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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 N2 fixation, 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. Plain Language Summary: 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, N2 fixation, 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

This review examines the current understanding of the global coastal ocean carbon cycle and provides a new quantitative synthesis of air-sea CO2 exchange. This reanalysis yields an estimate for the globally integrated coastal ocean CO2 flux of −0.25 ± 0.05 Pg C year−1, with polar and subpolar regions accounting for most of the CO2 removal (>90%). A framework that classifies river-dominated ocean margin (RiOMar) and ocean-dominated margin (OceMar) systems is used to conceptualizecoastal carbon cycle processes. The carbon dynamics in three contrasting case study regions, the Baltic Sea, the Mid-Atlantic Bight, and the South China Sea, are compared in terms of the spatio-temporal variability of surface pCO2. Ocean carbon models that range from box models to three-dimensional coupled circulation-biogeochemical models are reviewed in terms of the ability to simulate key processes and project future changes in different continental shelf regions. Common unresolved challenges remain for implementation of these models across RiOMar and OceMar systems. The long-term trends in coastal ocean carbon fluxes for different coastal systems under anthropogenic stress that are emerging in observations and numerical simulations are highlighted. Knowledge gaps in projecting future perturbations associated with before and after net-zero CO2 emissions in the context of concurrent changes in the land-ocean-atmosphere coupled system pose a key challenge. A new synthesis yields an estimate for a globally integrated coastal ocean carbon sink of −0.25 Pg C year−1, with greater than 90% of atmospheric CO2 removal occurring in polar and subpolar regions. The sustained coastal and open ocean carbon sink is vital in mitigating climate change and meeting the target set by the Paris Agreement. Uncertainties in the future coastal ocean carbon cycle are associated with concurrent trends and changes in the land-ocean-atmosphere coupled system. The major gaps and challenges identified for current coastal ocean carbon research have important implications for climate and sustainability policies. [ABSTRACT FROM AUTHOR]

Abstract This study examined Pu source terms in the western North Pacific (WNP) based on data collected in 2014 and 2015. The basin wide 240Pu/239Pu atom ratios ranged from 0.227 to 0.263 with an average value of 0.244 ± 0.011, consistently higher than that of global fallout (~0.180). The spatial distribution of 240Pu/239Pu atom ratios showed higher values within the Kuroshio region, the main western ocean boundary current, as compared to the zone off of the Kuroshio. There was also an overall decreasing trend of 240Pu/239Pu along the Kuroshio path to its extensions. 239+240Pu activities in surface seawater exhibited a wide range from 1.15 to 4.30 mBq m−3 and their spatial distribution showed an increasing trend with latitude. Unlike the 240Pu/239Pu atom ratios, which had heavier isotopic compositions in the Kuroshio mainstream compared to the zone off of the Kuroshio, the 239+240Pu activities were higher outside the Kuroshio than within the Kuroshio. These patterns in both Pu isotopic ratios and activities point towards a unique close-in fallout source, which levels down in its source term and has a high degree of scavenging during its transport along the Kuroshio, and can be traced back to a precursor, the North Equatorial Current, which originates near the Pacific Proving Grounds (PPG) with characteristically higher 240Pu/239Pu atom ratios. High 240Pu/239Pu atom ratios found in the zone outside the Kuroshio were transported from the Kuroshio via the formation and circulation of North Pacific Intermediate Water. We further revealed, using a simple two end-member mixing model, that the PPG source contributed 60 ± 13% of the Pu in the Kuroshio zone and 45 ± 10% in the zone off of the Kuroshio. Both the comparison of Pu isotopic composition in the WNP within a radius of 500 km or 1000 km off the Fukushima Daiichi Nuclear Power Plant (FDNPP) between prior to and post the accident and a simple first order mass balance calculation in terms of atmospheric deposition and release suggest that the Pu originating from the FDNPP accident, if any, was either negligible, or the input flux of 239+240Pu was too small to significantly alter the Pu isotopic composition in the ambient seawater. This article is part of a special issue entitled: "Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond" - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González. Highlights • Pu in the Kuroshio was featured by high 240Pu/239Pu atom ratios but low 239+240Pu activities. • Pu transport along Kuroshio showed higher degrees of scavenging than outside Kuroshio. • The fractional contribution of Pu from Pacific Proving Grounds was ~60% within Kuroshio and ~45% outside Kuroshio. • There is no evidence showing the western North Pacific basin was impacted by Fukushima. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to examine the sources of plutonium (Pu) and elaborate its scavenging and accumulation processes, 240 Pu/ 239 Pu atom ratios and 239 + 240 Pu activities in the water column of the South China Sea (SCS) were determined and compared with our previously reported data for the sediments. Consistently high 240 Pu/ 239 Pu atom ratios that ranged from 0.184–0.250 (average = 0.228 ± 0.015), indicative of non-global fallout Pu sources were observed both in the surface water and at depth during 2012–2014. The spatial distribution of the 240 Pu/ 239 Pu atom ratio in the SCS showed a decreasing trend away from the Luzon Strait, which was very consistent with the introduction pathway of the Kuroshio Current. The Kuroshio had an even heavier Pu isotopic ratio ranging from 0.250–0.263 (average = 0.255 ± 0.006), traceable to the non-global fallout Pu signature from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we further revealed that this PPG source contributed 41 ± 17% of the Pu in the SCS water column. The 239 + 240 Pu activities in the SCS surface seawater varied from 1.59 to 2.94 mBq m − 3 , with an average of 2.34 ± 0.38 mBq m − 3 . Such an activity level was ~ 40% higher than that in the Kuroshio. The distribution of 239 + 240 Pu in the surface seawater further showed a general trend of increase from the Kuroshio to the SCS basin, suggesting significant accumulation of Pu within the SCS. The 239 + 240 Pu inventory of the water column in the SCS basin at the SEATS station with a total depth of ~ 3840 m was estimated to be ~ 29 Bq m − 2 , which was substantially higher than the sediment core estimates made for the SCS basin (3.75 Bq m − 2 ) but much lower than the sediment core estimates made for the shelf of the northern SCS (365.6 Bq m − 2 ). Such differences were determined by the lower scavenging efficiency of Pu in the SCS basin compared to the northern SCS shelf. [ABSTRACT FROM AUTHOR]

The East China Sea (ECS) off the Changjiang (Yangtze River) Estuary, located around the near field of the Changjiang plume (CJP) is a hot spot where phytoplankton blooms in the surface water and hypoxias in the subsurface/bottom waters are frequently observed. Based on field observations conducted in summer 2009 and 2011, we examined non-local drivers associated with the initial dissolved oxygen (DO) levels that had significant impact on the development of summer hypoxias in the ECS off the Changjiang Estuary. The bottom water mass therein could be traced isopycnally at 24.2 < σ θ < 25.2 back to the vicinity of the Luzon Strait, ∼1300 km upstream, where subsurface Kuroshio water (∼220 m deep with ∼190 μmol DO kg −1 ) mixed with the South China Sea subsurface water (∼120 m deep with ∼130 μmol DO kg −1 ). Owing to the difference in DO of these two source water masses, their mixing ratio ultimately determined the initial DO supply to the ECS bottom water that eventually reached the hypoxic zone. This water mass mixture was also subject to biogeochemical alteration during its travel (∼60 days) after it intruded into the ECS at the northeastern tip of Taiwan. Along the pathway of the intruded bottom-hugging water, we found systematic increases in nutrient concentrations and apparent oxygen utilization, or drawdown in DO following Redfield stoichiometry as a result of marine organic matter decomposition. These non-local factors exerted a synergistic control on the initial DO of CJP bottom water promoting hypoxia formation, although the residence time of the CJP bottom water was relatively short (∼11 days). We contend that such far field drivers should be taken into account in order to better predict the future scenarios of coastal hypoxias in the context of global warming. [ABSTRACT FROM AUTHOR]

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−2 d−1 in the upwelling zone was comparable to that in the plume (7.1 ± 7.0 mmol C m−2 d−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. [ABSTRACT FROM AUTHOR]

