Eutrophication and hypoxia are intensifying in many estuarine systems globally, driven by complex interactions between physical and biogeochemical processes. In the Pearl River Estuary (PRE), the combined effects of large-scale river plume dispersion and wind-driven coastal upwelling remain poorly understood and rarely observed at high resolution. To address this gap, we conducted an intensive, high-resolution field campaign in the PRE in June 2021, capturing two contrasting hydrodynamic scenarios: high river discharge with plume-favorable northeasterly winds, and low river discharge with upwelling-favorable southwesterly winds. Using a well-validated three-endmember mixing model, we quantified surface nutrient consumption and bottom nutrient accumulation, and assessed the spatial extent of hypoxia under each scenario. In the high-discharge scenario, strong plume spreading enhanced water column stratification, promoting surface biological consumption of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). This resulted in elevated nutrient additions in bottom waters and a hypoxic area covering ∼1232 km2. In the low-discharge, upwelling-favorable scenario, the bottom-water hypoxic area contracted to ∼412 km2, while low-oxygen waters were brought to the surface and nutrient additions were moderated. Fitted slope values for DIN and DIP additions were 16.4 and 15.1 under the two scenarios, respectively, consistent with the Redfield ratio, indicating organic matter degradation dominated bottom-water nutrient addition and oxygen consumption. Our results reveal the dynamic interplay between river plume dispersion and coastal upwelling in shaping nutrient and dissolved oxygen distributions on intra-seasonal timescales, through coupled physical and biogeochemical processes, including surface primary production and bottom organic matter degradation. These findings provide critical new insights into biogeochemical modeling, coastal environmental forecasting and management.

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.

Rapid mariculture expansion has raised concerns about coastal eutrophication. This study assesses nutrient cycling in Sansha Bay, China, a eutrophic semi-enclosed bay with intensive mariculture. A two-endmember mixing model showed significant additions of dissolved inorganic nitrogen (DIN; 6.9 ± 4.1 μmol L-1) and phosphorus (DIP; 0.45 ± 0.29 μmol L-1) in May 2020, mainly from mariculture. Estimated N and P inputs from fish farming were 7789 ± 361 tons and 1497 ± 91 tons in spring, respectively, with N mainly in dissolved form and P in particulate form. And, trash fish feed caused higher nutrient release than formulated feed. Of the feed input, 52.8 ± 4.7 % of DIN and 33.0 ± 3.7 % of DIP were released into environment, exceeding riverine input and offshore exchanges. Co-culturing kelp and oysters removed 1079 ± 11 tons of N and 156 ± 8 tons of P. Therefore, adjusting feed types and planning co-cultivation strategies could alleviate eutrophication resulting from mariculture expansion.

The direct and selective coupling of benzenes with aliphatic hydrocarbons is a promising strategy for C(sp2)-C(sp3) bond formation using readily available starting materials, yet it remains a significant challenge. In this study, we have developed a simplified photochemical system that incorporates catalytic amounts of iron(III) halides as multifunctional reagents and air as a green oxidant to address this synthetic problem. Under mild conditions, the reaction between a strong C(sp2)-H bond and a robust C(sp3)-H bond has been achieved, affording a broad range of cross-coupling products with high yields and commendable chemo-, site-selectivity. The iron halide acts as a multifunctional reagent that responds to visible light, initiates C-centered radicals, induces single-electron oxidation to carbocations, and participates in a subsequent Friedel-Crafts-type process. The gradual release of radical species and carbocation intermediates appears to be critical for achieving desirable reactivity and selectivity. This eco-friendly, cost-efficient approach offers access to various building blocks from abundant hydrocarbon feedstocks, and demonstrates the potential of iron halides in sustainable synthesis.

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 CO 2 ( p CO 2 ), pH and the aragonite saturation state index (Ω arag ). Additionally, high surface phytoplankton production consumed DIC of 10.0±10.4 µmol kg −1 but did not significantly affect TA, which dominated p CO 2 drawdown in the coastal plume water and increased the pH and Ω arag 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 Ω arag and higher p CO 2 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 p CO 2 and increased pH and Ω arag . In the Pearl River Plume, the solubility-driven CO 2 sink exceeded biological CO 2 uptake, resulting in an additional decrease of pH and Ω arag and increase of seawater p CO 2 . 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 CO 2 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.

