Jiulong River Estuary Research Articles

Distribution, provenance, contamination, and probabilistic ecological risk of rare earth elements in surface sediments of Jiulong River estuary and adjacent watershed

Rare earth elements (REE) are increasingly recognized as emerging micropollutants in the environment, and concerns surrounding their presence are steadily growing. However, the understanding of possible pollution and ecological risks of REE in river sediments is relatively scarce. To address this, an investigation into the distribution, origins, pollution levels, and potential ecological risks of REE in sediments along the Jiulong River Estuary and adjacent watersheds was conducted. The results revealed that ΣREE in sediments ranged from 54.98 to 156.05 mg/kg along the Jiulong River and its estuary. The REE concentrations generally decreased from the Jiulong River to the estuary. Cluster analysis and geochemical parameter ratios suggested the high REE concentrations in Jiulong River may be derived from fertilizer and coal combustion. The sediments REE was normalized using the chondrites and exhibited apparent fractionation between LREE and HREE. Additionally, the REE in sediments was characterized by LREE enrichment and negative Ce and Eu anomalies. The evaluation of geo-accumulation and enrichment factors indicated that Gd, Yb, Lu, Tb and Dy were the elements most highly enriched in the sediments. Sites labeled S1–S3 and S5 were classified as moderately polluted, with Lu posing a moderate potential ecological risk, while other elements presented only mild ecological risks. Our findings suggest that anthropogenic activities significantly influence the pollution and potential ecological risks of REE in river sediments. Therefore, attention should be paid to the management of emerging contaminants in rivers and estuaries to protect ecosystems and local populations from potential risks associated with the accumulation of REE in sediments.

Hydrographic proxies for submarine groundwater discharge in the Jiulong River estuary and global perspectives

Submarine groundwater discharge (SGD) significantly impacts most coastal waters. However, its quantification, depending on chemical tracers/proxies, limits its parameterization in numerical models. This study explored the hydrographic proxies of SGD in the Jiulong River estuary (JRE) using 226Ra and 228Ra as SGD tracers. Our results showed significant monthly fluctuations in the flux of SGD, with a peak in June and a minimum in April. On average, the flux of SGD was equivalent to 10 ± 1.67 % of the concurrent river discharge, with the area-normalized rate of 0.007 ± 0.017 to 0.13 ± 0.04 m/day. Positive SGD response to river discharge implies a connection with the surface runoff of the shallow aquifers. Furthermore, the flux of SGD presented a significant negative correlation with the return flow factor and flushing time of the estuary. The radium activities in the estuary were positively correlated with water depth, indicating that SGD was not driven by tidal pumping. Instead, physical mixing in low to middle salinity regions predominated such behavior of radium. Our results indicate that river discharge, flushing time and return flow factor may serve as hydrographic proxies of SGD in the JRE and potentially be applicable in parameterization of SGD in numerical models in similar coastal ecosystems. Globally, a positive correlation between SGD flux and river discharge emphasizes the latter as a general proxy in estuaries.

Phosphorus starvation response genes and function coupling: A mechanism to regulate phosphorus availability in a subtropical estuary

Phosphorus (P) plays an important role in regulating primary production in estuarine environments. However, knowledge of the P-functional gene composition of microbial communities and the mechanisms of microbial adaptation to changes in available P in estuaries remain limited. This study coupling 16 s rDNA and metagenomics sequencing was conducted to reveal the relationship between P cycling functional genes, microbial interactions, and P availability in the Jiulong River Estuary. The results showed that the relative abundance of P cycling functions genes was highest in winter, and lowest in summer. Spatially, the total relative abundance of P cycling functions genes was higher in the riverward than that in the seaward. P cycling functional microbial interactions and P cycling gene coupling were strongest in summer and in the seaward. Changes in both temperature and salinity had significant direct and indirect effects on P cycling function, and the influence of salinity on P cycling function was greater than that on the microbial community in the estuary. Salinity had significant direct negative effects on inorganic P-solubilization (IP), organic P-mineralization (OP), and P uptake and transport functions (PT). Whereas, salinity had a significant positive effect on P-starvation response regulation (PR) function. Thus, salinity and microbial communities regulate the soluble reactive phosphate concentrations in estuarine environments by strengthening internal coupling among P cycling functions, promoting PR function, and facilitating PT gene expression. PR is the most important predictors, PR, PT, and PR-PT together explained 38.56 % of the overall soluble reactive phosphorus (SRP) variation. Over 66 % of the explained SRP variations can be predicted by the PR, PT, and PR-PT functional genes. This finding improves the knowledge base of the microbial processes for P cycling and provides a foundation for eutrophication management strategies in the estuary.

