Dinoflagellate harbors diverse associated bacterial communities (ABCs) that play crucial roles in host physiology and ecological interactions. Coolia malayensis is a benthic dinoflagellate distributed from temperate to tropical regions. Despite growing knowledge about the morphology, distribution, and toxicity of C. malayensis, little is known about its associated bacteria, particularly under changing environmental conditions. Given that climate change alters microbial interactions through temperature shifts, this study investigated the composition, diversity, and dynamics of ABCs across four C. malayensis strains, under varying temperatures and growth time. Despite differences in strain origin, a stable core microbiome, comprising Alteromonas, Marinobacter, Muricauda, and Ruegeria, was consistently observed, suggesting these taxa are functionally important members of the ABC. However, strain-specific differences and temperature-driven shifts were also detected, especially among low-abundance bacterial taxa. Negative interactions among abundant amplicon sequence variants, such as those between Ruegeria and Muricauda, and between Alteromonas and Marinobacter, were conserved across conditions, suggesting stable patterns of co-occurrence. Functional predictions and Kyoto Encyclopedia of Genes and Genomes pathway analysis suggested potential involvement of ABCs in chemoheterotrophy, fermentation, hydrocarbon degradation, energy metabolism, and the metabolism of cofactors and vitamins, implying potential diverse metabolic exchanges between the bacteria and C. malayensis. These findings highlight both the ecological stability and environmental sensitivity of C. malayensis culture-associated microbiomes, with important implications for host health and ecosystem dynamics under climate change.

Abstract Pulsed particulate organic carbon export plays a vital role in sustaining Arctic marine ecosystems and the biological pump. Wintertime particulate organic carbon export in ice-covered Arctic waters is generally considered negligible due to light-limited local primary production. However, using time-series sediment-trap samples, we observe a high wintertime particulate organic carbon flux lasting over three months at the ice-covered Northwind Abyssal Plain in the High Arctic. We examine its drivers and implications for zooplankton. Here, we show that elevated particulate organic carbon fluxes during winter coincide with increased fecal-pellet carbon and zooplankton swimmer fluxes. Fecal-pellet evidence and nitrogen isotope values indicate that the wintertime carbon export represents a potential food source for zooplankton during the dark season. This wintertime carbon flux is consistent with lateral input, plausibly sourced from shelf and slope regions. Our results demonstrate that such wintertime carbon export provides an important food source for overwintering zooplankton, highlighting its ecological role in ice-covered Arctic ecosystems.

The Kuroshio functions as a critical nutrient stream, transporting nutrient-rich intermediate waters that serve as a vital source of nutrients in the East China Sea. Although prior studies observed rising Chlorophyll a (Chl a) along the continental slope within the Kuroshio pathway, the underlying mechanisms remained unclear. Here, we analyzed annual variations in upwelling and vertical mixing to clarify nutrient supply processes sustaining this Chl a trend. Our findings revealed that the Kuroshio intermediate waters from 300 to 500 m upwelled and supplied over 60% of the shelf-bottom nutrients, yet upwelling intensity stayed stable from 1997 to 2019. Crucially, winter mixing was essential for transporting these upwelled waters to the euphotic layer, with the annual deepening of the mixed layer depth identified as the primary driver of rising Chl a. If this deepening trend continues, future Chl a within the euphotic layer increases could exceed current observations. Plain language summary: The localized primary production enhanced consistently around the continental slope of the East China Sea based on field observations and satellite ocean colour data, and this enhancement was associated with elevated dissolved inorganic nitrogen (DIN) input. However, the dynamics that stimulate the DIN input remain unclear. Mechanisms suggested to be driving the elevating production include strengthened up-welling intensity, enhanced Kuroshio transport, atmospheric deposition, vertical mixing and biological N2 fixation. To examine these hypotheses, we used field data at PN-line transect and remote sensing Chl a data along the Kuroshio pathway from 1997 to 2020. The results showed that the DIN in the upwelled waters was the main driver causing the elevating primary production. The upper mesopelagic waters from 200 to 500 m was transported to the bottom of the continental shelf by upwelling, and then the nutrient-enriched waters supported the production by vertical mixing especially in winter. Other processes such as atmospheric deposition, biological N2 fixation, continental shelf pump and upstream Kuroshio transport showed minor contributions to the Chl a increase. Under the deepening of mixed layer depth, more nutrients will supply to the euphotic layer to support more pronouncing Chl a increases in future.