The Kuroshio intrusion through the Luzon Strait into the northern South China Sea (SCS) is an important contributor to the heat and salt budgets of the SCS and is usually the strongest in winter. The extent of the intrusion in summer and the residence time of the very surface layer (0–5 m) in the northern SCS slope and basin areas, however, are seldom quantitatively evaluated. In this study we investigated surface distributions of radium isotopes (226Ra and 228Ra) across the Luzon Strait into the northern SCS in late spring to mid-summer to reveal the fraction of the Kuroshio water and the transport path of the Kuroshio intrusion in the northern SCS. The activity of 228Ra ranged from 0.67 to 23.5 dpm 100 L-1, with the maximum occurring at the lowest salinity of 28.8 on the northern SCS shelf and the minimum appearing east of the Luzon Strait characteristic of the more saline Ra-depleted Kuroshio water. The activity of 226Ra showed a similar pattern in a smaller range of 4.63–9.64 dpm 100 L-1. Distributions of 226Ra and 228Ra, combined with the distribution of salinity, demonstrate that three water masses contribute to the surface seawater in the northern SCS, the Kuroshio surface water, the plume water, and the island-influenced surface water. We quantified the fraction of the Kuroshio water in the northern SCS, considering the conservation of 226Ra and salinity, to be in the range of 4 ± 7%–51 ± 4% with an average of 23 ± 11%. The fraction decreased southward and westward from the northwest off the Luzon Strait. The intrusion of the Kuroshio reached as far west as 115° E and as far south as 14° N. The residence time of the very surface water in the northern SCS slope and basin areas was estimated using 228Ra as a timer to be 0.22 ± 0.59–9.98 ± 0.54 years with an average of 2.92 ± 2.20 years.

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.

We present tritium (H-3) and helium isotope (delta He-3) data from the South China Sea (SCS), to estimate apparent oxygen utilization rates (AOURs). The observed delta He-3 values are close to the theoretical solubility equilibrium value of -1.7% in the upper mixed layer, followed by an increase with depth down to similar to 1500 m. Below 1500 m depth, delta He-3 is homogenously distributed with a value of 20.9% +/- 1.0%. The distribution of delta He-3 reveals that a significant fraction of He-3 throughout the water column over the entire SCS basin is allochthonous, derived from mantle sources originated from the Pacific Ocean. By using the salinity-normalized "potential" alkalinity (NPA) as a conservative mixing tracer, He-3 produced by tritium decay (tritiogenic) was separated from mantle He-3, and the apparent H-3-He-3 ages of the SCS water masses were subsequently calculated to be 11 +/- 5 years at 100 m depth and 50 +/- 4 years at 1000 m. Together with the observed dissolved oxygen concentrations, we estimated the mean AOURs and obtained values of 4.15 +/- 0.27 mu mol kg(-1).yr(-1) for the depth range of 100-500 m, and 0.81 +/- 0.23 mu mol kg(-1).yr(-1) for that between 800 and 1000 m depth. The depth-integrated AOURs between 100 and 1000 m depth, yield a spatially and temporally averaged export flux of organic carbon of 1.96 +/- 0.16 mol C.m(-2).yr(-1), comparable with previous estimates based on either U-238/Th-234 disequilibrium or nutrient and oxygen mass balance calculations in the SCS.

