Eutrophication-Driven Hypoxia in the East China Sea off the Changjiang Estuary

Mitigation of hypoxia and ocean acidification on the inner East China Sea shelf impacted by the 2023 summer drought

Hypoxia and acidification are universal environmental issues in coastal seas, especially in large river dominated shelves, and the East China Sea shelf is a typical case among them. However, the responses of status of hypoxia and acidification in coastal seas to the extremes of river discharges are still to be revealed. This study surveyed the influences of a summer drought on the status of hypoxia and acidification on the inner East China Sea shelf off the Changjiang estuary. In August of 2023 during a summer drought, carbonate system parameters and dissolved oxygen (DO) were surveyed on the East China Sea shelf off the Changjiang estuary. As expected, dissolved inorganic carbon (DIC) removal (up to >40 μmol kg-1) and DO over-saturation (up to >110 %) accompanied by high pH (up to >8.15) in the surface water were observed. However, low DO (32-172 μmol kg-1), low pH (7.63-8.04) and low saturation state index of aragonite (ΩAr) (1.34-3.06) in the bottom water were observed. Relationships of Excess DIC with DO consumption, and pH and ΩAr with Excess DIC indicated that the hypoxia and acidification in the bottom water was due mainly to the remineralization of the marine-sourced organic matter. Nevertheless, both hypoxia and acidification were mitigated, i.e. the hypoxic area was smaller, the minimum DO concentration, pH and saturation state index of aragonite were higher in August of 2023 than under the general summer condition. The lower Changjiang discharge (∼60 % of the long-term monthly average) mitigated eutrophication of the East China Sea shelf and decreased the phytoplankton biomass in the surface water and subsequently the hypoxia and acidification in the bottom water. However, acidification of the bottom water on the East China Sea shelf was still severe even during the summer drought. Regulating the anthropogenic impact on the coastal marginal seas is still urgently needed to mitigate the acidification status.

Decoupling of high-resolution surface pH and DO reveals temporal algal bloom dynamics on the East China Sea

The stoichiometric ratio between seawater CO2 and dissolved oxygen (DO) during phytoplankton metabolism holds significant importance in evaluating ecological and biogeochemical processes. We collected high-resolution underway temperature, salinity, DO, and pH data in the East China Sea inner shelf in May 2017. Our results revealed high pH (8.36) and supersaturated DO (171 %) in the outer Changjiang Estuary, indicating the occurrence of an algal bloom event. They were significantly correlated with a regression slope of 0.0029, which roughly followed the Redfield ratio. In contrast, a much higher ratio (0.0088) manifested in a low-salinity patch on north of the Changjiang Estuary, featuring a pH of 8.40 and oxygen saturation of approximately 123 %. The substantially faster air-sea equilibrium rate of O2 than CO2 probably caused such decoupling, offering insight into the temporal evolutions of algal bloom. Theoretically, a steeper regression slope implies an earlier onset of algal bloom. An end-member mixing model was constructed to exclude the physical mixing influences on dissolved inorganic carbon (ΔDIC) and DO (ΔDO). Furthermore, we conducted simulations to explore the temporal variations of ΔDIC-ΔDO regression slope with time. Comparing slopes derived from simulation and mixing model suggested that the biological signal of the decoupled waters likely preceded our observations by 6-10 days. Satellite results captured high-Chl a waters southwest of the low-salinity patch a week before our observation, potentially transported northward by prevailing southwest wind. Given that oxygen and pH are frequently measured in aquatic environments, their combined assessment could be a valuable method for assessing temporal algal bloom dynamics.

Investigation of the causes and mechanisms of hypoxia in the central Bohai Sea in the summer of 2022

The deep-water area in the central Bohai Sea (BS) serves as a spawning ground and nursery for fish, shrimp, and crabs. Frequent hypoxia will affect the ecological environment in the central BS. Data from an on-site investigation of the central BS in the spring and summer of 2022 were used to analyze the relevant factors generating the occurrence of hypoxia in the central BS through the eutrophication index E, apparent oxygen consumption (AOU), and Spearman correlation. The hypoxia area was largely distributed in the study area's deep water section, and stratification was the main cause of hypoxia at the bottom. Organic matter mineralization, degradation, and biological respiration further exacerbated the hypoxia. In the summer of 2022, temperature stratification was the dominant factor influencing hypoxia.

The vulnerability of sharks, skates, and rays to ocean deoxygenation: Physiological mechanisms, behavioral responses, and ecological impacts

Levels of dissolved oxygen in open ocean and coastal waters are decreasing (ocean deoxygenation), with poorly understood effects on marine megafauna. All of the more than 1000 species of elasmobranchs (sharks, skates, and rays) are obligate water breathers, with a variety of life-history strategies and oxygen requirements. This review demonstrates that although many elasmobranchs typically avoid hypoxic water, they also appear capable of withstanding mild to moderate hypoxia with changes in activity, ventilatory responses, alterations to circulatory and hematological parameters, and morphological alterations to gill structures. However, such strategies may be insufficient to withstand severe, progressive, or prolonged hypoxia or anoxia where anaerobic metabolic pathways may be used for limited periods. As water temperatures increase with climate warming, ectothermic elasmobranchs will exhibit elevated metabolic rates and are likely to be less able to tolerate the effects of even mild hypoxia associated with deoxygenation. As a result, sustained hypoxic conditions in warmer coastal or surface-pelagic waters are likely to lead to shifts in elasmobranch distributions. Mass mortalities of elasmobranchs linked directly to deoxygenation have only rarely been observed but are likely underreported. One key concern is how reductions in habitat volume as a result of expanding hypoxia resulting from deoxygenation will influence interactions between elasmobranchs and industrial fisheries. Catch per unit of effort of threatened pelagic sharks by longline fisheries, for instance, has been shown to be higher above oxygen minimum zones compared to adjacent, normoxic regions, and attributed to vertical habitat compression of sharks overlapping with increased fishing effort. How a compound stressor such as marine heatwaves alters vulnerability to deoxygenation remains an open question. With over a third of elasmobranch species listed as endangered, a priority for conservation and management now lies in understanding and mitigating ocean deoxygenation effects in addition to population declines already occurring from overfishing.

Nutrient sources, phytoplankton blooms, and hypoxia along the Chinese coast in the East China Sea: Insight from summer 2014

Phytoplankton blooms are common along the Chinese coast in the East China Sea, driven by various nutrient sources including river discharge, bottom water regeneration, and Kuroshio subsurface water intrusion. A notable 2014 summer bloom off the Zhejiang coast, exhibiting a Chl a concentration of 20.1 μg L-1, was significantly influenced by Changjiang River discharge, and high nutrient concentrations are often observed in the region's surface water. During blooms, primary production peaks at 1686.3 mg C m-3 d-1, indicating substantial CO2 absorption, with surface water fCO2 declining to 299.5 μatm, closely linked to plankton activities. Hypoxia often coincides with these frequent bloom occurrences, implicating marine-derived organic matter decomposition as a pivotal factor. Elevated particulate organic carbon concentrations further support this assumption, alongside increased nutrient levels, fCO2, and low pH in hypoxic waters. These findings underscore the intricate interplay between phytoplankton, nutrient cycling, and hypoxia formation, essential for effective coastal ecosystem management.

Key drivers of hypoxia revealed by time-series data in the coastal waters of Muping, China

Coastal hypoxia (low dissolved oxygen in seawater) is a cumulative result of many physical and biochemical processes. However, it is often difficult to determine the key drivers of hypoxia due to the lack of frequent observational oceanographic and meteorological data. In this study, high-frequency time-series observational data of dissolved oxygen (DO) and related parameters in the coastal waters of Muping, China, were used to analyze the temporal pattern of hypoxia and its key drivers. Two complete cycles with the formation and destruction of hypoxia were captured over the observational period. Persistent thermal stratification, high winds and phytoplankton blooms are identified as key drivers of hypoxia in this region. Hypoxia largely occurs due to persistent thermal stratification in summer, and hypoxia can be noticeably relieved when strong wind mixing weakens thermal stratification. Furthermore, we found that northerly high winds are more efficient at eroding stratification than southerly winds and thus have a greater ability to relieve hypoxia. This study revealed an episodic hypoxic event driven by a phytoplankton bloom that was probably triggered by terrestrial nutrient loading, confirming the causal relationship between phytoplankton blooms and hypoxia. In addition, we found that the lag time between nutrient loading, phytoplankton blooms and hypoxia can be as short as one week. This study could help better understand the development of hypoxia and forecast phytoplankton and hypoxia, which are beneficial for aquaculture in this region.

