Spatiotemporal variations in the summer hypoxia in the Bohai Sea (China) and controlling mechanisms

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.

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.

Latitudinal pronounced variations in tintinnid (Ciliophora) community at surface waters from the South China Sea to the Yellow Sea: Oceanic-to-neritic species shift, biotic-abiotic interaction and future prediction

The oceanic-to-neritic species shift of microzooplanktonic tintinnids and their interaction with relevant abiotic variables are two crucial processes in the marine ecosystem. However, these processes remain poorly documented in China's marginal seas. In the summer of 2022, we investigated the community structure of pelagic tintinnids in surface waters from the South China Sea (SCS) to the Yellow Sea (YS), passing through the East China Sea (ECS). A number of 58 species from 23 genera were identified, with 36 and 22 species belonging to oceanic and neritic genera, respectively. The abundance proportion of oceanic and neritic genera exhibited a decreasing and increasing trend, respectively, from the SCS to YS. Furthermore, four distinctive tintinnid community groups were classified based on cluster analysis using tintinnid species and abundance data, and the position of southern Taiwan Strait was identified as the "Shift Point" for oceanic-to-neritic species dominance. The top two tintinnid species in each group showed distinct variations in body size. Additionally, multivariate biotic-abiotic statistical analyses revealed that temperature determined tintinnid species richness, while temperature, salinity, Si(OH)4, and Chl a determined tintinnid abundance. Our study provides a substantial foundation for recognizing the oceanic-to-neritic species shift of tintinnids in the China's marginal seas, and highlights the role of biotic-abiotic factors in driving biogeochemical fluxes and the potential response of microzooplankton to future climate change.

A simulation-optimization approach based on the compound eutrophication index to identify multi-nutrient allocated load

Eutrophication with nutrient enrichment is a global marine ecosystem concern that threatens human health, economic activities, and ecosystem functions. Therefore, a nutrient load optimization method is required to help control marine eutrophication. However, eutrophication-based nutrient allocated load optimization is a multi-objective project due to a series of eutrophication pressures, such as cross-regional land-based nutrient loads and multi-nutrient regimes and ratios. In this study, a synergistic multi-nutrient control method was developed for the Bohai Sea (BS), China, which links multi-nutrient pressures with eutrophication states. Based on the eutrophication control standard, which is the second level of compound eutrophication index (CEI), the total maximum allocated loads (CEI-based TMALs) of total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and the chemical oxygen demand (COD) were calculated by a simulation-optimization approach. Using the end year of China's 13th Five-Year Plan (2020) as the reference year, 154 high load pressure jurisdictions (HLPJs) that contribute to eutrophication response segments in the BS were identified. Accordingly, practiced the optimized annual reduction rates of TDN, TDP, and COD in the HLPJs at 15 %, 11 %, and 2 % according to CEIII, respectively, the proportion of eutrophicated areas gradually decreased from 32 % in 2020 to 15 % in 2025 and might be 0 % in 2035 with ecosystem resilience in 2035. In particular, under the annual reduction rates of TDN and TDP optimized based on CEIII, the DIN/DIP molar ratio in the BS decreased to 16:1 by 2035. The simulation-optimization approach associated with the CEI-based TMALs for multi-nutrient control in this study might make implementing land-sea coordination more efficiency and marine nutrient regime stably. This can provide scientific and technological support for improving the health of coastal ecosystems.

Molecular insights into nitrogen constraint for niche partitioning and physiological adaptation of coastal Synechococcus assemblages