The spatiotemporal variations of nitrous oxide (N 2 O) in the Pearl River Estuary, a large perturbed estuary, were investigated via six cruises covering both wet and dry seasons during 2007–2011. Significant spatial and temporal variabilities in N 2 O concentrations and N 2 O saturations were detected. Spatially, N 2 O was oversaturated in the entire estuary; ranging from 328 nmol L − 1 , or 38 times saturation in the O 2 -depleted Upper Estuary, down to 11–79 nmol L − 1 in the Middle Estuary (163–905% saturation), and to ~ 7 nmol L − 1 (slight supersaturation) in the Lower Estuary. Temporally, increased N 2 O up to 182 ± 82 nmol L − 1 (1800 ± 750% saturation) was observed in the Upper Estuary during winter at low river discharge in comparison to 76 ± 19 nmol L − 1 (1163 ± 287% saturation) in summer at high river discharge; whereas no significant seasonal difference was detected within the Middle and Lower Estuaries. The N 2 O fluxes decreased by 2 orders of magnitude from upstream to downstream (733 to lower than 5 μmol m − 2 d − 1 ). Seasonally, the higher N 2 O fluxes integrated across the estuary were in spring and winter, and lower fluxes were exhibited in summer and autumn. The annual water–air N 2 O flux was estimated to be 37 ± 15 μmol m − 2 d − 1 . This rendered a total emission of (1.67 ± 0.89) × 10 9 g N 2 O yr − 1 , which is equivalent to the revised total emission from 19 European inner estuaries (1.35 × 10 9 g N 2 O yr − 1 ). Moreover, this amount of N 2 O emission equals approximately 30% of reported CO 2 emission from the Pearl River Estuary in terms of greenhouse warming potential. The N 2 O production was predominantly modulated by nitrification in the Upper Estuary while in the Middle and Lower Estuaries, estuarine mixing appeared to dominate the N 2 O behavior. [ABSTRACT FROM AUTHOR]

Based on a large and high quality dataset of total organic carbon (TOC, an approximation of dissolved organic carbon) collected from three cruises in spring, fall and winter in 2009–2011, we examined the distribution of TOC and its seasonality in the oligotrophic regime of the Northern South China Sea (NSCS) as well as its exchanges with the West Philippine Sea (WPS) in the Northwest Pacific Ocean through the Luzon Strait, the only deep channel linking the South China Sea (SCS) and the Pacific Ocean. Surface TOC concentration in the slope and basin areas of the NSCS varied from 65 to 75 μmol L −1 with relatively high values in the northeast part (southwest of Taiwan Island) in spring, and in the eastern parts of the NSCS during fall and winter. The TOC inventory in the upper 100 m of the water column ranged from 6.0–7.5 mol m −2 with a similar distribution pattern as the surface TOC concentration. There were two most significant differences in the TOC profiles between the SCS and the WPS. One was in the upper 200 m, where more TOC was accumulated in the WPS; the other was in the intermediate layer at ~1000–1500 m, where the gradient of TOC concentration was still persistent below 1000 m in the SCS, a feature which did not exist in the WPS. At this intermediate layer, there also appeared an excess of TOC in the SCS as compared with that in the WPS. The TOC concentration below 2000 m in the SCS was identical to that in the Northwestern Pacific, both of which were ~40 μmol L −1 without significant difference among stations and seasons, suggesting that this deep water TOC was homogeneously distributed in the deep SCS basin owing to the fast replenishment of the deep water from the WPS. We adopted an isopycnal mixing model to derive the water proportion contributed respectively from the SCS and Kuroshio along individual isopycnal plane and examined the impact of the Kuroshio intrusion on the TOC in the NSCS. The upper 100 m TOC inventory in the NSCS was overall positively correlated with the Kuroshio water fraction, suggesting that the Kuroshio intrusion enhanced the TOC inventory thereby significantly influencing TOC distribution in the NSCS. Following the sandwich structure of water exchange through the Luzon Strait, with an inflow in the surface and deep layer but an outflow from the SCS in the intermediate layer, we conducted a first order estimation of the TOC transport fluxes based on the reported cross strait volume transport. The TOC transport flux was −107.1±54.6, 54.7±15.0 and −16.4±13.1 Tg C yr −1 at the upper, intermediate and deep layer, respectively. Note that the positive sign means that the flux was from the SCS to the WPS. By integrating the three-layers, the total net transport flux of TOC through the Luzon Strait would be −68.8±58.0 Tg C yr −1 . Because of the great spatial–temporal variability of the water flow across the Luzon Strait, these first order TOC flux estimates were subject to large uncertainty. Nevertheless, because the SCS is featured by higher DOC production, the exchange of these fluxes with the open ocean interior where DOC would have experienced more degradation would have important implications for both the microbial community in the ocean interior and overall carbon cycle in the SCS. [ABSTRACT FROM AUTHOR]

Based on our multiple year observations during 2000–2008 in the Pearl River Estuary, this study sought to synthesize the long-term pattern of hypoxia and its relationship to organic carbon and nutrient loading in this important world major estuary under significant human impacts. We confirmed previously observed year-round low dissolved oxygen (DO) of < 63 μmol kg − 1 reaching the threshold of hypoxia in the upper reaches of the Pearl River Estuary, extended from the Guangzhou Channel to downstream of the Humen Outlet, in the surface water, covering a water body of ~ 75 km length. The surface DO concentration had a significantly negative correlation with dissolved and particulate organic carbon, and NH 4 + concentrations. Both aerobic respiration and nitrification highly varied spatially in the water column. The highest rates of respiration and nitrification were observed in the Guangzhou Channel, which decreased downstream along with organic carbon and NH 4 + concentrations. Seasonally, the highest rates of total oxygen consumption upon normalization to the substrate (TOC, total organic carbon; and NH 4 + ) were observed in summer, suggesting that both the substrate availability and water temperature were major factors controlling the oxygen consumption rates. Oxygen mass balance calculations showed that in summer, the oxygen consumption rate in the water column by aerobic respiration (21.3 × 10 6 mol O 2 d − 1 ) and nitrification (14.5 × 10 6 mol O 2 d − 1 ) was almost balanced by the reaeration (32.6 × 10 6 mol O 2 d − 1 ) and net advective complement (2.3 × 10 6 mol O 2 d − 1 ). The contributions of other processes (e.g., sediment oxygen consumption and photosynthesis) appeared to be minor. We estimated that the on-site biogenically produced organic matter, or autochthonous organic material, contributed only 13% ± 10% of the TOC being respired in the hypoxic area, suggesting that the allochthonous organic material, primarily derived from sewage discharge, dominated aerobic respiration and the associated oxygen consumption. Meanwhile, NH 4 + which was clearly reflective of sewage loadings (if not all) dominated the nitrification process and the associated oxygen consumption. Taken together, the hypoxia in the studied area was profoundly anthropogenic and this conclusion should have many implications towards regional environmental management. [ABSTRACT FROM AUTHOR]

Abstract: It is increasingly recognized that mesoscale eddies play an important role in modulating the variability of ocean biogeochemistry. It is commonly believed that contrary to cyclonic eddies, anticyclonic eddies are characterized by downwelling at their core regime, which may suppress particle export. Here, by considering submesoscale domains we demonstrate that particle export might be alternatively enhanced in anticyclonic eddies on the basis of a study carried out in the oligotrophic northern South China Sea basin. We examined particle fluxes associated with three coherent anticyclonic eddies using the naturally occurring radionuclide 234Th. When applying a 1D steady-state model, 234Th and its derived particulate organic carbon (POC) fluxes in all three eddy cores were 1.9- and 1.6-fold higher, respectively, relative to those in the non-eddy region. However, an eddy-resolving circulation numerical model showed complex submesoscale circulations associated with the anticyclonic eddy. Notably, dynamic interactions occurred at submesoscales that might induce advection into the eddy core from the edge, where the 234Th deficit was elevated owing to higher particle production and export, probably stimulated by upwelling at the edges. We suggest therefore that enhanced particle fluxes derived from the 1D model along the vertical horizon at eddy cores only appeared to be changes, and that horizontal advection between the eddy core and edge should be taken into consideration in the flux estimation. Indeed, by integrating the 234Th deficit among multiple profiles in the entire anticyclonic eddy system, we derived an average 234Th flux of at the 100-m horizon, equivalent to a POC flux of . This export level was 1.6-fold higher than that from the reference sites. [Copyright &y& Elsevier]