Nutrient budgets and biogeochemical dynamics in the coastal regions of northern Beibu Gulf, South China Sea: Implication for the severe impact of human disturbance

This study examined the nutrient budgets and biogeochemical dynamics in the coastal regions of northern Beibu Gulf (CNBG). Nutrient concentrations varied spatially and seasonally among the different bays. High nutrient levels were found in the regions with high riverine inputs and intensive mariculture. Using a three end-member mixing model, nutrient biogeochemistry within the ecosystem was estimated separately from complex physical mixing effects. Nutrient consumption dominated in most bays in summer, whereas nutrient regeneration dominated in winter, likely due to phytoplankton decomposition, vertical mixing and desorption. Through the Land-Ocean Interaction Coastal Zone (LOICZ) model, the robust nutrient budgets were constructed, indicating that the CNBG behaved as a sink of dissolved inorganic nitrogen, phosphorus and silicon. River-borne nutrient inputs were the dominant nutrient source, while residual flows and water exchange flows transported nutrient off the estuaries. This study could help us better understand nutrient cycles and nutrient sources/sinks in the CNBG.

Spatial heterogeneity of sedimentary organic matter sources in the Yangtze River estuary: Implications from fatty acid biomarkers

This study investigated the sources of sedimentary organic matter (OM) in the Yangtze River estuary (YRE), using multiple biomarkers. The results of stable carbon isotope (δ13C) and total organic carbon to nitrogen ratio (TOC/TN) suggests the contribution of marine-derived OM significantly increased seawards, while fatty acid (FA) composition provides more specific information on OM sources. In total, 30 components of FAs were identified at the studied 17 sites, which mainly composed of phytoplankton FA, followed by ubiquitous FA and bacterial FA, while terrestrial FA contributed less to the total FAs. Under the strong impacts of the large physicochemical gradients in the YRE, TOC, TN and FA components showed higher concentrations in the estuary mixing zone (especially within the turbidity maximum zone), attributing to their strong binding with OM-enriched fine particles. The spatial heterogeneity of sedimentary OM sources was highly impacted by salinity and Chl-a, as well as bacteria-mediated OM degradation.

Nutrient characteristics driven by multiple factors in large estuaries during summer: A case study of the Yangtze River Estuary

Nutrients directly control the level of primary productivity and are crucial for the stability of marine ecosystems. Focusing on the survey results in August 2020 of the Yangtze River Estuary, this study elucidated the distribution characteristics and controlling factors of three nutrients: NO3-N, PO4-P, SiO3-Si. The results showed that the concentrations of NO3-N, PO4-P, SiO3-Si in the study area were generally higher near the shore than far shore, with average concentrations of 11.40, 0.70, and 23.73 μmol/L, respectively. The ocean currents drove the distribution of nutrients, and the transport of CDW and YSCC increased the nutrient levels. The resuspension of sediment caused by factors such as terrain and weather may lead to an abnormal increase in nutrients in the bottom waters. The main controlling factors of the three nutrients were different. NO3-N was significantly affected by human activities, PO4-P and SiO3-Si were mainly affected by natural factors.

Acidification Offset Warming-Induced Increase in N2O Production in Estuarine and Coastal Sediments

Global warming and acidification, induced by a substantial increase in anthropogenic CO2 emissions, are expected to have profound impacts on biogeochemical cycles. However, underlying mechanisms of nitrous oxide (N2O) production in estuarine and coastal sediments remain rarely constrained under warming and acidification. Here, the responses of sediment N2O production pathways to warming and acidification were examined using a series of anoxic incubation experiments. Denitrification and N2O production were largely stimulated by the warming, while N2O production decreased under the acidification as well as the denitrification rate and electron transfer efficiency. Compared to warming alone, the combination of warming and acidification decreased N2O production by 26 ± 4%, which was mainly attributed to the decline of the N2O yield by fungal denitrification. Fungal denitrification was mainly responsible for N2O production under the warming condition, while bacterial denitrification predominated N2O production under the acidification condition. The reduced site preference of N2O under acidification reflects that the dominant pathways of N2O production were likely shifted from fungal to bacterial denitrification. In addition, acidification decreased the diversity and abundance of nirS-type denitrifiers, which were the keystone taxa mediating the low N2O production. Collectively, acidification can decrease sediment N2O yield through shifting the responsible production pathways, partly counteracting the warming-induced increase in N2O emissions, further reducing the positive climate warming feedback loop.

Fate of carbon influenced by the in-situ growth of phototrophic biofilms at the soil-water interface of paddy soil

Phototrophic biofilms (PBs) are commonly found in the sediment/soil-water interface of paddy soils and have a significant impact on carbon cycles. However, the specific carbon fate influenced by the in-situ growth of PBs in paddy soil remains unclear. In this study, we investigated the effect of in situ PBs growth on methane and carbon dioxide emissions, as well as dissolved organic matter (DOM) transformation. Our findings demonstrated a negative correlation between PBs growth and methane and carbon dioxide emissions, while showing a positive correlation with DOM composition. The in-situ growth of PBs reduced methane emissions by approximately 79 % and carbon dioxide emissions by approximately 33 % in the daytime, and also slowed down the degradation rate of dissolved organic matter from over 30.4 % to <16 %. Microsensor measurements revealed that these changes were attributed to the increased concentration and penetration depth of oxygen, as well as variations in pH caused by the growth of in situ PBs. Co-occurrence analysis indicated a robust correlation between DOM transformation and the significantly suppressed methanogenesis by methanogens such as Methanosaeta, Methanomassiliicoccus and Methanosarcina, and also the notably enhanced methane oxidation by methanotrophs including Methylobacterium, Methyloversatilis and Methylomonas, in response to the growth of PBs. These findings shed light on the impact of in situ PBs on methane and carbon dioxide emissions and DOM transformation, providing new insights for understanding carbon cycling in paddy soils.

Particle-associated denitrification is the primary source of N2O in oxic coastal waters

The heavily human-perturbed coastal oceans are hotspots of nitrous oxide (N2O) emission to the atmosphere. The processes underpinning the N2O flux, however, remain poorly understood, leading to large uncertainties in assessing global N2O budgets. Using a suite of nitrogen isotope labeling experiments, we show that multiple processes contribute to N2O production throughout the estuarine-coastal gradient, sustaining intensive N2O flux to the atmosphere. Unexpectedly, denitrification, rather than ammonia oxidation as previously assumed, constitutes the major source of N2O in well-oxygenated coastal waters. Size-fractionated manipulation experiments with gene analysis further reveal niche partitioning of ammonia oxidizers and denitrifiers across the particle size spectrum; denitrification dominated on large particles and ammonia oxidizers on small particles. Total N2O production rate increases with substrate and particle concentrations, suggesting a crucial interplay between nutrients and particles in controlling N2O production. The controlling factors identified here may help understand climate feedback mechanisms between human activity and coastal oceans.

Organic enrichment reduces sediment bacterial and archaeal diversity, composition, and functional profile independent of bioturbator activity

Eutrophication is a worldwide issue that can disrupt ecosystem processes in sediments. Studies have shown that macrofauna influences sediment processes by engineering environments that constrain microbial communities. Here, we explored the effect of different sizes of the Sydney cockle (Anadara trapezia), on bacterial and archaeal communities in natural and experimentally enriched sediments. A mesocosm experiment was conducted with two enrichment conditions (natural or enriched) and 5 cockle treatments (small, medium, large, mixed sizes and a control). This study was unable to detect A. trapezia effects on microbial communities irrespective of body size. However, a substantial decrease of bacterial richness, diversity, and structural and functional shifts, were seen with organic enrichment of sediments. Archaea were similarly changed although the magnitude of effect was less than for bacteria. Overall, we found evidence to suggest that A. trapezia had limited capacity to affect sediment microbial communities and mitigate the effects of organic enrichment.

Dissolved organic matter distribution in the water column and sediment pore water in a highly anthropized coastal lagoon (Mar Menor, Spain): Characteristics, sources, and benthic fluxes

Coastal lagoons are among the most productive and biodiverse systems in the world and are important sentinels of climate change. The Mar Menor is one of the largest coastal lagoons in the Mediterranean, providing a variety of ecosystem services and resources to the community. However, in recent decades this lagoon has suffered drastic changes and degradation caused by human activities. We analyzed the concentration of dissolved organic carbon (DOC) and the optical properties of dissolved organic matter (DOM) in the water column and sediment pore water during the summer and winter of 2018 and during eighteen months from 2016 to 2018. Overall, we found that the composition of DOM is mainly related to and enhanced by anthropogenic activities and microbial metabolism. DOM enters the lagoon via urban and agricultural runoff, drainage systems, and wastewater treatment plants. Additionally, strong microbial metabolism in sediments leads to differences in DOM composition between water and sediments. In the water column, humic-like components accounted for 71 % of the total DOM, while protein-like compounds were most abundant in sediment pore water. We observed a strong seasonal variability associated with precipitation and the system collapse in 2016 (phytoplankton bloom), which resulted in the death of 80 % of macrophytes. The sediments act as a source of DOM to the overlying water, likely due to relatively high organic matter content and intense microbial activity, primarily through anaerobic pathways. Benthic fluxes of DOC ranged from 5.24 to 33.30 mmol m-2 d-1, being higher in winter than summer 2018 and decreasing from north to south, likely related to lower residence time in the northern basin, groundwater discharge and accumulation of organic matter from the dead meadows. We estimate a net flux of DOC from the Mar Menor toward the Mediterranean Sea of 1.57 × 107 mol yr-1.

