Nutrients serve as the material basis for marine primary production, whose dynamics in coastal oceans are influenced by both terrestrial inputs and oceanic processes. Based on a field cruise conducted in autumn 2022, this study examines the dynamics and diapycnal transport of nutrients in the Beibu Gulf (BG). Surface nutrient concentrations exhibited an east-west gradient, with the eastern BG showing much higher concentrations, while the western basin remained oligotrophic. Four distinct water masses were identified, influencing the nutrient distributions in the BG. Stoichiometric analysis revealed that N:P < 16 prevailed in northern BG Coastal Water and South China Sea Subsurface Water, whereas the eastern BG close to Qiongzhou Strait was characterized by higher values (N:P > 16). Diapycnal nutrient fluxes were 0.0-2.0 (0.23 ± 0.12), 0.0-0.3 (0.021 ± 0.011), and 0.0-3.0 (0.29 ± 0.16) mmol m-2 d-1 for NO3- + NO2-, soluble reactive phosphorus, and silicate, respectively. These fluxes were predominantly controlled by stratification intensity, with turbulent kinetic energy dissipation rates as a secondary factor. The constructed nutrient budget revealed that lateral advection, primarily via the Western Guangdong Coastal Current, constituted the dominant external nutrient source. Notably, diapycnal nutrient transport, representing the internal nutrient supply, was comparable to the combined inputs from rivers, the atmosphere, and sediments, contributing 28.4%, 123.6%, and 105.0% of the N, P, and Si requirements of net community production in the BG, respectively.

Nutrients play a critical role in oceanic primary productivity and the biological pump. However, compared to hydrographic parameters such as temperature and salinity, nutrient observations are limited due to their labor-intensive and costly measurements. Thus, nutrient observations are several orders of magnitude sparser than hydrographic observations. In this study, we first established a rigorous data quality control procedure to clean the hydrographic and nutrient (including NO3-, NO2-, DIP, and Si(OH)4) observations collected from World Ocean Database (WOD) and CLIVAR and Carbon Hydrographic Data Office (CCHDO) in the North Pacific. Subsequently, the cleaned and high-quality CCHDO dataset was used to train three machine learning models - Random Forest, Light Gradient Boosting Machine (LightGBM), and Gaussian Process Regression - to establish relationships between nutrient concentrations and key variables, including space coordinates (longitude, latitude, and depth), time variables (year and month), and water mass properties (indexed by potential temperature and salinity). Validation shows that the reconstruction closely matches the observations, with Root Mean Squared Errors (RMSEs) of <1.41, <0.071, <0.089 and <3.07 & micro;mol kg(-1) for NO3-, NO2-, DIP, and Si(OH)4, respectively. The validated models were then applied to reconstruct nutrient concentrations from the hydrographic observations in WOD, most of which lacked direct nutrient measurements. This resulted in similar to 473 million reconstructed nutrient data points across 1.92 million stations for each nutrient, spanning from 1895 to 2024, representing a 2127- to 2393-fold increase compared to the original nutrient observations in the North Pacific (197 539 to 222 234). This new dataset will be valuable for studying nutrient transport and budgets, spinning up and validating ocean biogeochemical models, assessing long-term nutrients and their stoichiometric changes driven by anthropogenic forcing and climate change. The dataset generated in this study is openly available via Zenodo (10.5281/zenodo.17451417) (Du et al., 2025).