The Central Arctic Ocean is undergoing rapid environmental change associated with sea-ice retreat. The sea-ice bottom and adjacent under-ice waters are key zones for particulate organic matter (POM) accumulation and zooplankton foraging, yet the characteristics of POM across this interface remain unclear. Here, we analyzed particulate organic carbon (POC) and fatty-acid (FA) profiles of POM from these two habitats to quantify organic matter enrichment and characterize POM sources, omega-3 FA availability, and potential trophic significance. The sea-ice bottom represented an organic-matter hotspot, where POC and essential omega-3 FAs (eicosapentaenoic acid, EPA, and docosahexaenoic acid, DHA) were enriched relative to the under-ice water, with station-specific enrichment factors approaching two orders of magnitude. FA signatures indicate predominantly algal-derived POM in both habitats; the elevated diatom index at the sea-ice bottom suggests a greater diatom contribution to ice-bottom POM. In contrast to the concentration enrichments at the sea-ice bottom, the sum of EPA and DHA proportions (EPA% + DHA%) was similar between the two habitats. Copepod-marker FA concentrations also peaked at the sea-ice bottom, coinciding with the maxima in organic matter and essential omega-3 FAs, suggesting the potential ecological relevance of this interface for Arctic copepods. Our findings suggest the sea-ice bottom is a hotspot of POC and essential omega-3 FAs, and may be ecologically significant for Arctic copepods. These results provide a reference point for evaluating how sea-ice loss may alter the availability of these resources and foraging conditions for consumers in the Central Arctic Ocean.

The article presents hydrological, hydrochemical, biological, and geological data obtained in the Sea of Okhotsk and Sea of Japan during cruise 77 of the R/V Akademik Oparin (2025), aimed primarily at assessing the effect of tides and continental runoff on nutrient fluxes in three areas: Penzhina Bay, Eastern Sakhalin, and the Peter the Great Bay.

Petrogenic organic carbon (OC) is an important component of terrestrial organic matter and has traditionally been regarded as excluded from active regional carbon cycling. Recent studies, however, have demonstrated that petrogenic OC can undergo active oxidation in river-dominated continental margins, implying that its burial and preservation would contribute to the global carbon budget. Nevertheless, the burial processes and their controlling factors of petrogenic OC remain poorly constrained. Here, we use hopanoids as source-specific biomarkers to reconstruct a 300-year burial history of petrogenic OC in the East China Sea (ECS), revealing the two burial stages under climatic and anthropogenic forcing. Prior to the 1950s, low hopanoid maturity and dominant biological hopanoid configurations indicated natural inputs from rock weathering and petrogenic residues in soils. Despite frequent flooding during the 1840s–1920s, the petrogenic OC gradually declined, in contrast to the high-energy hydrodynamic environment. We attribute this to rapid proximal deposition, which limits offshore transport on the ECS inner shelf. The low proportion of petrogenic OC indicated that enhanced soil OC delivery further diluted the natural petrogenic OC signal. After the 1950s, hopanoid maturity increased sharply, indicating the presence of anthropogenic petroleum inputs. In addition, natural petrogenic OC persisted or even rose in recent years, likely due to the reactivation of previously deposited petrogenic OC, driven by enhanced erosion in the lower Changjiang River basin and its delta after dam construction. Overall, this record reveals a transition from a climate-dominated regime to an anthropogenic one after the 1950s. Hopanoid-based reconstruction provides new insights into the fate of petrogenic organic carbon and advances our understanding of its role in the global carbon budget and carbon cycle.