Effects of dredging wastewater input history and aquaculture type on greenhouse gas fluxes from mangrove sediments along the shorelines of the Jiulong River Estuary, China

Dredging wastewater (DW) from aquaculture ponds is a major disturbance factor in mangrove management, and its effects on the greenhouse gas (GHG) fluxes from mangrove sediment remain controversial. In this study, we investigated GHG (N2O, CH4, and CO2) fluxes from mangrove sediment at typical aquaculture pond–mangrove sites that were stimulated by DW discharged for different input histories and from different farm types. The GHG fluxes exhibited differing cumulative effects with increasing periods of DW input. The N2O and CH4 fluxes from mangrove sediment that received DW inputs for 17 y increased by ∼10 and ∼1.5 times, respectively, whereas the CO2 flux from mangrove sediment that received DW inputs for 11 y increased by ∼1 time. The effect of DW from shrimp ponds on the N2O flux was significantly larger than those of DW from fish/crab ponds and razor clam ponds. Moreover, the total global warming potentials (GWPs) at the field sites with DW inputs increased by 29–129% of which the CO2 flux was the main contributor to the GWP (85–96%). N2O as a proportion of CO2-equivalent flux increased from 2% to 12%, indicating that N2O was an important contributor to the increase in GWP. Overall, DW increased the GHG fluxes from mangrove sediments, indicating that the contribution of mangroves to climate warming was enhanced under DW input. It also implies that the carbon sequestration potential of mangrove sediments may be threatened to some extent. Therefore, future assessments of the carbon sequestration capacity of mangroves at regional or global scales should consider this phenomenon.

Variations of activity and community structure of nitrite-driven anaerobic methanotrophs in soils between native and invasive species in China's coastal wetlands

Nitrite-driven anaerobic oxidation of methane (nitrite-driven AOM), mediated by ‘Candidatus Methylomirabilis oxyfera’ (M. oxyfera)-related bacteria, is a newly-discovered CH4 consumption process in coastal wetlands. Although Spartina alterniflora invasion significantly affects CH4 emissions from coastal wetlands, its impact on the nitrite-driven AOM process and the underlying mechanisms remain unknown. Here, we examined nitrite-driven AOM activity and M. oxyfera-related bacterial community in four coastal wetlands along the southeastern coast of China, under invasive Spartina alterniflora and native plants, including Kandelia candel, Avicennia marina or Phragmites australis. Linear mixed-effects models indicated that the Spartina alterniflora invasion stimulated the overall nitrite-driven AOM activity by an average of 61.5% in coastal wetlands (p < 0.05), but had no impact on the M. oxyfera-related bacterial abundance (p > 0.05). The nitrite-driven AOM activity was 7.1 times higher under Spartina alterniflora than under native species in Yueqing Bay (p < 0.05), and was 34.7%, 8.9% and 15.1% higher under Spartina alterniflora than under native species in Hengsha Island, Jiulong River and Zhanjiang, respectively (p > 0.05). Spartina alterniflora invasion increased the bacterial abundance in Yueqing Bay and Jiulong River Estuary by 6.8 and 7.6 times, respectively, while decreased the abundance by 34.4% and 51.4%, respectively, in Hengsha Island and Zhanjiang (p > 0.05). The partial least squares path model indicated an indirect impact of Spartina alterniflora invasion on the nitrite-driven AOM activity through its effect on soil properties, primarily including dissolved organic carbon content and nitrate content. The Spartina alterniflora invasion did not greatly alter M. oxyfera-related bacterial community. Overall, we shed new light on the potential impact of Spartina alterniflora invasion on CH4 cycling in coastal wetlands.