Hypoxia in aquatic invertebrates: Occurrence and phenotypic and molecular responses

Global deoxygenation in aquatic systems is an increasing environmental problem, and substantial oxygen loss has been reported. Aquatic animals have been continuously exposed to hypoxic environments, so-called "dead zones," in which severe die-offs among organisms are driven by low-oxygen events. Multiple studies of hypoxia exposure have focused on in vivo endpoints, metabolism, oxidative stress, and immune responses in aquatic invertebrates such as molluscs, crustaceans, echinoderms, and cnidarians. They have shown that acute and chronic exposure to hypoxia induces significant decreases in locomotion, respiration, feeding, growth, and reproduction rates. Also, several studies have examined the molecular responses of aquatic invertebrates, such as anaerobic metabolism, reactive oxygen species induction, increased antioxidant enzymes, immune response mechanisms, regulation of hypoxia-inducible factor 1-alpha (HIF-1α) genes, and differently expressed hemoglobin/hemocyanin. The genetic basis of those molecular responses involves HIF-1α pathway genes, which are highly expressed in hypoxic conditions. However, the identification of HIF-1α-related genes and understanding of their applications in some aquatic invertebrates remain inadequate. Also, some species of crustaceans, rotifers, sponges, and ctenophores that lack HIF-1α are thought to have alternative defense mechanisms to cope with hypoxia, but those factors are still unclear. This review covers the formation of hypoxia in aquatic environments and the various adverse effects of hypoxia on aquatic invertebrates. The limitations of current hypoxia research and genetic information about the HIF-1α pathway are also discussed. Finally, this paper explains the underlying processes of the hypoxia response and presents an integrated program for research about the molecular mechanisms of hypoxic stresses in aquatic invertebrates.

MeHg production in eutrophic lakes: Focusing on the roles of algal organic matter and iron-sulfur-phosphorus dynamics

The mechanisms by which eutrophication affects methylmercury (MeHg) production have not been comprehensively summarized, which hinders accurately predicting the MeHg risk in eutrophic lakes. In this review, we first discussed the effects of eutrophication on biogeochemical cycle of mercury (Hg). Special attentions were paid to the roles of algal organic matter (AOM) and iron (Fe)-sulfur (S)-phosphorus (P) dynamics in MeHg production. Finally, the suggestions for risk control of MeHg in eutrophic lakes were proposed. AOM can affect in situ Hg methylation by stimulating the abundance and activities of Hg methylating microorganisms and regulating Hg bioavailability, which are dependent on bacteria-strain and algae species, the molecular weight and composition of AOM as well as environmental conditions (e.g., light). Fe-S-P dynamics under eutrophication including sulfate reduction, FeS formation and P release could also play crucial but complicated roles in MeHg production, in which AOM may participate through influencing the dissolution and aggregation processes, structural order and surface properties of HgS nanoparticles (HgS_NP). Future studies should pay more attention to the dynamics of AOM in responses to the changing environmental conditions (e.g., light penetration and redox fluctuations) and how such variations will subsequently affect MeHg production. The effects of Fe-S-P dynamics on MeHg production under eutrophication also deserve further investigations, especially the interactions between AOM and HgS_NP. Remediation strategies with lower disturbance, greater stability and less cost like the technology of interfacial O₂ nanobubbles are urgent to be explored. This review will deepen our understanding of the mechanisms of MeHg production in eutrophic lakes and provide theoretical guidance for its risk control.

Distribution of nitrogen (N) and phosphorus (P) in seasonal low-oxygen marine ranching in northern Yellow Sea, China

Seasonal low-oxygen in marine ranching in the northern Yellow Sea has been one of the major environmental problems in coastal waters in recent years. Nitrogen (N) and phosphorus (P) are important nutrients, which are susceptible to the concentration of dissolved oxygen (DO). This article studied the effects of low-oxygen on nutrients represented by N and P fractions in marine ranching in the northern Yellow Sea. The results showed that there were significant layer differences in temperature and salinity during the low-oxygen period. In the seawater, the nutrient distribution in the death disaster zone of sea cucumbers and the non-disaster zone was similar, and DO had a strong positive correlation with dissolved inorganic nitrogen (DIN). In the sediment, significant regional differences existed in nutrient concentration, and the concentration of total phosphorus (TP) decreased significantly with the increase in DO content. The results showed that the sources and sinks of nitrogen and phosphorus nutrients were inconsistent in this zone, and migration and transformation of the existing form of nitrogen with the seasonal changes in the water environment was a main factor for N distribution. This study extended the understanding of the effects of seasonal low-oxygen on N and P.

What drove the nonlinear hypoxia response to nutrient loading in Chesapeake Bay during the 20th century?

Previous data analysis showed that the large expansion of hypoxia in Chesapeake Bay between 1950s and 1980s was correlated to the increased riverine nutrient loading, but the physical and biogeochemical processes driving this hypoxia response need to be better understood. Using a validated coupled hydrodynamic-biogeochemical model, we conducted a hindcast simulation of dissolved oxygen during the 40-year period (1950-1989) when the nutrient loading doubled. The model reproduced the observed decline in O₂ concentration at monitoring stations and the expansion of the hypoxic volume. The peak summer hypoxic volume expanded from ∼5 km³ during 1950-1969 to ∼10 km³ during 1970-1989. To discern how different physical and biochemical processes regulated dissolved O₂, we examined O₂ budget in a fixed control volume of the bottom water most susceptible to hypoxia. The increased water column respiration was found to be the dominant driver of the hypoxia expansion. Further analysis showed a nonlinear response to the nutrient loading. The accumulative hypoxia volume days per unit of nitrate load showed an abrupt (∼50 %) jump around 1968. The summer mean hypoxic volume increased with the winter-spring nutrient load, but it was 1.3 km³ (about 30 %) higher in 1968-1989 than in 1950-1967 at the same nutrient load. This upward shift in hypoxia was caused by the upward shift in the relationship between the water column respiration and winter-spring nutrient load. Hypoxia suppressed nitrification and denitrification processes in the sediment, amplifying nutrient recycling by 15 % and water column respiration by 12 %. Our modeling analysis demonstrated a feedback mechanism for driving the nonlinear hypoxia response to nutrient loading.

Increased Nitrogen Loading Facilitates Nitrous Oxide Production through Fungal and Chemodenitrification in Estuarine and Coastal Sediments

Estuarine and coastal environments are assumed to contribute to nitrous oxide (N₂O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N₂O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N₂O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N₂O molecule increased due to increasing nitrate concentration, suggesting the changes in N₂O production pathways. Enhanced N₂O production under high nitrate concentration was not mediated by bacterial denitrification, but instead was mainly regulated by fungal denitrification. Elevated nitrate concentration increased the contribution of fungal denitrification to N₂O production by 11-25%, whereas it decreased bacterial N₂O production by 16-33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13-28% of N₂O production. Elevated nitrate concentration significantly mediated nirK-type denitrifiers structure and abundance, which are the keystone taxa driving N₂O production. Collectively, these results suggest that increasing nitrate concentration can shift N₂O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N₂O production and have widespread implications for N₂O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.

Sediment diffusion is feasible to simultaneously reduce nitrate discharge from recirculating aquaculture system and ammonium release from sediments in receiving intensive aquaculture pond

Nitrogen accumulation has become one of the greatest unresolved challenges restricting the development of aquaculture worldwide. In recirculating aquaculture system (RAS), lack of organic matter (OM) and sensitive organisms makes it difficult to apply efficient denitrifying technology, thus leading to a high nitrate‑nitrogen (NO₃^--N) accumulation. In contrast, excess OM accumulation in intensive aquaculture pond sediments is associated with dissolved oxygen depletion and ammonium‑nitrogen (NH₄⁺-N) accumulation in the sediments. Based on the opposing effects of OM on the nitrogen accumulation in RAS and intensive aquaculture ponds, this study assessed the feasibility of simultaneously reducing NO₃^--N discharge from RAS and controlling NH₄⁺-N accumulation in intensive aquaculture ponds by in situ diffusing RAS tailwater containing NO₃^--N into intensive aquaculture pond sediments. The results showed that NO₃^--N diffusion strategy improved the native sediment denitrification capacity, thus increasing NO₃^--N removal efficiency from RAS tailwater and significantly decreasing the NH₄⁺-N concentration in interstitial water and the total organic carbon content in intensive aquaculture pond sediments. High-throughput sequencing and quantitative real-time polymerase chain reaction (qPCR) results revealed that NO₃^--N addition significantly increased both nitrifying bacteria and denitrifying bacteria abundance. These results implied that NO₃^--N diffusion strategy could effectively stimulate microbial decomposition of OM, thus relieving the hypoxia limitation of sediment nitrification. Overall, this study offers a feasible method for simultaneous reduction of NO₃^--N from RAS tailwater and NH₄⁺-N in intensive aquaculture ponds with low cost and high efficiency.