Organonitrogen pesticides (ONPs) are widely used in agriculture and can enter the ocean via surface runoff, posing potential threats to marine ecosystems. This study systematically investigated the occurrence and spatiotemporal distribution of 49 ONPs in seawater of the Beibu gulf, China, examined their key influencing factors, and conducted source apportionment and ecological risk assessment using multiple statistical models. The results showed that target ONPs were ubiquitously detected, with total concentrations (Σ49ONPs) ranging from 14.3 to 38.0 ng/L (mean: 20.2 ng/L). Spatially, higher concentrations were observed in the northern and central gulf, and lower in the southern area. Seasonally, the Σ49ONPs in summer 2022 (10.7-37.2 ng/L) were significantly higher than in winter (6.36-14.2 ng/L, p < 0.01), but comparable to summer 2021 (14.3-38.0 ng/L), with this seasonal pattern primarily linked to variations in riverine discharge. Correlation and random forest analyses identified terrestrial inputs and their interactions with suspended particulate matter (SPM) as critical drivers of ONP distribution and fate. Seawater Si:P ratios and Chl a concentrations serve as effective indicators of ONPs levels, while SPM acts as both a carrier and a sink. These findings underscore the necessity of integrating runoff management with pesticide application controls in coastal pollution mitigation. Multi-model source apportionment indicated that ONPs mainly originated from local agricultural applications and rubber industrial processing activities, with additional modulation by ocean currents and water exchange rates. Ecological risk assessment revealed that total ONPs concentrations posed moderate-to-high risks to marine organisms across trophic levels at most sampling sites, deserving particular attention.

Abstract Floods deliver large amounts of terrestrial organic carbon (TerrOC) and nutrients, potentially influencing long‐term organic carbon (OC) burial. Flood‐induced pulses of terrestrial materials could form distinct event layers in sediments, serving as valuable archives for understanding how floods regulate carbon sequestration over time. We investigate the multi‐proxy records (e.g., carbon isotopes and lipid biomarkers) from a 300‐year sediment core in the East China Sea inner shelf to explore the impacts of floods on terrestrial and marine OC burial. Flood signatures in distal muddy sediments are amplified by increased flood magnitude and frequency, particularly for floods originating upstream. Although the OC burial regime shifted from hydrological forcing (East Asian Monsoon and floods) to increasing anthropogenic forcing (e.g., dam construction and coastal eutrophication) after the 1970s, episodic extreme floods remained a critical mechanism coupling terrestrial and marine OC burial over the past 300 years, reflected by synchronous peaks in MultiTerrOC and MultiMarine OC indices during major flood intervals. These coincident OC burial peaks, together with coarser particles and high hypoxia intensity, suggest that flood‐driven nutrient pulses stimulated phytoplankton blooms, promoted hypoxic conditions, and consequently enhanced OC preservation. Overall, episodic extreme floods promote long‐term OC burial on large river‐dominated continental margins by redistributing land‐derived OC and stimulating atmospheric CO 2 fixation via enhanced marine OC burial. This century‐scale sedimentary record provides critical insights into the reconstruction of extreme hydrological events in ancient oceans and highlights the increasingly important role of marginal seas in carbon fixation under future scenarios of intensified flooding.

To investigate the migration and transformation of black carbon (BC) under rapid Arctic changes, we systematically quantified its abundance, distribution and particulate (PBC) flux in the Chukchi Shelf and Borderland, western Arctic Ocean. The average sedimentary BC concentration was 1.29 mg g-1, with spatial distributions controlled primarily by distance from the coast, water column stratification, and seafloor bathymetry. In the Chukchi Borderland, the mean PBC sinking fluxes reached 0.40 mg m-2 d-1, exhibiting pronounced seasonal variability, with fluxes during the melting season quadrupling those observed during periods of ice formation. Stable carbon isotopic signatures (delta 13CBC) revealed a tripartite BC source mixture throughout the study area: modern biomass combustion, pyrogenic fossil fuel combustion, and petrogenic (coastal erosion) inputs. Finally, we estimated annual BC fluxes of 61 and 655 Gg in the Chukchi Borderland and Central Arctic Ocean, respectively. These findings establish BC as an essential component of the Arctic carbon sink.