Dynamics and geochemical responses of dissolved metals (Mn and Cu) in a subtropical estuary, China.

Theresearch delved into the occurrence and dynamics of dissolved metals, specifically manganese (Mn) and copper (Cu), withinthe Jiulong River Estuary, South China,a medium-sized subtropical estuary. Our findingsunveiled a nuancedseasonal and spatial variability of dissolved metals throughout the entire estuarine system.Notably,dissolved Mnconcentrations peaked (~ 3.5μM) in the upper estuary, diminishingsharply along the salinity gradient, with a modest risein the middle estuary and outer Xiamen Bay. In the upper estuary, heightenedconcentrations of dissolved Mn occurred in spring due to augmented terrestrial particle inputs, followed by suboxically reductive releases; conversely, concentrations were low in summer, attributed to dilution fromincreased freshwater discharges and particle scavenging. In contrast, dissolved Cu exhibited differently, with elevated concentrations (29.2-37.5nM) in the upper and middle estuaries, driven byreductive dissolution of Mn particles and chloride-induced ion exchanges, respectively. Concurrently, heightenedinputs of nutrients and metalscorrelated with elevated phytoplankton productivity (indicated by chlorophyll a) in the upper andouter estuaryregions. Our analysis underscored the sensitivity ofdissolved metals to environmental parameters, including temperature, pH, and dissolved oxygen. The integration of compiled historical dataunderscored the dynamic nature of dissolved metals, particularly Cu, in response to geochemical processes.The elevated ion levels indicated intensified ion releasesfrom particles and sediments, attributable to increased anthropogenic perturbation and climatic changes(e. g. oceanwarming).

Sulfurovum mangrovi sp. nov., an obligately chemolithoautotrophic, hydrogen-oxidizing bacterium isolated from coastal marine sediments.

A novel mesophilic, chemolithoautotrophic, hydrogen-oxidizing bacterium, designated strain ST1-3T, was isolated from mud sediment samples collected from mangroves in Jiulong River estuary. The cells were Gram-stain-negative, non-motile and rod-shaped. The temperature, pH and salinity ranges for growth of strain ST1-3T were 4-45 °C (optimum, 35 °C), pH 5.0-8.5 (optimum, pH 7.0) and 0-8.0 % (w/v) NaCl (optimum, 4.0 %). The isolate was an obligate chemolithoautotroph capable of growth using hydrogen as the only energy source, and molecular oxygen, thiosulphate and elemental sulphur as electron acceptors. The major cellular fatty acids of strain ST1-3T were summed feature 3 (C16 : 1 ω7c and/or C16 : 1 ω6c), C16 : 0 and summed feature 8 (C18 : 1 ω7c). The major polar lipids were phosphatidylethanolamine, phosphatidyldimethyl ethanolamine and phosphatidylglycerol. The respiratory quinone was menaquinone-6. The genomic DNA G+C content was 43.6 mol%. Phylogenetic analysis based on 16S rRNA gene sequences and core genes showed that the novel isolate belonged to the genus Sulfurovum and was most closely related to Sulfurovum lithotrophicum 42BKTT (94.7 % sequence identity). The average nucleotide identity and digital DNA-DNA hybridization values between ST1-3T and S. lithotrophicum 42BKTT were 74.6 and 16.3 %, respectively. On the basis of the phenotypic, phylogenetic and genomic data presented here, strain ST1-3T represents a novel species of the genus Sulfurovum, for which the name Sulfurovum mangrovi sp. nov. is proposed, with the type strain ST1-3T (=MCCC M25234T=KCTC 25639T).