Based on large-scale surveys conducted during all four seasons from 2009 to 2011, we investigated the carbonate systems on the northern South China Sea (NSCS) shelf featuring much higher variations in both seasonality and spatiality on its inner-shelf (<40 m) as compared to the areas on the mid-outer shelf (>40 m). The most notable forcing on the mid-outer shelf include the intrusion of Kuroshio water leading to high surface salinity and high total alkalinity (TA) in winter, the impact of which is however limited to the northeastern part of the NSCS. The Pearl River Plume (PRP), a prominent feature in summer also has profound impact on the carbonate system on the mid-outer shelf. On the inner-shelf, the carbonate system was much more dynamic, featuring complex modulations by coastal upwelling associated with relatively high dissolved inorganic carbon (DIC) and TA in summer, and the China Coastal Current (CCC) of high DIC in winter, spring and fall. In addition, the influences of coastal plume water from local rivers were identifiable on the inner-shelf in both winter and spring. Such distinction between inner-shelf and mid-outer shelf in the dynamics of DIC, the partial pressure of CO2 (pCO2) and saturation state index of aragonite (Omega arag) is also obvious. On the mid-outer shelf, the salinity normalized DIC (nDIC) fluctuated seasonally between 1974 +/- 9 and 2001 +/- 9 mu mol kg-1. The decline of nDIC from winter to spring and spring to summer mainly results from CO2 outgassing, while the increase in nDIC from summer to fall and from fall to winter is due to entrainment of the carbon-enriched subsurface water. The pCO2 increases from a minimum of 344 +/- 9 mu atm in winter to a maximum of 387 +/- 14 mu atm in spring, which is in phase with temperature changes and the fluctuations of nDIC. The Omega arag ranged 3.28-3.68 with the highest value in summer but lowest value in winter, which is consistent with the seasonal cycles of the nDIC. Nearshore on the inner-shelf influenced by the CCC water in winter and the mid-outer shelf influenced by the PRP in summer, the spatial dynamics of sea surface pCO2 and Omega arag are modulated by both temperature and the water mass mixing between CCC, PRP, and shelf waters. Here, the high biological uptake sustained by nutrients in the CCC and PRP drawdown the pCO2 and augmented the Omega arag, while the CO2 sequestration enhanced the sea surface pCO2 but drawdown the Omega arag.

The Kuroshio intrusion through the Luzon Strait into the northern South China Sea (SCS) is an important contributor to the heat and salt budgets of the SCS and is usually the strongest in winter. The extent of the intrusion in summer and the residence time of the very surface layer (0-5 m) in the northern SCS slope and basin areas, however, are seldom quantitatively evaluated. In this study we investigated surface distributions of radium isotopes (Ra-226 and Ra-228) across the Luzon Strait into the northern SCS in late spring to mid-summer to reveal the fraction of the Kuroshio water and the transport path of the Kuroshio intrusion in the northern SCS. The activity of Ra-228 ranged from 0.67 to 23.5 dpm 100 L-1, with the maximum occurring at the lowest salinity of 28.8 on the northern SCS shelf and the minimum appearing east of the Luzon Strait characteristic of the more saline Ra-depleted Kuroshio water. The activity of Ra-226 showed a similar pattern in a smaller range of 4.63-9.64 dpm 100 L-1. Distributions of Ra-226 and Ra-228, combined with the distribution of salinity, demonstrate that three water masses contribute to the surface seawater in the northern SCS, the Kuroshio surface water, the plume water, and the island-influenced surface water. We quantified the fraction of the Kuroshio water in the northern SCS, considering the conservation of 226 Ra and salinity, to be in the range of 4 +/- 7%-51 +/- 4% with an average of 23 +/- 11%. The fraction decreased southward and westward from the northwest off the Luzon Strait. The intrusion of the Kuroshio reached as far west as 115 degrees E and as far south as 14 degrees N. The residence time of the very surface water in the northern SCS slope and basin areas was estimated using 228 Ra as a timer to be 0.22 +/- 0.59-9.98 +/- 0.54 years with an average of 2.92 +/- 2.20 years.

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.