The Arctic Ocean ecosystem is undergoing dramatic changes as the sea ice cover retreats, underscoring the need for technologies that can monitor biological responses in the upper ocean, particularly beneath sea ice, where traditional ship-based investigations are severely constrained. Fish such as polar cod, which depend strongly on under-ice habitats, are widely regarded as key indicator species of Arctic warming, making their observation crucial for understanding ecosystem changes. In this study, we developed and deployed an integrated, fully automated observation system for long-term fish monitoring under the Arctic sea ice. The system combines a custom-designed underwater multi-focal automatic camera system (UMACS), a robust computing platform, and a satellite communication module to realize an autonomous ‘detection-to-transmission’ workflow under extreme polar conditions. A deep learning-based detection model trained on a multi-source dataset was implemented to address challenges such as low illumination, turbidity, and blurred backgrounds. A three-month continuous deployment in the Central Arctic Ocean demonstrated the robust engineering performance of the system under realistic field conditions. Although no fish were unambiguously confirmed, highlighting the intrinsic difficulty of discriminating small biological targets against a pure water background in this region, the system successfully achieved persistent, unattended, under-ice visual observation with data return. Therefore, this study provides a practical and transferable engineering framework for the scalable, technology-driven ecological monitoring of one of the planet’s most remote and fragile marine environments.

Understanding past sea-ice variability in Antarctica is crucial for studying global climate change mechanisms. While a di-unsaturated C25 highly branched isoprenoid (IPSO25) and its combination with phytoplankton biomarkers (PIPSO25) are promising sea-ice proxies, their applicability in Antarctica is limited by an incomplete mechanistic understanding of the processes transferring these biomarkers from the surface ocean to the sediment. Here, to explore these critical export processes across different sea-ice seasons, we investigate the first time-series sediment-trap record from Prydz Bay, Antarctica. The record includes IPSO25, the tri-unsaturated C25 highly branched isoprenoid (HBI III), brassicasterol, and PIPSO25. Our findings reveal that the export of the sea-ice biomarker IPSO25 is regulated by both sea-ice dynamics and the ballast effect. During the marginal ice zone or polynya stage, ballast materials (e.g., lithogenic materials) enhance the export of particles, synchronizing the export of various biomarkers and organic matter. At other times, biomarker fluxes are very low. Given this transport process affecting single biomarkers, we then explored whether a ratio-based index could provide a more robust signal. Our results show that PIIIIPSO25 (the HBI III-based PIPSO25) reflects general seasonal co-variation with satellite-derived sea-ice conditions, indicating PIIIIPSO25 is less affected by sinking processes compared to single-biomarker-based sea-ice reconstructions. This study emphasizes the need to account for both ballast effects and the ice retreat processes in paleoclimate interpretations. We show that ratio-based proxies like PIIIIPSO25 are promising tools for Antarctic sea-ice reconstructions, and we recommend further mechanistic and spatial validation studies across other Antarctic regions.

ABSTRACT Pyrocystis spp. thrive in warm, high-salinity waters and are transported into mid-latitude and coastal seas by warm currents like the Kuroshio. However, their global taxonomic composition and distribution remain unclear, particularly in Chinese marginal seas. Here, we present the Pyrocystis population and controlling factors in the East China Sea and southern Yellow Sea (ECSYS) during summer 2013. Nine species were identified, with Pyrocistis noctiluca dominant (66.0% relative abundance). Average abundance was 660.4 ± 1055.1 cells/m3, peaking at a front formed by the Nearshore Kuroshio Branch Current (NKBC) and Changjiang Diluted Water. Multiple statistical analyses (Spearman correlation, redundancy analysis and generalized additive models) consistently showed significant (P < 0.01) relationships between Pyrocystis population and both temperature and salinity. These relationships, together with optimum multiparameter analysis, reflect warm, saline waters advected by the Kuroshio and NKBC, which also transport Pyrocystis cysts into the ECSYS. Additionally, vertical circulation at the NKBC intrusion front promoted convergence and proliferation of large-celled cysts under suitable physicochemical conditions, enhancing local abundance. Based on distinct distribution patterns, six species (e.g. P. gerbautii and P. acuta) serve as indicators of the Kuroshio and NKBC. These findings highlight the critical role of Kuroshio intrusion in shaping Pyrocystis composition and distribution in the ECSYS.

