density and revealed that different processes control the magnitude of fixed nitrogen loss in these two regions. We postulate that the depth of the upper ETNP ODZ water mass, the 13 ºC water, influences the organic carbon supply to the core ODZ and therefore its strength. We correlated the fixed nitrogen loss in the core ODZ with a nearby sedimentary nitrogen isotope record and found that this recent, rapid increase has only occurred a few times over the last 1200 years. Using this correlation, we derived the first confidence interval for the strength of the core ETNP ODZ, 9.2-12.5 µmol kg-1 of fixed nitrogen loss. While the current increase is comparable to only two previous events, it is still within this confidence interval. Nevertheless, climate driven intensification could lead to unprecedented changes within the next decade. The deep ODZ also strengthened from 2016-2019 by approximately 30%, even more rapidly than the core ODZ. This dramatic increase was not observed over the rest of the 50-year time series.
Triple-oxygen isotope (δ18O and Δ17O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which can rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18α and 17α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18α data and no 17α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18α and 17α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate isomers—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion “water droplets”; using the B3LYP/6-31G+(d,p) method; we additionally determine higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) and anharmonic zero-point energy (ZPE) scaling factors using a suite of gaseous sulfoxy compounds and test their impact on resulting sulfoxyanion fractionation-factor estimates. When including redox state-specific CCSD/aug-cc-pVTZ and anharmonic ZPE scaling factors, our theoretical 18α predictions for protonated isomers closely agree with all existing experimental data, yielding root-mean-square errors of 1.8 ‰ for SO3(OH)-/H2O equilibrium (n = 18 experimental conditions), 2.2 ‰ for SO2(OH)-/H2O (n = 27), and 3.9 ‰ for S2O2(OH)-/H2O (n = 3). This result supports the idea that oxygen exchange occurs via isomers containing oxygen-bound protons. By combining 18α and 17α predictions, we additionally estimate that SO3(OH)-, SO2(OH)-, SO(OH)-, and S2O2(OH) exhibit Δ17O values as much as 0.167 ‰, 0.097 ‰, 0.049 ‰, and 0.153 ‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples.
The transport of methane from deep sediments towards the seafloor is widespread in ocean margins and has important biogeochemical implications for the deep ocean . A significant portion (>80%) of methane entering the shallow sediments from below at present is oxidized by microbially-driven anaerobic oxidation of methane (AOM), which mainly involves a microbial consortium of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria. Isoprenoid Glycerol dialkyl glycerol tetraethers (GDGTs) derived from core lipid membranes of ANMEs are often well preserved in sediment records. Methane Index (MI) is an organic geochemical proxy for methane seepage intensity which weighs in the relative proportion of GDGTs (GDGT-1,-2, and -3) preferentially synthesized by ANMEs with that of non-methane-related biomarker contribution from planktonic and benthic sources (Crenarchaeols) . This study analyzed the GDGT composition of sedimentary core lipids from IODP Site 1230 (Peru Margin) using two silica columns and a high-resolution and accurate mass Orbitrap Fusion Mass Spectrometer. Our results report novel GDGT isomers with concentration peaking at the Sulfate-Methane Transition Zones (SMTZ) with the highest AOM activity around 8 mbsf. Further, these isomers were almost absent above and below the SMTZ. Our observations suggest that these characteristic isomers of GDGT compounds preserved at the SMTZ depth are sourced from ANMEs. Identification of these novel isomers has important implications in refining the MI and additional GDGT based palaeoceanographic proxies like TEX86. 1. Akam et al. (2020), Frontiers in Marine Science 7, 206. 2. Y. G. Zhang et al. (2011), Earth and Planetary Science Letters 307, 525-534.
Due to the complex physical and biogeochemical conditions, the adjacent South Yellow Sea (SYS) and East China Sea (ECS) are ideal sites for studying different carbonate characteristics in marginal seas. The distributions of carbonate system parameters were investigated in this region in early spring and summer. Overall, dissolved inorganic carbon (DIC) and alkalinity concentrations in the SYS were higher than those in the ECS due to the Yellow River runoff which was featured with intensive carbonate weathering and erosion. Low DIC, alkalinity and high pH values were observed in the Zhe-Min Coastal Current with intensive primary production in spring caused by the Changjiang River and Taiwan Warm Current. Temperature and biological activities were the primary drivers in controlling the partial pressure of CO2 (pCO2) variability in the SYS, whereas temperature was the only dominant factor in the outer shelf of the ECS, which was heavily impacted by the Kuroshio Current. The pCO2 dynamics was controlled by primary production and physical mixing in the Changjiang River plume and the inner and middle shelves of the ECS, due to the influence of the Changjiang River with high nutrient supply. Overall, strong CO2 sinks (-4.11 ± 5.28 mmol m-2d-1) turned into weak sources (0.88 ± 5.09 mmol m-2d-1) in the entire study area 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 always served as a CO2 sink.