Abstract: We investigated the distribution, degradation and dynamics of organic carbon and its major compound classes, carbohydrates and amino acids, based upon a cruise in the Pearl River estuary in April 2007. Dissolved oxygen (DO), nutrients, particulate organic carbon (POC), chlorophyll a (Chl a), dissolved organic carbon (DOC), total dissolved carbohydrates (TCHO, including monosaccharides, MCHO, and polysaccharides, PCHO) as well as total dissolved amino acids (TAA, both dissolved free, DFAA, and combined components, DCAA) were measured along a salinity gradient. Community respiration and biodegradable DOC were also determined via both short term (within 3days) and long term (lasting 30days) incubation. DOC, MCHO, TCHO, DFAA and TAA concentrations were high in the upper reach of the Pearl River estuary and decreased rapidly downstream. Anthropogenic sewage input appeared to be an important source of the DOC pool in the upper estuary. DOC distribution was non-conservative during the estuarine mixing, showing a net consumption of DOC in the upper reach and in the low salinity (S <20) region of the Pearl River estuary. Changes in the relative compositions of carbohydrates (MCHO vs. PCHO) and amino acids (DFAA vs. DCAA) along the salinity gradient further indicated that different processes (biodegradation, flocculation, and phytoplankton production) had different influences on distributions of organic compound classes in this estuarine system. Our one-month incubation experiment further revealed that a substantial portion (15–45%) of DOC from the estuary was biodegradable. Bacterial respiration rates were much higher (0.12–5.8μmol O2 L−1 h−1) than the DOC consumption rates, suggesting that there were other oxygen consumption processes, such as nitrification besides the aerobic respiration of organic matter in the Pearl River estuary, as inferred by the distribution of NH4 + and NO3 −. We estimated that 5.3×108 g C d−1 of DOC can be exported out from the Lingdingyang Bay (a major subestuary of the Pearl River estuary) to the continental shelf of the South China Sea during this low flow season. [Copyright &y& Elsevier]

Abstract: Based upon seven field surveys conducted during April 2005 – April 2008, we examined the surface partial pressure of CO2 (pCO2) and dissolved oxygen (DO) in the outer Changjiang (Yangtze River) Estuary, on the inner shelf of the East China Sea (ECS). This area represents a most dynamic zone of the ECS where high pCO2 riverine water meets with highly productive shelf waters, covering a 2°×3° area, ~10% of the surface area of the entire ECS. Surface pCO2 ranged 320 – 380 µatm (average ~345 µatm) in winter, 180 – 450 µatm (average ~330 µatm) in spring, 150 – 620 µatm (average ~310 µatm) in summer and 120 – 540 µatm (average ~375 µatm) in autumn. The seasonal variation pattern of surface DO generally mirrored that of pCO2, ranging 95% – 105% in winter, 96% – 142% (average 110%) in spring, 73% – 192% (average 118%) in summer and 81% – 178% (average 102%) in autumn. The dynamics of pCO2 drawdown and DO enhancement in the warm seasons (from April to October) appeared to be controlled by primary productivity and air – sea exchange, while mixing dominated the aqueous pCO2 in the cold seasons (from November to March of the following year). This study showed that the outer Changjiang Estuary served as a moderate or significant sink of atmospheric CO2 in winter, spring and summer, while it turned to a net source in autumn. The integrated sea – air CO2 flux in the outer Changjiang Estuary was estimated as −1.9±1.3 mol m− 2 year− 1, which is double the recent sea–air CO2 flux estimation for the northern ECS. [Copyright &y& Elsevier]

Abstract: We present a time-series analysis of nutrient and pCO2 (partial pressure of CO2) levels in an oligotrophic coastal ecosystem (Gan Bay), which was likely to be influenced by upwelled subsurface water. Gan Bay is off Currimao Harbor, in the northwest Philippines and is located at the boundary of the South China Sea (SCS). This 42-h time-series observation was conducted in December 2006. In addition to continuous observations of dissolved oxygen (DO) and pCO2, discrete samples were collected at a depth of 5m every 3h for measurements of nutrients, including soluble reactive phosphorus (SRP) and inorganic nitrogen (NO3 − +NO2 −) in order to examine their dynamics and possible physical and biological controls. We observed remarkably large short-term variations in the surface water, spanning a 10-fold change for SRP (32–330nM) and from <0.3μM to 4.3μM for NO3 − +NO2 −. DO also varied substantially from a lower end of 171 to 205μM O2. Surface water pCO2 changed from an equilibrium stage with the atmosphere (∼386μatm) to a stage where it was a significant source for the atmospheric CO2 (seawater pCO2 ∼469μatm). We found that the variation of nutrients was driven neither by tidal mixing nor by biological activities, as was suggested by the variations in the total bacterial abundance and chlorophyll a. Instead, our inverse T–S relationship suggested a two end-member mixing process during the observation period. The N:P ratio throughout the observation period was ∼13.2, which is characteristic of SCS subsurface and deep waters. Moreover, pCO2 was correlated inversely with the sea surface temperature. It is likely, therefore, that an upwelled subsurface cold water with high nutrients, low-temperature and high-pCO2 existed. It should be noted that this upwelled cold water did not appear to impact the entire observation period (approximately 35h of 42h), which might suggest an extremely dynamic nature for this upwelled cold water mass. [Copyright &y& Elsevier]

Abstract: A sensor system suitable for monitoring changes in partial pressure of carbon dioxide (pCO2) in surface seawater or in the atmosphere has been developed. Surface seawater samples are pumped into a PVC tube enclosing an inner Teflon AF tube, which served as a long pathlength gas-permeable liquid-core waveguide for spectrophotometry. The Teflon cell contains a pH-sensitive indicator-buffer solution consisting of bromothymol blue (BTB) and sodium carbonate. Carbon dioxide in the sample diffuses into the indicator-buffer solution to reach equilibrium, resulting in pH changes, which are detected by changes in the absorbance of BTB at wavelengths of 620 and 434nm. The pCO2 in the sample is then derived from the pH change. The sensor has a response time of 2min at the 95% equilibrium value and a measurement precision of 0.26–0.37% in the range 200–800μatm pCO2. This chemical sensor takes advantage of a combination of long pathlength, multiple wavelength detection, indicator solution renewal, and in situ automatic control technology, and has the feature of low power consumption (the average being ∼4W with a peak of ∼8W). [Copyright &y& Elsevier]

Abstract: We observed a phytoplankton bloom downstream of a large estuarine plume induced by heavy precipitation during a cruise conducted in the Pearl River estuary and the northern South China Sea in May–June 2001. The plume delivered a significant amount of nutrients into the estuary and the adjacent coastal region, and enhanced stratification stimulating a phytoplankton bloom in the region near and offshore of Hong Kong. A several fold increase (0.2–1.8μgChlL−1) in biomass (Chl a) was observed during the bloom. During the bloom event, the surface water phytoplankton community structure significantly shifted from a pico-phytoplankton dominated community to one dominated by micro-phytoplankton (>20μm). In addition to increased Chl a, we observed a significant drawdown of pCO2, biological uptake of dissolved inorganic carbon (DIC) and an associated enhancement of dissolved oxygen and pH, demonstrating enhanced photosynthesis during the bloom. During the bloom, we estimated a net DIC drawdown of 100–150μmolkg−1 and a TAlk increase of 0–50μmolkg−1. The mean sea–air CO2 flux at the peak of the bloom was estimated to be as high as ∼−18mmolm−2 d−1. For an average surface water depth of 5m, a very high apparent biological CO2 consumption rate of 70–110mmolm−2 d−1 was estimated. This value is 2–6 times higher than the estimated air–sea exchange rate. [Copyright &y& Elsevier]