Hypoxia has become a universal environmental and ecological problem in recent decades. The Pearl River estuary (PRE), the largest estuary in Southern China, is hypoxic year-round in the upper estuary. This study reports the inter-annual variation between 2005 and 2019 in the carbonate system of the hypoxic upper PRE in winter. In January 2005, both dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations were >3000 μmol kg–1 at the upstream-most station and decreased sharply downstream. However, DIC and TA were lower, with concentrations of 2300 and 1950 μmol kg–1, respectively, at the upstream-most in January 2019. At salinities >15, both DIC and TA were conservative and reached steady values at the downstream seawater end-member. The upstream-most station was taken as an example to quantify the influences of biogeochemical processes on DIC and TA, including CO2 degassing, organic carbon oxidation, pelagic nitrification, CaCO3 dissolution and benthic release. Among the biogeochemical process, a decrease in CaCO3 dissolution (from 734.4 μmol kg–1 in 2005 to 168.9 μmol kg–1 in 2019) was the major factor driving the decreases of DIC and TA in 2019. In the context of global change, inter-annual variability in biogeochemical process should receive more attention.

The Bohai Sea is a shallow-water, semi-enclosed marginal sea of the Northwest Pacific. Since the late 1990s, it has suffered from nutrient over-enrichment. To better understand the eutrophication characteristics of this important coastal sea, we examined four survey datasets from summer (June 2011), late autumn (November 2011), winter (January 2016), and early spring (April 2018). Nutrient conditions in the Bohai Sea were subject to seasonal and regional variations. Survey-averaged N/P ratios in estuarine and nearshore areas were 20-133. In contrast, the central Bohai Sea had mean N/P ratios of 16.9 ± 3.4 in late autumn, 16.1 ± 3.0 in winter and 13.5 ± 5.8 in early spring, which are close to the traditional N:P Redfield ratio of 16. In summer, both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) were used up in the surface waters of the central Bohai Sea, suggesting that the biological consumption of DIN and DIP may also follow the Redfield ratio. Wintertime nutrient budgets of the central Bohai Sea water were then established based on a mass balance study. Our results suggest that the adjacent North Yellow Sea supplied additional DIP to the central Bohai Sea via wintertime water intrusion, balancing terrigenous excess DIN that was introduced in summer. A water-mixing simulation combining these two nutrient sources with atmospheric nitrogen deposition suggests that eutrophication in the central Bohai Sea will likely be enhanced by the large-scale accumulation of anthropogenic nitrogen in adjacent open oceans. Such changes in nutrients may have fundamentally contributed to the recent development of algal blooms and seasonal hypoxia in the central Bohai Sea.

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

Dynamically distributed at trace levels in the open ocean, ammonium is one of the most important and reactive nitrogen compounds in the marine environment. Obtaining reliable measurements of ammonium concentrations is thus a prerequisite to fully understand its role in marine biogeochemical processes, but remains challenging. Among others, quantification and identification of different sources of blanks is an outstanding issue, due partly to the fact that ammonium‐free water is very difficult to prepare and/or preserve. Building on a recently developed method using solid phase extraction combined with fluorescence detection (SPE‐Flu), we examined the kinetics of the ortho‐phthaldialdehyde‐sulfite‐ammonium reaction and introduce a new approach to quantifying reagent blanks via a “reagents addition” method. The reagent blank of the method, equivalent to 6.7 ± 1.5 nmol L−1 of ammonium under our experimental settings, accounted for up to 27% of the ammonium background in seawater samples collected from the oligotrophic ocean. We also showed that the SPE‐Flu method is highly specific, with negligible interference from three types of amines and 15 types of amino acids at nanomolar concentrations, which are typical of open ocean regimes. The determination of the reagent blank allowed for optimized data reduction, which was applied to a study in the oligotrophic South China Sea. Water column profiles showed a very well‐defined structure and smooth distribution of ammonium concentrations, consistent with the distribution of other parameters. We thus contend that our proposed approach provides a way to further optimize the quantification of ammonium concentrations in natural seawater via the SPE‐Flu method.