Nitrogen loss from the coastal shelf of the East China Sea: Implications of the organic matter

Organic matter is a critical factor which regulates nitrogen loss pathways of denitrification and anammox for microbes in marine ecosystems. However, only a little attention has been paid to contrasting studies on denitrification and anammox in sandy and muddy sediments, especially in the coastal continental shelf dominated by sandy sediments. This study determined the bulk properties and associated microbial nitrogen transformation processes of surface sediments in the East China Sea coastal shelf, with the aim of gaining insight into the interaction of nitrogen loss with organic matter at the molecular level. The results illustrate that nitrogen loss dominates in organic-rich muddy sediments, and its denitrification rate (14.39 nmol N g-1 h-1) and anammox rate (2.73 nmol N g-1 h-1) are greater than those of sandy sediments (denitrification rate = 5.55 nmol N g-1 h-1, anammox rate = 1.57 nmol N g-1 h-1). Furthermore, determination of the mean summed ladderanes shows higher anammox generated in the muddy sediments with a value of 167.78 ng g-1dw. Quantitative analysis demonstrated that organic-rich muddy sediments enhanced the copy number of the denitrifying functional gene nosZ and anammox functional gene hzsB. We inferred that the greater rate of nitrogen loss in muddy sediments was due to the coupling relationship between anammox and denitrification. Overall, the community distribution and abundance of denitrifying bacteria and anammox bacteria changed intricately under the influence of organic matter. Moreover, this study further improves the understanding of nitrogen loss pathways and mechanistic factors from sediments.

Identification of nitrogen sources and cycling along freshwater river to estuarine water continuum using multiple stable isotopes

Nitrogen (N) transport from terrene to river water is a major source of N in estuarine water, contributing to eutrophication, harmful algal blooms and hypoxia. However, there is a lack of holistic and systematic research on N sources and transformation in the freshwater river-estuarine water continuum. In this study, multiple stable isotope signatures of nitrate (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-), ammonium (δ¹⁵N-NH₄⁺), and suspended particulate nitrogen (δ¹⁵N-PN) were employed to differentiate the sources and transformations of N and calculate the proportional contribution of NO₃^- sources by Bayesian model in Qiantang River (QTR)-Hangzhou Bay (HZB) during the dry season. The results showed that: (1) Evidences from isotopic signatures suggested the occurrence of N transformation instead of conservation mixing. (2) Negative correlations between the δ¹⁵N-NO₃^- and δ¹⁵N-NH₄⁺, the relationships between δ¹⁵N-NO₃^- and NO₃^--N concentrations, and smaller δ¹⁸O-NO₃^- values were found in almost all surface water, indicating that nitrification was the dominant N transformation. (3) In addition to the nitrification evidence, significant correlations between δ¹⁵N-PN and δ¹⁵N-NH₄⁺ revealed that assimilation and nitrification jointly affected the N transformation in the QTR's upstream, midstream and lower tributaries, which are unaffected or less affected by tides. (4) The lack of a relationship between δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- or ln(NO₃^-) indicated that denitrification was weakened in all surface waters. (5) Qualitative identification of N pollution sources and quantitative calculation of NO₃^--N potential sources revealed that sewage was the dominant source of N in the QTR and the HZB, while the internal nitrification was also important factor in determining N levels. This study provided evidence to further understand the sources, transport, and transformation of N in the river-estuary continuum, which deepens the understanding of the land-ocean integrated management of N contaminant.

Transport of trace metals and their bioaccumulation in zooplankton from Changjiang (Yangtze River) to the East China Sea

The present study conducted a comprehensive field investigation on the transport and bioaccumulation of six trace metals (Cr, Cd, Pb, Mn, Ni, Cu) along a transect from Changjiang (Yangtze River) to the East China Sea continental shelf, which exhibited large variations in physiochemical properties (salinity, turbidity, pH, chlorophyll a, total nitrogen, dissolved oxygen, dissolved and particulate organic matter). From riverine sites to marine sites, dissolved Cr and Cd significantly increased, dissolved Pb and Mn showed less variations, while dissolved Cu and Ni showed complex spatial distribution patterns. Particulate trace metals (for Cr, Mn, Ni and Cu) were significantly negatively correlated with salinity. As a result, partition coefficients of trace metals (except Pb) were all significantly negatively correlated with salinity, indicating high salinity facilitated desorption/dissolution of metals from particulate phase. Additionally, the Changjiang derived particulate Pb, Mn, Ni and Cu sharply decreased (particularly for Mn) at the downstream of turbidity maximum zone, suggesting the efficient trapping of metals within this region. We further investigated the site-specific bioaccumulation of trace metals in size-fractionated zooplankton. Metal contents in macro-zooplankton were lower than micro- and meso-zooplankton owing to size-dependent zooplankton communities, while site-specific metal bioaccumulation mainly driven by site-specific zooplankton communities and salinity. The bioaccumulation factors of metals (Cr, Cd, Ni and Cu) were significantly negatively correlated with salinity, indicating high salinity hampered metal uptake which might attribute to competition of cations and formation of less bioavailable inorganic complexes with anions. Overall, high salinity generated two-sided effects (elevated dissolved metal concentrations Vs. reduced metal bioaccumulation) on metal contents in zooplankton (especially for Cr, Cd, Ni, and Cu), resulting in metal- and site-specific metal contents. We noticed relatively higher metal contents in zooplankton at hypoxia sites which could further transfer to predators in the East China Sea, and the underlying mechanisms still require future investigation.

High importance of coupled nitrification-denitrification for nitrogen removal in a large periodically low-oxygen estuary

The coupling between nitrification and denitrification/anammox (nitrate/nitrite used in denitrification/anammox derives from nitrification) is a significant process of reactive nitrogen (N) removal that has attracted much attention. However, the dynamics of coupled nitrification-denitrification/anammox in the periodically low-oxygen estuaries and coasts remain unclear. Here, continuous-flow experiments combined with isotope tracing techniques were conducted in periodically low-oxygen areas of the Yangtze Estuary to reveal the changes in benthic sediment denitrification and anammox as well as their coupling with nitrification. Our results showed that denitrification increased but anammox decreased during low-oxygen summer. The occurrence of low oxygen also promoted coupled nitrification-denitrification but decreased coupled nitrification-anammox. These results implied that decreased dissolved oxygen in summer did not largely restrict nitrification activity, and anaerobic denitrification/anammox regulated the magnitude of coupled nitrification-denitrification/anammox rates. Denitrification (74.95-100 %) was the dominant process in total N removal, while coupled nitrification-denitrification accounted for a higher proportion (45.68-97.05 %) of denitrification, indicating that coupling between nitrification and denitrification played a dominant role in N removal. In addition to dissolved oxygen levels, carbon and N substrate availabilities were also important variables to regulate N transformations. Overall, this study advanced our knowledge of the distribution patterns and controlling factors of N removal processes and highlighted that coupled nitrification-denitrification might have a significant but neglected role in N removal from periodically low-oxygen estuaries.

Ecological dynamics and impacts of viruses in Chinese and global estuaries

Estuaries are important ecosystems providing irreplaceable services for humankind and, in turn, are extensively influenced by human activities and climate changes. Microbial processes, which are largely controlled by viruses, are always responsible for the ecological function and environmental problems in estuaries. However, we know little about the ecology and importance of viruses in estuarine systems. Here, we investigated viral ecological dynamics in estuarine systems on local (four largest estuaries in China in different seasons) and global scales. Viral production varied by almost 20-fold in Chinese estuaries with significant seasonality, being responsible for the removal of 1.41%-21.45% of the bacterioplankton standing stock each day, and contributed directly to the organic carbon pool by releasing an average of 3.57 µg of cellular carbon per liter per day. By compiling data from 21 estuaries across the world, we found for the first time that viral population size peaked at mid-latitude and viral production increased towards the equator in estuarine ecosystems. The results indicated the higher viral impact on microbial mortality and dissolved organic matter cycling in tropical estuaries. Our field investigation and global synthesized analysis provide compelling evidence of spatiotemporal variations in estuarine viral dynamics. The global view of viral impacts on estuarine microbial mortality offers important insight for incorporating viruses into ecological models and understanding the environmental implications of the tropicalization of temperate aquatic ecosystems under a scenario of climate warming.