Silicon (Si) plays a central role in regulating marine primary productivity and mediating interactions between carbon (C) and nitrogen (N) cycling through its control on diatom growth. The stable Si isotopic composition (δ30Si) of biogenic silica (BSi) preserved in sediments has therefore been widely used to reconstruct past nutrient utilization and productivity. However, its applicability in low-productivity ocean regions, where diatoms contribute only modestly to total primary production, remains poorly constrained. Here, we present the first high-resolution, seasonally resolved datasets of coupled Si, C, and N isotope systems from the upper 200 m at two stations in the oligotrophic South China Sea (SCS) basin.Surface waters exhibit relatively heavy δ30Si signatures of dissolved silicic acid (DSi; δ30SiDSi; +2.6‰ to +3.2‰) and of BSi (δ30SiBSi; +1.9‰ to +2.1‰), reflecting near-complete DSi utilization by diatoms. Both Rayleigh-derived fractionation factors (30ℇDSi) based on δ30SiDSi data and apparent fractionation factors (Δ30Si, δ30SiBSi_obs. − δ30SiDSi_obs.) show a strong seasonal variability. 30ℇDSi in the euphotic zone ranging from −1.1‰ to −1.3‰ in summer and −0.6‰ to −0.8‰ in winter, and Δ30Si in the surface mixed layer ranging from −1.0‰ to −1.1‰ in summer and −0.5‰ to −0.8‰ in winter, with no clear spatial variability. Seasonal shifts in diatom productivity influence the distribution and export of biogenic particles. While the overall contribution to bulk primary production remains limited in the picophytoplankton-dominated SCS basin, diatoms can enhance BSi export and particulate organic carbon and nitrogen (POC and PON) production in winter. Coupled analyses of δ30SiBSi and C and N isotopic compositions of POC (δ13CPOC) and PON (δ15NPON) in the euphotic zone reveal a significant positive correlation between δ30SiBSi and δ13CPOC during the winter mixing period, indicating a strong linkage between the Si and C cycles under favorable environmental conditions. In contrast, no correlations are observed between δ30SiBSi and δ15NPON in either season, suggesting that Si and N cycles are largely decoupled due to complex N cycling processes. These findings indicate that δ30SiBSi holds potential as a proxy for tracing primary production in oligotrophic settings and highlight the value of integrating multiple isotope systems to disentangle cycling of different nutrients and improve palaeoceanographic reconstructions in low-productivity ocean regions.

The supply of nutrients via diapycnal processes from depth to the euphotic zone (EZ) is thought to be a main source sustaining new production in oligotrophic oceans. However, such diapycnal fluxes of nutrients remain insufficiently constrained due to limited observations and the dynamic nature of ocean turbulence. In this study, we present a comprehensive dataset of diapycnal fluxes of nutrients, including diapycnal diffusive (Fdiff_NOx) and effective diapycnal fluxes (Fe_NOx, representing the net diapycnal influx to the upper water column) of NO3 -+ NO2 - (NOx- ), based on measurements of turbulence microstructure and nutrients with high vertical resolutions from two cruises conducted during summer and winter in the oligotrophic western North Pacific Subtropical Gyre (NPSG). The Fdiff_NOx (Fe_NOx) exhibits evident spatial variations, with higher values observed at the south boundary of the NPSG near the North Equatorial Current and at the northern NPSG influenced by the North Pacific Tropical Subtropical Mode Water. In contrast, lower values are found in the central NPSG. These spatial variations are primarily attributable to the vertical concentration gradient of NOx- . At the base of the EZ, the cruise-averaged Fdiff_NOx (Fe_NOx) are 11.7 + 9.6 (11.9 + 8.6) and 8.5 + 6.1 (11.3 + 9.3) mu mol m-2 d- 1 in summer and winter, respectively, displaying insignificant seasonal variations. Moreover, we observed significantly higher flux ratios of Fdiff_NOx to diapycnal diffusive flux of phosphate (Fdiff_DIP), which were 18.2 + 2.0 and 13.9 + 2.0, compared to the N/P concentration ratios of 10.4 + 1.1 and 7.4 + 1.2 at the base of the EZ during summer and winter cruises, respectively, suggesting that the diapycnal transport could relieve nitrogen limitation in the upper NPSG. Notably, we identified strong linear relationships between the logarithm of Fdiff_NOx (Fe_NOx) and the NOxgradient. Leveraging these relationships, we estimate the climatological distributions of Fdiff_NOx (Fe_NOx) utilizing nutrient data from the World Ocean Atlas (WOA23). The Fe_NOx is estimated to be 20.2 + 16.6 mu mol m- 2 d- 1 and contributes to 8.5 + 8.3 % of the nitrogen required for new production in the NPSG. These estimates are slightly lower than previous studies, but highlight that diapycnal fluxes play a less important role on nitrogen budget compared to N2-fixation and atmospheric deposition in the oligotrophic NPSG. In contrast, the effective diapycnal diffusive flux of phosphate (Fe_DIP) is 1.5 + 1.3 mu mol m-2 d- 1, contributing to 18.1 + 17.9 % of the phosphorus required by new production, and is roughly ten times larger than the atmospheric phosphorus deposition in the NPSG.