Coastal nitrogen input has substantially increased due to human activity. However, much remains unknown about the nitrogen-driven patterns and the underlying genetic basis of coastal picoplankton. To investigate the response and mechanisms of picoplankton induced by nitrogen variation, we conducted in-situ investigations using high-throughput sequencing in the Bohai Sea and performed laboratory nitrogen simulation experiments accompanied by physiological, genomic, and transcriptomic analyses, with Synechococcus as a representative. The results of in-situ investigation revealed that Synechococcus clades I, III, WPC1, and VI of subcluster 5.1 (S5.1) are prevalent in strait areas characterized by robust water exchange with the North Yellow Sea, while clades II, VIII, and IX of S5.1, as well as subcluster 5.2 (S5.2) and subcluster 5.3 (S5.3) are more abundant in central and bay areas experiencing elevated nitrate and nitrite loads. The laboratory experiments further confirmed that inorganic nitrogen is a crucial determinant of diversity and niche partitioning of Synechococcus lineages. Besides, the raising inorganic nitrogen concentration within the current in-situ range (0.1-10 μmol L-1) enhances photosynthesis and carbon fixation of Synechococcus, however further escalation of inorganic nitrogen (100 μmol L-1) may hinder these processes instead. The phenomenon could be associated with the differential expression of genes in metabolic pathways regulating nitrogen metabolism, photosynthetic system II, and photosynthesis-antenna proteins in response to nitrogen concentration and type variation. These findings expand our understanding of the impact of macronutrient variation resulting from human activities on marine picoplankton and biogeochemical cycles.

Living in a hypoxic world: A review of the impacts of hypoxia on aquaculture

Hypoxia is a harmful result of anthropogenic climate change. With the expansion of global low-oxygen zones (LOZs), many organisms have faced unprecedented challenges affecting their survival and reproduction. Extensive research has indicated that oxygen limitation has drastic effects on aquatic animals, including on their development, morphology, behavior, reproduction, and physiological metabolism. In this review, the global distribution and formation of LOZs were analyzed, and the impacts of hypoxia on aquatic animals and the molecular responses of aquatic animals to hypoxia were then summarized. The commonalities and specificities of the response to hypoxia in aquatic animals in different LOZs were discussed lastly. In general, this review will deepen the knowledge of the impacts of hypoxia on aquaculture and provide more information and research directions for the development of fishery resource protection strategies.

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.

Shifts in the spatiotemporal distribution and sources of nitrous oxide in sediment cores from the Bohai Sea and South Yellow Sea

N₂O is among the most potent greenhouse gases. In this study, we investigated one of the important N₂O production hotspots, the continental margins. We looked at N₂O spatiotemporal distributions in situ as well as the potential contributions of nitrification and denitrification to N₂O production in sediment cores from the Bohai and South Yellow Seas. Real-time PCR and shotgun metagenomics sequencing were used to analyze the microbial communities related to N₂O production. The results showed that N₂O concentrations roughly decreased with depth-a trend that was consistent throughout the year and showed no significant seasonal variations. When all the research stations along the continental margin were considered, the estuary exhibited the lowest average N₂O concentration. Moreover, nitrification was identified as the main process responsible for N₂O production in estuary areas. This study demonstrates that spatial, as opposed to temporal, heterogeneity is the primary factor influencing N₂O concentration differences in sediments.

Pathogenic bacteria significantly increased under oxygen depletion in coastal waters: A continuous observation in the central Bohai Sea

The spread of pathogenic bacteria in coastal waters endangers the health of the local people and jeopardizes the safety of the marine environment. However, their dynamics during seasonal hypoxia in the Bohai Sea (BHS) have not been studied. Here, pathogenic bacteria were detected from the 16S rRNA gene sequencing database and were used to explore their dynamics and driving factors with the progressively deoxygenating in the BHS. Our results showed that pathogenic bacteria were detected in all samples, accounting for 0.13 to 24.65% of the total number of prokaryotic sequences in each sample. Pathogenic Proteobacteria was dominated in all samples, followed by Firmicutes, Actinobacteria, Tenericutes, and Bacteroidetes, etc. β-diversity analysis showed that pathogenic bacteria are highly temporally heterogeneous and regulated by environmental factors. According to RDA analysis, these variations may be influenced by salinity, ammonia, DO, phosphate, silicate, and Chl a. Additionally, pathogenic bacteria in surface water and hypoxia zone were found to be significantly separated in August. The vertical distribution of pathogenic bacterial communities is influenced by several variables, including DO and nutrition. It is noteworthy that the hypoxia zones increase the abundance of certain pathogenic genera, especially Vibrio and Arcobacter, and the stability of the pathogenic bacterial community increased from May to August. These phenomena indicate that the central Bohai Sea is threatened by an increasingly serious pathogenic community from May to August. And the developing hypoxia zone in the future may intensify this phenomenon and pose a more serious threat to human health. This study provides new insight into the changes of pathogenic bacteria in aquatic ecosystems and may help to make effective policies to control the spread of pathogenic bacteria.