Microbial interactions strengthen deterministic processes during community assembly in a subtropical estuary

Systematic studies on the assembly process and driving mechanisms of microbial communities in estuaries with diverse seasonal and spatial scales are still limited. In this study, high-throughput sequencing, and microbial network analysis were combined to decipher the impact of environmental changes and biological interactions on the maintenance of microbial diversity patterns in the Jiulong River Estuary (JRE). The results showed that overall, stochastic processes dominated the bacterioplankton community assembly in the estuary, accounting for 49.66–74.78 % of the total. Additionally, bacterioplankton community diversity varied significantly across seasons and subzones. Specifically, the concentration of soluble reactive phosphorus (SRP) in the estuary steadily reduced from winter to summer, and the corresponding bacterioplankton community interactions gradually shifted from the weakest interaction in winter to the strongest in summer. The deterministic processes contributed more than half (50.34 %) to microbial assembly in the summer, but only 25.22 % in winter. Deterministic processes prevailed in the seaward with low SRP concentrations and strong bacterioplankton community interactions, while stochastic processes contributed 70.14 % to the assembly of microbial communities riverward. Biotic and abiotic factors, such as nutrients and microbial interactions, jointly drove the seasonal and spatial patterns of bacterioplankton community assembly, but overall, nutrients played a dominant role. Nevertheless, the contributions of nutrients and microbial interactions were equivalent in spatial assembly processes, albeit nutrients were the primary seasonal driver of the bacterioplankton community assembly process. This study emphasizes the significance of microbial interactions in the bacterioplankton community assemblage. These findings provide new and comprehensive insights into the microbial communities' organization in estuaries.

Effect of typhoons on spatiotemporal patterns of multi-group persistent organic pollutants in sediment of Chinese southeastern coastal estuaries

Estuaries are susceptible to both anthropogenic disturbances and global climate changes. Impacts may be discriminated by pollution patterns of widely quantified persistent organic pollutants (POPs), though data are scarce for extreme climate events. This study quantified four groups of POPs, i.e., polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethanes (DDTs), and hexachlorocyclohexanes (HCHs), in sediments from seven Chinese coastal estuaries across a gradient of socioeconomic development in their watersheds with comparisons among the pre-typhoon, typhoon, and post-typhoon periods during 2016–2019. The maximal average concentrations, up to 1561 μg PAHs kg−1, 235 μg DDTs kg−1, and 38.9 μg HCHs kg−1, were quantified in the Jiulong River estuary and 7.61 μg PCBs kg−1 in the Jiao River estuary. Anthropogenic activities contributed to the distinctive spatial distributions of four groups of POPs in estuaries with non-agricultural gross domestic product (NAGDP) per capita significantly relating to sedimentary concentrations of PAHs and PCBs and agricultural gross domestic product (AGDP) per capita relating to DDTs and HCHs. Seasonality and typhoons led to less temporal variations in sedimentary POPs concentrations, whose spatial heterogeneity was remarkably reduced in the post-typhoon period rather than in the pre-typhoon and typhoon periods. The results of this study suggested that fingerprinting legacy POPs in spatial and temporal distributions contributed to identifying the effects of anthropogenic disturbances and climate changes on estuarine sediment quality.

Permeability decides the effect of antibiotics on sedimentary nitrogen removal in Jiulong River Estuary.