Abstract: We examined the carbonate system, mainly the partial pressure of CO2 (pCO2), dissolved inorganic carbon (DIC) and total alkalinity (TAlk) in the Changjiang (Yangtze) River Estuary based on four field surveys conducted in Sep.–Oct. 2005, Dec. 2005, Jan. 2006 and Apr. 2006. Together with our reported pCO2 data collected in Aug.–Sep. 2003, this study provides, for the first time, a full seasonal coverage with regards to CO2 outgassing fluxes in this world major river–estuarine system. Surface pCO2 ranged 650–1440 μatm in the upper reach of the Changjiang River Estuary, 1000–4600 μatm in the Huangpujiang River, an urbanized and major tributary of the Changjiang downstream which was characterized by a very high respiration rate, and 200–1000 μatm in the estuarine mixing zone. Both DIC and TAlk overall behaved conservatively during the estuarine mixing, and the seasonal coverage of these carbonate parameters allowed us to estimate the annual DIC export flux from the Changjiang River as ∼1.54×1012 mol. The highly polluted Huangpujiang River appeared to have a significant impact on DIC, TAlk and pCO2 in the lower reaches of the inner estuary. CO2 emission flux from the main stream of the Changjiang Estuary was at a low level of 15.5–34.2 mol m−2 yr−1. Including the Huangpujiang River and the adjacent Shanghai inland waters, CO2 degassing flux from the Changjiang Estuary may have represented only 2.0%–4.6% of the DIC exported from the Changjiang River into the East China Sea. [Copyright &y& Elsevier]

Abstract: We examined dissolved oxygen (DO), the carbonate system and nutrients in the upper reach of the Pearl River estuary in a very dry season in February of 2004. We observed very low DO, down to <12–30 μmol O2 kg−1 in the surface water, upstream of the vicinity of the Humen outlet, one of the major water channels into the estuary. The oxygen depleted water body encompasses a surface area of >20 km2 within a salinity range between 1 and ∼5. Accompanied with the low DO were extremely high pCO2 (up to >7000 μatm) and nutrients (ammonia >600 μmol kg−1 and nitrate >200 μmol kg−1). In addition to the aerobic respiration, processes such as nitrification substantially contributed to the consumption of DO, and may have a significant impact on the distribution pattern of the carbonate species including pH. Oxygen depletion was also observed in this area during our prior cruises in other seasons, signifying an alarming environmental condition in the region. [Copyright &y& Elsevier]

Abstract: The recently developed 2–5 L small-volume MnO2 coprecipitation method for determining 234Th in seawater has provided a new way to substantially increase, both temporally and spatially, the sampling resolution of 234Th and 234Th-based particulate organic carbon export estimates in the upper ocean. In this study, we further optimize the technique by reducing reagent quantities and the use of an additional water bath heating step. This optimization allows the filtration of the MnO2 precipitate onto a 25-mm diameter, 1.0 μm pore size QMA filter to be completed within <30 min for unfiltered waters from the South China Sea. In addition, we have modified the MnO2 purification procedure to allow for alpha spectrometric measurements of 234Th recoveries. Results from recovery experiments suggest that reagent amounts can be reduced to 0.0375 mg KMnO4 and 0.1 mg MnCl2·4H2O per liter of sample, while still maintaining high 234Th recovery. This study further confirms that the addition of a yield monitor is necessary for the application of the small-volume method. [Copyright &y& Elsevier]

Abstract: The distribution of pCO2 in the surface waters of the northern South China Sea (NSCS) was examined in the summer of 2000, the spring of 2001 and the late fall of 2002. For the offshore region >100 km away from the coastline, surface water pCO2 varied within a range of 360–450 μatm during all the surveys. Nevertheless, they were generally higher than atmospheric pCO2. Sea–air ΔpCO2 ranged in 50–100 μatm in the summer, 0–50 μatm in the spring and 0–90 μatm in the late fall. Average sea-to-air CO2 flux was 7 mmol CO2 m−2 day−1 in the summer and 1–3 mmol CO2 m−2 day−1 in the spring and fall. Nearshore pCO2 showed a very dynamic pattern likely associated with the regional hydrodynamic settings, yet again pCO2 in the surface water overall exceeded the air pCO2. Data from this study thus suggests that the NSCS served as a source of atmospheric CO2. Seasonal variations of the pCO2 in the NSCS outer shelf and slope surface waters were significantly influenced by sea surface temperature. [Copyright &y& Elsevier]

Abstract: We examined the concentration, size distribution, redox state and isotopic composition of plutonium (Pu) in groundwater at the 100K-Area at the U.S. Department of Energy''s (DOE) Hanford Site. Total concentrations of Pu isotopes were extremely low (10−4 to 10−6 pCi/kg, ≈104 to 106 atoms/kg) but measurable for the first time in the 100K-Area wells using mass spectrometric analyses that are much more sensitive than alpha spectroscopy methods used previously. Size fractionation data from two wells suggest that 7–29% of the Pu is associated with colloids, operationally defined here as particles between 1 kDa–0.2 μm in size. These colloids were collected using a 1 kDa cross-flow ultrafiltration (CFF) system developed specifically for groundwater actinide studies to include careful controls both in the field and during processing to ensure in situ geochemical conditions are maintained and size separations can be well characterized. Pu in this colloidal fraction was exclusively in the more reduced Pu(III/IV) form, consistent with the higher affinity of Pu in the lower oxidation states for particle surfaces. While the overall concentrations of Pu were low, the Pu isotopic composition suggests at least two local sources of groundwater Pu, namely, local Hanford reactor operations at the 100K-Area and spent nuclear fuel from the N-reactor, which was stored in concrete pools at this site. Differences between this site and the Savannah River Site (SRS) are noted, since groundwater Pu at the F-Area seepage basin at SRS has been found using these same methods, to be characterized by lower colloidal abundances and higher oxidation states. This difference is not directly attributable to groundwater redox potential or geochemical conditions, but rather the physical–chemical difference in Pu sources, which at SRS appear to be dominated downstream from the seepage basins by decay of 244Cm, resulting in more oxidized forms of 240Pu. There is no clear evidence for colloid facilitated transport of Pu in groundwater at the Hanford Site, since downstream wells have both an order of magnitude lower concentrations of Pu and a lower fractional colloidal distribution. [Copyright &y& Elsevier]

Abstract: We investigated distributions of surface water CO2 partial pressure (pCO2), dissolved oxygen (DO) and associated carbonate parameters in the Pearl River estuary, a large subtropical estuary under increasingly anthropogenic pressure in China, in the summer of 2000 and late spring of 2001. pCO2 levels, measured underway using a continuous measurement system, were high during both seasons, with levels of >4000 μatm at salinity <0.5. pCO2 distribution overall mirrored DO across the salinity gradient. Using the linear relationship between excess CO2 and apparent oxygen utilization (AOU) in surface water, we conclude that aerobic respiration is the most important process in maintaining such high pCO2 measured upstream. The material being respired is likely in a close association with the organic pollutants discharged into the system. Based on the measured excess CO2 vs. AOU plots, we estimate that the upper limit of pCO2 should be ∼7000 μatm in the Pearl River estuary assuming that CO2 was produced solely by aerobic respiration. [Copyright &y& Elsevier]

Chromophoric dissolved organic matter (CDOM) fluorescence and dissolved organic carbon (DOC) were measured in the Lingdingyang Estuary, a major component of the Pearl River Delta, China, in May 2001 and November 2002. Measurements of CDOM and DOC suggest multiple sources of dissolved organic matter (DOM) in the low-salinity region of the estuary, due to the mixing of four freshwater outlets, Humen, Jiaomen, Honqimen and Hengmen, all with different freshwater endmembers and all flowing into the head of the Lingdingyang. Our 2002 cruise expanded efforts to characterize these four major outlets, allowing for the quantification of CDOM inputs within this complex watershed.DOC and CDOM appear conservative in mid-salinity (5–20) waters, suggesting that mixing dominates production and removal processes over time scales on the order of 3 days, the residence time of water in this region of the estuary. DOC and fluorescence are linearly correlated in the mid-salinity region of the estuary north of Shenzhen, suggesting that in this region, terrestrial inputs dominate both signals. However, this relationship does not continue out onto the shelf, where water masses of differing optical properties mix. The CDOM/DOC ratio decreases with increasing salinity as would be expected through phototrasformation of CDOM into non-fluorescent DOC. Additionally, this ratio at low salinity appears to vary seasonally, with a higher value in Spring (high flow) compared to Fall (low flow). [Copyright &y& Elsevier]

