Coastal eutrophication and hypoxia remain a persistent environmental crisis despite the great efforts to reduce nutrient loading and mitigate associated environmental damages. Symptoms of this crisis have appeared to spread rapidly, reaching developing countries in Asia with emergences in Southern America and Africa. The pace of changes and the underlying drivers remain not so clear. To address the gap, we review the up-to-date status and mechanisms of eutrophication and hypoxia in global coastal oceans, upon which we examine the trajectories of changes over the 40 years or longer in six model coastal systems with varying socio-economic development statuses and different levels and histories of eutrophication. Although these coastal systems share common features of eutrophication, site-specific characteristics are also substantial, depending on the regional environmental setting and level of social-economic development along with policy implementation and management. Nevertheless, ecosystem recovery generally needs greater reduction in pressures compared to that initiated degradation and becomes less feasible to achieve past norms with a longer time anthropogenic pressures on the ecosystems. While the qualitative causality between drivers and consequences is well established, quantitative attribution of these drivers to eutrophication and hypoxia remains difficult especially when we consider the social economic drivers because the changes in coastal ecosystems are subject to multiple influences and the cause–effect relationship is often non-linear. Such relationships are further complicated by climate changes that have been accelerating over the past few decades. The knowledge gaps that limit our quantitative and mechanistic understanding of the human-coastal ocean nexus are identified, which is essential for science-based policy making. Recognizing lessons from past management practices, we advocate for a better, more efficient indexing system of coastal eutrophication and an advanced regional earth system modeling framework with optimal modules of human dimensions to facilitate the development and evaluation of effective policy and restoration actions.

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.

This review examines the current understanding of the global coastal ocean carbon cycle and provides a new quantitative synthesis of air-sea CO 2 exchange. This reanalysis yields an estimate for the globally integrated coastal ocean CO 2 flux of −0.25 ± 0.05 Pg C year −1 , with polar and subpolar regions accounting for most of the CO 2 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 pCO 2 . 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 CO 2 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 CO 2 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.

We examined the evolution of intermittent hypoxia off the Pearl River estuary based on three cruise legs conducted in July 2018: one during severe hypoxic conditions before the passage of a typhoon and two post-typhoon legs showing destruction of the hypoxia and its reinstatement. The lowest ever recorded regional dissolved oxygen (DO) concentration of 3.5 µmol kg−1 (∼ 0.1 mg L−1) was observed in bottom waters during leg 1, with an ∼ 660 km2 area experiencing hypoxic conditions (DO < 63 µmol kg−1). Hypoxia was completely destroyed by the typhoon passage but was quickly restored ∼ 6 d later, resulting primarily from high biochemical oxygen consumption in bottom waters that averaged 14.6 ± 4.8 µmol O2 kg−1 d−1. The shoreward intrusion of offshore subsurface waters contributed to an additional 8.6 ± 1.7 % of oxygen loss during the reinstatement of hypoxia. Freshwater inputs suppressed wind-driven turbulent mixing, stabilizing the water column and facilitating the hypoxia formation. The rapid reinstatement of summer hypoxia has a shorter timescale than the water residence time, which is however comparable with that of its initial disturbance from frequent tropical cyclones that occur throughout the wet season. This has important implications for better understanding the intermittent nature of hypoxia and predicting coastal hypoxia in a changing climate.

基于多年观测研究,南海CO,源汇及其时空格局的总体特征是:南海海盆是大气CO2的弱源区,年均海-气CO2通量为2.1±0.3 mmol·m-2·d-1;而南海北部陆架是碳汇区,年均CO,通量为-2.2±3.5 mmol·m-2·d-1;南海总体上每年向大气释放的碳量为1330万±1880万to由于南海位于陆地-大洋交界带,存在多个界面过程,根据物质交换发生的不同界面,可将南海海盆和北部陆架视为大洋主控型边缘海(OceMar)和河流主控型陆架海(RiOMar).这两类系统分别接受大洋和河流输入的外源无机碳和营养盐,经由一系列动力过程进入真光层后同时被生物消耗,无机碳和营养盐之间的“竞争”最终决定CO2源汇格局.在南海海盆,无机碳相对过剩,部分以CO2形式向大气释放,即为源;而在南海北部陆架,无机碳相对不足,系统需从大气补充CO2,即为汇.南海碳循环机理及其框架对于更好地理解全球其他陆架边缘海系统具有重要的借鉴意义.

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.

We examined the distribution and seasonality of dissolved organic carbon (DOC) based on a large data set collected from the northern South China Sea (NSCS) shelf under complex circulation schemes influenced by river plume, coastal upwelling, and downwelling. The highest surface values of similar to 117 mu mol L-1 were observed nearshore in summer suggesting high DOC supplies from the river inputs, whereas the lowest surface values of similar to 62 mu mol L-1 were on the outer shelf in winter due to entrainment of DOC-poor subsurface water under strengthened vertical mixing. While the summer coastal upwelling brought lower DOC from offshore depth to the nearshore surface, the winter coastal downwelling delivered higher surface DOC to the midshelf deep waters from the inner shelf fueled by the China Coastal Current (CCC) transporting relatively high DOC from the East China Sea to the NSCS. The intensified winter downwelling generated a cross-shelf DOC transport of 3.1 x 10(12) g C over a large shelf area, which induced a significant depression of the NSCS DOC inventory in winter relative to in autumn. In addition to the variable physical controls, net biological production of DOC was semiquantified in both the river plume (2.8 +/- 3.0 mu mol L-1) and coastal upwelling (3.1 +/- 1.3 mu mol L-1) in summer. We demonstrated that the NSCS shelf had various origins of DOC including riverine inputs, inter-shelf transport and in situ production. Via cross-shelf transport, the accumulated DOC would be exported to and stored in the deep ocean, suggesting that continental shelves are a potentially effective carbon sink.