Marine nutrient cycling, particularly of carbon (C), nitrogen (N), phosphorus (P), and silicon (Si), is intricately linked to phytoplankton metabolism, with the Redfield ratio (106:16:1, extended to 15–20 for Si) traditionally serving as a benchmark for nutrient stoichiometry. However, tropical coastal ecosystems experience significant spatial and temporal heterogeneity due to anthropogenic activities, geographic variability, and seasonal shifts, exacerbating imbalances in carbon and nutrient dynamics.Blue-carbon ecosystems, as a "natural solution," offer the potential to mitigate eutrophication and acidification. These highly productive systems can transform CO₂ sources into carbon sinks, contributing to carbon neutrality and improving coastal ecosystem resilience. Xiaohai Lagoon, the largest lagoon in Hainan, China, represents a successful case study of blue-carbon restoration. Over three years of comprehensive restoration measures, including large-scale seagrass and seaweed planting, the lagoon achieved Class I water quality through substantial government investment.Using high-resolution field surveys and real-time water quality monitoring, this study demonstrates how blue-carbon ecosystems dynamically regulate lagoon health through in situ metabolism. During the rainy season (October–December), blue-carbon species rapidly absorbed excess nutrients from land sources, and by November, shifted nutrient dynamics from nitrogen (N) limitation to phosphorus (P) limitation. This transformation converted the lagoon from a CO₂ emission source to a CO₂ sink through photosynthesis. During this process, the combined CO₂ equivalents of three typical greenhouse gases—CO₂, CH₄ (methane), and N₂O (nitrous oxide)—turned negative, −617 g CO₂e m⁻² annually under mean conditions and up to −1,800 g CO₂e m⁻² annually under optimal conditions, underscoring the substantial role of blue-carbon systems in mitigating climate change. In addition, dissolved oxygen (DO) levels increased (107%–136%), and acidification was alleviated (pH 8.41 ± 0.14). However, the decomposition of organic matter from declining blue-carbon species disrupted stoichiometry and caused water quality to deteriorate again, underscoring the critical need for sustained ecological governance.Our findings highlight the pivotal role of blue-carbon restoration in regulating offshore nutrient stoichiometry, mitigating greenhouse gas fluxes, and enhancing coastal ecosystem health. Scaling these results to 10 Hainan lagoons reveals a mitigation potential of ~310,000–500,000 tons CO₂e annually. These insights provide a scientific foundation for advancing Hainan’s ecological civilization pilot zone and offer practical strategies for global coastal management and achieving carbon neutrality.

