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We examined the spatial variation in size‐fractionated (0.8–51 and > 51 μm) particulate cadmium (Cd) and phosphorus (P) based on a large dataset collected during a GEOTRACES Section cruise (GP09) in the North Pacific Subtropical Gyre (NPSG) to better understand the interrelationship between Cd and P biogeochemical cycles. Concentrations of particulate Cd (0.06–2.15 pmol L−1) and P (0.33–10.19 nmol L−1) showed an initial increase with depth followed by a decrease, and were among the lowest observed in the global ocean. Bulk particulate Cd : P ratios in the euphotic zone, indicative of phytoplankton Cd assimilation, showed strong geographic variability averaging 0.05 ± 0.02 within the NPSG interior vs. 0.14 ± 0.04 pmol nmol−1at the southern boundary. Cadmium to P remineralization ratio in the mesopelagic zone had a roughly similar stoichiometry as what was produced in the euphotic zone, being ~ 0.05 ± 0.01 in the NPSG interior vs. 0.21 ± 0.04 pmol nmol−1at the southern boundary. Cd–P decoupling was reflected in the elements' vertical distribution, showing unsynchronized changes through the water column, consistent with Cd–P differential remineralization resulting from multiple Cd and P pools. The Cd–P relationship also differed between small and large particles, suggesting differences in Cd assimilation among phytoplankton assemblages as well as particle dynamic processes. Our results highlight complex processes fractionating Cd from P in the oligotrophic ocean and complicate the use of Cd as a palaeo‐phosphate proxy.

The impact of environmental factors on nitrogen (N) and oxygen (O) isotope effects during algal nitrate assimilation causes uncertainty in the field application of sedimentary N isotope records and nitrate isotopes to understand the marine nitrogen cycle. Ocean acidification is predicted to change nitrogen cycling including nitrate assimilation, but how N and O isotope effects during algal nitrate assimilation vary in response to changes in seawater pH and partial pressure CO2(pCO2) remains unknown. We measured N and O isotope effects during nitrate assimilation and physiological states of the marine diatom Thalassiosira weissflogiiand Synechococcusunder different pH (8.1 or 7.8) and pCO2(400 or 800 μatm) conditions. Low pH and/or high pCO2equally decreased N and O isotope effects during nitrate assimilation by diatoms possibly due to reducing cellular nitrate efflux/uptake ratio and decreased isotope effects for nitrate uptake, whereas they did not affect those by Synechococcuswith low intracellular nitrate concentration and limited nitrate efflux. Our results provide compelling experimental evidence showing different changes in N and O isotope effects during nitrate assimilation by marine eukaryotic and prokaryotic phytoplankton at low pH and/or high pCO2. These findings suggest new insight into environmental controls on variability in the isotope effect during algal nitrate assimilation, and have implications for improving a predictive understanding of N and O isotope tools in acidified oceans.

Atmospheric aerosol deposition into the low latitude oligotrophic ocean is an important source of new nutrients for primary production. However, the resultant phytoplankton responses to aerosol deposition events, both in magnitude and changes in community composition, are poorly constrained. Here, we investigated this with 19 d of field and satellite observations for a site in the subtropical North Atlantic. During the observation period, surface dissolved aluminum concentrations alongside satellite‐derived aerosol and precipitation data demonstrated the occurrence of both a dry deposition event associated with a dust storm and a wet deposition event associated with strong rainfall. The dry deposition event did not lead to any observable phytoplankton response, whereas the wet deposition event led to an approximate doubling of chlorophyll a, with Prochlorococcusbecoming more dominant at the expense of Synechococcus. Bioassay experiments showed that phytoplankton were nitrogen limited, suggesting that the wet deposition event likely provided substantial aerosol‐derived nitrogen, thereby alleviating the prevalent nutrient limitation and leading to the rapid observed phytoplankton response. These findings highlight the important role of wet deposition in driving rapid responses in both ocean productivity and phytoplankton community composition.

The South China Sea (SCS) is one of the largest marginal seas in the world. It includes a river-dominated, highly productive ocean margin on the northern shelf and an oligotrophic ocean-dominated basin along with other subregions with various features. It was a challenge to estimate the air–sea CO2 flux in this area. We developed a retrieval algorithm for sea surface partial pressure of CO2 ( ${p}$ CO2) by a combination of our previously established mechanistic semianalytical method (MeSAA) and machine learning (ML) method, named MeSAA-ML-SCS, built upon a large dataset of sea surface ${p}$ CO2 collected from in situ measurements during 44 cruises /legs to the SCS in the last two decades. We set several semianalytical parameters, including ${p}$ CO2_therm represented the combined effect of thermodynamics and the atmospheric CO2 forcing on seawater ${p}$ CO 2; upwelling index (UISST) and mixing layer depth (MLD) to characterize the mixing processes; and chlorophyll-a concentration (Chl-a) with remote sensing reflectance at 443 and 555 nm [Rrs(443) and Rrs(555)], which were proxies of biological effects and other characteristics for distinguishing shelf, basin, and subregions. We set the difference between seawater ${p}$ CO2 and atmospheric ${p}$ CO2( $\boldsymbol {\Delta p}\mathbf {C}\mathbf {O}_{\mathbf {2}}^{\mathbf {Sea{-}Air}}$ ) as the output, and the seawater ${p}$ CO2 was finally obtained by summing atmospheric ${p}$ CO2 and $\boldsymbol {\Delta p}\mathbf {C}\mathbf {O}_{\mathbf {2}}^{\mathbf {Sea{-}Air}}$ . We compared several ML models, and the XGBoost model was confirmed as the best. Independent cruise-based datasets that are not involved in the model training were used to validate the satellite products, with low root-mean-square error (RMSE = $11.69~\mu $ atm) and mean absolute percentage deviation (APD = 1.59%). The increasing trend of time-series satellite-derived ${p}$ CO2 (2.44 ± $0.24~\mu$ atm/year) was validated by the in situ data at the Southeastern Asia Time-series Study (SEATS) station, showing good consistency. Results indicate that the SCS as a whole is a source of atmospheric CO2, releasing an average of 12.34 ± 3.11 Tg C/year from a total area of $2.87\times106$ km2, while the northern shelf acts as a sink (2.02 ± 0.64 Tg C/year). With the forcing of increasing atmospheric CO2, the area-integrated CO2 efflux over the entire SCS is decreasing with a rate of 0.41 Tg C/year during 2003–2019. This shared long time series, high-accuracy dataset (1 km) can be helpful to further improve our understanding of the air–sea CO2 exchange dynamics in the SCS.

This study examined carbonate dynamics in the northwestern South China Sea (NWSCS), an area jointly influenced by upwelling, river plumes and submarine groundwater discharge. Data were obtained from two cruises conducted in summer 2009 and 2012. In 2009, a high salinity-low temperature water mass occurred nearshore off northeastern Hainan Island, indicative of upwelling, commonly referred to as HNEU. A river plume fueled primarily by local rivers and characterized by low salinity and high temperature was observed in the NWSCS off the mainland roughly along the 30 m isobath. In 2012, coastal upwelling off northeastern Hainan Island was not detectable at the surface, but was observed at a different location off eastern Hainan Island (HEU). River plume waters in 2012 were patchily distributed, with a low salinity zone further westerly than that in 2009 and another on the mid-shelf of the NWSCS sourced from the Pearl River which reached out ∼250 km from the mouth of the Pearl River Estuary. In 2009, elevated dissolved inorganic carbon (DIC) and total alkalinity (TA) occurred in the coastal plume, where submarine groundwater discharge contributed DIC and TA additions of 38.9±20.5 and 42.5±22.3 µmol kg−1, respectively, with a DIC/TA ratio of ∼0.92, which made a minor contribution to the variation of seawater partial pressure of CO2(pCO2), pH and the aragonite saturation state index (Ωarag). Additionally, high surface phytoplankton production consumed DIC of 10.0±10.4 µmol kg−1but did not significantly affect TA, which dominated pCO2drawdown in the coastal plume water and increased the pH and Ωaragat surface. Submarine groundwater discharge was also observed in the region influenced by upwelling, but to a lesser degree than that impacted by coastal plume. Lower pH and Ωaragand higher pCO2values than in offshore waters were observed downstream of the upwelling system, attributable largely to organic matter remineralization with a DIC addition of 23.8±8.4 µmol kg−1. In 2012, submarine groundwater discharge was not detected but high phytoplankton production dominated carbonate dynamics in the coastal plume water with a net DIC consumption of 104.2 µmol kg−1, which markedly drew down sea surface pCO2and increased pH and Ωarag. In the Pearl River Plume, the solubility-driven CO2sink exceeded biological CO2uptake, resulting in an additional decrease of pH and Ωaragand increase of seawater pCO2. Taken together, this study demonstrated complex spatial and year-to-year variability, and the controls of the carbonate system under the joint modulations of upwelling, river plumes and submarine groundwater discharge. A first order estimate that considered the rise of atmospheric CO2and seawater temperature further suggested a high risk of ocean acidification in this coastal area by the end of this century, which could be amplified under the stresses of river plumes, submarine groundwater discharge and organic matter remineralization.