Sedimentary denitrification takes place beneath the oxic layer at the sediment-water interface, where nitrate and antibiotics need to diffuse through the overlying water. However, the antibiotics' effect on sedimentary N removal and associated N2O production has not been adequately investigated under in situ conditions. Here, isotope pairing techniques, including slurry incubations (potential) and intact core incubations (in situ), combined with metagenomic analysis were applied to investigate the impacts of two protein-inhibiting antibiotics (oxytetracycline and thiamphenicol) on sediment nitrogen removal in a subtropical estuary. Slurry incubations showed that the two antibiotics significantly inhibited denitrification (67-98%) and anammox (49-99%), while intact core incubations presented no antibiotic effect at upstream but significant inhibition (23%-52%) at downstream. Meanwhile, N2O yields were stimulated up to 20 folds in slurry incubations yet showing insignificant response in intact cores. Such contrasting results between up- and down-stream and between slurry and intact core incubations strongly indicated that permeability, which determines diffusion of antibiotics to microbes, is the key to exert the effect of antibiotics on in situ sedimentary nitrogen removal processes regardless the existence of antibiotics resistance genes. This diffusive obstruction may mitigate the toxic effect of antibiotics on nitrogen removal related microbes in natural environments.

Lignin analysis of sediments from around 2,000 to 1,000 years ago (Jiulong River estuary, southeast China)

Abstract In this work, a sediment core collected from the Jiulong River estuary in southeastern China was subjected to 14C dating of foraminifera, as well as lignin, total organic carbon, and stable carbon isotope (δ13C) analyses in order to determine the impacts of climate change and human activities on the sedimentary organic matter in this area from around 2,000 to 1,000 years ago. The ratios of lignin parameters syringyl/vanillyl and cinnamyl/vanillyl ranged from 1.60 to 8.63 and 0.11 to 0.45, respectively, and the lignin phenol vegetation index ranged from 25.14 to 1740.14, indicating the presence of non-woody angiosperms. The ratio of vanillic acid/vanillin ranged from 0.42 to 2.04, indicating medium to high degrees of oxidative degradation. The vertical distribution profile showed a similar historical sedimentary trend with locations at higher latitudes along the Zhejiang-Fujian Mud Area, with the lower abundance of total lignin from around 2400 to 2000 BP being attributed to the decreasing temperature during this period. However, all the lignin parameters showed higher values and greater fluctuations due to increasing temperatures after 2000 BP, and human activity has probably had the greatest impact in the most recent 1,000 years.

Characteristics and provenances of rare earth elements and Nd isotopes in surface sediments of mangrove wetlands in the Jiulong River Estuary, China.

Rare earth elements (REEs) and Nd isotopes are frequently employed to determine provenance, although their characteristics and provenances in the surface sediments of mangrove wetlands are rarely analyzed. In this study, a thorough analysis of the characteristics and provenances of REEs and Nd isotopes in the surface sediments of mangrove wetland in the Jiulong River Estuary was carried out. According to the results, the mean concentration of REEs in the surface sediments was 290.9mg·kg-1, which was greater than the background value. Unpolluted to moderately polluted for La and Ce, as well as a moderate ecological risk for Lu, were indicated by the geoaccumulation index (Igeo) and potential ecological risk of individual factors ([Formula: see text]), respectively. The surface sediments showed substantial negative Eu anomalies but no significant Ce anomalies. The enrichments in LREE and flat HREE patterns are visible in the chondrite-normalized REE patterns. REEs in the surface sediments might be attributed to both natural sources (granite and magmatic rocks) and anthropogenic activities, including coal combustion, vehicle exhaust, steel smelting, and fertilizer, based on the (La/Yb)N-∑REE and ternary (La/Yb)N-(La/Sm)N-(Gd/Yb)N plots. The three-dimensional ∑LREE/∑HREE-Eu/Eu*-εNd(0) plot, when combined with the Nd isotope, further demonstrated that the REEs in the surface sediments appeared to have come from additional nonlocal potential sources.