Coastal eutrophication driven by anthropogenic nutrient inputs and climate change is a growing threat to marine ecosystems. This study examined the nutrient dynamics in Haikou Bay and its adjacent waters based on observational data collected from 24 cruises conducted from 2010 to 2021. The dissolved inorganic nitrogen (DIN) concentrations ranged from 3.00 μmol L-1 to 19.3 μmol L-1, with an average value of 10.8 ± 0.2 μmol L-1 in Haikou Bay. Three distinct spatial patterns of DIN concentrations were identified: river mouth enrichment, a west-to-east decreasing gradient, and a homogeneous distribution. A significant negative correlation between DIN concentrations and salinity highlighted the influence of freshwater on DIN distributions. Seasonally, DIN concentrations showed a declining trend from May to September, with values decreasing from 12.4 ± 0.6 to 8.89 ± 0.64 μmol L-1, then increased slightly thereafter. The decadal variations of DIN concentrations displayed a two-phase pattern: they increased from 2010 to 2013, then declined from 2013 to 2021. This pattern correlated with variations of freshwater input and agricultural fertilizer application from the landward side. In contrast, dissolved inorganic phosphorus (DIP) showed less pronounced spatial and decadal variabilities. The mean DIN/DIP ratio was 98.5 ± 9.0, substantially higher than the canonical Redfield ratio of 16, suggesting potential phosphorus limitation. These findings provide important insights into the decadal variations of nutrient dynamics under the combined influences of climate change and anthropogenic pressures in tropical bays.

Horizontal/Vertical nutrient supply in the upper ocean of the North Pacific Subtropical Gyre (NPSG) plays a pivotal role in biogeochemical cycling and CO2 uptake. However, research quantifying water/nutrient transport based on direct chemical observations and measurements is limited. Based on observations made during three GEOTRACES cruises in spring, summer, and winter, we identified horizontal and vertical water sources and quantified the water and nutrient supply, applying modified Optimum Multiparameter (OMP) analysis based on iterative calculation, in which rare earth elements (REEs) were used as quasi-conservative chemical tracers. The mean quantification results with a depth of <= 200 m show that Equator-derived water (Nutrient fraction: 51% +/- 37%) and vertical supply (31% +/- 33%) are the dominant nutrient sources; northern NPSG-derived water (0%+/- 1%) has little influence; North Equatorial Current-derived water shows a higher contribution at 200-300 m (38% +/- 26%) than the shallow layers (10% +/- 19%); coast-derived water (7% +/- 15%) contributes to NPSG in an inconsistent way. In addition, the enhanced vertical nutrient supply during the sampling period, which is more significant in spring, is likely to be attributed to the influence of what are considered different types of eddies based on the sea surface height. The vertical fluxes of dissolved inorganic nitrogen in the bottom eupho-tic layer at stations near warm, cold, and no eddies were estimated to be 0.10-0.76, 0.21-2.13, and 0.066-0.53 mmol m(-2)d(-1), respectively, which are 1-100 times the supply from nitrogen fixation. These nutrient fluxes could explain 5-169 mg C m(-2)d(-1) of the carbon fixation in the euphotic zone.

Nitrite, an intermediate product of the oxidation of ammonia to nitrate (nitrification), accumulates in upper oceans, forming the primary nitrite maximum (PNM). Nitrite concentrations in the PNM are relatively low in the western North Pacific subtropical gyre (wNPSG), where eddies are frequent and intense. To explain these low nitrite concentrations, we investigated nitrification in cyclonic eddies in the wNPSG. We detected relatively low half-saturation constants (i.e., high substrate affinities) for ammonia and nitrite oxidation at 150 to 200 meter water depth. Eddy-induced displacement of high-affinity nitrifiers and increased substrate supply enhanced ammonia and nitrite oxidation, depleting ambient substrate concentrations in the euphotic zone. Nitrite oxidation is more strongly enhanced by the cyclonic eddies than ammonia oxidation, reducing concentrations and accelerating the turnover of nitrite in the PNM. These findings demonstrate a spatial decoupling of the two steps of nitrification in response to mesoscale processes and provide insights into physical-ecological controls on the PNM.