Nitrification, the stepwise oxidation of ammonia to nitrate via nitrite, is a key process in the marine nitrogen cycle. Reported nitrite oxidation rates frequently exceed ammonia oxidation rates below the euphotic zone, raising the fundamental question of whether the two steps are balanced and if alternative sources contribute to nitrite production in the dark ocean. Here we present vertically resolved profiles of ammonia, urea, and nitrite oxidation rates and their kinetic traits in the oligotrophic Subtropical North Pacific. Our results show active urea‐derived nitrogen oxidation (urea‐N oxidation) in the presence of experimental ammonium amendment, suggesting direct urea utilization. The depth‐integrated rates of urea‐N oxidation and ammonia oxidation are comparable, demonstrating that urea‐N oxidation is a significant source of nitrite. The additional nitrite from urea‐N oxidation helps to balance the two steps of nitrification in our study region. Nitrifiers exhibit high affinity for their substrates, and the apparent half‐saturation constants for ammonia and nitrite oxidation decrease with depth. The apparent half‐saturation constant for urea‐N oxidation is higher than that for ammonia oxidation and shows no clear vertical trend. Such kinetic traits may account for the relatively higher urea concentration than ammonium concentration in the ocean's interior. Moreover, a compilation of our results and reported data shows a trend of increased urea‐N oxidation relative to ammonia oxidation from the eutrophic coastal zone to the oligotrophic open ocean. This trend reveals a substrate‐dependent biogeographic distribution of urea‐N oxidation across marine environments and provides new information on the balance and flux of the marine nitrification process. Active urea‐derived nitrogen oxidation in the presence of added ammonium suggests direct urea utilization by marine ammonia oxidizersSubstrate affinity regulates the vertical distribution of ammonia, urea, and nitrite in the ocean's interiorNitrite production from urea is comparable to that from ammonia oxidation in the energy‐starved mesopelagic ocean Active urea‐derived nitrogen oxidation in the presence of added ammonium suggests direct urea utilization by marine ammonia oxidizers Substrate affinity regulates the vertical distribution of ammonia, urea, and nitrite in the ocean's interior Nitrite production from urea is comparable to that from ammonia oxidation in the energy‐starved mesopelagic ocean

The removal of carbon dioxide from the atmosphere by the marine biological pump is a key regulator of Earth’s climate; however, the ocean also serves as a large source of nitrous oxide, a potent greenhouse gas and ozone-depleting substance. Although biological carbon sequestration and nitrous oxide production have been individually studied in the ocean, their combined impacts on net greenhouse forcing remain uncertain. Here we show that the magnitude of nitrous oxide production in the epipelagic zone of the subtropical ocean covaries with remineralization processes and thus acts antagonistically to weaken the radiative benefit of carbon removal by the marine biological pump. Carbon and nitrogen isotope tracer incubation experiments and nitrogen isotope natural abundance data indicate enhanced biological activity promotes nitrogen recycling, leading to substantial nitrous oxide production via both oxidative and reductive pathways. These shallow-water nitrous oxide sources account for nearly half of the air–sea flux and counteract 6–27% (median 9%) of the greenhouse warming mitigation achieved by carbon export via the biological pump.

Mesoscale eddies may enhance nutrient injection into the photic zone and ultimately the magnitude and composition of particle export to depth. Using satellite altimetry, we identified 38 cyclonic eddies that passed in close proximity to the Hawaii Ocean Time‐series (HOT) Station ALOHA, located in the North Pacific Subtropical Gyre, from 1993 to 2018. Particulate carbon (C), nitrogen (N), and biogenic silica (Si) export rates, measured using free floating sediment traps deployed at 150 m as part of HOT, were then associated with either the eddy core or edge based on distance to the eddy center and time of eddy evolution. Elemental fluxes varied significantly within and among individual eddies depending on season and eddy age. Spatially, biogenic Si fluxes were enhanced relative to particulate C and N fluxes at both the cores and edges, with temporally highest particulate C, N and biogenic Si fluxes occurring during the mature stage (3–8 weeks). On average, biogenic Si fluxes were 200 ± 80% (30–270% increase) higher relative to non‐eddy and during non‐bloom periods, with modest enhanced particulate C (10–30% increase) and N (10–20% increase) fluxes. In contrast, during the bloom season (July and August), elemental fluxes were all reduced by 20% relative to non‐eddy references, suggesting that cyclonic eddies depress export during the bloom period. Our results indicate that cyclonic eddies not only increase, but differentially impact the sinking export of critical biological elements, thereby contributing to long term ecological changes in foodwebs that rely on silica as well as carbon for growth.

Addition of the increased anthropogenic nitrogen (NOxand NHy) emitted from northeast Asian countries to the Yellow and East China seas and coastal waters around Korea has resulted in an unparalleled increase in the nitrate (N) concentration relative to the phosphate (P) and silicate (Si) concentrations in the upper ocean. We found that for the Yellow Sea the increase in N over P was largely explained by increased atmospheric nitrogen deposition, whereas for the northern East China Sea, downstream of the Changjiang River plume, the trend in N increase relative to P was more associated with a change in the combined input of nutrients from atmospheric deposition and riverine discharges. In contrast, the dynamics of the N to P relationship in the southern East China Sea was largely controlled by a change in the intrusion intensity of the Kuroshio Current, which has a low N : P ratio. The disproportionate and persistent input of nutrients to the marine waters of this region over the past four decades has transformed extensive areas from being N deficient to being P deficient, and has concurrently decreased the concentration of Si relative to N. In coastal waters around Korea in particular, these shifts in the nutrient regime have been accompanied by a change from diatom‐dominated to dinoflagellate‐dominated blooms. Given the complexity of coastal ecosystems, the associations between changes in nutrient regimes and biological changes need to be investigated in other coastal areas receiving increasing loads of anthropogenic nitrogen.

Carbon dioxide partial pressure (pCO2) in surface water was continuously measured every 3 h from July 2012 to June 2013 using an autonomous pCO2system (MAPCO2) deployed on a moored buoy on the East China Sea shelf (31°N, 124.5°E). Sea surface pCO2and 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 pCO2was respectively 335 ± 70 μatm, 422 ± 43 μatm, 362 ± 11 μatm, 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 pCO2in summer was below the atmospheric level, the net CO2flux has suggested a CO2source status due to the influence of typhoon. Our observation thus demonstrated the significant, even dominant impact of episodic typhoon events on surface ocean CO2chemistry and air–sea CO2gas 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 differed in pre‐, onset, and post‐typhoon stages. Based on our estimates, the degassing fluxes during typhoon reached as high as 82 mmol m−2CO2and 242 mmol m−2CO2in summer and autumn, respectively, accounting for twice as large as the summer CO2sink during non‐typhoon period, and 28% of the total CO2source in autumn.

Globally expanding hypoxia in estuaries and coastal oceans has largely been attributed to the elevated river nutrient inputs, whereas the role of river‐delivered terrestrial organic matter (OMterr) in hypoxia formation has been less investigated. This study uses a coupled physical‐biogeochemical model and observations to investigate how OMterrdirectly (via remineralization) and indirectly (via the nutrients released from OMterrremineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OMterrremineralization to total oxygen consumption by terrestrial and marine organic matter negatively correlates with salinity, decreasing from over 60% in the upper estuary to nearly 0% in the far reaches of the river plume, and is higher in the upstream (average 30%) than the downstream region (average 18%). Nevertheless, the nutrients released from OMterrremineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OMterrto hypoxia along the plume path is a combined result of the wind‐driven eastward shelf current and the OMterr‐released nutrients being advected farther downstream than the sinking OMterr. This highlights that without including the indirect effect of OMterrmay underestimate the role of OMterrin hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OMterrloads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream.

We surveyed the carbonate system along the main channel of the Chesapeake Bay in June 2016 to elucidate carbonate dynamics and the associated sources of oxygen‐consuming organic matter. Using a two endmember mixing calculation, chemical proxies, and stoichiometry, we demonstrated that in early summer, dissolved inorganic carbon (DIC) dynamics were controlled by aerobic respiration in the water column (43%), sulfate reduction in the sediment (39%), atmospheric CO2invasion (13%), and CaCO3dissolution (5%). A mass balance of the DIC concentration and its stable isotope suggested that the apparent δ13C of oxygen‐consuming organic matter was −19.4 ± 0.3‰. The bulk composition of particulate organic matter also reflected a dominance of algal material (C/N = ~ 6, δ13C > −25‰). Therefore, we concluded that the decomposition of autochthonous organic matter (i.e., eutrophication‐stimulated primary production) was the dominant process consuming oxygen, while allochthonous organic matter (terrestrially derived) made minor contributions to oxygen consumption in the hypoxic zone in June 2016. These findings in the Chesapeake Bay contrast with another hypoxic estuarine ecosystem, the Pearl River Estuary in China where allochthonous organic matter contributed significantly to oxygen consumption. The differences between these two systems in terms of hydrology, quantity and quality of organic matter, and physical characteristics are discussed to yield new insights on the formation and maintenance of hypoxia. In both systems, autochthonous organic matter dominates oxygen depletion, indicating that nutrient management and reduction are useful actions to control and mitigate the occurrence of hypoxia for the restoration of ecosystem.

We examined the dynamics of dissolved inorganic carbon (DIC) and pH in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS) shelf in summer, aiming for a better understanding of the interaction between eutrophication, hypoxia, and ocean acidification. Using a semi‐analytical diagnostic approach based on validated multiple end‐member water mass mixing models, we showed a −191 ± 54 μmol kg−1deficit in DIC concentrations in an extensive surface plume bulge, corresponding to a significant pH increase of ∼ 0.57 ± 0.19 units relative to conservative mixing. In contrast, DIC additions in the bottom hypoxic zone reached ∼ 139 ± 21 μmol kg−1, accompanied by a decrease in pH of −0.30 ± 0.04 units. In combination with stable carbon isotopic compositions, we found biological production and CO2outgassing to be responsible for DIC deficits in surface waters, while degradation of organic matter (OM) accounted for DIC additions in bottom waters. The PRE‐NSCS plume system as a whole served as a net source of atmospheric CO2from the perspective of Lagrangian observations, because strong CO2outgassing in the inner estuary overwhelmed the CO2uptake in the plume despite strong phytoplankton blooms. Using a two‐layer box model, we further estimated that at least ∼ 45 ± 13% of eutrophication‐driven OM production in the surface plume accounted for 67 ± 18% of the DIC addition and oxygen consumption in bottom waters. Eutrophication also buffered ocean acidification in surface waters while hypoxia enhanced it in bottom waters, but their effects on acid‐base buffering capacity were secondary to the amplification of coastal ocean acidification caused by freshwater inputs.

Uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has acidified the ocean and threatened the health of marine organisms and their ecosystems. In coastal waters, acidification is often enhanced by CO2and acids produced under high rates of biological respiration. However, less is known about buffering processes that counter coastal acidification in eutrophic and seasonally hypoxic water bodies, such as the Chesapeake Bay. Here, we use carbonate chemistry, mineralogical analyses and geochemical modelling to demonstrate the occurrence of a bay-wide pH-buffering mechanism resulting from spatially decoupled calcium carbonate mineral cycling. In summer, high rates of photosynthesis by dense submerged aquatic vegetation at the head of the bay and in shallow, nearshore areas generate high pH, an elevated carbonate mineral saturation state and net alkalinity uptake. Calcium carbonate particles produced under these conditions are subsequently transported downstream into corrosive subsurface waters, where their dissolution buffers pH decreases caused by aerobic respiration and anthropogenic CO2. Because this pH-buffering mechanism would be strengthened by further nutrient load reductions and associated submerged aquatic vegetation recovery, our findings suggest that the reduction of nutrient inputs into coastal waters will not only reduce eutrophication and hypoxia, but also alleviate the severity of coastal ocean acidification.

Global coastal oceans as a whole represent an important carbon sink but, due to high spatial–temporal variability, a mechanistic conceptualization of the coastal carbon cycle is still under development, hindering the modelling and inclusion of coastal carbon in Earth System Models. Although temperature is considered an important control of sea surface pCO2, we show that the latitudinal distribution of global coastal surface pCO2does not match that of temperature, and its inter-seasonal changes are substantially regulated by non-thermal factors such as water mass mixing and net primary production. These processes operate in both ocean-dominated and river-dominated margins, with carbon and nutrients sourced from the open ocean and land, respectively. These can be conceptualized by a semi-analytical framework that assesses the consumption of dissolved inorganic carbon relative to nutrients, to determine how a coastal system is a CO2source or sink. The framework also finds utility in accounting for additional nutrients in organic forms and testing hypotheses such as using Redfield stoichiometry, and is therefore an essential step toward comprehensively understanding and modelling the role of the coastal ocean in the global carbon cycle.

Biogeochemical silicon (Si) cycling in coastal systems is highly influenced by anthropogenic perturbations in recent decades. Here, we present a systematic study on the distribution of stable Si isotopes of dissolved silicate (δ30SiDSi) in a highly eutrophic coastal system, the Baltic Sea. Besides the well‐known processes, diatom production and dissolution regulating δ30SiDSivalues in the water column, we combined field data with a box model to examine the role of human disturbances on Si cycling in the Baltic Sea. Results reveal that (1) damming led to increased δ30SiDSivalues in water but had little impacts on their vertical distribution; (2) decrease in saltwater inflow due to enhanced thermal stratification had negligible impacts on the δ30SiDSidistribution. An atypical vertical distribution of δ30SiDSiwith higher values in deep water (1.57–1.95‰) relative to those in surface water (1.24–1.68‰) was observed in the central basin. Model results suggest the role of enhanced biogenic silica (BSi) deposition and subsequently regenerated dissolved silicate (DSi) flux from sediments. Specifically, eutrophication enhances diatom production, resulting in elevated exports of highly fractionated BSi to deep water and sediments. In situ sedimentary geochemical processes, such as authigenic clay formation, further fractionate Si isotopes and increase pore‐water δ30SiDSivalues, which then leads to pore‐water DSi flux carrying higher δ30SiDSicompositions into deep water. Our findings provide new quantitative information on how the isotope‐based Si cycle responds to human perturbations in coastal seas and shed lights on shifts of Si export to open ocean.

The N2fixation and primary production rates were measured simultaneously using 15N2and 13C incubation assays in the northern South China Sea influenced by the Kuroshio intrusion (KI) seasonally. The degree of KI (KI index, range from 0 to 1) was assessed by applying an isopycnal mixing model. The water column integrated N2fixation and primary production for stations with KI index larger than 0.5 were 463 ± 260 μmol N·m−2·day−1and 62 ± 19 mmol C·m−2·day−1, respectively, significantly higher than those for stations with KI index lower than 0.5 (50 ± 10 μmol N·m−2·day−1and 28 ± 10 mmol C·m−2·day−1, respectively). Trichodesmiumwas the dominant diazotroph at stations with KI index larger than 0.5, with 2 orders of magnitude higher nifHgene abundance than that at stations with KI index lower than 0.5. However, the highest N2fixation rates were found in waters with moderate KI index around 0.6, suggesting that frontal zone mixing might stimulate N2fixation. Our results demonstrated that diazotrophs (mainly Trichodesmium) were tightly associated with the KI, which modulated the biogeographic distribution of N2fixers. In summary, we found the transportation of Trichodesmiumby KI, then, we quantified the fraction of KI and N2fixation rates in the northern South China Sea. The results suggested that KI generated a new biogeographic regime which could significantly influence the carbon and nitrogen cycles far away from the main stream. The oligotrophic Kuroshio Current intrusion into the South China Sea generates a new biogeographic regime characterized by high N2fixationTrichodesmiumcontributes <10% of primary production yet creates a positive correlation between N2fixation and primary production

The Indonesian Throughflow (ITF), the only oceanic tropical pathway linking the Pacific and Indian Oceans, plays a critical role in the redistribution of heat and mass, affecting both the regional and global climate systems. Based on the distributions and changes in the tritium inventories, and tritium budgets in and between the South Pacific and South Indian Oceans from the Geochemical Ocean Sections Study and World Ocean Circulation Experiment programs, the long‐term mean water mass transport of the ITF is estimated. The total throughflow of the ITF is estimated as 16.2 ± 3.5 Sv. The North Pacific contributes 14.1 ± 2.7 Sv to the total throughflow, of which the layers between 0–250, 250–650, and 650–1,000 m carry 7.3 ± 0.6, 5.4 ± 1.3, and 1.4 ± 3.0 Sv, respectively. Along with the water transport, the North Pacific component of the ITF transfers 0.84 ± 0.14 PW of heat into the Indian Ocean, of which almost 90% exits from the Indian Ocean at 30°S. The net nitrate flux into the photic zone associated with the ITF is 2.71 ± 2.60 mmol·m−2·d−1in the Indonesian Seas, ~90% of which is induced by vertical diffusion. The water transfer between the Pacific and Indian Oceans, commonly referred to as the Indonesian Throughflow (ITF), is the only tropical interbasin oceanic pathway, which plays a critical role in the redistribution of heat and mass, affecting both the regional and global climate systems. Due to the strong variability at both seasonal and interannual‐interdecadal timescales and the multipathway of the ITF, constraining the long‐term mean ITF volume transport is challenging. Chemical tracer method is a good complement for physical observations to constrain the temporally integrated result. In this work, we use tritium data from Geochemical Ocean Sections Study and World Ocean Circulation Experiment to quantify the long‐term averaged volume transport of the ITF based on the difference in tritium inventory between the South Indian Ocean and the South Pacific Ocean. The total throughflow of the ITF is estimated as 16.2 ± 3.5 Sv. Along with the water transport, the North Pacific component of the ITF transfers 0.84 ± 0.14 PW of heat into the Indian Ocean, of which almost 90% exits from the Indian Ocean at 30°S. The net nitrate flux into the photic zone associated with the ITF is 2.71 ± 2.60 mmol·m−2d−1in the Indonesian Seas, 90% of which is induced by vertical diffusion. The total long‐term mean water transport of the ITF is 16.2 ± 3.5 Sv derived from tritium budgets in the South Pacific and South Indian OceansThe North Pacific component of the ITF transfers 0.84 ± 0.14 PW heat into the Indian OceanVertical diffusion dominates nutrient supply to the photic zone in the Indonesian Seas

Nitrogen (N), as a critical element for microbial metabolisms, recycles rapidly in the euphotic ocean. Oxidation by nitrifiers is a competing pathway for phytoplankton assimilation of regenerated N (NH4+and urea). Sharing the overlapping substrates may result in competitive exclusion, thus, niche separation for the two assemblages. Both pathways are sensitive to light, but whether light intensity will intensify or alleviate such resource competition in the euphotic zone remains poorly explored in the field at the community level. By using 15N labeling techniques, paired kinetic responses of uptake and oxidation were conducted in single bottles under manipulated light intensities for both NH4+and urea. We found light stimulated the maximum rate (Rm) and specific affinities (αU) of both NH4+and urea uptake. In contrast, light effects were opposite for oxidation kinetics (Rmand αO). As irradiance increased, the rapid increase in αUand concomitant decrease in αOimply a distinctive competition advantage of photosynthetic organisms over oxidizers under substrate‐limited environments. The ratio of αU/αOfor NH4+ranged from 0.8 to 3089 (5.8–46,788 for urea) showing a distinct increasing pattern as ambient light increases, demonstrating that phytoplankton overwhelms nitrifiers throughout the oligotrophic euphotic zone, driving down concentrations and maintaining short turnover times of the two regenerated N substrates. Moreover, phytoplankton relied equally on NH4+and urea; yet, nitrifiers preferred NH4+to urea. In the nitrate‐depleted euphotic ocean, light acts as a crucial driver for utilization pathways of regenerated N and vertical niche separation.

The chlorophyll in the northern South China Sea (NSCS) shows strong interannual variability between different phases of the El Niño‐Southern Oscillation (ENSO), primarily due to the influence from Kuroshio intrusion. Chlorophyll observations also reveal that significant year‐to‐year variation remains in the same ENSO phase, but its controlling mechanism is unknown. In this study, we examined driving mechanisms for such regional ecosystem variability on a year‐to‐year timescale. Using observational data and modeling results, we found that both cyclonic eddies (CEs) and wind‐induced mixing affect phytoplankton variability, but the former is the dominant factor regulating the interannual variability of chlorophyll during La Niña years, while the latter becomes the dominant one during El Niño years. The underlying mechanisms leading to the contrast in biological responses are attributable to changes in background nutricline depth and mixed‐layer depth (MLD) during different ENSO phases. During La Niña, both thermocline and nutricline are deepened in the NSCS. Thus, wind mixing is less effective in non‐eddy areas due to the deepened nutricline, while the CE‐induced subsurface nutrient anomaly associated with increased MLD is able to control the interannual variability of chlorophyll. During El Niño, wind‐induced mixing is more effective than CEs because the surface wind can influence a larger area.

