Roles of photochemical consumption of VOCs on regional background O3 concentration and atmospheric reactivity over the pearl river estuary, Southern China

Understanding of the photochemical ozone (O3) pollution over the Pearl River Estuary (PRE) of southern China remains limited. We performed an in-depth analysis of volatile organic compounds (VOCs) data collected on an island (i.e., the Da Wan Shan Island, DWS) located at the downwind of Pearl River Delta (PRD) from 26 November to 15 December 2021. Abundances of O3 and its precursors were measured when the air masses originated from the inland PRD. We observed that the VOCs levels at the DWS site were lower, while the mixing ratio of O3 was higher, compared to those reported at inland PRD, indicating the occurrence of photochemical consumption of VOCs during the air masses transport, which was further confirmed by the composition and diurnal variations of VOCs, as well as ratios of specific VOCs. The simulation results from a photochemical box model showed that the O3 level in the outflow air masses of inland PRD (O3(out-flow)) was the dominant factor leading to the intensification of O3 pollution and the enhancement of atmospheric radical concentrations (ARC) over PRE, which was mainly contributed by the O3 production via photochemical consumption of VOCs during air masses transport. Overall, our findings provided direct quantitative evidence for the roles of outflow O3 and its precursors from inland PRD on O3 abundance and ARC over the PRE area, highlighting that alleviation of O3 pollution over PRE should focus on the impact of photochemical loss of VOCs in the outflow air masses from inland PRD.

Distribution and typologies of anthropogenic seafloor litter in the Pearl River Estuary and adjacent coastal waters, China

Marine litter pollution poses a significant threat to offshore ecosystems, eliciting widespread concern. We investigated seafloor litter patterns in the Pearl River Estuary and adjacent coastal waters of China in 2023 via bottom trawl survey. Average number and weight densities were found to be 154.34 ± 30.95 n/km2 and 2384.63 ± 923.98 g/km2, respectively. Plastic was the most abundant material by number density (79.07 %), and rubber the highest by weight density (22.93 %). Overall number density varied from 40.50 ± 22.50 to 221.13 ± 52.44 n/km2, with the highest in Daya Bay and the lowest in Guanghai Bay. Weight density varied from 189.93 ± 71.94 to 5386.70 ± 3050.30 g/km2, with the highest in Qiao Island and the lowest in Honghai Bay. The main source was plastic bags and wrappers. The Pearl River Delta and Daya Bay were identified as seafloor litter distribution hotspots. Controlling plastic waste input is crucial for reducing seafloor litter in the Pearl River Estuary.

Hydrodynamics and dissolved organic matter components shaped the fate of dissolved heavy metals in an intensely anthropogenically disturbed estuary

Anthropogenic activities and natural erosion caused abundant influx of heavy metals (HMs) and organic matter (OM) into estuaries characterized by the dynamic environments governed by tidal action and river flow. Similarities and differences in the fate of HM and OM as well as the influences of OM on HMs remain incomplete in estuaries with seasonal human activity and hydrodynamic force. To address this gap, dissolved HMs (dHMs) and fluorescence dissolved OM (FDOM) were investigated in the Pearl River Estuary, a highly seasonally anthropogenic and dynamic estuary. It aimed to elucidate the effects of hydrodynamic conditions and DOM on the seasonal fate of dHMs via the multivariate statistical methods. Our findings indicated dHMs and FDOM exhibited consistently higher levels in the upper estuarine and coastal waters in both seasons, predominantly controlled by the terrestrial/anthropogenic discharge. In the wet season, dHMs and humic-like substances (HULIS) were positively correlated, showing that dHMs readily combined with HULIS. This association led to a synchronous decrease offshore along the axis of the estuary and the transport following the river plume in the surface affected by the salt wedge. Contrarily, dHMs were prone to complex with protein-like components impacted by the hydrodynamics during the dry season. Principal component analysis (PCA) results revealed the terrestrial/anthropogenic inputs and the fresh-seawater mixing process were the most crucial factors responsible for the fate of dHM in wet and dry seasons, respectively, with DOM identified as a secondary but significant influencing factor in both seasons. This study holds significance in providing valuable insights into the migration, transformation, the ultimate fate of dHMs in anthropogenically influenced estuaries, as well as the intricate dynamics governing coastal ecosystems.