Nutrient stoichiometry (e.g., nitrate + nitrite to soluble reactive phosphorus, refer to N + N/SRP, N/P hereafter) governs growth, competition and niche partitioning of phytoplankton in the illuminated oceans. The N/P, however, varies widely across the ocean and the underlying mechanisms remain unclear. Here, we report direct observations of significant variations in N/P in response to different life stages of two cyclonic eddies observed in the western South China Sea. High N/P (19.1 ± 6.9) values were observed around the nitracline in a mature-stage eddy, whereas a decay-stage eddy was characterized with low N/P (14.4 ± 4.1). The elevated N/P ratios accompanied by enriched fucoxanthin (pigment for diatom) and biogenic silica around the nitracline suggest that eddy pumping enhanced the growth of diatom which preferentially uptakes P relative to N in the mature stage of the eddy. Such high N/P ratios in the upper ocean could be reproduced if diatom uptake ratio was set between 10 and 16 in a data constrained numerical model. The preferential P uptake by enhanced diatom growth might reduce the P supply to the surface ocean, which is critical for N 2 -fixers. The transient changes in nutrient stoichiometry associated within the life cycle of cyclonic eddies also challenges the parameterization of physical–biogeochemical models with fixed phytoplankton uptake stoichiometry ratios, which could lead to bias of the model output for phytoplankton dynamics in oligotrophic ocean, where eddies frequently occur.

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.

In regions of the nitrogen limited low latitude ocean, phosphate can also be depleted to levels initiating stress responses in marine microbes. Here, we associate a broad region of phosphate depletion in the subtropical North Pacific with different levels of phosphorus stress. Nutrient and aerosol addition experiments demonstrated primary nitrogen limitation of the bulk phytoplankton community, with supply of aerosols relieving this limitation. At northern sites with depleted phosphate, alkaline phosphatase activities were enhanced, indicating elevated phosphorus stress. Analysis of satellite- and model-derived aerosol loading showed that aerosol deposition was elevated in these regions. Surface rate measurements suggested that the regional enhancement in phosphate depletion was predominantly driven by elevated nitrogen fixation, likely stimulated by the coincident supply of aerosol iron. Such observations are important for predicting future biogeochemical responses in the subtropical North Pacific to changing aerosol supply.

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 N2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N2 fixation 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.

Reconstructing past changes in the oceanic nitrate inventory with sedimentary N records in the South China Sea (SCS), which is the terminal of North Pacific Intermediate Water (NPIW), has regional and global implications. However, water-column nitrate cycling that affects N isotope preservation remains poorly understood in the SCS. We present a new data set of nitrate isotopes (delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$) to elucidate nitrate dynamics in the SCS and the adjoining western North Pacific Ocean (wNPO). Greater increases in delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ than in delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ are observed in the SCS euphotic zone, suggesting a combined effect of partial nitrate assimilation and nitrification. In the subsurface and thermocline waters of both regions, upward disproportional decreases in delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ accompanied by elevated nitrate anomalies (N*) indicate an accumulation of external N. Such changes are less significant in the SCS due to higher nitrate concentrations therein, although external N influxes are comparable in both regions. High delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ values in the wNPO intermediate water result from the lateral transport of NPIW with isotopically more enriched nitrate from the remote denitrification zones followed by mixing with overlying water containing isotopically depleted nitrate. As NPIW flows into the SCS, its isotopically enriched signal is further diluted by strong vertical mixing with overlying and underlying waters in the interior. Compared to its source water from the wNPO, the SCS deep water has consistent nitrate isotopic compositions but significantly lower N*, indicating increased benthic denitrification at the wNPO margins with an estimated rate of 0.26-0.41 mmol N m(-2) day(-1).

Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N2 fixation across the tropical ocean.