The Zhu-jiang (Pearl River) estuary and its adjacent continental shelf in the Northern South China Sea (SCS) is unique in that its drainage basin is located entirely in a subtropical zone with heavy population development, and therefore represents an important regime for biogeochemical studies on how large rivers influence continental shelves. The near-zero salinity end member has high nutrient concentrations (silicate 130–140 μM, nitrate 75–100 μM and phosphate 0.2–1.2 μM) and relatively high total dissolved inorganic carbon (DIC) (1500 μM) and alkalinity (∼1650 μM) values. Water column DIC, alkalinity, and nutrient in the estuary are largely controlled by mixing of waters from different tributaries with different drainage basin chemistry, anthropogenic influence, and degree of estuarine recycling. Biological uptake of nutrients and inorganic carbon occur in the outer estuary and inner shelf areas supported by riverine nutrients. The N/P and Si/P ratios are generally very high within the estuary. The summertime area-integrated biological production rate of 0.8 gC m-2 d-1 is estimated based on the depletion of DIC and alkalinity relative to the conservative mixing line and a plume travel time. This estimate agrees reasonably well with 14C based primary production rates (PP) and with that from effective river phosphate flux. Biological production decreases about 10-fold in the open continental shelf and slope and is largely supported by mixing with subsurface water. A comparison of DIC, phosphate, and nitrate concentrations in the surface mixing layer and at the bottom of the euphotic zone with the 14C-based PP (0.13 gC m-2 d-1) suggests that the surface water residence time in the Northern SCS is ∼1.3 years. The N/P, Si/P, and Si/C ratios are 15, 25, and 0.15, respectively. The subtropical Pearl River study is also compared to other large rivers with regard to differences in both natural processes (i.e., weathering rates) and anthropogenic influences (i.e., nutrient input) between these different river–estuary systems. [Copyright &y& Elsevier]

The Indo-Pacific Intersection (IPI), centered in the Indonesian Seas (IS) and South China Sea (SCS), is a convergence zone of both hydrological and atmospheric circulations and plays an important role in the water cycle and global climatic system. This study presents a unique dataset of both 239+240Pu activities and 240Pu/239Pu atom ratios to trace Plutonium (Pu) source terms and their transport in the IPI, to examine constituent water masses, notably freshwater fluxes. The 240Pu/239Pu atom ratios in surface IPI seawater ranged from 0.194 to 0.258 with an average of 0.223 ± 0.021, values that were consistently higher than global fallout (~0.18). 239+240Pu activities ranged widely from 0.55 to 2.8 mBqm−3 with a generally accumulating trend from the western Pacific Ocean (WP) to the IPI as previously observed. We further reveal, using a two end-member mixing model between global fallout and Pacific Proving Grounds (PPG), that the PPG source contributes 34 ± 8% of the Pu to the IS. In the IS, both 239+240Pu activities and 240Pu/239Pu atom ratios further show values between those in the WP and SCS, indicating a two-endmember mixing scheme between the SCS (characteristically lower-salinity but higher PPG Pu-fraction) and the WP (higher-salinity and higher PPG Pu-fraction). We estimate the fractional contribution of the SCS water mass as 35 ± 16% in the surface mixed layer and 49 ± 14% in the IS 50–235 m depth interval. This water mass transport is equivalent to a freshwater influx of 80 ± 47 mSv from the SCS to the IS (known as the South China Sea Throughflow (SCSTF), a branch of the Indonesian Throughflow (ITF)). It accounts for over half (53 ± 31%) of the total freshwater flux transported by the ITF into the Indian Ocean and supplies ~24% the freshwater net evaporation in the Indian Ocean over the 8°N – 32°S latitudinal range, and it is thus crucial in maintaining the fresh water balance therein. This freshwater influx to the IS from the SCS also leads to a ~ 0.2 kg m−3 decrease in the mean density of the upper 235 m water layer, which serves to inhibit the transport of upper layer warm water from the western tropical Pacific warm pool into the Indian Ocean. Therefore, the SCS, together with the SCSTF, play a critical role in regulating freshwater and heat balance in the IPI and Indian Ocean. These findings have strong implications for the integrative role of the SCS and SCSTF on the Indo-Pacific climate and water cycle. • The isotopic ratio of 240Pu/239Pu in the Indo-Pacific Intersection (IPI) ranges 0.194–0.258. • Pu isotopes in the IPI suggest mixed sources from both global and Pacific Proving Grounds close-in fallouts. • The freshwater flux from the SCS to the IS amounts to 80 ± 47 mSv, contributing over half of the total ITF freshwater flux. [ABSTRACT FROM AUTHOR]

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−1 at 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−1 at 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. [ABSTRACT FROM AUTHOR]

• We show spatial and seasonal patterns of the carbonate system in the northern basin of the South China Sea (SCS) based, on a thus far, the largest dataset in a major world marginal sea. • Exchange fluxes of the carbonate variables between the northern SCS and West Philippine Sea (WPS) via the Luzon Strait are constrained in the upper layer by Kuroshio intrusion and over the full depth range. • We reveal significant spatial and seasonal changes in the carbonate chemistry of the deep SCS below 2000 db, attributable likely to spatial and seasonal variations in organic matter decomposition. Using a most comprehensive, high quality, high-resolution dataset for any marginal sea up to depths >2000 db, we examined the seasonality of the carbonate system in the northern South China Sea (nSCS) and exchange with the West Philippine Sea (WPS) during 2009–2011. The carbonate system dynamics demonstrated evident spatial and seasonal variations. Winter exhibited the highest average surface dissolved inorganic carbon (DIC) concentrations (1936 ± 15 μmol kg−1), and summer had the lowest (1882 ± 12 μmol kg−1), primarily associated with more abundant freshwater inputs in summer. At 100 db depth, decreased DIC and total alkalinity (TA) values were observed within the Luzon Strait vicinity due to the influence of WPS waters. Higher DIC and TA concentrations were found within the central nSCS basin. The average Kuroshio contribution to the DIC inventory in the upper 150 db was seasonally significant, ranging between 11 and 32%, with the highest contributions during spring and winter. Below 2000 db, nSCS basin-averaged DIC was significantly higher than WPS-averaged DIC (∼23 µmol kg−1 difference) due to more organic matter decomposition in the nSCS basin. Within the basin, average deep water DIC values were highest in autumn, and averaged concentrations at >18.5°N were lower than at ≤18.5°N. Our datasets and analysis imply that (i) the significant seasonal and spatial patterns of carbonate chemistry in the nSCS are controlled by a combination of large-scale and smaller mesoscale physical processes; (ii) extrinsically from Asian monsoons via seasonal freshwater discharge and dynamic exchanges with open ocean waters; and (iii) intrinsically, through seasonal vertical mixing as well as mesoscale processes and their subsequent new productions. The seasonal and spatial variability in carbonate parameters established here serves as an essential baseline to monitor future changes to the nSCS and to compare with other marginal sea systems. [ABSTRACT FROM AUTHOR]