IntroductionRapid Arctic sea-ice loss is reshaping deep particle export by lengthening the open-water season, altering ice-associated export, and enhancing lateral shelf-to-basin transport. However, it remains unclear whether annual variations in the flux and biochemical lability of sinking particles are synchronized.MethodsWe present a one-year record of amino acid and amino sugar compositions in sinking particles collected by a moored sediment trap at 870 m depth at station DM on the southern Northwind Ridge from August 2008 to September 2009. We used amino acid- and amino sugar-based indicators to assess the reactivity of sinking particles.ResultsParticulate organic carbon (POC) flux ranged from 0.91 to 30.53 mg C m-2 d-1, and total hydrolysable amino acid carbon (THAA-C) accounted for 10.5-38.5% of POC. During spring under heavy ice cover, sinking particles showed a moderate POC flux but the highest reactivity. In contrast, during the ice-free season, POC flux was highest whereas particle reactivity was only moderate. This indicates a seasonal decoupling between export magnitude and biochemical reactivity.DiscussionThis decoupling may reflect efficient transport of relatively fresh organic matter to the deep sea by ice-algal aggregates in spring, whereas summer export likely includes a larger contribution of laterally transported and more reworked material. These findings indicate that evaluation of the Arctic Ocean biological pump should consider not only carbon export fluxes but also the biochemical reactivity of sinking particles.

Global climate change has increased the frequency of extreme climatic events, complicating the interactions among nutrient dynamics, organic carbon cycling, and oxygen depletion in estuarine systems. However, the impacts of drought on particulate organic carbon (POC) inputs, transformations, and associated ecosystem responses remain poorly understood. This study conducted multi-proxy geochemical surveys to investigate 2022 drought-induced changes in POC sources, distribution, and ecological effects in the Changjiang Estuary (CE). The results show that the maximum POC concentration (878.3 μg∙L-1) was an order of magnitude lower than that in the normal year (2021, 5161.7 μg∙L-1), and net community production (NCP) declined to 138-151 mg C∙m-2∙d-1, accounting for only 37%-41% of the normal year. Drought reduced POC input by approximately 63% and altered the positions of turbidity and salinity fronts as well as upwelling intensity, thereby reshaping the spatial distribution of POC from different sources. Despite the substantial reduction in POC input, bottom hypoxia (dissolved oxygen <2 mg∙L-1) still developed during the drought year. Statistical analyses indicate that the degradation of in-situ produced marine POC contributes more substantially to oxygen depletion in the water column than terrigenous POC. Mixing model results further suggest that enhanced intrusion of low-oxygen shelf bottom water during the drought year established a low oxygen background in the CE, enabling limited oxygen consumption from POC decomposition to trigger hypoxia under strong summer stratification. Our findings advance understanding of drought-driven changes in POC cycling and oxygen consumption in estuarine waters, with implications for coastal environmental management.

Deep-sea REE-rich sediments represent a potential marine mineral resource, but the mechanisms controlling their enrichment remain controversial. Previous studies have emphasized the roles of paleoceanographic factors, particularly enhanced primary productivity and Antarctic Bottom Water (AABW)-related processes, in REE accumulation, based largely on tropical Pacific settings. However, the mechanisms of REE-rich sediments in the temperate North Pacific remain poorly constrained.Here, we present a systematic chronological, mineralogical, and geochemical investigation of REE-rich sedimentary intervals from core NP-GC01 recovered from the Pacific Basin east of Hess Rise. The core contains a REE-rich layer overlain by a Ba-rich interval, with age constraints of ∼ 29.1 Ma and ∼ 26.3 Ma, respectively. Distinct geochemical characteristics of bulk sediments, fish teeth, and micronodules reveal contrasting enrichment processes between these intervals. The Ba-rich interval reflects enhanced primary productivity, whereas the REE-rich layer is characterized by extremely low sedimentation rates, intensified bottom-current activity, and oxic depositional conditions, as indicated by abundant micronodules with hydrogenetic signatures. Furthermore, authigenic carbonate fluorapatite (CFA) particles enclosed within phillipsite are identified as important REE-bearing phases.Our results demonstrate that REE enrichment in deep-sea sediments is not simply controlled by enhanced primary productivity. Instead, prolonged sediment starvation, strong bottom-current ventilation, and oxic conditions promote authigenic CFA formation and REE sequestration. These findings provide new insights into the mineralization mechanisms of deep-sea REE-rich sediments beyond tropical oceanic settings.