Risk assessment of e-waste - Liquid Crystal Monomers re-suspension caused by coastal dredging operations

The Pearl River Estuary (PRE), one of the primary e-waste recycling centers in the world, has been suffering from the pollution of Liquid Crystal Monomers (LCMs), critical materials with persistent, bio-accumulative, and toxic substances used in electronic devices. It has been detected in seabed sediment with both high frequency and concentration near PRE - Hong Kong (HK) waters. In the same area, dredging operations with in-situ sediment have been frequently used in the last decades for coastal land reclamation projects. Dredging is known to cause a huge amount of sediment re-suspension into water columns, with potential damage to marine ecosystems and biodiversity. In this study, we proposed a new risk assessment strategy to estimate the secondary pollution due to the re-suspension sediment highly contaminated by LCMs. We formulate a robust and reliable probabilistic approach based on unsupervised machine learning and hydrodynamic and sediment transport numerical simulation. New risk indexes were also proposed to better quantify the impact of contaminated sediments. We applied the methodology to assess the potential impact of dredging operations in the PRE and Hong Kong waters on the local marine ecosystem. The results of the analysis showed how the potentially contaminated areas depended on the dredging locations.

Labyrinthulomycetes thrives in organic matter-rich waters with ecological partitioning in the Pearl River Estuary

Labyrinthulomycetes play an important role in marine biogeochemical cycles, but their diversity, distribution patterns, and key regulatory factors remain unclear. This study measured the abundance and diversity of Labyrinthulomycetes in the Pearl River Estuary (PRE) to understand its distribution pattern and relationship with environmental and biological factors. The abundance of Labyrinthulomycetes ranged from 24 to 500 cells·mL-1, with an average of 144.37 ± 94.65 cells·mL-1, and its community composition showed obvious ecological partitioning in the PRE. The results of statistical analysis indicated that CDOM, salinity, and chlorophyll a contributed significantly (P < 0.01) to the community composition, explaining 46.59%, 11.34%, and 4.38% of the variance, respectively. The Labyrinthulomycetes distribution pattern combined with the niches of dominant species was revealed; low-salinity species mainly use terrigenous organic matter occupied dominant positions in the upper estuary and showed the highest abundance; moderate-salinity species that can use phytoplankton-derived resources thrived in the middle estuary; and seawater species dominated the lower estuary with the highest diversity but the lowest abundance. In addition, the results of phylogenetic tree analysis indicated that the existence of a novel lineage, and further study on the diversity and ecological functions of Labyrinthulomycetes is needed.IMPORTANCELabyrinthulomycetes play important roles in organic matter remineralization, carbon sinks, and food webs. However, the true diversity of Labyrinthulomycetes is still unclear due to limitations in isolation and culture methods. In addition, previous studies on their relationship with environmental factors are inconsistent and even contradictory, and it is speculated that their community composition may have spatial heterogeneity along the environmental gradient. In this study, the distribution pattern and key regulators of Labyrinthulomycetes in the PRE were revealed. Combining the niche of dominant species, it is suggested that salinity determines the spatial differences in Labyrinthulomycetes diversity, and the resources of substrate (terrestrial input or phytoplankton-derived) determine the dominant species, and its abundance is mainly determined by organic matter concentrations. Our study provided new information on the Labyrinthulomycetes diversity and verified the spatial heterogeneity of Labyrinthulomycetes community composition, providing reliable explanations for the inconsistencies in previous studies.

Occurrence, distribution, and ecological risks of polyhalogenated carbazoles in sediments from Daya Bay and Pearl River Estuary, China

Polyhalogenated carbazoles (PHCZs) are a group of emerging organic pollutants attracting increasing concern. In this study, 32 sediment samples were collected from the Pearl River Estuary (PRE) and adjacent Daya Bay (DYB) in China and were investigated for the occurrence and distribution of PHCZs. Total concentration of sedimentary PHCZs (∑PHCZs) ranged from 0.79 to 3.08 ng/g in PRE and 0.89 to 1.95 ng/g in DYB, both containing 3,6-dichlorocarbazole as the main component. Higher concentrations of ∑PHCZs were found in the rivers-mouth and inner part of the PRE indicating their main origins from anthropogenic activities. Notably, concentrations of brominated carbazoles (BCZs) gradually increased offshore, which suggests the potential bio-transformation of BCZs under a saline environment. The toxic equivalent of PHCZs was estimated at 0.13-0.34 pg TEQ/g suggesting limited dioxin-like effects on local organisms.