• Our model well simulated the Pearl River Plume (PRP) and the spatiotemporal distribution of surface p CO 2. • Controls of surface p CO 2 differ substantially in near-, mid- and far-field PRP sub-regions. • Primary production, plume current and CO 2 buffer capacity jointly determine the PRP's role as an atmospheric CO 2 sink. River-dominated ocean margins (RiOMars), characterized by river plumes and abundant riverine nutrient inputs, are especially critical in determining the oceanic uptake of atmospheric CO 2. Using a well validated three-dimensional, coupled physical-biogeochemical model, we examined the dynamics of the carbonate system in the Pearl River Plume (PRP) during summer over a typical RiOMar in the northern South China Sea (NSCS). Sea surface p CO 2 in the PRP was mainly influenced by a combination of physical processes, air-sea exchange, and biological activity. The interplay between these complex processes differed spatially and temporally depending on the evolution of the PRP. The latter was divided into three sub-regions: near-, mid- and far-field. In the near-field PRP, the evolution of surface p CO 2 was primarily influenced by biological activity. Surface p CO 2 decreased substantially at the initial stage as a result of phytoplankton blooms, and then increased due to the reduction in the phytoplankton and increase of zooplankton and detritus. In the mid-field, surface p CO 2 was initially dominated by air-sea exchange. Subsequently, the rates of biological processes exceeded the rate of air-sea exchange, resulting in a strong drawdown of surface p CO 2 or a strong sink for atmospheric CO 2. The far-field of the PRP acted as a weak CO 2 sink, where surface p CO 2 was dominated by air-sea exchange as biological processes were fairly weak. In addition, given that the air-sea CO 2 equilibrium time is much longer than water residence time of the PRP, the biologically-mediated low p CO 2 surface water was enabled to be transported far away from estuary. Taken together, the combined effect of enhanced primary production, strong plume current and strong seawater carbonate buffering capacity were responsible for maintaining low surface p CO 2 levels in this subtropical RiOMar system. [ABSTRACT FROM AUTHOR]

• Particle export in the SEATS station was influenced by Asian monsoon. • Particle export was related with mixed layer depth and mesoscale features. • Particle export was modulated by phytoplankton biomass and its composition. This study examines temporal variations of 234Th-based particle export based on time-series observations conducted from 2004 to 2014 at the South-East Asian Time-series Study (SEATS) site in the basin area of the Northern South China Sea. It shows significant variability in particle export at both seasonal and interannual time scales. A significant feature of our data as observed by prior studies is that particle export was high during the NE monsoon in winter, but low during the inter-monsoon and SW monsoon. Particle flux increased with increasing mixed layer depth (MLD), primarily in winter due to the presence of a strong NE monsoon, in addition to influence by mesoscale features quantified by eddy kinetic energy values. Particle export was generally elevated when phytoplankton biomass was abundant, as shown by the significant correlation between particle fluxes and concentrations of chlorophyll (Chl) a and/or particulate organic carbon (POC) between 0 and 100 m. This is also supported by the negative correlation between 234Th residence time and Chl a concentration, suggesting that increased phytoplankton biomass accelerated downward particle sinking. The correlation disappeared however at Chl a values < 0.11 mg m−3, implicating other factors (e.g., phytoplankton community composition) as playing a more important role when biomass is relatively low. Satellite Chl a- based POC export estimated using various established algorithms agreed well with our measured fluxes when the export horizon was defined as the bottom of the mixed layer, but differed compared to estimated fluxes at 100 m depth. This is likely due to the fact that satellite-derived Chl a measurements only reflect surface values, suggesting that reliable algorithms must be used after considering the biological processes in subsurface waters. [ABSTRACT FROM AUTHOR]

Aim: In microbial biogeography, it is crucial to link spatial patterns with underlying drivers in natural ecosystems. Bacterial communities driving key biogeochemical processes in coastal zones, which are important interfaces between terrestrial and marine ecosystems, are affected by perturbations due to both natural and anthropogenic factors. However, the assembly of bacterioplankton communities, either free‐living (FL) or particle‐associated, in coastal ecosystems is still poorly understood. Location: Coastal South China Sea influenced by the Pearl River (SCSPR). Methods: In this study, we investigated FL, nanoparticle‐associated (NA) and microparticle‐associated (MA) bacterial communities in the SCSPR, using environmental DNA metabarcoding based on the V4 region of the 16S rRNA gene. We assessed the relative importance of ecological processes using null model analyses based on a two‐step framework. Results: We found that the observed amplicon sequence variants (ASVs) increased from FL, NA to MA communities, and a remarkably pervasive core set of ASVs closely belonged to potential hydrocarbonoclastic bacteria with wide habitat niche breadths. Analyses of similarity tests revealed that FL, NA and MA communities differed significantly but weakly, based on unweighted (R = 0.27, p <.001) and weighted (R = 0.18, p <.001) dissimilarities. Fundamental regulation of bacterial communities via homogeneous selection was most prominent in FL (90%), followed by NA (86.8%) and MA (73.3%), whereas the relative importance of dispersal limitation was most pronounced in MA (13.2%), followed by NA (11.7%) and FL (8.6%). Main conclusions: Bacterial communities in the SCSPR can be predominantly influenced by human activities, as indicated by the core ASVs closely related to hydrocarbonoclastic bacteria. Importantly, anthropogenic influences can temporally overwhelm the large environmental heterogeneities in coastal ecosystems. This study fills knowledge gaps in bacterial community assembly, which may facilitate future studies regarding anthropogenic influences on coastal ecosystems. [ABSTRACT FROM AUTHOR]

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 weissflogii and Synechococcus under different pH (8.1 or 7.8) and pCO2 (400 or 800 μatm) conditions. Low pH and/or high pCO2 equally 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 Synechococcus with 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. [ABSTRACT FROM AUTHOR]

Concentrations of bioavailable inorganic nitrogen (N) and phosphorus (P) are simultaneously depleted in the (sub)tropical North Atlantic Ocean, but it remains unclear if phytoplankton growth rates are N limited or N–P co‐limited. Here we present findings from three bottle‐scale experiments using a four‐by‐four matrix of low‐level N and P additions, conducted at one site in the subtropical North Atlantic Ocean. Phytoplankton responses were assessed both in terms of bulk chlorophyll a (Chl a) concentrations and intracellular Chl a of dominant Prochlorococcus and Synechococcus groups. Two matrix experiments suggested that N was independently limiting in situ growth, with no co‐limiting role for P, while the third showed co‐limitation by both N and P in this region. This switch from N limitation to N–P co‐limitation was attributed to an episodic wet deposition event that supplied N, thereby stimulating phytoplankton growth and consuming available P. Such rapid transitions in nutrient limitation in response to environmental forcing might be common in oceanic systems with multiple depleted nutrients. [ABSTRACT FROM AUTHOR]

Nutrient transfer into the sunlit surface ocean by cyclonic eddies is potentially crucial for sustaining primary productivity in the stratified subtropical gyres. However, the nature of productivity enhancements, including the flow of matter to higher trophic levels and its impact on carbon fluxes, remain poorly resolved. Here, we report a detailed assessment of the biogeochemical response to a cyclonic eddy in the subtropical Northwest Pacific via a combination of ship‐based and autonomous platforms. Primary production was enhanced twofold within the eddy core relative to reference sites outside, whereas phytoplankton biomass even decreased. Pico‐phytoplankton (< 2 μm) dominated (> 80%) both phytoplankton biomass and primary production inside and outside the eddy. The stimulated primary production in the eddy core was accompanied by an approximately twofold increase in mesozooplankton abundance, an approximately threefold increase in particle formation in the deep chlorophyll maximum layer, as well as significantly enhanced surface oceanic CO2 uptake and net community production. We suggest these observations carry important implications for understanding carbon export in the subtropical ocean and highlight the need to include such subtropical eddy features in ocean carbon budget analyses. [ABSTRACT FROM AUTHOR]

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 OMterr directly (via remineralization) and indirectly (via the nutrients released from OMterr remineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OMterr remineralization 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 OMterr remineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OMterr to 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 OMterr may underestimate the role of OMterr in hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OMterr loads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream. [ABSTRACT FROM AUTHOR]