Per- and polyfluoroalkyl substances (PFASs) in water and sediment from a temperate watershed in China: Occurrence, sources, and ecological risks

Per- and polyfluoroalkyl substances (PFASs) are a class of synthetic organic fluorides that have been widely used in various industrial and consumer applications. However, their potential ecological risks have raised concerns. In this study, PFASs were investigated in different environmental media in the Jiulong River and Xiamen Bay regions of China, revealing widespread contamination of PFASs in the watershed. PFBA, PFPeA, PFOA, and PFOS were detected in all 56 sites, with short-chain PFASs dominating (72 % of the total). Novel PFAS alternatives, including F53B, HFPO-DA, and NaDONA, were detected in >90 % of the water samples. Seasonal and spatial variations in PFAS concentrations were observed in the Jiulong River estuary, while Xiamen Bay was not significantly affected by seasonal changes. In sediment, PFSAs were dominant with long-chains while PFCAs with short-chains, and the occurrence was influenced by water depth and salinity. PFSAs were more inclined to be adsorbed in sediments than PFCAs, and log Kd of PFCAs increased with the numbers of -CF2-. Paper packaging, machinery manufacturing, WWTP discharge, airport and dock activities were the dominant sources of PFASs. Risk quotient showed that PFOS or PFOA may pose high toxicity to Danio rerio and Chironomus riparius. Although the overall ecological risk in the catchment is still low, the hazard of bio-concentration under long-term exposure and multi-pollutant synergistic toxicity cannot be ignored.

Interspecies calibration for biomonitoring metal contamination in coastal waters using oysters and mussels

Differences in metal bioaccumulation among species make it difficult to compare biomonitoring results obtained using different marine bivalve species. To address this challenge and improve the interpretation of biomonitoring data, we studied the toxicokinetic mechanisms underlying these differences and developed a method to estimate seawater metal concentrations based on metal concentrations in the organisms. We transplanted six common species of oysters and mussels found in Chinese coastal waters into the Jiulong River estuary and monitored metal concentrations in the organisms, water, and suspended particles every three days over a six-week period. A one-compartment first-order toxicokinetic model was used to describe the relationship between metal bioaccumulation and metal concentrations in the environment. The model parameters, including aqueous uptake (ku) and dietary assimilation (kp) rate constants, and elimination rate constant (ke), were estimated using a Markov Chain Monte Carlo fitting method with a priori information from a systematic review we conducted. The toxicokinetic model successfully fitted the temporal changes in metal bioaccumulation in all six bivalve species and explained the interspecies differences. Using the calibrated models, we were able to calculate metal concentrations in the seawater at the bivalve collection sites and enable comparisons of biomonitoring data across multiple species. In conclusion, we have established a toxicokinetic framework to explain interspecies differences in metal bioaccumulation in six commonly found bivalves and provided a useful tool for interpreting biomonitoring data in coastal environments.

Real-time underway measurement of ammonium in coastal and estuarine waters using an automated flow analyzer with hollow fiber membrane contactor

Ammonium (NH4+) is an important parameter for aquatic ecosystems. To date, continuous and underway acquisition of NH4+ in coastal and estuarine waters has been challenged by the strongly varying salinity and complex matrices in these waters. To address these issues, a hollow fiber membrane contactor (HFMC) was constructed and incorporated in flow injection analysis (FIA) to achieve online separation/preconcentration of NH4+ in water. In the FIA-HFMC system, NH4+ in the water sample was converted into NH3 under alkaline conditions in the donor channel. The generated NH3 diffused across the membrane and was absorbed in an acid solution in the acceptor channel. The resultant NH4+ in the acceptor was then quantified based on a modified indophenol blue (IPB) method. Parameters affecting the performance of the FIA-HFMC-IPB system were evaluated and optimized. Under the optimized conditions, the proposed system exhibited a limit of detection of 0.11 μmol L−1, with relative standard deviations of 1.0–1.9 % (n = 7), and a good linear response (R2 = 0.9989) for the calibration in the field with NH4+ standards in the range of 0.40–80 μmol L−1. The proposed system was applied to a shipboard underway measurement of NH4+ in a two-day cruise in the Jiulong River Estuary–Xiamen Bay, China. A good agreement was observed between measurements from the proposed system and those from manual sampling and laboratory analysis. Both laboratory and field results demonstrated that the system was free of salinity effect and interference from organic nitrogen compounds. The system also showed excellent stability and reliability during a 16-day observation. This work suggests that the proposed FIA-HFMC-IPB system is applicable for the underway measurement of NH4+ in water, especially for estuarine and coastal waters with varying salinity and complex matrices.