Alternatives Exert Higher Health Risks than Bisphenol A on Indo-Pacific Humpback Dolphins

The detrimental effects of bisphenol (BP) exposure are a concern for vulnerable species, Indo-Pacific humpback dolphins (Sousa chinensis). To investigate the characteristics of BP profiles and their adverse impact on humpback dolphins, we assessed the concentrations of six BPs, including bisphenol A (BPA), bisphenol S (BPS), bisphenol F (BPF), bisphenol AF (BPAF), bisphenol B (BPB), and bisphenol P (BPP) in blubber (n = 26) and kidney (n = 12) of humpback dolphins stranded in the Pearl River Estuary, China. BPS accounted for the largest proportion of the total bisphenols (∑BPs) in blubber (55%) and kidney (69%). The concentration of ∑BP in blubber was significantly higher than that in the kidney and liver. The EC50 values of five BPA alternatives were lower than those of BPA in humpback dolphin skin fibroblasts (ScSF) and human skin fibroblasts (HSF). ScSF was more sensitive to BPS, BPAF, BPB, and BPP than HSF. The enrichment pathway of BPA was found to be associated with inflammation and immune dysregulation, while BPP and BPS demonstrated a preference for genotoxicity. BPA, BPP, and BPS, which had risk quotients (RQs) > 1, were found to contribute to subhealth and chronic disease in humpback dolphins. According to the EC50-based risk assessment, BPS poses a higher health risk than BPA for humpback dolphins. This study successfully evaluated the risks of bisphenols in rare and endangered cetacean cell lines using a noninvasive method. More in vivo and in field observations are necessary to know whether the BPA alternatives are likely to be regrettable substitutions.

Spatial and temporal dynamics and fluxes estimation of manganese fractions in sediments from the Pearl River Estuary, southern China

Sequential extraction was used to study the historical dynamics and fluxes of the chemical fractions of manganese (Mn) in sediments collected from the Pearl River Estuary (PRE), southern China. Results revealed that the proportion of Mn associated with different fractions decreased in the order of acid-soluble fraction (F1) > reducible fraction (F2) > residual fraction (F4) > oxidizable fraction (F3). F1 (47%) was the dominant Mn fraction, indicating the strong bioavailability of Mn to aquatic organisms in the PRE. In addition, the Mn fraction F2 was present at an average rate of 27.93 % in surface sediments, indicating that it could be mobilized when environmental conditions become increasingly reducing or oxidizing. The decline in Mn fraction fluxes after 2006 indicated that the region has partially decreased due to the removal of heavily polluting industries and effective control of sewage discharge.

The distinct phases of fresh-seawater mixing intricately regulate the nitrogen transformation processes in a high run-off estuary: Insight from multi-isotopes and microbial function analysis

Excessive anthropogenic nitrogen inputs lead to the accumulation of nitrogen, and significantly impact the nitrogen transformation processes in estuaries. However, the governing of nitrogen during its transport from terrestrial to estuary under the influence of diverse human activities and hydrodynamic environments, particularly in the fresh-seawater mixing zone, remains insufficient researched and lack of basis. To address this gap, we employed multi-isotopes, including δ15N-NO3-, δ18O-NO3-, δ15N-NH4+, and δ15N-PN, as well as microbial function analysis, to investigate the nitrogen transformation processes in the Pearl River Estuary (PRE), a highly anthropogenic and terrestrial estuary. Principle component analysis (PCA) confirmed that the PRE could clearly partitioned into three zone, e.g., terrestrial area (T zone), mixing area (M zone) and seawater area (S zone), in terms of nitrogen transportation and transformation processes. The δ15N-NO3- (3.38±0.60‰) and δ18O-NO3- (6.35±2.45‰) results in the inner estuary (T area) indicate that NO3-attributed to the domestic sewage and groundwater discharge in the river outlets lead to a higher nitrification rate in the outlets of the Pearl River than in the reaching and seawater intrusion areas, although nitrate is rapidly diluted by seawater after entering the estuary. The transformation of nitrogen in the T zone was under significant nitrogen fixation (0.61 ± 0.22 %) and nitrification processes (0.0043 ± 0.0032 %) (presumably driven by Exiguobacterium sp. (14.1 %) and Cyanobium_PCC-6307 (8.1 %)). In contrast, relatively low δ15N-NO3- (6.83 ± 1.24‰) and high δ18O-NO3- (22.13±6.01‰) imply that atmospheric deposition has increased its contribution to seawater nitrate and denitrification (0.53±0.13 %) was enhanced by phytoplankton/bacterial (such as Psychrobacter sp. and Rhodococcus) in the S zone. The assimilation of NH4 results from the ammonification of NO3- reduces δ15N-NH4+ (5.36 ± 1.49‰) and is then absorbed by particulate nitrogen (PN). The retention of nitrogen when fresh-seawater mixing enhances the elevation of δ15N-NH4+ (8.19 ± 2.19‰) and assimilation of NH4+, leading to an increase in PN and δ15N-PN (6.91 ± 1.52‰) from biological biomass (mainly Psychrobacter sp. and Rhodococcus). The results of this research demonstrate a clear and comprehensive characterization of the nitrogen transformation process in an anthropogenic dominated estuary, highlighting its importance for regulating the nitrogen dissipation in the fresh-seawater mixing process in estuarine ecosystems.