Production and transformation of dissolved and particulate organic matter (DOM, POM) in estuaries regulate the carbon export from land to ocean, yet tracing their source and diagenetic status is challenging in these dynamic environments. Here we study the production, transport, and diagenetic status of DOM in the Pearl River Estuary (PRE), China, based on total dissolved amino acids, their enantiomers (D/L ratios), and other ancillary biogeochemical parameters with a complete coverage of upper, middle, and lower estuary. Inferred from amino acid composition and D/L ratios, DOM in the upper PRE was highly altered by bacteria, while the carbon yields of total dissolved amino acids were relatively high showing a conflicting pattern likely contributed by multiple sources including planktonic production, soil leachates, and sewage discharge. Conservative mixing between freshwater and seawater predominantly controlled DOM dynamics in the middle PRE. In contrast, the lower PRE was characterized by high planktonic production, leading to the accumulation of labile DOC in the surface water. The compositional pattern of amino acids differed significantly between dissolved and particulate phases, yet the diagenetic indices of POM and DOM were both relatively lower in the upper and middle PRE compared to the lower PRE, suggesting that primary production is a major driving factor in the lower PRE. Overall, our study emphasizes the highly dynamic and spatially heterogeneous nature of the PRE, and the results from molecular level characterization provide new insights into the complex sources, diagenesis, and transformation processes of DOM in different regions of the estuary, as well as its connection with POM. Amino acids and their enantiomers were applied as tracers for diagenetic status of organic materials in the Pearl River EstuaryDiagenetic status of DOM was regulated by the extent of bacterial alteration, mixing, and primary production in the estuaryThe covaried diagenetic indices of DOM and POM implied that the two organic pools were somewhat coupled in the Pearl River Estuary

Natural and anthropogenic radionuclides are used to study a suite of environmental processes. Yet their applications in aquatic systems are hindered by a general lack of knowledge regarding the underlying concepts of radioactivity, the occurrence of radionuclides in ecosystems, and the equations used to describe their decay mechanisms in environmentally applicable ways. The goal of this lecture is to provide upper level undergraduate and graduate students with a basic understanding of the fundamentals of radiochemistry, including the origin and stability of elements, radioactive decay mechanisms, and the fundamental equations that govern radioactive decay. This lecture is the first of a four-part lecture series on radionuclides in the marine environment.

Natural and anthropogenic radionuclides are used to study a suite of environmental processes. Yet their applications in aquatic systems are hindered by a general lack of knowledge regarding the underlying concepts of radioactivity, the occurrence of radionuclides in ecosystems, and the equations used to describe their decay mechanisms in environmentally applicable ways. The goal of this lecture is to provide upper level undergraduate and graduate students with a basic understanding of how the naturally occurring uranium-thorium radioactive decay series can be used to address a range of environmentally relevant questions in marine systems. The lecture begins with a brief introduction to uranium-thorium series decay patterns and their distribution in the marine environment. The remaining lecture focuses on four case studies that cover a range of applications where uranium-thorium series radionuclides are used and includes: scavenging, air-sea gas exchange, tracing groundwater, and sedimentation/age dating. This lecture is the second of a four-part lecture series on radionuclides in the marine environment.

Natural and anthropogenic radionuclides are ubiquitous in the marine biosphere. They are used to study a suite of environmental processes, including those related to marine food webs, yet they also potentially negatively impact marine biota and humans. The goal of this lecture is to provide upper level undergraduate and graduate students with a basic understanding of marine radioecology and how marine organisms bioaccumulate and influence the cycling of radionuclides in the environment. The lecture begins with a brief introduction to the methods and models used to understand biological radionuclide uptake and loss, followed by how organisms biogeochemically and physically transfer radioactive substances throughout the ocean. The remaining lecture focuses on current methods for assessing potential radiological impacts on marine biota and risks associated with contaminated seafood consumption. This is the last lecture of a four-part lecture series on radionuclides in the marine environment.

Natural and anthropogenic radionuclides are used to study a suite of environmental processes. Yet their applications in aquatic systems are hindered by a general lack of knowledge regarding the underlying concepts of radioactivity, the occurrence of radionuclides in ecosystems, and the equations used to describe their decay mechanisms in environmentally applicable ways. The goal of this lecture is to provide upper level undergraduate and graduate students with a basic understanding of how cosmogenic and anthropogenically produced radionuclides can be used to address a range of environmentally relevant questions in marine systems. The lecture begins with a brief introduction to cosmogenic and anthropogenic radionuclide production and sources to the marine environment. The remaining lecture focuses on specific case studies using these radionuclides in a range of applications including: ocean circulation and mixing, particle scavenging, and sedimentation/age dating. This lecture is the third of a four-part lecture series on radionuclides in the marine environment.

The eastern equatorial Pacific (EEP) is a source of atmospheric CO2during the last deglaciation, but the associated oceanic dynamics in the broader low‐latitude Pacific is not fully understood. Here, we report 30,000‐year‐long surface and subsurface pCO2records for the western equatorial Pacific (WEP), based on boron isotopes in two planktonic foraminiferal species from core MD10‐3340. Our results show that the WEP surface became a significant atmospheric CO2sink despite that its subsurface waters were enriched by CO2during the last deglaciation to early Holocene. Combined with EEP proxy data and model results, we suggest that a deglacial‐early Holocene zonal seesaw of sea‐air CO2exchange across the equatorial Pacific led to a net CO2outgassing much greater than the modern situation. This can be ascribed to strengthened Subtropical‐Tropical Circulation, resulting in stronger upper ocean stratification in the WEP concurrent with enhanced upwelling of CO2‐rich subsurface waters in the EEP. The modern equatorial Pacific is one of the main sources of CO2released from the ocean into the atmosphere, with the strongest outgassing in the central‐eastern part and nearly neutral state in the western part. However, according to our boron isotope data of planktonic foraminifera, the western equatorial Pacific was turned into a significant atmospheric CO2sink rather than a source between 16,000 and 7,000 years ago. At the same time, the eastern equatorial Pacific released more CO2, resulting in a zonal seesaw of sea‐air CO2exchange across the equatorial Pacific that generally accelerated the CO2outgassing and favored global warming. We further argue that these deglacial‐early Holocene CO2changes were ascribed to the strengthened Subtropical‐Tropical Circulation, in which the accelerated eastward Equatorial Undercurrent leads to stronger upper ocean stratification in the WEP, and also converges seawater CO2into the upper thermocline and finally releases at the surface EEP. This finding helps us understand and quantitatively assess future global carbon budget, particularly how the upper ocean circulation modulates the sea‐air CO2flux in a manner similar to the past. Strengthened upper ocean stratification led to significant atmospheric CO2sink in the western equatorial Pacific during ∼16–7 kaThe zonal seesaw of sea‐air CO2exchange in the equatorial Pacific resulted in a net CO2outgassing much greater than the modern situationThe sea‐air CO2exchange across the equatorial Pacific was modulated by the Subtropical‐Tropical Circulation on orbital timescales Strengthened upper ocean stratification led to significant atmospheric CO2sink in the western equatorial Pacific during ∼16–7 ka The zonal seesaw of sea‐air CO2exchange in the equatorial Pacific resulted in a net CO2outgassing much greater than the modern situation The sea‐air CO2exchange across the equatorial Pacific was modulated by the Subtropical‐Tropical Circulation on orbital timescales

Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N2fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N2fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region. Cyclonic eddies are oceanographic features leading to upward transfer of deep‐water nutrients that support phytoplankton growth in the sunlit surface ocean. In the low‐nutrient Northwest Pacific, cyclonic eddies are considered to be an extremely important nutrient source in alleviating nutrient limitation; however, their biogeochemical effects are not well‐examined, in particular in the upper layers where nutrients are below the detection limit of conventional techniques. By examining a cyclonic eddy with four sequential transect observations, our results showed that the eddy core had higher nutrient concentrations in the lower euphotic zone compared to the eddy edge, consistent with previous studies; in contrast, concentrations of phosphate in the upper surface layer had significantly lower values within the core relative to those at the edge. Surface rate measurements suggested that this lower phosphate was potentially driven by elevated N2fixation, likely due to enhanced iron supply from depth, thereby resulting in additional consumption of excess surface phosphate. However, this increased nutrient supply did not lead to the enhancement of total phytoplankton biomass across the entire euphotic zone. Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edgeEnhanced N2fixation within the eddy core is proposed to have driven increased phosphate consumptionNo substantial total phytoplankton biomass increase was found within the eddy core Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edge Enhanced N2fixation within the eddy core is proposed to have driven increased phosphate consumption No substantial total phytoplankton biomass increase was found within the eddy core

Global carbon dioxide (CO2) evasion from inland waters (rivers, lakes, and reservoirs) and carbon (C) export from land to oceans constitute critical terms in the global C budget. However, the magnitudes, spatiotemporal patterns, and underlying mechanisms of these fluxes are poorly constrained. Here, we used a coupled terrestrial–aquatic model to assess how multiple changes in climate, land use, atmospheric CO2concentration, nitrogen (N) deposition, N fertilizer and manure applications have affected global CO2evasion and riverine C export along the terrestrial‐aquatic continuum. We estimate that terrestrial C loadings, riverine C export, and CO2evasion in the preindustrial period (1800s) were 1,820 ± 507 (mean ± standard deviation), 765 ± 132, and 841 ± 190 Tg C yr−1, respectively. During 1800–2019, multifactorial global changes caused an increase of 25% (461 Tg C yr−1) in terrestrial C loadings, reaching 2,281 Tg C yr−1in the 2010s, with 23% (104 Tg C yr−1) of this increase exported to the ocean and 59% (273 Tg C yr−1) being emitted to the atmosphere. Our results showed that global inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans, highlighting the important role of inland waters in the global C balance, an amount that should be taken into account in future C budgets. Our analysis supports the view that a major feature of the global C cycle–the transfer from land to ocean–has undergone a dramatic change over the last two centuries as a result of human activities. Despite occupying only 1% of the Earth's surface, inland waters (rivers, lakes, and reservoirs) play a critical role in global carbon (C) cycling by linking two of the Earth's largest C pools, terrestrial and marine ecosystems, as well as by exchanging CO2with the atmosphere. Inland waters emit and bury C before it reaches the oceans, with important implications for the global C budget. Although global estimates of lateral C fluxes have been made previously, much uncertainty exists in their magnitudes, spatiotemporal patterns, and underlying controls (anthropogenic vs. natural processes). By improving a coupled terrestrial–aquatic model, we assess how climate, land use, atmospheric CO2, and nitrogen enrichment affected global CO2evasion and riverine C export along the terrestrial–aquatic continuum since the 1800s. We estimate a 25% increase in terrestrial C loading since the 1800s, of which 59% was emitted to the atmosphere and 23% was exported to the ocean. The increased riverine C exports were primarily due to increasing atmospheric CO2level and nitrogen inputs; additionally, climate and land conversion dominated interannual and decadal variations in CO2evasion. Our findings indicate that anthropogenic‐induced climate change and multiple environmental stresses since the preindustrial era have resulted in significant increases in terrestrial C export to oceans and CO2evasion. Global inland water recycles and exports nearly half of the net land C sink into the atmosphere and ocean, underscoring the importance of inland waters for closing the global carbon budget. Terrestrial carbon loading since 1800 has increased by 25%, with 23% of this increase exported to the ocean and 59% being emitted to the atmosphereAtmospheric CO2and N inputs dominated C export increase, while the climate and land use change dominated the decadal variations in CO2evasionGlobal inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans in the 2010s Terrestrial carbon loading since 1800 has increased by 25%, with 23% of this increase exported to the ocean and 59% being emitted to the atmosphere Atmospheric CO2and N inputs dominated C export increase, while the climate and land use change dominated the decadal variations in CO2evasion Global inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans in the 2010s