Distinct Features of Sedimentary Archaeal Communities in Hypoxia and Non-Hypoxia Regions off the Changjiang River Estuary

Water hypoxia (DO < 2 mg/L) is a growing global environmental concern that has the potential to significantly influence not only the aquatic ecosystem but also the benthic sedimentary ecosystem. The Changjiang River Estuary hypoxia, classified as one of the world's largest seasonal hypoxic water basins, has been reported to be expanding rapidly in recent decades. However, the microbial community dynamics and responses to this water hypoxia are still unclear. In this study, we examined the abundance, community composition, and distribution of sedimentary archaea, one important component of microbial communities in the Changjiang River Estuary and the East China Sea (ECS). Our results indicated that Thaumarchaeota and Bathyarchaeota were predominant archaeal groups in these research areas, with their 16S rRNA gene abundance ranged from 8.55 × 106 to 7.51 × 108 and 3.18 × 105 to 1.11 × 108 copies/g, respectively. The sedimentary archaeal community was mainly influenced by DO, together with the concentration of ammonium, nitrate, and sulfide. In addition, distinct differences in the archaeal community's composition, abundance, and driving factors were discovered between samples from hypoxia and non-hypoxia stations. Furtherly, microbial networks suggest various microbes leading the different activities in hypoxic and normoxic environments. Bathyarchaeota and Thermoprofundales were "key stone" archaeal members of the low-DO network, whereas Thaumarchaeota constituted a significant component of the high-DO network. Our results provide a clear picture of the sedimentary archaeal community in coastal hypoxia zones and indicates potential distinctions of archaea in hypoxia and non-hypoxia environments, including ecological niches and metabolic functions. IMPORTANCE In this study, the sedimentary archaeal community composition and abundance were detailed revealed and quantified based on 16S rRNA genes off the Changjiang River Estuary. We found that the community composition was distinct between hypoxia and non-hypoxia regions, while Thaumarchaeota and Bathyarchaeota dominated in non-hypoxia and hypoxia samples, respectively. In hypoxia regions, the sedimentary archaea were mainly affected by salinity, ammonium, and nitrate, whereas total organic carbon, total nitrogen, and sulfide were major influencing factors in non-hypoxia regions. The distinct microbial network may suggest the niche difference of archaeal community under various oxygen level.

Unexpected high indirect impacts of riverine organic matter to coastal deoxygenation

Hypoxia has become a serious threat to the sustainability of marine ecosystems globally. As one of the leading components of eutrophication, river-delivered organic matter (ROM) regulates estuarine/coastal dissolved oxygen condition and contributes to hypoxia formation both directly via remineralization and indirectly via the nutrients released during remineralization. However, the impacts of ROM on the coastal environment have long been underestimated because the indirect contribution has been overlooked and left largely unknown. This study used the Changjiang River, the second-largest ROM source in the world, as a prototype. A coupled physical-biogeochemical model was used to explore the indirect contribution of Changjiang ROM to marginal sea deoxygenation and hypoxia formation. Here I show that ROM remineralization contributed 5.4% directly and 7.3% indirectly to the total oxygen consumption in the off-estuary hypoxic zone. The indirect contribution persistently sustained in a large part of the northwest Pacific marginal seas. Strict mitigation is required to manage the ecosystem impacts of deoxygenation. The findings of this study also indicate that river discharge, freshwater turbidity, and interactions between freshwater and shelf circulation are fundamental factors that regulate the contributions of ROM to hypoxia formation and shelf deoxygenation.

Exploring origin of oxygen-consuming organic matter in a newly developed quasi-hypoxic coastal ocean, the Bohai Sea (China): A stable carbon isotope perspective

The Bohai Sea, adjacent to the Northwest Pacific, is a semi-enclosed shallow-water marginal sea that was considered on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism-induced summertime dissolved oxygen (DO) decline, five field surveys were conducted between July 2019 and July 2021 to investigate the seasonal/interannual and spatial variations in DO, dissolved inorganic carbon (DIC), and stable isotopic composition of DIC (δ¹³C_DIC). Although the water-mixing scheme was subject to spatial variation, a uniform apparent ratio of δ¹³C_DIC versus apparent oxygen utilization was estimated at -0.0122‰ per μmol O₂ kg^-1 in the Bohai Sea in summer. Based on a three-endmember water-mixing model and the mass balance of DIC and its stable isotopic composition, the assumed uniform δ¹³C values of oxygen-consuming organic matter in the Bohai Sea DO-deficient areas was estimated to be -19.47 ± 1.85‰ in 2020 and between -20.6‰ and - 18.1‰ in 2021. This isotopic composition is very similar to the δ¹³C value of organic matter from marine diatoms, but different to that of terrestrial organic matter sources surrounding the Bohai Sea. Our results indicate that nearly all the organic matter consumed by community respiration in the Bohai Sea is produced in situ by marine plankton. To mitigate the seasonal DO shortage in the Bohai Sea, reduction of allochthonous nutrients is crucial.

Performance evaluations and applications of a δ13C-DIC analyzer in seawater and estuarine waters

Dissolved inorganic carbon (DIC) and its stable isotope (δ¹³C-DIC) are important parameters for studying carbon cycling in aquatic environments. Traditional methods based on isotope-ratio mass spectrometers are labor-intensive and not easily deployable at field sites. Here we report the performance of a method that simultaneously measures DIC concentration and its stable isotope by using a CO₂ extraction device and a Cavity Ring-Down Spectroscopy (CRDS) detector. A multi-port valve is used to increase sample throughput and improve precision. The instrument achieves average precisions of better than ±1.95 μmol kg^-1 and ±0.06‰, respectively, for DIC and δ¹³C-DIC in seawater based on three injections for each sample. We also provide recommendations on how to precisely determine δ¹³C-DIC samples with a wide range of DIC content in different types of waters by examining injection volume and concentration effects. This technique was applied to study carbon cycling in the Delaware Estuary. It demonstrates that a simultaneous and precise determination of both DIC and δ¹³C-DIC is a powerful and effective approach for constraining the processes controlling aquatic carbon cycling and CO₂ fluxes. Both laboratory tests and field applications confirmed that this system can be used with high precision to study carbon cycling in various aquatic environments.

Improvement of Water Quality by Light-Emitting Diode Illumination at the Bottom of a Field Experimental Pond

Remediation of water quality by stimulating algal photosynthesis using light-emitting diodes (LEDs) has attracted attention, but few studies have examined this in outdoor freshwater environments. To understand the effects of LED illumination on water quality, the dissolved oxygen (DO), temperature, pH, and electric conductivity were monitored over 5 months in three depressions with or without a red/blue LED light at the bottom of an experimental pond. The effects of the blue LED on water quality were evident in the period with less rainfall after the change of water quality to an equilibrium state; DO and pH were higher, and EC was lower for the blue LED than for the control. The diel changes of these variables were also lower for the blue LED. The effects of the red LED on DO and pH were also evident, but to a lesser extent compared to those of the blue LED. A vertical mixing of water associated with a nighttime cooling of the surface water was suggested by a rapid DO increase after a temperature decrease in the control. Such internal water circulation and an inflow of water after rainfall might have obscured the LED effects in the rainy period. The bottom water of the blue LED had a higher density and species richness of phytoplankton than that of the control at the end of the experiment. A lower density of phytoplankton and higher nutrient concentrations in the red LED might have been due to a higher density and feeding activity by zooplankton. Our results confirmed the applicability of LED illumination in stimulating algal photosynthesis, and in improving the oxygen condition of the bottom water in freshwater ponds.

Phytoplankton assemblage and chlorophyll a along the salinity gradient in a hypoxic eutrophic tropical estuary-Ulhas Estuary, West Coast of India

Phytoplankton assemblage and chlorophyll a along the salinity gradient and oxygen zonation were studied in Ulhas Estuary. Oxic condition exist in euhaline region of the estuary that receives oxygenated coastal waters, while, poly-meso-oligohaline region is hypoxic due to the accumulation of voluminous amount of pollutants from industrial effluents, domestic/sewage and agricultural discharges. Additionally, anthropogenically generated nutrients enhanced phytoplankton biomass (chlorophyll a) and their senescence also led to hypoxia. The dominance of diatoms (>70%) was observed in the euhaline-oxic region, while a tremendous decrease in their contribution (18%) was observed in the oligohaline region. Meso to oligohaline region of the estuary is governed by cyanophytes and chlorophytes due to their affinity towards low salinity and high dissolved inorganic nutrients. Carlson's Trophic State Index (TSI) further substantiated the estuary as eutrophic. Additionally, the study could identify the adaptation of cyanophytes and chlorophytes in oxygen deficient water.