We examine the current status of dissolved oxygen (DO) and its trend over the past 25 years in the lower Pearl River Estuary, a large eutrophic estuary located in Southern China and surrounded by large cities including Hong Kong, Shenzhen and Guangzhou. Monthly cruises conducted from April 2010 to March 2011 clearly show that DO depletion began to emerge in the bottom layer of the lower estuary off Hong Kong in June, and became fully developed in July and August when oxygen-deficient water occupied similar to 1000 km(2) before gradually becoming re-oxygenated in September and October. The development of the low oxygen zone was closely coupled with phytoplankton blooms in the surface water, which was supersaturated with respect to DO suggesting the importance of autochthonous organic matter in fueling bottom DO consumption after settling through the pycnocline. Long-term monitoring data collected in the study area adjacent to Hong Kong by the Hong Kong Environmental Protection Department showed a decreasing trend of similar to 20 +/- 0.9 mu mol kg(-1) yr(-1) in the annual minimum DO concentration in bottom water over the past 25 years. Associated with the decrease in DO was an increase in the annual maximum surface concentration of dissolved inorganic nitrogen (DIN) at a rate of -1.4 +/- 0.3 mu mol kg(-1) yr(-1), suggesting again that eutrophication is the most plausible driver of oxygen deficiency in this region. Therefore, our monthly cruises, along with the decadal monitoring data, reveal a large low oxygen zone, likely developing into a large hypoxic zone driven primarily by anthropogenic eutrophication. This new development suggests environmental stressors such as eutrophication may have a cascading effect, with important and expensive consequences for the regional environment. (C) 2018 Elsevier Ltd. All rights reserved.

The spatiotemporal variations of nitrous oxide (N2O) 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 N2O concentrations and N2O saturations were detected. Spatially, N2O was oversaturated in the entire estuary; ranging from 328nmolL−1, or 38 times saturation in the O2-depleted Upper Estuary, down to 11–79nmolL−1 in the Middle Estuary (163–905% saturation), and to ~7nmolL−1 (slight supersaturation) in the Lower Estuary. Temporally, increased N2O up to 182±82nmolL−1 (1800±750% saturation) was observed in the Upper Estuary during winter at low river discharge in comparison to 76±19nmolL−1 (1163±287% saturation) in summer at high river discharge; whereas no significant seasonal difference was detected within the Middle and Lower Estuaries. The N2O fluxes decreased by 2 orders of magnitude from upstream to downstream (733 to lower than 5μmolm−2d−1). Seasonally, the higher N2O fluxes integrated across the estuary were in spring and winter, and lower fluxes were exhibited in summer and autumn. The annual water–air N2O flux was estimated to be 37±15μmolm−2d−1. This rendered a total emission of (1.67±0.89)×109gN2Oyr−1, which is equivalent to the revised total emission from 19 European inner estuaries (1.35×109gN2Oyr−1). Moreover, this amount of N2O emission equals approximately 30% of reported CO2 emission from the Pearl River Estuary in terms of greenhouse warming potential. The N2O production was predominantly modulated by nitrification in the Upper Estuary while in the Middle and Lower Estuaries, estuarine mixing appeared to dominate the N2O behavior.

An improved four-step approach for the stereoselective synthesis of long-chain anti-2-amino-3-alkanols is described. Using this method, the syntheses of antiproliferative (antitumoral) compounds, spisulosine (ES-285, 2), clavaminols A and B (3 and 4), the deacetylated products of clavaminols H and N (7 and 8), as well as (2S,3R)-2-aminododecan-3-ol (9) and xestoaminol C (10), have been achieved in excellent diastereoselectivities. In vitro study showed that these compounds induced cell death and dose-dependently inhibited cell proliferation in human glioblastoma cell line SHG-44, indicating the anti-tumor property of this series of compounds.

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 (0.2–1.8 μg Chl L−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 (>20 μm). 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 100–150 μmol kg−1 and a TAlk increase of 0–50 μmol kg−1. The mean sea–air CO2 flux at the peak of the bloom was estimated to be as high as ∼−18 mmol m−2 d−1. For an average surface water depth of 5 m, a very high apparent biological CO2 consumption rate of 70–110 mmol m−2 d−1 was estimated. This value is 2–6 times higher than the estimated air–sea exchange rate.