Subtropical gyres cover 26%–29% of the world's surface ocean and are conventionally regarded as ocean deserts due to their permanent stratification, depleted surface nutrients, and low biological productivity. Despite tremendous advances over the past three decades, particularly through the Hawaii Ocean Time‐series and the Bermuda Atlantic Time‐series Study, which have revolutionized our understanding of the biogeochemistry in oligotrophic marine ecosystems, the gyres remain understudied. We review current understanding of upper ocean biogeochemistry in the North Pacific Subtropical Gyre, considering other subtropical gyres for comparison. We focus our synthesis on spatial variability, which shows larger than expected dynamic ranges of properties such as nutrient concentrations, rates of N2fixation, and biological production. This review provides new insights into how nutrient sources drive community structure and export in upper subtropical gyres. We examine the euphotic zone (EZ) in subtropical gyres as a two‐layered vertically structured system: a nutrient‐depleted layer above the top of the nutricline in the well‐lit upper ocean and a nutrient‐replete layer below in the dimly lit waters. These layers vary in nutrient supply and stoichiometries and physical forcing, promoting differences in community structure and food webs, with direct impacts on the magnitude and composition of export production. We evaluate long‐term variations in key biogeochemical parameters in both of these EZ layers. Finally, we identify major knowledge gaps and research challenges in these vast and unique systems that offer opportunities for future studies. Vast subtropical oceans feature basin‐wide anticyclonic gyres, which restrict vertical supplies of nutrients, resulting in low surface nutrient concentrations and generally low rates of biological production. The subtropical gyres have therefore traditionally been regarded as ocean deserts. Through a comprehensive data re‐analysis focusing on the North Pacific Subtropical Gyre, we find larger than expected spatiotemporal variability in biogeochemical properties in subtropical gyres, including the distribution of nutrients, N2fixation, and biological production. Such variations are most pronounced through the sunlit water column (euphotic zone [EZ]), including sharp gradients in nutrient concentrations and sources and microbial community structure. Based on analysis and synthesis conducted in this study, we show evidence for the functioning of a two‐layer EZ habitat, with each layer differentiated primarily by inputs of sunlight and nutrients. Our understanding of long‐term biogeochemical variability is relatively restricted, owing to a lack of reliable long‐term observations at adequate spatial scales. We therefore urge the development and use of higher temporal and spatial resolution sampling strategies, for example, through the use of autonomous sampling platforms, that will allow for a greater understanding of biogeochemical processes and facilitate improvements in numerical modeling capabilities. Subtropical gyres display larger spatiotemporal dynamics in biogeochemical properties than previously consideredAn improved two‐layer framework is proposed for the study of nutrient‐driven and biologically mediated carbon export in the euphotic zoneFuture research will benefit from high‐resolution samplings, improved sensitivity of nutrient analyses, and advanced modeling capabilities Subtropical gyres display larger spatiotemporal dynamics in biogeochemical properties than previously considered An improved two‐layer framework is proposed for the study of nutrient‐driven and biologically mediated carbon export in the euphotic zone Future research will benefit from high‐resolution samplings, improved sensitivity of nutrient analyses, and advanced modeling capabilities

This paper aims to provide an overview of regional carbon fluxes and budgets in the marginal seas adjacent to China. The “China Seas” includes primarily the South China Sea, East China Sea, Yellow Sea, and the Bohai Sea. Emphasis is given to CO2fluxes across the air-sea interface and their controls. The net flux of CO2degassing from the China Seas is estimated to be 9.5±53 Tg C yr−1. The total riverine carbon flux through estuaries to the China Seas is estimated as 59.6±6.4 Tg C yr−1. Chinese estuaries annually emit 0.74±0.02 Tg C as CO2to the atmosphere. Additionally, there is a very large net carbon influx from the Western Pacific to the China Seas, amounting to ∼2.5 Pg C yr−1. As a first-order estimate, the total export flux of particulate organic carbon from the upper ocean of the China Seas is 240±80 Tg C yr−1. This review also attempts to examine current knowledge gaps to promote a better understanding of the carbon cycle in this important region.

The China Seas include the South China Sea, East China Sea, Yellow Sea, and Bohai Sea. Located off the Northwestern Pacific margin, covering 4700000 km2from tropical to northern temperate zones, and including a variety of continental margins/basins and depths, the China Seas provide typical cases for carbon budget studies. The South China Sea being a deep basin and part of the Western Pacific Warm Pool is characterized by oceanic features; the East China Sea with a wide continental shelf, enormous terrestrial discharges and open margins to the West Pacific, is featured by strong cross-shelf materials transport; the Yellow Sea is featured by the confluence of cold and warm waters; and the Bohai Sea is a shallow semi-closed gulf with strong impacts of human activities. Three large rivers, the Yangtze River, Yellow River, and Pearl River, flow into the East China Sea, the Bohai Sea, and the South China Sea, respectively. The Kuroshio Current at the outer margin of the Chinese continental shelf is one of the two major western boundary currents of the world oceans and its strength and position directly affect the regional climate of China. These characteristics make the China Seas a typical case of marginal seas to study carbon storage and fluxes. This paper systematically analyzes the literature data on the carbon pools and fluxes of the Bohai Sea, Yellow Sea, East China Sea, and South China Sea, including different interfaces (land-sea, sea-air, sediment-water, and marginal sea-open ocean) and different ecosystems (mangroves, wetland, seagrass beds, macroalgae mariculture, coral reefs, euphotic zones, and water column). Among the four seas, the Bohai Sea and South China Sea are acting as CO2sources, releasing about 0.22 and 13.86–33.60 Tg C yr−1into the atmosphere, respectively, whereas the Yellow Sea and East China Sea are acting as carbon sinks, absorbing about 1.15 and 6.92–23.30 Tg C yr−1of atmospheric CO2, respectively. Overall, if only the CO2exchange at the sea-air interface is considered, the Chinese marginal seas appear to be a source of atmospheric CO2, with a net release of 6.01–9.33 Tg C yr−1, mainly from the inputs of rivers and adjacent oceans. The riverine dissolved inorganic carbon (DIC) input into the Bohai Sea and Yellow Sea, East China Sea, and South China Sea are 5.04, 14.60, and 40.14 Tg C yr−1, respectively. The DIC input from adjacent oceans is as high as 144.81 Tg C yr−1, significantly exceeding the carbon released from the seas to the atmosphere. In terms of output, the depositional fluxes of organic carbon in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea are 2.00, 3.60, 7.40, and 5.92 Tg C yr−1, respectively. The fluxes of organic carbon from the East China Sea and South China Sea to the adjacent oceans are 15.25–36.70 and 43.93 Tg C yr−1, respectively. The annual carbon storage of mangroves, wetlands, and seagrass in Chinese coastal waters is 0.36–1.75 Tg C yr−1, with a dissolved organic carbon (DOC) output from seagrass beds of up to 0.59 Tg C yr−1. Removable organic carbon flux by Chinese macroalgae mariculture account for 0.68 Tg C yr−1and the associated POC depositional and DOC releasing fluxes are 0.14 and 0.82 Tg C yr−1, respectively. Thus, in total, the annual output of organic carbon, which is mainly DOC, in the China Seas is 81.72–104.56 Tg C yr−1. The DOC efflux from the East China Sea to the adjacent oceans is 15.00–35.00 Tg C yr−1. The DOC efflux from the South China Sea is 31.39 Tg C yr−1. Although the marginal China Seas seem to be a source of atmospheric CO2based on the CO2flux at the sea-air interface, the combined effects of the riverine input in the area, oceanic input, depositional export, and microbial carbon pump (DOC conversion and output) indicate that the China Seas represent an important carbon storage area.