Sedimentary processes dominate nitrous oxide production and emission in the hypoxic zone off the Changjiang River estuary

Coastal oceans, known as the major nitrous oxide (N₂O) source to the atmosphere, are increasingly subject to eutrophication and concurrent near-bottom hypoxia. The natural nitrogen cycle is likely to be altered markedly in hypoxic coastal oceans. However, the processes responsible for N₂O production and emission remain elusive because of lacking field rate measurements simultaneously conducted in the water column and sediment. Here, we quantified N₂O production rates using a ¹⁵N-labeled technique in the water-column and surface sediments off the Changjiang (Yangtze) River estuary, the largest hypoxic zone in the Pacific margins. Our results showed that the estuarine surface sediments were the major source for N₂O production, accounting for approximately 90% of the total water-column accumulation and consequent efflux of N₂O in the hypoxic zone, whereas the water-column nitrification and denitrification combined only contributed <10%. More importantly, the coupling of nitrification and denitrification at the presence of abundant supply and remineralization of labile organic matter was the main driver of the N₂O release from the sediment-water interface in this region. This study highlights the dominant role of benthic processes occurring at the sediment-water interface controlling the coastal N₂O budget, as the anthropogenic eutrophication and hypoxia are expanding in coastal oceans.

Significance of nutrients in oxygen-depleted bottom waters via various origins on the mid-outer shelf of the East China Sea during summer

East China Sea (ECS) is considered one of the largest dissolved oxygen (DO) depleted areas in the world's oceans. To assess the relative importance of water sources and biological processes to modulate low DO water over the ECS shelf, we conducted 7 cruises in the summers between 2004 and 2015. To cover a broad study area, observations were taken by both Chinese and Japanese research vessels in 2013, the consistent DO values were obtained in the intercalibration station from China and Japan. The subsurface/bottom water DO depletion was observed over both the inner and mid-outer shelves. In 2009 and 2013, the low DO (3-4.2 mg L^-1) area covered ca. 4 × 10⁴ km² on the mid-outer shelf, comparable with the reported area of summer hypoxia off the Changjiang estuary. On the basis of a seven endmember mixing model using heavy rare earth elements, temperature and salinity data collected in 2013 and 2015, we determined that on the southern shelf the low DO water mainly originated from Kuroshio Subsurface Water (28-72%). Both the DO level in the dominant source water and organic matter (OM) remineralization modulated the formation and expansion of low DO waters. Oxygen-depleted bottom waters featured with high nutrients were both transported from the water's source regions and produced by OM remineralization on the mid-outer shelf. The estimated regenerated nutrient fluxes derived from OM respiration in the bottom water of the mid-outer shelf were equivalent to 18-37% of the nitrate and nitrite, and 2 to 5-fold the phosphorus, delivered from the Changjiang River in summer. The large quantity of regenerated nutrients from oxygen-depleted bottom waters on the mid-outer shelf could be utilized and support primary production in the adjacent oceans. Our findings provide valuable observation for simulation models of nutrient cycles and budgets in the ECS and adjacent oceans.

A multi-stable isotopic constraint on water column oxygen sinks in the Pearl River Estuary, South China

Bottom water oxygen depletion is a central concern in estuaries and coastal oceans worldwide. However, a mechanistic understanding and quantitative diagnosis of different oxygen-consuming processes are less clear. In this study, a multi-stable isotope approach is developed to delineate the role of oxygen respiration and nitrification contributing to total oxygen consumption in the Pearl River Estuary (PRE), a large eutrophic estuary in south China. The approach highly couples with analysis of the carbon isotope composition of dissolved inorganic carbon (δ¹³C-DIC) and with stable nitrogen isotope analysis in ammonium (δ¹⁵N-NH₄⁺) and nitrate (δ¹⁵N-NO₃^-). In all seasons, relatively low DO concentrations were observed in the upper reach and, to some extent, in the outer estuary during summer, while high concentrations of DO were found in the transition zone between the inner and outer estuary. On the basis of isotopic differentiation, our data reveal that much more depleted δ¹³C-DIC is coincident with DIC additions and low oxygen in the upper reach and inner estuary during most seasons. This is most likely a consequence of organic carbon (OC) degradation via aerobic respiration. Based on the carbon isotopic mass balance of DIC and the stoichiometry ratio of -ΔDO/ΔDIC, we found that the OC degradation dominates the total oxygen consumption in the upper reach, as well as in the inner estuary during summer (48.3%-93.5%). In addition, nitrification is another key process in contributing to total oxygen loss in the upper reach, as supported by the well-coupled variations of δ¹⁵N of NH₄⁺ and NO₃^- and apparent oxygen utilization (AOU). Using the formerly determined N isotopic fractionation and observed δ¹⁵N variation, we estimated that nitrification could account for 35.3%-44.1% and 28.5%-31.6% of the total oxygen consumption in the upper reach during winter and summer, respectively, while its contribution to total oxygen loss is minor in the inner and outer estuary. Overall, this study demonstrates the potential of the multi-stable isotopic approach to assess oxygen sink partitioning in large human-perturbed estuaries.

Nitrogen isotope constraint on the zonation of multiple transformations between dissolved and particulate organic nitrogen in the Changjiang plume

Information on the sources and transformations of particulate organic N (PN) and dissolved organic N (DON) at the coastal interface remains insufficient due to technological difficulties and complicated features of intensive physical mixing and rapid biological activities. Here, we investigated the spatial distribution of concentrations and isotopic compositions of PN and DON in the Changjiang plume during the summer flood period. In average, DON and PN accounted for 25.6 ± 12.1% and 8.1 ± 9.1% (n = 55), respectively, of the total N pool, with the remaining N primarily in the form of nitrate (NO₃^-). Mean δ¹⁵N values were the lowest for DON (-0.1 ± 2.7‰, n = 58) and slightly higher for PN (2.0 ± 1.6‰, n = 101), and the highest for NO₃^- (6.5 ± 2.2‰, n = 67), suggesting multiple transformations had occurred to differentiate isotopic characteristics among the three N pools. By applying a conservative mixing model, we found DON deficits (-3.5 ± 3.7 μmol L^-1, n = 43) and negative shift in δ¹⁵N_DON (-3.6 ± 2.2‰, n = 43) in the Changjiang plume, revealing nonconservative DON behaviors. In the offshore surface plume where Chlorophyll a was high, the most likely cause is the DON uptake by phytoplankton with a strong inverse isotope effect (around -40‰). This DON assimilation by phytoplankton contributed to ~16 ± 12% of the PN production, with the remaining supported by NO₃^- assimilation, producing an overall isotope effect of 4-9‰. However, in waters near the river mouth and at the bottom of the offshore plume where total suspended matter concentrations were high (>5 mg L^-1), the DON deficit was most likely induced by the selective adsorption of ¹⁵N enriched moieties of DON onto particulate surfaces (with an isotope effect of -20‰ to -5‰). Unlike dissolved organic carbon to behave conservatively in most estuaries, our results show that active transformations had occurred between the DON and PN pools in the Changjiang plume.

Annual hypoxia causing long-term seawater acidification: Evidence from low-molecular-weight organic acids in the Changjiang Estuary and its adjacent sea area

In this study, components, concentrations, distribution characteristics, sources of low-molecular-weight organic acids (LMWOAs) and relationships among the annual hypoxia, LMWOAs and seawater acidification were investigated in the Changjiang Estuary and its adjacent sea area in July 2015. Lactic, acetic and formic acids were detected in the seawater samples in the study area, and their total concentrations (ΣLMWOAs) varied from 0 to 262.6 μmol·L^-1, with an average value of 39.2 μmol·L^-1. In the surface seawater, high concentration areas of ΣLMWOAs occurred in the sea area near the Changjiang Estuary and the Hangzhou Bay, and north of study area. In the sampling stations along transect A6, high concentration areas of ΣLMWOAs appeared in the bottom seawater of nearshore stations and middle seawater of offshore stations. The terrigenous inputs, especially the Changjiang runoff, were the dominant sources for LMWOAs in the sampling period. The consistency of hypoxia areas, high concentration areas of ΣLMWOAs and low pH value areas in winter and summer suggested that annual hypoxia could cause the long-term seawater acidification by producing LMWOAs in the Changjiang Estuary and its adjacent sea area.