Deep ocean microorganisms consume particulate organic matter that is produced in the surface ocean and exported to deeper depths. Such consumption not only enriches inorganic carbon in the deep ocean but also transforms organic carbon into recalcitrant forms, creating an alternative type of carbon sequestration. However, estimates of deep microbial carbon demand substantially exceed the available particulate organic carbon exported from the euphotic zone, resulting in an unbalanced dark ocean carbon budget. Here, we combined field‐based microbial activity parameters, integrated multiyear particle export flux data, sinking particle fluxes measured by sediment traps, and optical data from Biogeochemical‐Argo floats to quantify the main sources of organic carbon to the dark ocean. Laterally transported particles (including sinking and suspended particles) serve as a major energy source, which directly provide organic carbon and enhance new organic carbon production by dark carbon fixation, reconciling the mismatch in the regional carbon budget. Plain Language Summary: Particulate organic matter, produced by phytoplankton in the upper ocean, can sink through the water column and act as a source of organic matter to the deep ocean. These particles are decomposed to carbon dioxide by microorganisms, resulting in dissolved inorganic carbon and organic carbon resistant to decomposition in the deeper ocean. This process controls the biological sequestration of CO2 by the oceans. However, there is an imbalance between the low amount of organic carbon exported from the photic zone and the high microbial demand for carbon in the dark ocean. We attempted to explain how the deep ocean carbon and energy supply can meet the microbial metabolic demand. Four main organic carbon sources were measured and quantified in the South China Sea: particles that come from the photic zone, particles that move laterally through the ocean, dark carbon fixation, and dissolved organic carbon. We found that laterally transported particles from the surrounding margins provide a direct source of organic carbon and also allow for much new organic carbon production through dark carbon fixation. These particles, which provide a major energy source to dark ocean ecosystems, help resolve the mismatch in the regional carbon budget. Key Points: Data from Biogeochemical‐Argo floats showed direct evidence of episodic pulses of laterally transported particles into the deep seaLaterally transported particles are a direct organic carbon source and also enhance new organic carbon production by dark carbon fixationLaterally transported particles provide major carbon and energy for deep ecosystems, reconciling the mismatch in the regional carbon budget [ABSTRACT FROM AUTHOR]

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 δ30SiDSi values 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 δ30SiDSi values 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 δ30SiDSi distribution. An atypical vertical distribution of δ30SiDSi with 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 δ30SiDSi values, which then leads to pore‐water DSi flux carrying higher δ30SiDSi compositions 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. [ABSTRACT FROM AUTHOR]

The eastern equatorial Pacific (EEP) is a source of atmospheric CO2 during 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 pCO2 records 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 CO2 sink despite that its subsurface waters were enriched by CO2 during 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 CO2 exchange across the equatorial Pacific led to a net CO2 outgassing 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. Plain Language Summary: The modern equatorial Pacific is one of the main sources of CO2 released 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 CO2 sink 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 CO2 exchange across the equatorial Pacific that generally accelerated the CO2 outgassing and favored global warming. We further argue that these deglacial‐early Holocene CO2 changes 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 CO2 into 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 CO2 flux in a manner similar to the past. Key Points: Strengthened upper ocean stratification led to significant atmospheric CO2 sink in the western equatorial Pacific during ∼16–7 kaThe zonal seesaw of sea‐air CO2 exchange in the equatorial Pacific resulted in a net CO2 outgassing much greater than the modern situationThe sea‐air CO2 exchange across the equatorial Pacific was modulated by the Subtropical‐Tropical Circulation on orbital timescales [ABSTRACT FROM AUTHOR]

• METAL and MTAG could boost statistic power for mbGWAS. • Genus of Bacteroides and psychiatric diseases had causal relationships. • Expression of genes of contributing to microbiomes are enriched in brain tissues. • Annotation for IVs in MR showed association with hippocampal volume. The correlation between human gut microbiota and psychiatric diseases has long been recognized. Based on the heritability of the microbiome, genome-wide association studies on human genome and gut microbiome (mbGWAS) have revealed important host-microbiome interactions. However, establishing causal relationships between specific gut microbiome features and psychological conditions remains challenging due to insufficient sample sizes of previous studies of mbGWAS. Cross-cohort meta -analysis (via METAL) and multi-trait analysis (via MTAG) were used to enhance the statistical power of mbGWAS for identifying genetic variants and genes. Using two large mbGWAS studies (7,738 and 5,959 participants respectively) and 12 disease-specific studies from the Psychiatric Genomics Consortium (PGC), we performed bidirectional two-sample mendelian randomization (MR) analyses between microbial features and psychiatric diseases (up to 500,199 individuals). Additionally, we conducted downstream gene- and gene-set-based analyses to investigate the shared biology linking gut microbiota and psychiatric diseases. METAL and MTAG conducted in mbGWAS could boost power for gene prioritization and MR analysis. Increases in the number of lead SNPs and mapped genes were witnessed in 13/15 species and 5/10 genera after using METAL, and MTAG analysis gained an increase in sample size equivalent to expanding the original samples from 7% to 63%. Following METAL use, we identified a positive association between Bacteroides faecis and ADHD (OR, 1.09; 95 %CI, 1.02–1.16; P = 0.008). Bacteroides eggerthii and Bacteroides thetaiotaomicron were observed to be positively associated with PTSD (OR, 1.11; 95 %CI, 1.03–1.20; P = 0.007; OR, 1.11; 95 %CI, 1.01–1.23; P = 0.03). These findings remained stable across statistical models and sensitivity analyses. No genetic liabilities to psychiatric diseases may alter the abundance of gut microorganisms. Using biological annotation, we identified that those genes contributing to microbiomes (e.g., GRIN2A and RBFOX1) are expressed and enriched in human brain tissues. Our statistical genetics strategy helps to enhance the power of mbGWAS, and our genetic findings offer new insights into biological pleiotropy and causal relationship between microbiota and psychiatric diseases. [ABSTRACT FROM AUTHOR]

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 Prochlorococcus becoming 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. [ABSTRACT FROM AUTHOR]

The land-to-ocean export of particulate organic carbon (POC) connects carbon flow from the atmosphere through land to the ocean, of which the contemporary fraction that reaches the deep sea for burial may effectively affect atmospheric CO2. In this regard, small mountainous rivers (SMRs) in Oceania, a global erosion hotspot driven by torrential typhoon rain and active earthquakes are potentially important. Here we measured typhoon lignin discharges for Taiwan SMRs. We found that the particulate lignin export in 96 hours by a single SMR amounting to ~20% of the annual export by Mississippi River. The yearly particulate lignin discharge from Taiwan Island (35,980 km2) is governed by the frequency and magnitude of typhoon; thus, the historical lignin export ranged widely from 1.5 to 99.7 Gg yr−1, which resulted in a 10-100 times higher areal yield relative to non-Oceanian rivers. The lignin-derived modern POC output from Oceania region is 37 ± 21 Tg C yr−1, account for approximately 20% of the annual modern POC export from global rivers. Coupled with the hyperpycnal pathway, the forested watersheds of SMRs in Oceania may serve as a giant factory to rapidly produce and efficiently convey modern POC into deep sea for sequestration. [ABSTRACT FROM AUTHOR]