Phosphorus distribution in the water and sediment of Laizhou Bay and sediment phosphorus release potential

Phosphorus is an integral component of marine biogeochemistry. This research investigated the environmental behavior of P in Laizhou Bay using high-resolution sampling, P fractionation, and isotherm adsorption. The total dissolved P (TDP) concentration ranged from 8.4 to 61.0 μg/L in the bay water, while total P (TP) concentration ranged from 311.6 to 654.5 mg/kg in the sediment. The TDP concentration in the water was high in the estuarine area of the Yellow River and the southwestern bay under the combined effects of riverine inputs, direct wastewater discharge, and limited water exchange ability. High TP concentrations in the sediment were observed near the mouth of the Yellow River and central bay, mainly due to the movement and settlement of fine suspended particles under the influence of ocean currents. The P in the bay sediment was predominantly in the calcium-bound fraction and was associated with small particles such as silt and clay. The equilibrium P concentration (EPC₀) ranged from 1.6 to 131.4 μg/L, and P partition coefficient or buffer intensity (K_d) ranged from 104 L/kg to 880 L/kg. The EPC₀ decreased from the northeastern to southwestern area, while K_d showed an inverse distribution; therefore, the southwestern bay sediment had high buffer intensity for external P loads. Additionally, ECP₀ increased linearly, and K_d decreased with exchangeable P (Exc-P) and Fe-bound P (Fe-P) concentrations in the sediment, demonstrating that P sediment-water exchange in LZB was dominated by contributions from Exc-P and Fe-P. These results can aid the understanding of the P sources and geochemistry of coastal ecosystems, particularly sediment P release potential.

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.

Understanding the Variation of Bacteria in Response to Summertime Oxygen Depletion in Water Column of Bohai Sea

Aiming to reveal the variation in bacteria community under oxygen depletion formed every summer in water column of central Bohai Sea, a time-scenario sampling from June to August in 2018 at a 20-day interval along one inshore-offshore transect was settled. Water samples were collected at the surface, middle, and bottom layer and then analyzed by high-throughput sequencing targeting both 16S rRNA and nosZ genes. Compared to the surface and middle water, oxygen depletion occurred at bottom layer in August. In top two layers, Cyanobacteria dominated the bacterial community, whereas heterotrophic bacteria became dominant in bottom water of Bohai Sea. Based on the time scenario, distinct community separation was observed before (June and July) and after (August) oxygen depletion (p = 0.003). Vertically, strict stratification of nosZ gene was stably formed along 3 sampling layers. As a response to oxygen depletion, the diversity indices of both total bacteria (16S rRNA) and nosZ gene-encoded denitrification bacteria all increased, which indicated the intense potential of nitrogen lose when oxygen depleted. Dissolved oxygen (DO) was the key impacting factor on the community composition of total bacteria in June, whereas nutrients together with DO play the important roles in August for both total and denitrifying bacteria. The biotic impact was revealed further by strong correlations which showed between Cyanobacteria and heterotrophic bacteria in June from co-occurrence network analysis, which became weak in August when DO was depleted. This study discovered the variation in bacteria community in oxygen-depleted water with further effort to understand the potential role of denitrifying bacteria under oxygen depletion in Bohai Sea for the first time, which provided insights into the microbial response to the world-wide expanding oxygen depletion and their contributions in the ocean nitrogen cycling.