Metagenomic profiles of planktonic bacteria and resistome along a salinity gradient in the Pearl River Estuary, South China

Estuarine ecosystems undergo pronounced and intricate changes due to the mixing of freshwater and saltwater. Additionally, urbanization and population growth in estuarine regions result in shifts in the planktonic bacterial community and the accumulation of antibiotic resistance genes (ARGs). The dynamic changes in bacterial communities, environmental factors, and carriage of ARGs from freshwater to seawater, as well as the complex interrelationships among these factors, have yet to be fully elucidated. Here, we conducted a comprehensive study based on metagenomic sequencing and full-length 16S rRNA sequencing, covering the entire Pearl River Estuary (PRE) in Guangdong, China. The abundance and distribution of the bacterial community, ARGs, mobile genetic elements (MGEs), and bacterial virulence factors (VFs) were analyzed on a site-by-site basis through sampling along the salinity gradient in PRE, from upstream to downstream. The structure of the planktonic bacterial community undergoes continuous changes in response to variations in estuarine salinity, with the phyla Proteobacteria and Cyanobacteria being dominant bacterial throughout the entire region. The diversity and abundance of ARGs and MGEs gradually decreased with the direction of water flow. A large number of ARGs were carried by potentially pathogenic bacteria, especially in Alpha-proteobacteria and Beta-proteobacteria. Multi-drug resistance genes have the highest abundance and subtypes in PRE. In addition, ARGs are more linked to some MGEs than to specific bacterial taxa and disseminate mainly by HGT and not by vertical transfer in the bacterial communities. Various environmental factors, such as salinity and nutrient concentrations, have a significantly impact on the community structure and distribution of bacteria. In conclusion, our results represent a valuable resource for further investigating the intricate interplay between environmental factors and anthropogenic disturbances on bacterial community dynamics. Moreover, they contribute to a better understanding of the relative impact of these factors on the dissemination of ARGs.

Source apportionment of nitrate in the Pearl River Estuary using δ15N and δ18O values and isotope mixing model

The mitigation of eutrophication in the Pearl River Estuary (PRE) has encountered numerous challenges in regards to source control. Herein, the isotope mixing model (SIAR) was used to quantify the primary nitrate sources in the PRE. The results showed that the nitrate levels were significantly higher in the high-flow season than in the low-flow season. Meanwhile, we found the most important nitrate sources were manure and sewage during the high-flow season, with a contribution ratio of 47 % in the low salt area (LSA) and 29 % in the high salt area (HSA). During the low-flow season, the primary nitrate sources were identified as reduced nitrogen fertilizer in the LSA and manure and sewage in the HSA, which accounted for 52 % and 44 %, respectively. Furthermore, we also suggest that a feasible measure might be to control the pollution caused in the PRE by manure and sewage as well as reduced nitrogen fertilizer.

Hypoxia and warming take sides with small marine protists: An integrated laboratory and field study