Oceanic submesoscale flows are considered to be a crucial conduit for the downscale transfer of oceanic mesoscale kinetic energy and upper-ocean material exchange, both laterally and vertically, but defining observations revealing submesoscale dynamics and/or transport properties remain sparse. Here, we report on an elaborate observation of a warm and fresh filament intruding into a cyclonic mesoscale eddy. By integrating cruise measurements, satellite observations, particle-tracking simulations, and the trajectory of a surface drifter, we show that the filament originated from an anticyclonic eddy immediately to the west of the cyclonic eddy, and the evolution of the filament was mainly due to the geostrophic flows associated with the eddy pair. Our observations reveal the mass exchange of the eddy pair and suggest that submesoscale flows can degrade the coherence of mesoscale eddies, providing important implications for the transport properties of mesoscale eddies. Vigorous submesoscale turbulence was found within the eddy core region, due to filamentous intrusion and frontogenesis. Our findings have thus offered novel insights into the dynamics and transport properties of oceanic submesoscale flows, which should be taken into account in their simulation and parameterization in ocean and climate models. Significance Statement: Mesoscale eddies, with a spatial scale from tens to hundreds of kilometers, are ubiquitous in the global ocean. Carrying the largest proportion of oceanic kinetic energy, mesoscale eddies play a key role in ocean dynamics and have important applications for marine biology. Although mesoscale eddies have been studied extensively over the past decades, there are two major issues that remain inconclusive: (i) How do mesoscale eddies dissipate? (ii) Can eddies coherently trap waters when moving over a long distance? Recent studies, mostly through computer simulations, suggest that oceanic submesoscale processes, with a typical scale of a few kilometers, are highly relevant to the above two issues. This study presents a rare observation of a filament intruding into a cyclonic eddy. Because of this filament intrusion, submesoscale activities are enhanced near the eddy core area, in contrast to previous observations that normally suggest weaker submesoscale activities in the eddy core area than at eddy peripheries. Such dedicated process-oriented observations provide unique opportunities for better understanding the dynamics and transport properties of mesoscale eddies and submesoscale processes. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) is the largest marginal sea of the North Pacific Ocean, where intensive field observations, including mappings of the sea surface partial pressure of CO 2 (p CO 2), have been conducted over the last 2 decades. It is one of the most studied marginal seas in terms of carbon cycling and could thus be a model system for marginal sea carbon research. However, the cruise-based sea surface p CO 2 datasets are still temporally and spatially sparse. Using a machine-learning-based method facilitated by empirical orthogonal function (EOF) analysis, this study provides a reconstructed dataset of the monthly sea surface p CO 2 in the SCS with a reasonably high spatial resolution (0.05 ∘ × 0.05 ∘) and temporal coverage between 2003 and 2020. The data input to our model includes remote-sensing-derived sea surface salinity, sea surface temperature, and chlorophyll, the spatial pattern of p CO 2 constrained by EOF, atmospheric p CO 2 , and time labels (month). We validated our reconstruction with three independent testing datasets that are not involved in the model training. Among them, Test 1 includes 10 % of our in situ data, Test 2 contains four independent in situ datasets corresponding to the four seasons, and Test 3 is an in situ monthly dataset available from 2003–2019 at the South East Asia Time-series Study (SEATs) station located in the northern basin of the SCS. Our Test 1 validation demonstrated that the reconstructed p CO 2 field successfully simulated the spatial and temporal patterns of sea surface p CO 2 observations. The root mean square error (RMSE) between our reconstructed data and in situ data in Test 1 averaged ∼10 µatm , which is much smaller (by ∼50 %) than that between the remote-sensing-derived data and in situ data. Test 2 verified the accuracy of our retrieval algorithm in months lacking observations, showing a relatively small bias (RMSE of ∼8 µatm). Test 3 evaluated the accuracy of the reconstructed long-term trend, showing that, at the SEATs station, the difference between the reconstructed p CO 2 and in situ data ranged from -10 to 4 µatm (-2.5 % to 1 %). In addition to the typical machine learning performance metrics, we assessed the uncertainty resulting from reconstruction bias and its feature sensitivity. These validations and uncertainty analyses strongly suggest that our reconstruction effectively captures the main spatial and temporal features of sea surface p CO 2 distributions in the SCS. Using the reconstructed dataset, we show the long-term trends of sea surface p CO 2 in five subregions of the SCS with differing physicobiogeochemical characteristics. We show that mesoscale processes such as the Pearl River plume and China coastal currents significantly impact sea surface p CO 2 in the SCS during different seasons. While the SCS is overall a weak source of atmospheric CO 2 , the northern SCS acts as a sink, showing a trend of increasing strength over the past 2 decades. The data used in this article are available at 10.57760/sciencedb.02050 (Wang and Dai, 2022). [ABSTRACT FROM AUTHOR]

Salinity, as one of the essential physical properties of seawater, is a common tracer differentiating water masses in the ocean, which often require relatively high-resolution datasets. Limited by the coverage of direct observations, however, high-resolution spatial and temporal salinity data are not always available, which hinders the fine application of salinity data in discerning ocean processes and improved modeling of ocean physics and biogeochemistry. To supplement the salinity database, we reconstructed sea surface salinity (SSS) with reasonably high spatial resolution (0.05° × 0.05°) over 2003–2020 in the South China Sea (SCS) with a machine learning algorithm based on a combination of MODIS-Aqua remote sensing data and a large cruise observation-based dataset. The reconstructed SSS has a mean absolute error (MAE) of 0.2 when compared with our underway observations with a corresponding root mean square error (RMSE) of 0.3. The MAE between station-based observations and our reconstruction was 0.5, and the RMSE was 0.7. These validations strongly suggest that our reconstruction is highly adequate, representing at most a quarter of the identified discrepancies compared to the remote sensing SSS or two other prevalent model-derived datasets. Based on our reconstruction, the SSS in the SCS is relatively low in coastal waters, but high in the ocean basin, with a seasonal pattern with a minimum in the summer and a maximum in the winter. This spatio-temporal distribution is well consistent with the observations and is affected by the Pearl River plume, sea surface circulation, and precipitation. Using our reconstructed SSS, we were able to successfully characterize the spreading of the Pearl River and Mekong River plumes and the intrusion of the Kuroshio Current from the Pacific Ocean into the SCS. [ABSTRACT FROM AUTHOR]

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−1 but did not significantly affect TA, which dominated pCO2 drawdown in the coastal plume water and increased the pH and Ωarag at 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 Ωarag and higher pCO2 values 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 pCO2 and increased pH and Ωarag. In the Pearl River Plume, the solubility-driven CO2 sink exceeded biological CO2 uptake, resulting in an additional decrease of pH and Ωarag and 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 CO2 and 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. [ABSTRACT FROM AUTHOR]

The particulate organic carbon (POC) to 234 Th ratio, or POC/ 234 Th, is crucial to constrain the 234 Th-derived downward export flux. Marine particles for 234 Th based export studies are typically collected via two sampling modes: filtration of Niskin or GO-FLO bottle water samples and filtration of in situ pumped samples. Large discrepancies have been frequently observed between these bottle and pump filtration methods and attributed to the difference in adsorption of dissolved material. We noted however, that simultaneous measurements of carbon and 234 Th have not been made to evaluate such a putative effect on the POC/ 234 Th ratio. In this study, we adopted a “two filters in-line” approach method with one stacked on another and we measured both carbon and 234 Th to compare their adsorption behavior in bottle and pump sampling modes. We proposed that the 234 Th and carbon on the second filter (filter-B) might be attributable to direct adsorption from the dissolved phase (including the partial capture from the submicron particles) and/or breakdown of particles initially collected on the first filter (filter-A). The relative importance of these two processes differed between the two filtration modes. For bottle sampling, we observed significantly positive correlation (R 2 = 0.54) between the 234 Th recorded on the second filter and dissolved 234 Th activities, suggesting that adsorption was the dominant contributor to the second filter. The overestimation due to adsorption (filter-B/filter-A) is estimated to be 22 ± 7% for POC and 25 ± 17% for particulate 234 Th. The effect of adsorption on the POC/ 234 Th, on the other hand, would be negligible since the adsorption-corrected and -uncorrected POC/ 234 Th on filter-A were similar. For the large volume pumping mode, the absorbed 234 Th on the filters appeared to reach the saturation state as evidenced by the fact that the amount of particulate 234 Th (dpm cm − 2 ) on filter-B was independent of the filtered sample volume. We justified that the saturation amount should be > 0.21 dpm cm − 2 which was the highest unsaturated value from the bottle sample, but < 0.31 dpm cm − 2 which was the upper value on filter-B taken from pump samples. The calculated adsorption still dominated for 234 Th on filter-B and particle breakdown might be important in the euphotic zone. The collected POC on filter-B tended to increase with the filtration volume which implied that DOC adsorption remained under-saturated. Similar to bottle filtration, the corrected POC/ 234 Th was comparable to the uncorrected POC/ 234 Th on filter-A. We thus conclude that adsorption did not appear to influence the POC/ 234 Th ratio in either sampling mode. We suggested that the positive offset in POC/ 234 Th between bottle and pump sampling, if any, cannot be caused by adsorption. Other mechanisms such as zooplankton invasion and/or artificial particle formation need to be examined in order to resolve the discrepancy. [ABSTRACT FROM AUTHOR]