We examined response of phytoplankton total chlorophyll a(TChl a) and community composition to three coherent anticyclonic eddies (ACEs) observed during a cruise to the northern South China Sea on 28 July to 7 August 2007. Photosynthetic pigments were measured to estimate the contribution of nine phytoplankton groups to TChl a. Although the water column‐integrated TChl ainventory in the upper 100 m was very similar among the three ACEs (17.6–18.9 mg/m2) we observed during the cruise, there were remarkable enhancements in biomasses at the eddy edges. TChl ainventory was 20.8 ± 3.0 mg/m2at the edge, which was 33% or 60% higher than at the center and reference. The greatest enhancement of the TChl aat edge was attributed to haptophyte‐8, which was 1.6 and 2.2 times the analogous concentrations at the center and reference sites. The ProchlorococcusChl awas ~50% greater at the edge relative to the reference and was intermediate at the center. Diatom Chl aat the edge was ~2.5 times the concentrations at the center and reference sites. The positive correlation between particulate organic carbon flux and haptophyte‐8 Chl aat the edge implied an important role of haptophyte‐8 in particle export productivity. It is interesting to note that there occurred higher fluxes of biogenic Si at the center of the ACEs due likely to lateral transport of diatoms from the edge. The phenomenon of higher TChl aat the edge but higher export at the center may have been the combined result of vertical convection and lateral transport within the eddies. The response of different phytoplankton groups to three anticyclonic eddies (ACEs) and the relationship between the carbon export and phytoplankton community was studied in the northern South China Sea on July 28‐August 7, 2007. The phytoplankton TChl a concentration inventory in the upper 100 m was very similar among the three ACEs. But the biomass and community composition varied in different parts of the ACEs, including the ACEs' centers, edges and outside the ACEs. The enhancement of TChl a at edge attributed to haptophyte‐8 was siginificantly.The Prochlorococcus Chl a was ~50% greater at the edge relative to the reference and was intermediate at the center. The haptophyte‐8 played an important role of in particle export. The phenomenon of higher TChl a at the edge but higher export at the center may have been the combined result of vertical convection and lateral transport within the eddies The phytoplankton community composition had similarity among different anticyclonic eddiesEddies edge effect was obvious and discrepant for different phytoplankton groupsThe physical‐nutrients‐phytoplankton‐POC fluxdynamic process reflected both the vertical and horizontal inside/outside the eddies

Using field measurements and a process‐oriented three‐dimensional coupled physical‐biogeochemical numerical model, we investigated the physical and biogeochemical processes governing the bottom hypoxic zone west off the Pearl River estuary. The intensity and area of the hypoxia grew with increasing total nutrient input from the Pearl River that has increased continuously in recent decades. The hypoxic zone was formed and maintained largely associated with the stable water column where the stability was provided simultaneously by wind stress and freshwater discharge, favorable local hydrodynamics for flow convergence, and westward organic matter transport. Wind stress altered the stratification, while freshwater discharge changed the stratification and baroclinic velocity shear simultaneously. Two‐layered flow with a cyclonically rotating current around a coastal salient edge of the western shelf off the estuary hydrodynamically enhanced the local convergence, allowing sufficient residence time in the bottom for the remineralization of organic matter produced in the hypoxic zone and organic matter transported into the region. Our results suggest that a combination of unique local hydrodynamic feature and decomposition of organic matter in water column (and possibly in the sediment) are the cause of the formation and maintenance of the bottom hypoxia on the western shelf of the estuary during summer. Hypoxia results in dead zonesin the ocean. It often occurs in the bottom waters below surface eutrophication due to decomposition of organic matter in the water column and also possibly in sediment. Both physical and biogeochemical processes in the ocean control the formation of the eutrophication and hypoxia. This study investigates these processes for an observed strong hypoxia zone off Pearl River estuary using a numerical model and field measurement data. Through comprehensive analyses, we found that the intensity and area of the hypoxia grew with increasing total nutrient input from Pearl River. The hydrodynamic conditions of ocean flow field such as stability of the water column associated with wind forcing and freshwater buoyancy discharge, local flow convergence, and external organic matter input provide favorable conditions for the formation of hypoxia. The hypoxia west off Pearl River estuary is extrinsically controlled by biophysical fluxes and intrinsically by local hydrodynamicsRemineralization of organic matter in the water column and sediment and local unique hydrodynamics formed and maintained the bottom hypoxiaWater column stability was provided by wind and freshwater discharge while local rotating current provided favorable residence time

Lateral input of dissolved organics may play a significant role to support productivity in oligotrophic ocean although associated biogeochemical evidences are lacking in the field. Ammonia oxidation (AO), the first step of nitrification that bridges organic remineralization and nitrate, is potentially an immediate responder. By using 15N‐NH4+, the spatial distribution of AO was investigated in the northern South China Sea, where Kuroshio Current intrudes frequently. AO ranged widely (0.001 to 134 nmol · L−1· day−1) in space and the depth‐integrated (200 m) AO peaked where the Kuroshio influence is moderate suggesting that enhanced AO had occurred due to lateral mixing. Since oligotrophic Kuroshio is characterized by high dissolved organic nitrogen (DON), such lateral mixing not only introduces external DON into the northern South China Sea but also enhances NH4+regeneration and subsequent oxidation to complicate the conventional new production in the boundary zone with DON gradient. The horizontal mixing is widespread in oligotrophic ocean; however, the mixing‐induced biogeochemical response is hard to detect and remains less explored. We measured ammonia oxidation (AO), a critical process immediately responds to remineralization of dissolved organic matter (DOM), along the pathway of Kuroshio intrusion into the South China Sea. We found AO peaked at stations where the influence of Kuroshio Current intrusion is moderate. We hypothesized that intrusion of oligotrophic water brings on‐site recalcitrant DOM into marginal seas and enriches ammonium level via bacteria decomposition of the foreign DOM. Accompanied is the enhancement of AO, which transfers DOM into nitrate, further complicating the conventional concept of N‐based new production. Such lateral intrusion or transport appears also along a meridional direction. For example, the western boundary current (warm and saline) carrying DOM‐rich water flows toward higher latitudes where nutrients are high. According to our findings, such hemispheric scale transport may exert considerable influence on high‐latitude nitrogen biogeochemistry. Knowledge of the impact of Kuroshio intrusion on AO in the northern South China Sea was a step forward in documenting this type of response in frontal zone, but more studies will be needed to validate the underlying mechanisms. The spatial distribution of ammonia oxidation was reported for the first time in the northern South China SeaEnhanced ammonia oxidation was associated with the Kuroshio Current intrusionRemineralization of DON introduced by the Kuroshio Current was likely a stimulator to enhance the ammonia oxidation in the northern South China Sea

To evaluate biogeochemical impacts of shelf‐scale submarine groundwater discharge (SGD), the northern South China Sea (NSCS) and the East China Sea (ECS) shelf were chosen for this study. Based on a three end‐member mixing model and a Ra box model, SGD fluxes on NSCS (west and east of the Pearl River Estuary, WPRE and EPRE, respectively) and ECS shelf in winter were estimated to be 3.8–9.5 × 109, 1.4–2.2 × 109and 0.7–2.2 ×1010m3d−1, respectively. Our results were equivalent to the SGD flux to the entire Mediterranean Sea, and were an order of magnitude greater than fluxes to the South Atlantic Bight. SGD associated nutrient fluxes into WPRE, EPRE and ECS were almost in the same order of magnitude, 0.2–2.4 × 103mol m−1d−1for dissolved inorganic nitrogen, 1.2–9.8 mol m−1d−1for soluble reactive phosphorus, and 0.4–3.4 × 103mol m−1d−1for dissolved silicate. Moreover, SGD delivered 1.1–2.8 × 104, 0.6–0.9 × 104, 1.7–5.1 × 104mol m−1d−1dissolved inorganic carbon and 1.1–2.7 × 104, 0.5–0.8 × 104, 1.6–4.8 × 104mol m−1d−1total alkalinity to WPRE, EPRE and ECS, respectively. Shelf‐scale SGD is a significant source of nutrients and carbon, and may pose great impacts on regional marine ecosystems. Shelf‐scale submarine groundwater discharge was estimated using RaSubmarine groundwater discharge is an important source of nutrients and carbon on shelvesSubmarine groundwater discharge must be considered in shelf biogeochemical studies considering their wide occurrences

We examined the distribution and seasonality of dissolved organic carbon (DOC) based on a large data set collected from the northern South China Sea (NSCS) shelf under complex circulation schemes influenced by river plume, coastal upwelling, and downwelling. The highest surface values of ∼117 μmol L−1were observed nearshore in summer suggesting high DOC supplies from the river inputs, whereas the lowest surface values of ∼62 μmol L−1were on the outer shelf in winter due to entrainment of DOC‐poor subsurface water under strengthened vertical mixing. While the summer coastal upwelling brought lower DOC from offshore depth to the nearshore surface, the winter coastal downwelling delivered higher surface DOC to the midshelf deep waters from the inner shelf fueled by the China Coastal Current (CCC) transporting relatively high DOC from the East China Sea to the NSCS. The intensified winter downwelling generated a cross‐shelf DOC transport of 3.1 × 1012g C over a large shelf area, which induced a significant depression of the NSCS DOC inventory in winter relative to in autumn. In addition to the variable physical controls, net biological production of DOC was semiquantified in both the river plume (2.8 ± 3.0 μmol L−1) and coastal upwelling (3.1 ± 1.3 μmol L−1) in summer. We demonstrated that the NSCS shelf had various origins of DOC including riverine inputs, inter‐shelf transport and in situ production. Via cross‐shelf transport, the accumulated DOC would be exported to and stored in the deep ocean, suggesting that continental shelves are a potentially effective carbon sink. Seasonal dynamics of DOC were primarily controlled by the water circulationNet production of DOC was comparable in both the river plume and coastal upwelling in summerDOC cross‐shelf transport driven by winter downwelling was observed, suggesting an effective pathway of the continental shelf pump