Role of hydrodynamics in shaping chemical habitats and modulating the responses of coastal benthic systems to ocean global change

Marine coastal zones are highly productive, and dominated by engineer species (e.g. macrophytes, molluscs, corals) that modify the chemistry of their surrounding seawater via their metabolism, causing substantial fluctuations in oxygen, dissolved inorganic carbon, pH, and nutrients. The magnitude of these biologically driven chemical fluctuations is regulated by hydrodynamics, can exceed values predicted for the future open ocean, and creates chemical patchiness in subtidal areas at various spatial (µm to meters) and temporal (minutes to months) scales. Although the role of hydrodynamics is well explored for planktonic communities, its influence as a crucial driver of benthic organism and community functioning is poorly addressed, particularly in the context of ocean global change. Hydrodynamics can directly modulate organismal physiological activity or indirectly influence an organism's performance by modifying its habitat. This review addresses recent developments in (i) the influence of hydrodynamics on the biological activity of engineer species, (ii) the description of chemical habitats resulting from the interaction between hydrodynamics and biological activity, (iii) the role of these chemical habitat as refugia against ocean acidification and deoxygenation, and (iv) how species living in such chemical habitats may respond to ocean global change. Recommendations are provided to integrate the effect of hydrodynamics and environmental fluctuations in future research, to better predict the responses of coastal benthic ecosystems to ongoing ocean global change.

Nutrient budgets for the Bohai Sea: Implication for ratio imbalance of nitrogen to phosphorus input under intense human activities

Eutrophication is a global problem for coastal ecosystems, one that the Bohai Sea (BHS), China, is severely afflicted by due to rapid economic and social development over the last forty years. For sustainable nutrients management in the BHS, comprehensive budgets for Nitrogen (N) and Phosphorus (P) was characterized in 2017, and the relative contributions of river input, submarine fresh groundwater discharge, atmospheric deposition, sediment diffusion, and exchange with the Yellow Sea were quantified. The annual N and P fluxes into the BHS were 362 × 10³ t and 10.4 × 10³ t, respectively. The terrigenous N inputs occupied the highest proportion, while the largest P input was from sediment diffusion. The ratio of N:P was 77 for total external inputs, while that of the Yellow River was 680; both exceeded the Redfield ratio, indicating an imbalance in the nutrient structure and a P limitation in the BHS.

Regional distribution and environmental regulation mechanism of nitrous oxide in the Bohai Sea and North Yellow Sea: A preliminary study

Nitrous oxide is one of the most powerful greenhouse gases and can destroy the ozone layer through photochemical reactions. In 2019, we conducted three cruises to study the spatial and temporal variability of N₂O distribution and emissions in the Bohai Sea (BS) and North Yellow Sea (NYS), and analyzed the regional sources and sinks. The maximum average N₂O concentrations were observed in the summer, followed by autumn, while the minimum was observed in the spring. The N₂O concentration decreased in a gradient from the estuary to the continental shelf, particularly in summer, which indicated that the riverine input from the estuary was a strong source of N₂O in the Bohai Sea. Due to the vertical mixing of the water column, the vertical distribution of N₂O was moderate in autumn, while the bottom remained a hotspot for N₂O emissions in spring and summer. The generalized additive model (GAM) showed that the temperature, salinity, DO and pH were strong predictors of N₂O in the BS and NYS. Excess N₂O concentrations were positively linearly correlated with the apparent oxygen utilization and NO₃^- concentrations, which suggested that nitrification was the dominant process of in situ N₂O production in the BS and NYS. The mixing of water masses, especially DW (diluted water) and BCW (Bohai Sea coastal water), provided a significant amount of N₂O to the entire shelf area of the BS. In addition, the coastal input was a dominate pusher of N₂O emissions in the estuarine region. Overall, the annual N₂O emissions from BS and NYS were approximately 1.72 × 10^-2 Tg yr^-1, which accounted for 0.51% of the annual global marine N₂O emissions, but only 0.04% of the total area of the world's oceans. Hence, both the BS and NYS acted as N₂O sources to the atmosphere.

Methane emission via sediment and water interface in the Bohai Sea, China

Sediment is recognized as the largest reservoir and source of methane (CH₄) in the ocean, especially in the shallow coastal areas. To date, few data of CH₄ concentration in sediment have been reported in the China shelf seas. In this study, we measured CH₄ concentration in sediment and overlying seawater columns, and conducted an incubation experiment in the Bohai Sea in May 2017. CH₄ concentration was found to be ranged from 3.075 to 1.795 μmol/L in sediment, which was 2 to 3 orders of magnitude higher than that in overlying seawater columns. The surface sediment was an important source of CH₄, while bottom seawater acted as its sink. Furthermore, the net emission rate via sediment water interface (SWI) was calculated as 2.45 μmol/(m²∙day) based on the incubation experiment at station 73, and the earthquake may enhance CH₄ release from sediment to seawater column in the eastern Bohai Sea.