Geochemical records of Lake Erhai (South-Western China) reveal the anthropogenically-induced intensification of hypolimnetic anoxia in monomictic lakes

In monomictic lakes, hypolimnetic anoxia is becoming severe in extent and duration over the past few decades. Understanding historical trends in hypolimnetic dissolved oxygen (DO) levels and the factors controlling them is crucial for effective protection and management of monomictic lakes everywhere, but the issue remains little studied in China. Here, our study elucidated the variation of hypolimnion DO and organic matter (OM) input in Lake Erhai (a typical monomictic lake in South-Western China) during the past 200 years, by using the geochemical profiles of elements (C, N, P, S, Mo, Ca, and Al) and aliphatic hydrocarbons in a dated sediment core. The values of element proxies (S concentrations, S/Al ratios, Mo enrichment factor, and total organic carbon/total P ratios) and pristane/phytane (Pr/Ph) ratios reflect relatively limited development of anoxia in the lake hypolimnion before 1990. Meanwhile, the n-alkane proxies (short-chain, middle-chain, and long-chain n-alkane abundances, n-C₁₇/n-C₁₆ alkane ratios, and Paq) indicate relatively scant inputs of OM from phytoplankton and relatively high inputs of OM from terrestrial plants or from submerged macrophytes. Taken together the results show that OM supplied in this period did not deteriorate hypolimnion DO in Lake Erhai. The element proxies and Pr/Ph ratios point to that the lake had experienced a pronounced intensification of hypolimnetic anoxia after 1990, and the n-alkane proxies indicate that the lake was susceptible to severe eutrophication and phytoplankton blooms in this period. The synchronous sharp variation implies the decay of massive phytoplankton OM had severely consumed oxygen in the lake hypolimnion. The large surface area/depth ratio in Lake Erhai is conducive for an overturn of the water column during wind disturbance, which allowed the water column stratification and relating effects (e.g., hypolimnetic anoxia) less vulnerable to some aspects of climate change.

Satellite Observed Spatial and Temporal Variabilities of Particulate Organic Carbon in the East China Sea

In this study, we investigate spatial and temporal variabilities of particulate organic carbon (POC) in the East China Sea (ECS) and explore the dominant influencing factors for its three subregions—coastal, continental shelf and open sea—by analyzing satellite derived POC from 2001 to 2011 after calibration against in situ samplings. The analysis reveals that the largest POC occurs in spring at the coastal region with a value of 297.5 mg m−3, and the smallest in summer at the open sea region with a value of 108.9 mg m−3. POC in the coastal region is dominated by biological activity related to river discharge of nutrients and organic matter, and is regulated by sediment discharge variation due to the Three Gorge Project and chemical fertilizer application along the Changjiang River watershed; the open sea region is dominated by water exchange with the Kuroshio current, which results in the highest sea surface temperature and the lowest chlorophyll and POC among the three subregions; POC in the continental shelf region is driven by the seasonal competition between the influence of the coastal ocean and the Kuroshio current through biological activity and water exchange. A method to estimate the organic matter reserve in the ECS was developed based on satellite data and in situ samplings. Organic carbon storage was estimated at 4.08 × 1013 g, which could be used as a baseline for future estimates. An improved estimation could be found with additional in situ data and with the use of a more sophisticated algorithm for satellite POC analysis.

Hypoxia formation in the East China Sea by decomposed organic matter in the Kuroshio Subsurface Water

Although internal decomposition of organic matter (OM) in the Kuroshio Subsurface Water (KSSW) is a crucial factor for hypoxia formation in the East China Sea (ECS), the seasonal source and contributions of this OM remain debated. This study applied datasets collected in June and October 2015 to discuss these issues qualitatively and quantitatively. According to the variations in several parameters along the KSSW route, N₂ fixation signals related to decomposed OM were apparent in the southern ECS during June, while terrestrial input signals were revealed in the northern ECS during June and most of the ECS during October. The terrestrial input contributed 47% of the decomposed OM near the historic hypoxic area in June, indicating that the terrestrial and marine sources contributed almost equally to the development of ECS hypoxia. These results provide vital information for understanding the mechanism of hypoxia formation driven by eutrophication and oceanic circulation.

Response mechanism of microbial community to seasonal hypoxia in marine ranching

Seasonal hypoxia, as an increasingly recognized environmental issue, frequently occurred in marine ranching from northern Yellow Sea, China. Although microorganisms play an important ecological role in marine ecosystems, but little is known on the response mechanism of microbial community to seasonal hypoxia in marine ranching. A total of 132 seawater samples and 47 sediment samples were collected from the marine ranching, both in the death disaster zone of sea cucumbers and the non-disaster zone, and in different months. 16S rRNA gene high-throughput sequencing was used to explore the microbial community and its influencing factors. The results showed that the stratification in community composition and dissolved oxygen content appeared in August. The Alpha diversity in seawater was higher in summer than in winter, and significant differences in Beta diversity appeared between the death disaster zone of sea cucumbers and the non-disaster zone in sediments. In addition, environmental effects explained more of the variation in bacterial community composition in seawater as compared with spatial effects did, whereas, sedimentary bacterial communities were more closely related to spatial effects. The present results could provide fundamental data for understanding the response mechanism of the microbial community to seasonal hypoxia in marine ranching and are of great significance for the management and protection of marine ranching.

Wind induced algal migration manipulates sediment denitrification N-loss patterns in shallow Taihu Lake, China

Driven by winds, the distribution of algae is often noticeably patchy at kilometer scales in shallow lakes. The decomposition of the settled algal biomass may affect nitrogen (N) biogeochemical cycles and thereby N loss in sediments. In this study, we investigated sediment denitrification N-loss patterns along algal migration pathway in Taihu Lake, a shallow and eutrophic lake in China, and found that wind-induced algal migration in the overlying water manipulated the temporal and spatial patterns of denitrification N-loss in sediments. A N loss hotspot in sediments was created in the algae concentrated zone, where N loss was, however, temporarily inhibited during algal bloom seasons and generally exhibited a negative relationship with algal biomass. In the zone where algae have left, sediment N loss rate was relatively low and positively correlated with algal biomass. The decay of algal biomass generated organic carbon and created anoxia, favoring denitrification, while excessive algal biomass could deplete oxygen and inhibit nitrification, causing nitrate limitation for denitrification. Piecewise linear regression analysis indicated that algal biomass of Chl-a > 73.0 μg/L in the overlying water could inhibit denitrification N-loss in sediments. This study adds to our understanding of N biogeochemical cycles in shallow eutrophic lakes.

Heterogeneity of the sediment oxygen demand and its contribution to the hypoxia off the Changjiang estuary and its adjacent waters

The severe hypoxia off the Changjiang estuary (CE) has a dual-core structure, and the two hypoxic zones exhibit behavioural, physical and biochemical differences. Currently, few studies have revealed straightforward differences regarding the key biochemical processes between these two hypoxic zones. In this study, the phytoplankton sinking rate (PSR) and sediment oxygen demand (SOD) were measured by field experiments and compared between the two hypoxic regions. PSR and SOD ranged from 0.75-3.34 m day^-1 and 5.67-16.19 mmol m^-2 day^-1, respectively. Interestingly, PSR and SOD were higher in the southern region than in the northern region, implying stronger pelagic-benthic biogeochemical coupling in the southern region. SOD accounted for approximately 44% and 51% of DO net consumption in the northern and southern regions, respectively, from July to August. The southern hypoxic region appeared to exhibit intense DO consumption and fast DO supplementation, while the northern hypoxic region seemed to exhibit slow DO consumption and supplementation.

Low diffusive nitrogen loss of urban inland waters with high nitrogen loading

Little is known about the exchange of gaseous nitrogen (N2) with the atmosphere from urban inland waters, which are characterized by low carbon-to‑nitrogen ratios and low nitrogen-to‑phosphorus ratios. Here, we studied diffusive nitrogen loss based on the measurement of dissolved N2 concentrations and related gene abundance of N2 production and fixation in rivers and lakes in the megacity of Beijing, China, between 2018 and 2020. The excess dissolved N2 (△N2) ranged from -51.2 to 56.8 μmol L-1 (average - 0.03 ± 13.8 μmol L-1), and approximately 43% of the river samples and 72% of the lake samples being undersaturated with N2, suggesting that the lakes mainly acted as a role of N2 sink. The N2 removal fraction (△N2/DIN, average 3.5 ± 4.3%) at the sites of rivers with positive △N2 was lower than that in other rivers around the world. The average N2 flux (0.8 ± 23.9 mmol m-2 d-1) in the urban rivers was also lower than that in other rivers. The low carbon-to‑nitrogen ratios in Beijing inland waters are not beneficial for N2 production during denitrification, and low nitrogen-to‑phosphorus ratios potentially favor N2 fixation with a high abundance of the nitrogenase nifH gene in the sediment, resulting in low net N2 production. The traditional paradigm is that rivers constantly lose vast N to the atmosphere via denitrification and anammox, but this study indicates that urban inland rivers emit negligible N even under high nitrogen loading.