Acidification and hypoxia are universal environmental issues in coastal seas, especially in large river estuaries such as the Pearl River estuary. In July and August of 2015, two legs of a field survey were conducted in the Pearl River plume. Leg 1 was sampled during the influence of upwelling favorable winds, while Leg 2 was during downwelling favorable winds. During both legs, instead of the typically observed dissolved inorganic carbon (DIC) consumption and dissolved oxygen (DO) over‐saturation, upwelling‐induced high DIC (>2,000 μmol kg−1), low pH (7.7–7.8) and low DO (140–150 μmol kg−1) values were observed in surface waters at the estuary mouth and the area off Hong Kong. In the bottom waters, hypoxia, acidification (pH 7.6–7.8) and DIC accumulation (DIC addition of ∼100–180 μmol kg−1) were observed. Hypoxia was less severe during Leg 2 compared to Leg 1. The stoichiometry of oxygen depletion to DIC addition was 0.89 for bottom water, suggesting remineralization was dominated by marine sourced organic matter. However, a comparison of data from the two legs showed that the stoichiometry of oxygen consumption to DIC accumulation was significantly higher during Leg 2 (0.73 ± 0.03 for Leg 1 vs. 0.80 ± 0.05 for Leg 2), although N/P ratios were the same (13.54 ± 1.93 for Leg 1 vs. 13.51 ± 2.04 for Leg 2). This phenomenon was attributed mainly to enhanced ventilation (re‐oxygenation) under the influence of the downwelling favorable winds during Leg 2. Although ventilation relieves some hypoxia, it might enhance acidification in bottom waters after a short‐term ventilation event. The enhanced acidification after short‐term ventilation is worthy of further study considering that most hypoxia and acidification are found in shallow coastal seas. Acidification and hypoxia are universal environmental issues in coastal seas. In the summer of 2015, we conducted two survey legs in the Pearl River plume of the northern South China Sea. Legs 1 and 2 were conducted under the influence of upwelling favorable and downwelling favorable winds, respectively. During both legs, upwelling induced subsurface water characterized by high dissolved inorganic carbon (DIC), low pH and low dissolved oxygen to the sea surface of the coastal area. In the bottom waters, hypoxia, acidification and DIC accumulation were observed. Hypoxia was less severe during Leg 2 compared to Leg 1. Comparison between the two legs showed that the stoichiometry of oxygen consumption to DIC accumulation was significantly higher during Leg 2, which was attributed mainly to the enhanced ventilation under the influence of the downwelling favorable winds. Although ventilation relieves hypoxia, it might enhance acidification after short‐term ventilation. This phenomenon should get more attention as most hypoxia and acidification are observed in shallow coastal areas. Coastal upwelling induced acidification accompanied by oxygen consumption in surface water was observed for the first time in the Pearl River estuaryDifferent stoichiometry of oxygen to carbon under influence of upwelling and down‐welling favorable winds were observed Coastal upwelling induced acidification accompanied by oxygen consumption in surface water was observed for the first time in the Pearl River estuary Different stoichiometry of oxygen to carbon under influence of upwelling and down‐welling favorable winds were observed

Nutrients from depth have been hypothesized as a primary source of new nutrients that sustain new productivity in oligotrophic oceans; however, the flux is challenging to quantify. Here we show for a first time in the oligotrophic South China Sea an extremely low diapycnal dissolved inorganic nitrogen (DIN) flux as 1.8 × 10−4mmol m−2d−1in the nutrient‐depleted layer (NDL) above the nutricline, where other nutrient supplies sustain the new production. Here higher phosphate and silicate fluxes relative to DIN than Redfield stoichiometry further indicate N‐limited biological productivity and additional removal of DIN by diatoms. Below the NDL across the nutricline to the base of euphotic zone, termed as nutrient replete layer, the DIN flux is three orders of magnitude larger and sufficient in supporting the export production therein. Here higher DIC flux relative to DIN than Redfield stoichiometry further infers DIC excess in the upper ocean. The commonly ignored diapycnal advection induced by turbulent mixing is important in calculation of diapycnal nutrient fluxesThe euphotic zone should be categorized into two distinct layers in terms of vertical nutrient fluxes in oligotrophic oceanic regimesThe modulation of diapycnal nutrient fluxes on biological carbon pump in the upper ocean and air‐sea CO2exchange was inferred

Nitrogen sources and dynamics, one of the key issues in marine biogeochemical cycles, remain poorly constrained in marginal seas. Here we examine the nitrogen cycle in the northern South China Sea (SCS) by combining data from previous reports with a new data set of N isotopic compositions (δ15N) of nitrate, zooplankton, and sinking particles. Average δ15N in subsurface nitrate is 4.8 ± 0.3‰, similar to that of sinking particles (δ15Nsinkof 4.4‰) through the euphotic zone (EZ) collected by floating traps and to documented mean (4.6‰) for long‐term mooring traps at 200 m. This along with oft‐observed shallow nitracline (<100 m) suggests that subsurface nitrate is the primary source of new N to support export production. Moreover, δ15Nsinkat the bottom of the EZ resembles those of suspended particles (4.2 ± 1.0‰) and zooplankton (5.4 ± 1.0‰) inside the EZ. High similarity in δ15N among various types of particles including zooplankton in different size fractions in the EZ implies rapid N turnover in the ecosystem. In deep waters at 2000–3000 m, however, additional particulate N fluxes are found due to lateral transport, which contain 15N‐depleted particles, resulting in a downward decreasing trend of δ15Nsink. Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ15Nsinkin the deep waters. The significant δ15Nsinkchange in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ15N to reconstruct paleonitrogen processes. High similarity in δ15N is found between subsurface nitrate and particles regardless of size fractions in the euphotic zoneLateral transport of 15N‐depleted resuspended sediments contributes to a downward decreasing trend of δ15N in sinking particlesThe significant change of δ15N in sinking particles in the water column makes the South China Sea different from most other marginal seas

This study shows that the response of satellite‐observed chlorophyll (CHL) to eddy motion varies seasonally in the South China Sea (SCS). The spatial pattern of the CHL anomaly composite for eddies is a dipole in summer and a monopole in winter, indicating that sea surface CHL is largely regulated by the horizontal rotational velocity of the eddy in summer and by eddy pumping and trapping in winter. The dipole pattern for anticyclonic eddies was confirmed by in situ observations, which also show that the dipole pattern is mainly restricted to the mixed layer. The underlying mechanism was further investigated with a coupled physical–biogeochemical model. The key driver leading to the seasonal variation of the eddy effect is found to be the seasonal variation of the mixed layer depth. In summer when the mixed layer is shallow, the monopole nutrient change induced by eddy is restricted to the subsurface. The sea surface CHL distribution is thus mostly affected by eddy advection. In winter, the deepening of the mixed layer mixes the nutrients from subsurface where eddy significantly changes the nutrient levels, allowing the monopole pattern to be observed in the sea surface CHL anomaly. Dipole and monopole patterns of sea surface chlorophyll for eddies in the South China Sea were found in summer and winter, respectivelyIn‐situ observations in summer show that the dipole pattern mainly retains in the mixed layerThe key driver leading to the seasonal variation of eddy effect is the seasonal variation of the mixed layer depth

We examined the dynamics and production of dissolved organic carbon (DOC) on a large continental shelf in the northern South China Sea, which is largely shaped by a river plume and coastal upwelling, based on a cruise in summer 2008. The plume water extended from the mouth of the Pearl River estuary to the middle shelf and was characterized by high DOC concentrations, while the upwelled water occupying the nearshore area featured low DOC concentrations. Biological production of DOC was observed in both the river plume and the coastal upwelling zones with different behavior between regions. The system appeared to be autotrophic in terms of DOC throughout the plume, while in the upwelling circulation, the metabolism of DOC was mixed trophic. Nevertheless, the integrated net DOC production rate of 11.5 ± 6.9 mmol C m−2d−1in the upwelling zone was comparable to that in the plume (7.1 ± 7.0 mmol C m−2d−1). The net DOC production correlated strongly with net consumption of dissolved inorganic carbon (DIC) and inorganic nutrients, suggesting that the net DOC production was highly coupled to net community production (NCP) in both the plume and upwelling zones. Both regimes had similar DOC/NCP partitioning, with 19–27% of NCP in the plume and 24–26% of NCP in the upwelling zones converted to DOC. A positive correlation was also found between particulate organic carbon (POC) and net DIC consumption, with higher POC production in the upwelling zones where large phytoplankton prevailed. Most NCP removal occurred through POC sinking and/or the diffusion and horizontal transport of DOC.

The events that followed the Tohoku earthquake and tsunami on March 11, 2011, included the loss of power and overheating at the Fukushima Daiichi nuclear power plants, which led to extensive releases of radioactive gases, volatiles, and liquids, particularly to the coastal ocean. The fate of these radionuclides depends in large part on their oceanic geochemistry, physical processes, and biological uptake. Whereas radioactivity on land can be resampled and its distribution mapped, releases to the marine environment are harder to characterize owing to variability in ocean currents and the general challenges of sampling at sea. Five years later, it is appropriate to review what happened in terms of the sources, transport, and fate of these radionuclides in the ocean. In addition to the oceanic behavior of these contaminants, this review considers the potential health effects and societal impacts.

Biogenic silica (BSi) is a key component of the marine silicon cycle, which is mainly driven by diatom metabolism notably contributing to primary production and export of particulate organic carbon (POC), particularly in ocean margins. Based on a large data set collected in the East China Sea (ECS), including BSi, fucoxanthin (Fuco), POC, and total suspended matter (TSM), we systematically explored the distribution and control of BSi in a typical eutrophic marginal sea. Spatially, BSi concentrations generally decreased from the shelf to the slope during all seasons, because the former is largely fed by river plumes and/or coastal currents enriched in nutrients favoring diatom growth. Abundant BSi was also observed in nearshore bottom waters probably influenced by sediment resuspension indicated by high TSM concentrations. Seasonally, BSi concentrations were on average higher in summer and autumn than in spring and winter, which reflects elevated diatom productivity during warm seasons. The BSi standing stock in the shallow water column of the ECS was significantly correlated with that of Fuco demonstrating diatoms’ dominant control on BSi dynamics. In addition, significant relationships between BSi and POC standing stocks were observed in summer and autumn, indicating the major role of diatoms in C fixation. By comparing with the northern South China Sea (NSCS), we suggested relatively small seasonal variability of BSi on the ECS shelf but significant decrease during cold seasons in the ECS slope. In the latter case, diatoms are ecologically and biogeochemically more important and more sensitive to the changing physical and/or chemical conditions than in the oligotrophic NSCS slope.