Seasonal evolution and controlling factors of bottom oxygen depletion in the Bohai Sea

A coupled physical-biogeochemical model is used to investigate the seasonal evolution and controlling factors of oxygen depletion in the Bohai Sea (BS). Comparisons show that the model reproduces observed spatiotemporal variations of important physical and biogeochemical variables well. Bottom oxygen in the BS shows an annual cycle with significant drawdown in summer and enhanced replenishment in fall. Two oxygen-depleted regions off Qinhuangdao (QHD) and the Yellow River estuary (YRE) develop separately and experience higher oxygen depletion rates and longer durations of low-oxygen conditions. The evolution of oxygen depletion is primarily controlled by stratification and biological oxygen consumption but is also modulated by lateral transport. Strong stratification is established earlier than oxygen depletion and maintains its development. The biological oxygen consumption determines the two oxygen-depleted regions under stratified conditions. Lateral transport influenced by anticyclonic circulations favors an expansion of oxygen depletion off QHD but alleviates oxygen depletion off the YRE.

Dynamics of bacterial communities during a seasonal hypoxia at the Bohai Sea: Coupling and response between abundant and rare populations

Marine bacterial community plays a vital role in the formation of the hypoxia zone in coastal oceans. Yet, their dynamics in the seasonal hypoxia zone of the Bohai Sea (BHS) are barely studied. Here, the 16S rRNA gene-based high-throughput sequencing was used to explore the dynamics of their diversity, structure, and function as well as driving factors during the gradual deoxygenation process in the BHS. Our results evinced that the bacterial community was dominated by Proteobacteria, followed by Bacteroidetes, Firmicutes, Actinobacteria, and Cyanobacteria, etc. The abundant subcommunity dominated in the number of sequences (49%) while the rare subcommunity dominated in the number of species (99.61%). Although abundant subcommunity accounted for most sequences, rare subcommunity possessed higher diversity, richness and their population dramatically changed (higher turnover) during the hypoxia transition. Further, co-occurrence network analysis proved the vital role of rare subcommunity in the process of community assembly. Additionally, beta diversity partition revealed that both subcommunities possessed a higher turnover component than nestedness and/or richness component, implying species replacement could explain a considerable percentage of community variation. This variation might be governed by both environmental selection and stochastic processes, and further, it influenced the nitrogen cycle (PICRUSt-based prediction) of the hypoxia zone. Overall, this study provides insight into the spatial-temporal heterogeneity of bacterial and their vital role in biogeochemical cycles in the hypoxia zone of the BHS. These findings will extend our horizons about the stabilization mechanism, feedback regulation, and interactive model inside the bacterial community under oxygen-depleted ecosystems.

Summer deoxygenation in a bay scallop (Argopecten irradians) farming area: The decisive role of water temperature, stratification and beyond

During 2015-2020, 26 cruises were carried out in a bay scallop farming area, North Yellow Sea, to study the dissolved oxygen (DO) dynamics and its controlling factors. Significant DO depletion (deoxygenation) was observed in the summertime with the decrease rates of 0.31-0.55 and 0.96-2.10 μmol d^-1 in the surface and bottom waters, respectively, which were comprehensively forced by temperature, photosynthesis and microbial respiration. Seasonally, temperature was the main driver of the deoxygenation processes. In the surface water, DO dynamics were dominated by temperature-induced solubility changes, while the photosynthesis offset the effects of physical processes to a certain extent; in the bottom water, its dynamics were mainly attributed to the comprehensive control of temperature-induced solubility changes and biological respiration. Overall, the results suggested that the occurrence of hypoxia and acidification in the coastal waters were highly associated with the formation of temperature-induced stratification under complex hydrodynamic processes.