Physiological and behavioral responses of the copepod Temora turbinata to hypoxia

Hypoxia is intensified in tropical and subtropical estuarine and coastal waters and brings about lethal and sublethal effects to marine copepods. The physiological and behavioral responses of the subtropical calanoid copepod Temora turbinata were tested after short-term exposure to hypoxia. The LD₅₀ values were 3.02 ± 0.21, 2.00 ± 0.35, and 3.11 ± 0.31 mg L^-1 for nauplii (II-III), copepodites (II-III), and female adults, respectively. With a decrease in the ambient dissolved oxygen (DO) level from 8 to 0.5 mg L^-1, the ingestion rates decreased significantly at all life stages, as did oxygen consumption in female adults. In an artificial stratification column with a DO gradient, female adults exhibited an obvious avoidance response to the hypoxic bottom layer. Our study provides preliminary evidence for high hypoxia sensitivity in T. turbinata and implies that the DO level may be the main factor controlling the distribution of this species in tropical and subtropical coastal and estuarine waters.

Body size affects lethal and sublethal responses to organic enrichment: Evidence of associational susceptibility for an infaunal bivalve

Eutrophication is an increasing problem worldwide and can disrupt ecosystem processes in which macrobenthic bioturbators play an essential role. This study explores how intraspecific variation in body size affects the survival, mobility and impact on sediment organic matter breakdown in enriched sediments of an infaunal bivalve. A mesocosm experiment was conducted in which monocultures and all size combinations of three body sizes (small, medium and large) of the Sydney cockle, Anadara trapezia, were exposed to natural or organically enriched sediments. Results demonstrate that larger body sizes have higher tolerance to enriched conditions and can reduce survival of smaller cockles when grown together. Also, large A. trapezia influenced sediment organic matter breakdown although a direct link to bioturbation activity was not clear. Overall, this study found that intraspecific variation in body size influences survival and performance of bioturbators in eutrophic scenarios.

Distinctive Patterns and Controls of Nitrous Oxide Concentrations and Fluxes from Urban Inland Waters

Inland waters are significant sources of nitrous oxide (N₂O), a powerful greenhouse gas. However, considerable uncertainty exists in the estimates of N₂O efflux from global inland waters due to a lack of direct measurements in urban inland waters, which are generally characterized by high carbon and nitrogen concentrations and low carbon-to-nitrogen ratios. Herein, we present direct measurements of N₂O concentrations and fluxes in lakes and rivers of Beijing, China, during 2018-2020. N₂O concentrations and fluxes in the waters of Beijing exceeded previous estimates of global rivers due to the high carbon and nutrient concentrations and high aquatic productivity. In contrast, the N₂O emission factor (N₂O-N/DIN, median 0.0005) was lower than global medians and the N₂O yield (ΔN₂O/(ΔN₂O + ΔN₂), average 1.6%) was higher than those typically observed in rivers and streams. The positive relationship between N₂O emissions and denitrifying bacteria as well as the Michaelis-Menten relationship between N₂O emissions and NO₃^--N concentrations suggested that bacteria control the net production of N₂O in waters of Beijing with N saturation, leading to a low N₂O emission factor. However, low carbon-to-nitrogen ratios are beneficial for N₂O accumulation during denitrification, resulting in high N₂O yields. This study demonstrates the significant N₂O emissions and their distinctive patterns and controls in urban inland waters and suggests that N₂O emission estimates based on nitrogen loads and simple emission factor values are not appropriate for urban inland water systems.

Investigating the link between Pearl River-induced eutrophication and hypoxia in Hong Kong shallow coastal waters

We present geochemical analysis of 75 surface water samples collected in 2016 in Hong Kong coastal waters. We found that nitrogen distribution around Hong Kong can be characterized by two regimes driven by the influence of the Pearl River: 1) a regime where nitrate is the dominant species of nitrogen, associated with lower salinity and more faecal coliform and 2) a regime where dissolved organic nitrogen is dominant, associated with higher salinity and fewer faecal coliform. While the impact of the Pearl River on Hong Kong coastal waters is well characterized, we used the sharp contrast between the nitrogen regimes to produce new evidence about the role of the Pearl River on the generation of local hypoxia in Hong Kong. The impact of nitrate originating from the Pearl River on the generation of hypoxia in Hong Kong might be less important than previously thought, as no sign of eutrophication was found within the zones dominated by dissolved organic nitrogen and an historical decoupling of surface processes and bottom water oxygenation was observed. Moreover, we measured elevated ammonium levels and rapid cycling of ammonium and dissolved organic nitrogen in Victoria Harbour suggesting local sources, such as wastewater, might be rapidly oxidized and thus play an important role in the consumption of oxygen locally. A first-order calculation highlighted the potential for wastewater to drive the observed seasonal decline in oxygen. Taken together, these evidences suggest that eutrophication might not be the primary driver in the generation of seasonal hypoxia and that oxidation of ammonium released locally might play a bigger role than initially thought.

Impact of upwelling on phytoplankton blooms and hypoxia along the Chinese coast in the East China Sea

This study evaluates the rarely observed phenomenon of the simultaneous occurrences of phytoplankton blooms, hypoxia, and upwelling along the Zhejiang coast in the East China Sea. Results show that the upwelling uplifted bottom water to 5-10 m below the surface. In the upwelling region, phytoplankton blooms (Chl a = 10.9 μg L^-1) occurred and hypoxia or low-oxygen appeared below the surface water. High concentrations of nitrate and phosphate were regenerated in the hypoxic regions, corresponding with mean values (± SD) of 16.9 (± 1.5) and 0.90 (± 0.14) μM, respectively. The upwelling expanded the region of hypoxic water, which nearly reached the surface, thereby increasing the threat to marine life. In addition to fluvial nutrients, the upwelling of water with high nutrient levels, especially phosphates, can enhance phytoplankton blooms. The results suggest that hypoxia can become more severe due to further decomposition of bloom-derived organic matter after blooms crash.

Contribution of the offshore detached Changjiang (Yangtze River) Diluted Water to the formation of hypoxia in summer

The Changjiang (Yangtze River) Diluted Water (CDW) plume substantially impacts the biogeochemical processes off the estuary and its adjacent area, resulting in considerable environmental and ecological effects. Based on survey data in the northeastern area off the Changjiang Estuary (CE) obtained in the summers of 2008 and 2013, the hypoxia induced by the offshore detached CDW plume and the associated controlling mechanisms were investigated. The results show that the offshore transport of the CDW plume caused a dispersed low-salinity area in the northeastern area off the CE during summer, in sharp contrast with the surrounding high-salinity and high-density waters. There was a hypoxic area with low-pH (i.e., acidification) near the 40-m isobath in bottom waters in the northeastern area off the CE, and its position generally corresponded to the surface offshore CDW plume. In the area affected by the offshore low-salinity water, the surface patch-like phytoplankton bloom and the organic debris produced in situ were the material drivers of the bottom oxygen consumption and led to the corresponding relationship between the bottom hypoxic zone and the high chlorophyll-a (Chl-a) area at the surface. We consider that the local stratification caused by the offshore low-salinity water and the stable environment within the detached CDW plume constituted the external dynamic conditions for maintaining the bottom hypoxia. Our results demonstrate that the offshore detached CDW plume in the northeastern area off the CE may contribute to the formation of a local hypoxic center with low pH. This study would provide basis for understanding of the physical-biogeochemical processes and environmental responses in the offshore areas of the CDW plume.

Intense methane ebullition from urban inland waters and its significant contribution to greenhouse gas emissions

The evasions of methane (CH₄) and carbon dioxide (CO₂) from inland waters represent substantial fluxes of greenhouse gases into the atmosphere, offsetting a large part of the continental carbon sink. However, the CH₄ and CO₂ emissions from urban inland waters are less constrained. In particular, ebullitive CH₄ emissions from these waters are poorly understood. Here, we measured the concentrations and fluxes of CH₄ and CO₂ in rivers and lakes in the megacity of Beijing, China, between 2018 and 2019. The CH₄ concentration ranged from 0.08 to 70.2 µmol L^-1 with an average of 2.5 ± 5.9 µmol L^-1. The average CH₄ ebullition was 11.3 ± 30.4 mmol m^-2 d^-1 and was approximately 6 times higher than the global average. The average total CH₄ flux (14.2 ± 35.1 mmol m^-2 d^-1) was 3 times higher than the global average, with ebullition accounting for 80% of the flux. The high surface water CH₄ concentrations and ebullitive fluxes were caused by high sediment organic carbon/dissolved organic carbon contents, high aquatic primary productivity and shallow water depths in the urban inland waters. The CH₄ emissions accounted for 20% of CO₂ emissions in terms of the carbon release and were 1.7 times higher in terms of CO₂ equivalent emissions from Beijing inland waters. Furthermore, the CH₄ ebullition and its contribution to the total carbon gas emissions increased exponentially with the water temperature, suggesting a positive feedback probably occurs between the greenhouse gas emissions from urban inland waters and climate warming. This study confirms the major role of CH₄ ebullition from urban inland waters in the global carbon budget under the rapid progress of global urbanization.