Productive shelf seas are often considered hotspots for absorbing atmospheric CO 2 . However, the contribution of biological processes to carbon source or sink remains unclear in global large river‐dominated shelf seas. As a case study, we applied a three end‐members mixing model to quantify the contribution of biological processes (including the release of CO 2 from organic matter degradation by microbes and CO 2 uptake by phytoplankton) to the variations of air‐sea CO 2 flux (F) in the Changjiang plume‐impacted shelf area (CPS). We also established a 1‐D mass budget model to quantitatively assess the factors controlling of the partial pressure of carbon dioxide ( p CO 2 ) during seasonal transitions. Our results showed that CPS acted as a source of atmospheric CO 2 in summer (2.0 ± 13.9 mmol m −2 d −1 ) and autumn (5.4 ± 8.6 mmol m −2 d −1 ), but as a sink in winter (−6.8 ± 9.4 mmol m −2 d −1 ) and spring (−1.0 ± 4.8 mmol m −2 d −1 ). Biological processes significantly influence p CO 2 variability. It is worth noting that during the winter‐to‐spring transition, biological processes contributed 36% (the largest among the four transition periods) to the p CO 2 decrease. In the CPS, the air‐sea CO 2 flux caused by biological processes in winter, spring, summer, and autumn are 2.6 ± 4.1, −15.1 ± 22.0, 2.2 ± 8.2, and −2.0 ± 4.9 mmol m −2 d −1 , respectively, with relative contributions of biological processes to F of 13.5%, −53.9%, 5.0%, and −11.0%, respectively. These findings suggest biological uptake of dissolved inorganic carbon in global continental shelf seas significantly enhance carbon sequestration and contribute to mitigating global warming.

The complex hydrodynamic conditions in the Yangtze River Estuary play a pivotal role in shaping the distribution, transport, and transformation of nutrients, thereby influencing phytoplankton growth and ecosystem stability in the region. This study, based on hydrographic, biological, and chemical data collected during a mid-August 2023 cruise survey in the Yangtze River Estuary and the adjacent East China Sea, integrates nitrate nitrogen (delta 15NNO3) and oxygen isotope (delta 18ONO3) tracing techniques to investigate the synergistic effects of physical-biogeochemical processes during water mass convergence. The findings reveal that high concentrations of terrestrial nitrate, transported by the Changjiang Diluted Water (CDW), serve as the primary inorganic nitrogen source, driving rapid phytoplankton growth. Meanwhile, the Yellow Sea Coastal Current (YSCC) and Taiwan Warm Current (TWC) regulate nutrient transport and redistribution through mixing and convergence processes. The high phosphate input from the TWC optimizes the regional nutrient structure, alleviating phosphorus limitation caused by excessive nitrogen input from the CDW. However, the intrusion of low-oxygen YSCC waters may worsen bottom-water hypoxia. The frontal systems and water column stratification, induced by the convergence of CDW, YSCC, and TWC, control the spatial distribution of nitrogen and phosphorus nutrients, shape phytoplankton distribution patterns, and foster the formation of localized high-productivity zones. The synergistic effects of physical and biogeochemical processes, driven by water mass interactions, can significantly reduce bottom-water dissolved oxygen concentrations, thereby increasing the risk of localized hypoxia and threatening regional ecosystem stability.