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 75μmolL−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 100m of the water column ranged from 6.0–7.5molm−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 200m, where more TOC was accumulated in the WPS; the other was in the intermediate layer at ~1000–1500m, where the gradient of TOC concentration was still persistent below 1000m 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 2000m in the SCS was identical to that in the Northwestern Pacific, both of which were ~40μmolL−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 100m 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±13.1TgCyr−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±58.0TgCyr−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.

On the basis of most up-to-date literature data, this study evaluated the influxes of dissolved organic carbon (DOC) to the ocean. Using the DOC concentrations in 118 world rivers and long-term average river discharges, we estimated the river influxes of DOC to the coastal seas as 0.21 Pg C yr(-1). This flux was reduced to 0.19 Pg C yr(-1) when we took into account DOC removal during its transport into the Arctic. When we further adopted an average removal rate of 10% for the rest of the river DOC input into the coastal ocean, we obtained an estimate of the global river DOC flux of 0.17 Pg C yr(-1), which is at the lower end of prior estimations. Considering the seasonal variation of the river end-member DOC concentration, our current estimate of the global river DOC discharge is subject to an uncertainty of similar to 30%.DOC fluxes into the ocean have significant spatial variations in terms of their continents of origin, recipient coastal seas, ocean basins and latitudinal zones. The highest DOC flux was from South America into the western ocean boundaries and eventually into the Atlantic Ocean. The most abundant riverine DOC discharge was in the low latitudinal zones with 38.0 and 90.0 Tg C yr(-1) in the 0-30 degrees N and 0-30 degrees S zones, respectively, the combination of which accounted for similar to 62% of the global DOC input. On the basis of these updated fluxes, we estimated a global river mean DOC concentration of 5.29 mg L-1.

We examined the dynamics of the carbonate system in a complex mixing scheme with enhanced biological consumption modulated by both a river plume and summer coastal upwelling in a large shelf system, the northern South China Sea (NSCS) shelf. The plume waters originated from a large flooding upstream the Pearl River, and extended from the mouth of the Pearl River estuary to the middle shelf and were characterized by low dissolved inorganic carbon (DIC) and total alkalinity (TAlk), and a high aragonite saturation state (Omega(arag)). In contrast, the upwelled water occupying the nearshore area was distinguished by high DIC and TAlk and a low Omega(arag). While the dynamics of the carbonate system were largely shaped by physical mixing through plume and upwelling processes between the plume water, the offshore subsurface water and the offshore surface water, biological consumption of DIC was observable in both the river plume and the coastal upwelling areas and contributed to the elevated Omega(arag) during their pathway. Correlations between salinity normalized TAlk and DIC indicated that organic carbon production rather than biocalcification exclusively induced the DIC removal. By using a three end-member mixing model, we estimated the net community productivity in the plume water and the upwelled water to be 36 +/- 19 mmol C m(-2) d(-1) and 23 +/- 26 mmol C m(-2) d(-1), respectively. With the combination of stoichiometric relationship analysis of the carbonate system and applying the three end-member mixing model, we successfully differentiated semiquantitatively the biologically mediated DIC variations from its overall mixing control. We also attempted to link this natural process to the carbonate saturation on the NSCS shelf, contending that at present natural factors associated with the river plume and the coastal upwelling largely modulate the dynamics of the carbonate system on the NSCS shelf, whereas anthropogenic stressors such as ocean acidification currently play a relatively minor role.

Depth profiles of total organic carbon (TOC) were measured in spring (2005) and winter (2006) in the South China Sea (SCS), the largest marginal sea adjacent to the North Western Pacific (NWP). Compared to TOC profiles in the NWP, excess TOC (3.2 ± 1.1 μmol kg−1) was revealed in the intermediate layer of the SCS at σθ ∼ 27.2–27.6 (∼1000–1500 m). Below the depth of 2000 m, TOC concentrations were identical between the SCS and the NWP. Based on a one‐dimensional steady state diffusion advection model constrained by potential temperature, we estimated a net TOC production rate of 0.12 ± 0.04 μmol kg−1 yr−1 to maintain this excess. A positive relationship between TOC and apparent oxygen utilization in the SCS deep water lent support to such a model‐derived TOC production. This excess TOC in the out‐flowing intermediate water may carry 3.1 ± 2.1 Tg C yr−1 of organic carbon out from the SCS and potentially into the deep open ocean. In light of the short residence time of the SCS deep water, the exported TOC was likely from the recently fixed organic carbon within the SCS. The export of such organic carbon, thereby less likely to return to the atmosphere may therefore contribute significantly to the carbon sequestration in the SCS.