Modeling Suitable Habitats of Indo-Pacific Humpback Dolphins (Sousa chinensis) in a Highly Urbanized Bay

Indo-Pacific humpback dolphins (Sousa chinensis) live in coastal waters that have experienced considerable anthropogenic disturbances. Specifically, along the coasts of highly urbanized regions, available habitats have decreased in both space and scale. This study used Maxent software to simulate the distribution of suitable habitats for S. chinensis during the dry and wet seasons in Xiamen Bay. We also aimed to identify key environmental factors influencing habitat distribution, with the goal of using our results to analyze conservation gaps and propose adjustments. We found that during the dry season, S. chinensis were mainly distributed in the Western Harbor, Jiulong River Estuary, and the mouth of Tongan Bay. During the wet season, they were distributed in the south of Western Harbor, Jiulong River Estuary, and around the Wuyu and Dadeng-Xiaodeng Islands. Nekton density, chlorophyll-a concentration, sea surface temperature, slope, and salinity are the primary factors affecting S. chinensis distribution in Xiamen Bay. To better protect the species, the existing Tongan Bay reserve should be expanded during dry seasons, and suitable habitats in the Jiulong River Estuary should be turned into a seasonal protected region. Similarly, during wet seasons, the original Western Harbor portion of the reserve should increase to include the Jiulong River Estuary, northwest of the Wuyu Islands and south of the Dadeng-Xiaodeng Islands. In addition to this dynamic seasonal management of reserve size, we also recommend developing multiple-use protected areas and establishing a joint protection mechanism between local governments. In conclusion, our model offers important guidance on improving conservation measures of S. chinensis in Xiamen Bay.

Bioaccumulation and human health implications of trace metals in oysters from coastal areas of China

This study recompiled a national dataset to characterize the pollution level and health risk of cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) in oysters along the coastal areas of China. Results showed that the median concentrations of Cd, Cu, Pb and Zn in nationwide oysters were 5.5, 335, 1.3 and 1280 mg/kg dry weight, respectively. Generally, oysters from the north coasts presented lower metal pollution and higher quality than those from the south. The regional characteristics of trace metals in oysters might be contributed by the interspecific differences. Nationally, the noncarcinogenic risk posed by these four metals in oysters was relatively low, with the risk only occurring in a few hotspots such as the Pearl River Estuary and the Jiulong River Estuary. However, more attention should be paid to the carcinogenic risk of Cd, and priority should be given to formulating control measures to mitigate Cd pollution.

The biological transformation of ammonium and urea in a eutrophic estuarine system in Southern China