Globally consistent assessment of coastal eutrophication

Eutrophication is an emerging global issue associated with increasing anthropogenic nutrient loading. The impacts and extent of eutrophication are often limited to regions with dedicated monitoring programmes. Here we introduce the first global and Google Earth Engine-based interactive assessment tool of coastal eutrophication potential (CEP). The tool evaluates trends in satellite-derived chlorophyll-a (CHL) to devise a global map of CEP. Our analyses suggest that, globally, coastal waters (depth ≤200 m) covering ∼1.15 million km² are eutrophic potential. Also, waters associated with CHL increasing trends-eutrophication potential-are twofold higher than those showing signs of recovery. The tool effectively identified areas of known eutrophication with severe symptoms, like dead zones, as well as those with limited to no information of the eutrophication. Our tool introduces the prospect for a consistent global assessment of eutrophication trends with major implications for monitoring Sustainable Development Goals (SDGs) and the application of Earth Observations in support of SDGs.

Disproportionate responses between free-living and particle-attached bacteria during the transition to oxygen-deficient zones in the Bohai Seawater

The Bohai Sea has recently suffered several seasonal oxygen-deficiency, even hypoxia events during the summer. To better understand effects of dissolved oxygen (DO) concentration on the bacterial composition in particle attached (PA) and free living (FL) fractions during the transition from oxic water to low oxygen conditions, the bacterial communities under three different oxygen levels, i.e., high oxygen (HO, close to 100% O₂ saturation), medium oxygen (MO, close to 75% O₂ saturation), and low oxygen (LO, close to 50% O₂ saturation) in the Bohai Sea were investigated using 16S rRNA amplicon sequencing. Fourteen water samples from 5 stations were collected during a cruise from August to September in 2018. The results showed that the sequences of Proteobacteria and Actinobacteriota jointly accounted for up to 74% across all 14 samples. The Shannon index in HO samples were significantly higher than in LO samples (P < 0.05), especially in PA communities. The composition of bacterial communities varied by oxygen concentration in all samples, and the effect was more pronounced in the PA fraction, which indicates that the PA fraction was more sensitive to the change in oxygen concentration, possibly due to the tighter interactions in this community than in the FL fraction. This study provides novel insights into the distribution of bacterial communities, and clues for understanding the responses of bacterial communities in the Bohai Sea during the transition from the oxic to oxygen-deficient zones.

Photosensitized formation of sulfate and volatile sulfur gases from dissolved organic sulfur: Roles of pH, dissolved oxygen, and salinity

The photodegradation of dissolved organic sulfur (DOS) is a potential source of aqueous sulfate and its chemical precursors in surface water. However, the photochemical fate of DOS and factors that control its fate still remain unclear. Herein, we employed a DOS model featuring a photosensitizer (humic acids, HA) to investigate the photochemical degradation pathways of DOS in various natural water sources, from which we observed the substantial photosensitized formation of sulfate, methanesulfonic acid (MSA), carbonyl sulfide (COS), and carbon disulfide (CS₂). However, the photochemical production of sulfate and MSA tends to be more efficient than COS and CS₂. The formation of sulfur-containing photodegradation products was also strongly affected by the identity of the organic sulfur precursor, the oxygen concentration, and the pH, while the salinity did not significantly influence the production ratios. Our results revealed that the photosensitization of DOS contributed significantly to the overall production of sulfate and MSA production, especially in acidic and oxygen-enriched environments, which was attributed to the photochemical production of reactive intermediates, such as excited CDOM (³CDOM*) and reactive oxygen species (ROS). Considering the coexistence of DOS and photosensitizers in aquatic environments, photochemistry may play an essential role in the fate of aquatic DOS.

Oxygen decline in a temperate marginal sea: Contribution of warming and eutrophication