The coastal ocean and marginal sea play a disproportionally important role in the release of nitrous oxide (N2O) into the atmosphere. The spatial and temporal distribution of N2O in these important source regions remains highly uncertain due to the scarcity of N2O measurements. Here we present a large data set of N2O concentrations and fluxes obtained from 10 cruises covering four seasons in the Northern South China Sea (NSCS). The study area is overall a net source of atmospheric N2O with an annual flux of 1.9 ± 1.2 × 108, 0.8 ± 0.5 × 108and 1.2 ± 0.7 × 108mol N2O yr−1in the shelf, slope and basin regions, respectively. In terms of global warming potentials, the N2O emissions offset 27.8% of the CO2sink on the shelf, and are equivalent to 3.5 and 0.2‐fold of the CO2emission in the slope and basin of the NSCS. On the seasonal time scale, N2O flux was significantly higher in autumn and winter than in the warm seasons. The spatial variability was contrastingly less pronounced. The seasonality of N2O distribution in the shelf region was modulated by the riverine discharge, while intrusion of the Kuroshio Current exerted profound control on N2O distribution in the open waters of the NSCS. The variable relationships between N2O excess, apparent oxygen utilization, and nitrate in the shelf, NSCS basin and the Luzon Strait indicated a regional difference in N2O cycling pathways along with the impact of water mass mixing. Our study establishes a robust baseline to understand N2O distribution and flux in the NSCS. N2O dynamics demonstrate significant seasonal, but much less pronounced spatial variations in the Northern South China SeaAnnual N2O emmision offset 28% of the CO2sink on the shelf, and are equivalent to 249% and 23% of the CO2emission in the slope and basinRiverine discharge and Kuroshio Current intrusions are the main drivers of N2O dynamics in the Northern South China Sea N2O dynamics demonstrate significant seasonal, but much less pronounced spatial variations in the Northern South China Sea Annual N2O emmision offset 28% of the CO2sink on the shelf, and are equivalent to 249% and 23% of the CO2emission in the slope and basin Riverine discharge and Kuroshio Current intrusions are the main drivers of N2O dynamics in the Northern South China Sea

Many recent models for retrieval of primary production in the sea from ocean‐color data are temperature based. But previous studies in low latitudes have shown that models that include phytoplankton community structure can have improved predictive capability. In this study, we measured photosynthetic parameters from photosynthesis‐irradiance (P‐E) experiments, phytoplankton absorption coefficients, and phytoplankton community structure derived from algal pigments during four cruises in the northern South China Sea (NSCS). The maximum quantum yield of CO2( ΦmC) and the chlorophyll a‐normalized P‐E curve light‐limited slope (αB) varied significantly with the blue‐to‐red ratio of phytoplankton absorption peaks (aph(435)/aph(676)) (p< 0.001, r= −0.459 and −0.332, respectively). The unexplained variability could be due in part to the absorption associated with nonphotosynthetic pigments. The chlorophyll a‐normalized light‐saturated photosynthetic rate ( PmB) at the surface showed a unimodal distribution over the chlorophyll arange during the spring and summer, and significantly increased when Prochlorococcuswas outcompeted by other picophytoplankton (p< 0.01). Almost 60% of the variance of PmBcould be explained by a piecewise regression with phytoplankton absorption coefficients and pigment markers. Unlike previous studies, our data showed that changes of PmBwere unrelated to the size structure of phytoplankton. Although a temperature‐based approach could not effectively predict αBand PmBin the NSCS, a trophic‐based approach can be used for assignment of these parameters in a regional primary production model using ocean‐color data. A trophic‐based approach for assignment of photosynthetic parametersEffects of photo‐acclimation on seasonal and vertical changes of PmBVariation of PmBwas related to change in phytoplankton community structure

Anaerobic ammonium oxidation (anammox) plays an important role in the marine nitrogen cycle. The Pearl Estuary, a typical subtropical estuary characterized by hypoxia upstream and high loads of organic matter and inorganic nutrients caused by anthropogenic activities, has received extensive attention. In this study, anammox bacterial community structures in surface sediments along the Pearl Estuary were investigated using 16S rRNA and hydrazine oxidoreductase (HZO) genes. In addition, abundance of anammox bacteria in both water and surface sediments was investigated by quantitative PCR. Obvious anammox bacterial community structure shift was observed in surface sediments, in which the dominant genus changed from “CandidatusBrocadia” or “CandidatusAnammoxoglobus” to “CandidatusScalindua” along the salinity gradient from freshwater to the open ocean based on 16S rRNA gene and HZO amino acid phylotypes. This distribution pattern was associated with salinity, temperature, pH of overlying water, and particularly C/N ratio. Phylogenetic analysis unraveled a rich diversity of anammox bacteria including four novel clusters provisionally named “CandidatusJugangensis,” “CandidatusOceanicum,” “CandidatusAnammoxidans,” and “CandidatusAestuarianus.” The abundance of anammox bacteria in surface sediments, bottom and surface waters ranged from 4.22 × 105to 2.55 × 106copies g−1, 1.24 × 104to 1.01×105copies L−1, and 8.07×103to 8.86×105copies L−1, respectively. The abundance of anammox bacteria in the water column was positively correlated with NO2−and NO3−, and negatively correlated with dissolved oxygen, although an autochthonous source might contribute to the observed abundance of anammox bacteria. Shift in anammox bacterial community was observed along the Pearl EstuaryFour novel genera of anammox bacteria were identifiedAnammox bacterial abundance related positively with NO2−and inversely with DO

In this study, we utilize 234Th/238U disequilibrium to determine particulate organic carbon (POC) export from the euphotic zone in the South China Sea. Depth profiles of 234Th, total chlorophyll, pigments, and POC were collected during four cruises from August 2009 to May 2011, covering an entire seasonal cycle of spring, summer, autumn, and winter. The extensive data set that was acquired allows for an evaluation of the seasonal variability of upper ocean POC export and its controls in a large marginal sea. The results show that 234Th fluxes from the euphotic zone fall in the range of 528−1550, 340−2694, and 302–2647 dpm m−2d−1for the coastal, shelf, and basin regimes, respectively. In these regimes, POC/234Th ratios at the base of the euphotic zone fall in the range of 5.7–58.2, 4.6–44.0, and 2.5–15.5 μmol dpm−1, respectively. Accordingly, for the coastal, shelf, and basin regimes, the mean POC export fluxes from the euphotic zone are 24.3, 18.3, and 6.3 mmolC m−2d−1, respectively. Seasonal variations in POC export flux are remarkable in the study area, and POC export peaks were generally observed in autumn. We use a simple linear regression (LLS) method to examine the correlation of POC export versus POC stock and versus plankton community structure. We found a strong correlation (R2= 0.73, p< 0.005) between POC export flux and the fraction of diatom in the coastal area, indicating that POC export flux in this province is driven by large phytoplankton, in particular, diatoms. In the shelf area, a relatively strong correlation (R2= 0.54, p< 0.0001) was noted for POC export flux and POC stock in the euphotic zone. This indicates that POC export flux in the South China Sea shelf is primarily controlled by POC stock. In contrast, in the South China Sea basin, we identified a weak but intriguing correlation (R2= 0.26, p< 0.0001) between POC export flux and the fraction of haptophytes and prasinophytes that are typically < 5 μm in size. This suggests that mechanisms controlling POC export flux in the South China Sea basin are complicated. However, small phytoplankton may play a significant role in controlling POC export flux since they dominate the phytoplankton community structure in this region. POC fluxes are highly variable in the South China SeaDiatoms drive POC export in the coastal areaPOC stock regulates POC fluxes in the shelf

While satellite remote sensing has become a very useful tool contributing to assessments of sea surface partial pressure of carbon dioxide (pCO2) that subsequently allow quantification of air‐sea CO2flux, the application of empirical approaches in coastal oceans has proven challenging owing to the interaction of multiple controlling factors. We propose a “mechanistic semi‐analytic algorithm” (MeSAA) to estimate sea surface pCO2in river‐dominated coastal oceans using satellite data. Observed pCO2can be analytically expressed as the sum of individual components controlled by major factors such as thermodynamics (or temperature), mixing, and biology. With marine carbonate system calculations, temperature and mixing effects can be predicted using thermodynamic principles and by assuming conservative two end‐member mixing of total dissolved inorganic carbon and total alkalinity (e.g., the Changjiang River and Kuroshio water in the East China Sea, ECS). Next, an integral expression for pCO2drawdown due to biological effects can be parameterized using the chlorophyll aconcentration (chla). We demonstrate the validity and applicability of the algorithm in the ECS during summertime. Sensitivity analysis shows that errors in empirical coefficients and three input satellite parameters (salinity, SST, chla)have limited influence on the algorithm, and satellite‐derived pCO2is consistent with underway data, even though no in situ pCO2data from the ECS shelves was used to train the algorithm. Our algorithm has more physical and biogeochemical mechanistic meaning than empirical methods, and should be applicable to other similar systems. A semi‐analytic algorithm for satellite‐derived pCO2in the East China SeaCarbonate calculations with satellite‐derived salinity used as a mixing indexBiological effect on pCO2is parameterized by an integral expression of chla

The effect of phosphorus limitation on the Pearl River plume ecosystem, where large gradients in both nitrogen (N) and phosphorus (P) concentrations exist, is investigated in this process‐oriented study by coupling the Regional Ocean Modeling System (ROMS) model with a new nitrogen, phosphorus, phytoplankton, zooplankton, and detritus (NPPZD) ecosystem model. The results of the N‐based only model of Gan et al. (2010) were compared with those of the new NP‐based model for the plume. The inclusion of P‐limitation noticeably reduces the total phytoplankton production in the plume in the P‐limited near and midfield regions of the plume. However, the nitrate in the plume extends farther downstream and forms a broad area of phytoplankton bloom in the N‐limited far field. Moreover, it changes the photosynthetically active radiation and strengthens the subsurface chlorophyll maximum in the near and midfields, but weakens it in the far field. A high N:P ratio of ∼120 in the near field decreases quickly to a low N:P ratio of <13.3 in the far field due to a higher N:P consumption ratio and mixing with ambient waters with a lower N:P ratio. Mortality and coagulation acts as major sinks for phytoplankton production in the near and midfield during the developmental stage of the bloom, but grazing gradually becomes the most important sink for phytoplankton production in the entire plume during the mature stage. It was shown that the magnitudes of the difference between the NP‐based and N‐based cases decrease sequentially for nutrients, phytoplankton, and zooplankton. A new ecosystem model with nitrogen and phosphorus limitation was developedThe effect of phosphorus limitation on a plume ecosystem was investigatedThe N:P ratio and the effect of phosphorus limitation vary along the plume