Human activity and global climate change increasingly affect marine environments, leading to increases in harmful algal blooms (HABs) caused by phytoplankton. These blooms pose significant threats to public health, tourism, fisheries, and ecosystems. As an important fishing ground and tourist destination, the Beibu Gulf faces growing environmental pressure. This study sought to assess the phytoplankton community structure and status of HABs, with a focus on potential HAB species. Using environmental DNA (eDNA) metabarcoding, summer and winter surveys at both coastal and offshore waters revealed 66 potential HAB species, 23 of which were newly recorded in the Beibu Gulf. The potential HAB species exhibited greater richness and relative abundance in summer than in winter. Offshore areas showed greater diversity, whereas coastal areas showed greater relative abundance. Temperature emerged as the most influential factor shaping phytoplankton composition, and pH was found to play an important role in coastal areas. Nutrients such as silicate and ammonium are critical for the distribution of potential HAB species. Among the potential HAB species, Cyclotella cryptica predominated in coastal areas during winter, whereas Chaetoceros tenuissimus predominated in summer. Some species that caused severe HAB events in other oceanic regions were first detected in this study, including Margalefidinium polykrikoides, Karlodinium veneficum, and Prorocentrum concavum. This study revealed the diversity and complexity of the phytoplankton community in the Beibu Gulf, emphasizing the critical importance of monitoring and early warning of potential HAB species, particularly those driven by human activities and climate change.

Acidification and hypoxia present significant ecological and environmental challenges for coastal oceans, particularly estuarine systems with high nutrient inputs such as the Changjiang Estuary (CJE). We conducted three replicate cruises from July to September in 2020 along the same transects off the CJE to investigate dynamic changes of coastal acidification and hypoxia. Bottom hypoxia expanded and intensified from July to August and alleviated in September. Changes in pH(T) generally followed those of dissolved oxygen (DO), although the spatial and temporal patterns did not entirely align. In August, the tight connection between hypoxia and acidification differed between the northern and southern regions. The northern region experienced widespread hypoxia and acidification with DO and pH(T) as low as 39 mu mol kg(-1) and 7.66, whereas the southern region maintained similar pH(T) values despite higher DO (>94 mu mol kg(-1)). By September, hypoxia was alleviated, and pH(T) showed a more pronounced rise, with pH(T) increasing by similar to 0.02 at the same DO level. Aerobic remineralization emerged as the primary driver of bottom acidification and hypoxia off the CJE during summer, peaking in August and diminishing by September. In August, the northern region experienced synchronous and severe hypoxia and acidification, whereas intermittent localized mixing in the south alleviated hypoxia. By September, enhanced mixing in the CJE increased the buffering capacity of subsurface waters, mitigating pH decline despite ongoing hypoxia. These findings improve our understanding of short-term dynamics in estuarine carbon cycling, acidification and hypoxia.

Field observations are often inadequate and numerical simulations are not sufficiently accurate for determining carbon source-sink trends in estuaries, particularly those in frontal zones. To address these challenges, herein, we develop a carbon assimilation model that integrates the advantages of the total alkalinity diagnostic formula from field survey data, high-accuracy partial pressures of CO2 (pCO2) from remote sensing data, and representations of near-coast and frontal carbon processes from a FVCOM-NPZC model. In the regions between the 15 and 30 isohalines on the East China Sea shelf, the Changjiang River plume converges with offshore high-salinity waters, thereby forming a typical frontal zone of a mid-latitude estuary. This model captures pCO2 and air-sea CO2 flux trends in the frontal zone along with their underlying mechanisms. The frontal zone exhibits a unique pCO2 decreasing trend, thus forming a spatial "increasing-decreasing-increasing" structure in the near-coast, frontal, and offshore areas. The unique decreasing trend in the frontal zone shows a three-branched structure, which is associated with the reduction in the plume's expansion pathways. A rapid carbon source-sink trend (-0.28 mmol/m2/d/y) is observed in the frontal zone, which is considerably faster than the increasing sink trend (-0.10 mmol/m2/d/y) in the offshore area. Enhanced shoreward wind and increased chlorophyll a concentration contribute to the pCO2 decreasing trend and the rapid carbon source-sink trend in the frontal zone. The enhanced shoreward wind in the estuary increases salinity and total alkalinity, ultimately decreasing pCO2 levels. Enhanced phytoplankton growth in the frontal zone decreases both dissolved inorganic carbon concentration and pCO2 levels.