Dissolved oxygen (DO) decline (i.e., deoxygenation) is an ongoing process in parts of the coastal and open oceans as a result of increased greenhouse gas emissions and nutrient discharges. Yet its controlling mechanisms remain unclear and patchy. Based on continuous observational data collected in a temperate margin, the southern Yellow Sea (SYS), we quantitatively evaluate how deoxygenation responds to warming and eutrophication in different seasons by using an evaluation method that allows us to distinguish the effects of temperature, salinity and biological activities. Results show that during winter, when the water column is vertically well-mixed, and in summer surface waters, deoxygenation is dominated by warming-induced decreases of O₂ solubility due to a quick exchange of O₂ between the ocean and atmosphere. Moreover, we find a regionally accelerated deoxygenation with enhanced warming along the pathway of the YSCC (Yellow Sea Coastal Current) in winter. In contrast, for bottom waters in summer when O₂ exchange is inhibited due to high stratification, deoxygenation appears to be dominated by biological respiration associated with eutrophication. Also, we find the summer bottom deoxygenation can be accelerated by warming, indicating that the bottom waters or the hypolimnion may be vulnerable to deoxygenation in the future. Our study further demonstrates that the deoxygenation mechanisms in shallow coastal oceans are associated with water column structures, i.e., well-mixed vs. stratified water column. Information is assembled into a conceptual model to provide an overview of deoxygenation in temperate marginal systems.

Main factors dominating the development, formation and dissipation of hypoxia off the Changjiang Estuary (CE) and its adjacent waters, China

Hypoxia off the Changjiang Estuary (CE) and its adjacent waters is purported to be the most severe in China, attracting considerable concern from both the scientific community and the general public. Currently, continuous observations of dissolved oxygen (DO) levels covering hypoxia from its appearance to disappearance are lacking. In this study, twelve consecutive monthly cruises (from February 2015 to January 2016) were conducted. The consecutive spatiotemporal variations in hypoxia throughout the annual cycle were elucidated in detail, and the responses of annual variations in hypoxia to the different influential factors were explored. Overall, hypoxia experienced a consecutive process of expanding from south to north, then disappearing from north to south. The annual variations in hypoxia were mainly contingent on stratification variations. Among different stages, there was significant heterogeneity in the dominant factors. Specifically, low-DO waters initially appeared from the intrusion of nearshore Kuroshio branch current (NKBC), as NKBC intrusion provided a low-DO background and triggered stratification. Thereafter, stratification was enhanced and gradually expanded northward, which promoted the extension of low-DO areas. The formation of hypoxia was regionally selective, and more intense organic matter decomposition at local regions facilitated the occurrence and discontinuous distribution of hypoxia. Hypoxic zones were observed at the Changjiang bank and Zhejiang coastal region from August (most extensively at 14,800 km²) to October. Thereafter, increased vertical mixing facilitated the dissipation of hypoxia from north to south.

Multiple factors dominate the distribution of methane and its sea-to-air flux in the Bohai Sea in summer and autumn of 2014

The Bohai Sea is well-known as a source of atmospheric methane (CH₄). However, the main regulate factors of the spatiotemporal distribution of CH₄ and its sea-to-air flux remain largely unknown. In this study, the observed CH₄ concentration ranged from 4.8 to 32.7 nmol/L and 3.1 to 15.2 nmol/L in August and November of 2014, respectively. The main factors that influence the distribution of CH₄ and its sea-to-air flux were stratification, solubility, and current structure for the mid-west depression basins, the permanent well-mixed seawater column and CH₄ source strength for the centre shallow ridge zone, and the upwelling for the east depression basin, respectively. Meanwhile, wind also plays an important role in sea-to-air CH₄ flux in the study area except the centre shallow ridge zone. Upwelling made the east depression basin the most intensive source of CH₄, with a flux of 2 to 4 times higher than the other sub-regions.

Long-term changes in nutrient regimes and their ecological effects in the Bohai Sea, China

The nutrient regime has changed significantly in the Bohai Sea (BS) during the past six decades because of anthropogenic perturbations. Specifically, the concentration of DIN increased by about 7-fold from the end of the 1950s to the mid-2010s, while DIP and DSi concentrations decreased from the end of the 1950s to the beginning of the 1990s, and have since increased again. Unsynchronized changes in nutrient levels have led to changes in the nutrients structure, which has caused a series of ecological effects. Phytoplankton biomass increased by 6-fold from the 1960s to the mid-2010s. Additionally, phytoplankton composition shifted from a diatom-dominated to a dinoflagellate-dominated system, and the dominant species of macrozoobenthos changed. Red tides rarely occurred before the 1980s, but have occurred periodically and frequently since the 1990s. Finally, the BS ecosystem has shifted from an N-limited oligotrophic state before the 1990s to a potentially P-limited eutrophic state.