Estuaries channel large amounts of anthropogenic nitrogen (N) from continents to the offshore where productivity is widely limited by N and phosphorus. Ammonium and urea, two reduced forms of anthropogenic N commonly observed, are the preferred substrates for various microorganisms (e.g., uptake by phytoplankton or bacteria and oxidation by nitrifier). Yet, it remains underexplored how they transform and their concentrations vary during transport in the estuary which may influence the offshore microbial community. We applied 15N isotope tracer incubation techniques to determine the two main bio-consumption processes, i.e., uptake and oxidation, of ammonium and urea, in the dark for the Jiulong River Estuary, a eutrophic estuary in southeastern China. Results showed that light penetration depth ranged from 0.8-3.3 m leaving 76% of estuary water bodies to stay in dark situations throughout a day. Ammonia oxidation, which favors dark conditions, dominates the estuarine regenerated-N cycle, showing the rank: ammonia oxidation &amp;gt; ammonium uptake &amp;gt;&amp;gt; urea uptake ≈ urea oxidation. By compiling the reported case studies, we found the relatively low ratio of urea oxidation to ammonia oxidation was accompanied by a relatively high ammonium concentration. Microorganisms’ high preference for ammonium over urea may lead to an inhibitory-like phenomenon. An analogous effect was evident by the increased urea uptake at downstream when ammonium was down to a few µM. The obstructed urea utilization resulted in 10 times longer lifetime for urea relative to ammonium (surface: 19 ± 9 days; bottom 12 ± 7 days). Such an inhibitory-like effect allows urea to be preserved in the estuary and allows urea to be transported farther offshore to stimulate microorganisms capable of utilizing urea, which may have significant impacts on offshore ecology.

Impacts of a subtropical storm on nitrogen functional microbes and associated cycling processes in a river-estuary continuum

Storms, in subtropical regions such as S.E. China, cause major changes in the physical and biogeochemical fluxes of anthropogenic N species through the river-estuary continuum to the coast. Two weeks continuous observations at a sampling station (Station E) in the upper Jiulong River Estuary (S.E. China) were conducted to track the changes of physical and biogeochemical parameters together with genomic identification of nitrogen cycling microbes through a complete storm event in June 2019. In conjunction with previous N flux measurements, it was found that there was greatly increased flux of N to and through the upper estuary during the storm. During the storm, the freshwater/brackish water boundary moved downstream, and previously deposited organic rich sediment was resuspended. During baseflow, anthropogenically derived ammonium was oxidised dominantly by the marine nitrifying (AOA) microbe Nitrosopelagicus. However, during the storm, the dominant ammonia-oxidizing archaea (AOA) at Station E changed to the riverine genus (Nitrosotenuis) while the marine genus, Nitrosopumilus decreased. At the same time the dominant ammonia-oxidizing bacteria (AOB) was still the marine genus (Nitrosomanas). Estuarine nitrifiers had higher abundance, weighted entropy and diversity during the Flood, suggesting that the high NH4-N and DO during the Rising period of the Flood resulted in a bloom of nitrifiers. The changing gene abundances of nitrifiers were reflected in changes in the concentration and isotopic composition of DIN confirming active nitrification in the oxygen-rich water column. During the storm the numbers of denitrifiers (narG, nirS and nod), DNRA (nrfA) and anammox (hzsB) were found in the water column increased, and the larger fraction was associated with the <22 μm free-living fraction. However it was not possible with the data obtained to estimate what fraction of these anaerobic bacteria were active in the dominantly oxic water column.

Spatiotemporal patterns of different forms of nitrogen in a coastal mangrove wetland invaded by Spartina alterniflora

The invasion by smooth cordgrass (Spartina alterniflora) has a serious impact on ecosystem function and the ecological environment in the coastal region of subtropical China. However, little is known about the fluctuation of different forms of nitrogen in mangrove wetlands invaded by cordgrass. In this study, we compared the spatial and temporal distribution of different forms of nitrogen with or without invasion by S. alterniflora in a coastal wetland in the Jiulong River estuary, Fujian province, China. The results revealed that the variation of nitrogen concentration in the wetland was closely related to the main intertidal vegetation type, season and sampling location. Organic nitrogen accounted for the majority of the deposited nitrogen and exhibited the highest values in the mangrove zone. The content of ammonium nitrogen and nitrate nitrogen was higher in cordgrass zone and lower in the mangrove, reaching a maximum in summer. Total nitrogen concentration was significantly influenced by the tidal water level which was highest in high tide zone and lowest at low tide. Invasion of S. alterniflora significantly altered nitrogen stocks in the sediment, including the contents of total nitrogen, total organic nitrogen, ammonium nitrogen and nitrate nitrogen.