Nutrients are typically the most important determinant of the productivity of marine ecosystems. Hence, nutrients have been an essential variable of ocean observations in the modern oceanography era. Understanding of marine ecosystems and biogeochemistry, however, is largely limited by the spatiotemporal coverage of nutrient data. Herein, we developed a novel algorithm based on a large observational dataset of nutrients and their relationship with water masses indexed by temperature and salinity in the South China Sea (SCS). The algorithm yielded errors of <= 1.3, <= 0.10 and <= 3.5 mu mol L (-1) for NO3 (-) + NO2 (-) (N + N), phosphate and silicate, respectively. It is then applied to reconstruct nutrient concentrations primarily using temperature and salinity data archived in the World Ocean Database and Argo database during 1940-2018. It increases the nutrient data to similar to 5 million, by ca. three orders of magnitude compared to direct measurements in the SCS. This allows for a full examination of the seasonal climatology of nutrients in the SCS. In summer, in the upper 200 m, nutrient concentrations in the northwest and southernmost SCS are higher than in the rest of the SCS. An overall reversed pattern is revealed in winter, when higher nutrient concentrations are found in the central basin rather than basin margins. The Kuroshio intrusion and the vertical displacement of the nutricline, driven by upwelling/downwelling induced by horizontal convergences/divergences at both meso- and basin scales, determine the spatial and seasonal variation of nutrients. Seasonally, the Kuroshio intrusion and vertical displacements of the nutricline tend to offset the spatial variation of nutrient concentrations in spring, while they show an additive effect in the fall.

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 mu mol kg(-1)), and summer had the lowest (1882 +/- 12 mu mol 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 umol 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 degrees N were lower than at <18.5 degrees 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.

Ammonium (NH4+) dynamics in the oligotrophic South China Sea (SCS) were examined using data collected during summer 2014 and spring 2016. A sensitive fluorometric technique was used to measure NH4+ concentrations (detection limit of 0.7 nmol L-1). TheNH(4)(+) inventory of the upper 100 m showed considerable variation between two samplings, averaging 3.46 mmol m(-2) in 2014 and almost doubling in 2016. This could be attributed to seasonality or more likely to influence of Kuroshio Current intrusion, prominent within El Nino years such as 2016. Kuroshio-influenced stations exhibited elevated NH4+ and decreased NO3- inventories. Higher NH4+ concentrations at the Kuroshio-SCS frontal zone were consistent with prior observations of enhanced microbial consumption of Kuroshio-derived dissolved organic matter. Two patterns in vertical NH4+ distributions were observed: profiles with subsurface NH4+ maximum (30.1-241 nmol L-1), usually occurring at depths of 50-100 m that were closely coupled with the deep chlorophyll maximum and nitrite maximum, and profiles with low and generally uniform NH4+ concentrations with depth. The depth of the NH4+ maximum appears to be controlled by the euphotic depth, position of the nitracline, as well as the difference in the affinity to NH4+ between phytoplankton and nitrifiers. We demonstrated that precise measurements of NH4+ concentrations at nanomolar levels facilitate improved understanding of oceanic nitrogen biogeochemistry.

海水营养盐自动观测技术是海洋观测技术的核心之一,也是世界各国海洋观测技术发展的重点.本文概述了海水营养盐自动观测技术在海洋环境监测中的研究进展,并基于航次的现场实践,比较了海水营养盐自动观测设备的应用情况,归纳了不同设备的优缺点.基于紫外光谱法的硝酸盐传感器(in-situ ultraviolet spectroscopy,ISUS)具有不需化学试剂、响应速度快、适合连续长期观测、耐压深度深的优点,可广泛应用于海水和淡水环境的走航观测、现场剖面观测和浮标定点观测;其缺点是测定参数少、灵敏度低.基于湿化学法的营养盐剖面自动分析仪(Autonomous Profiling Nutreint Analyzer,APNA)与连续流动在线分析仪(型号为QuAAtro)具有测定精度和准确度高、多营养盐参数同步测定的优点,可用于浅水剖面和短时间连续观测,但存在操作复杂、工作时间短、试剂用量大的缺点,不适合长期时间序列或深水观测;APNA能进行原位观测,QuAAtro基于船载,需要加载样品采集过滤系统.本研究基于以上实践与经验,进一步探讨了现有海水营养盐自动观测技术目前所面临的技术瓶颈及未来发展趋势,旨在为海洋现场自动监测仪器的选用提供参考.