Hypoxia and ocean warming are two mounting global environmental threats influencing marine ecosystems. However, the interactive effects of rising temperature and depleted dissolved oxygen (DO) on marine protists remains unknown. Here, we conducted a series of laboratory experiments on four protozoa with distinct cell sizes to investigate the combined effects of temperature (19, 22, 25, 28, and 31 °C) and oxygen availability (hypoxia, 2 mg DO L^-1 and normoxia, 7 mg DO L^-1) on their physiological performances (i.e., growth, ingestion, and respiration rates). The hypoxia-induced inhibition in three physiological rates increased with the biovolume of the protists. As the larger surface area to volume (SA/V) quotients of smaller protists facilitate higher capabilities of oxygen absorption and utilization, the smaller protists suffered less inhibitions induced by hypoxia. Moreover, the hypoxia-induced inhibition in physiological rates was exacerbated by increasing temperature, which can be verified by the reductions in the temperature sensitivities (represented by the activation energy, E). These results suggest that hypoxia could lead to a shift of protistan community with enhanced domination of small protists, and warming could exacerbate such a trend. We further examined our laboratory results in the Pearl River Estuary, where extensive bottom hypoxia often occurs in summer. We found the mean protist biovolume in hypoxic waters was significantly lower than that at normal stations. Also, the mean protist biovolume decreased with declining DO concentration and rising temperature, indicating the interactive effect of temperature and oxygen availability. Collectively, we suggest that hypoxia could cause a higher proportion of small-sized cells in the marine protistan community, and the projected ocean warming could intensify the tendency, which could undermine the capacity of oceanic carbon sequestration.

nov., a novel bacterium isolated from Pearl River Estuary

A Gram-staining-positive, aerobic, motile, and rod-shaped strain, designated SYSU M60031^T, was isolated from a Pearl River Estuary sediment sample, Guangzhou, Guangdong, China. The isolate could grow at pH 5.0-8.0 (optimum, pH 7.0), 25-37 °C (optimum, 28 °C) and in the presence of 0-1 % (w/v) NaCl (optimum, 0 %). The predominant respiratory menaquinone of SYSU M60031^T was MK-7. The cellular polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, one unidentified aminophospholipid, and one unidentified aminolipid. The major fatty acids (>10 % of total) were iso-C_14 : 0, iso-C_15 : 0, anteiso-C_15 : 0, iso-C_16 : 0, and C_16 : 0. The genomic DNA G+C content was 51.2 %. Phylogenetic analyses based on 16S rRNA gene sequences and core genes indicated that strain SYSU M60031^T belonged to the genus Ectobacillus and showed the highest sequence similarity to Ectobacillus funiculus NAF001^T (96.16%), followed by Ectobacillus antri SYSU K30001^T (95.08 %). Based on the phenotypic, genotypic, and phylogenetic data, strain SYSU M60031^T should be considered to represent a novel species of the genus Ectobacillus, for which the name Ectobacillus ponti sp. nov. is proposed. The type strain of the proposed novel isolate is SYSU M60031^T (=CGMCC 1.19243^T =NBRC 115614^T).

Variations in the concentration, source and flux of polycyclic aromatic hydrocarbons in sediments of the Pearl River Estuary: Implications for anthropogenic impacts

Variations in the distribution, source composition, mass inventory and burial flux of polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the Pearl River Estuary (PRE) collected in 2011 and 2019 were analyzed to investigate the influence of the anthropogenic activities. Total concentrations of 16 priority PAHs in 2019 (200.40 ± 188.86 ng g^-1 dry weight on average) were at the medium level among global bays/estuaries/coastal areas. In 2019, PAH concentrations have decreased by about 50% compared to 2011 and the dominant composition has changed from low- to high-molecular-weight PAHs. The qualitative and quantitative source apportionment analysis indicates that the dominant source of PAHs has shifted from petroleum (40.33%) in 2011 to traffic emission (44.17%) in 2019. The source variation in the PRE can be attributed to the transformation of the energy source structure from petrogenic to pyrogenic in the Pearl River Delta. The estimated PAH mass inventory of the top 5-cm sediment was 38.70 metric tons in 2019, which was about 41 metric tons lower than that in 2011. The average deposition fluxes have dropped from 418.91 ± 261.02 ng cm^-2 yr^-1 in 2011 to 215.52 ± 246.63 ng cm^-2 yr^-1 in 2019. The decreasing PAH concentration is related to the sediment coarsening and decline of total organic carbon. These findings in the PRE can be applied to other estuarine environments influenced by anthropogenic activities.

Evaluation of environmental impact of red tide around Pearl River Estuary, Guangdong, China

This paper presents a case study of red tide hazards around the Pearl River Estuary (PRE). Red tide hazards, meteorological data, and seawater monitoring data were collected from 1996 to 2020 at different locations around the PRE to investigate the internal and external factors influencing the occurrence of red tides. The enhancement of the assessment of estuarine trophic status (ASSETS) method enables us to evaluate the effects of meteorological factors and seawater eutrophication status on the red tide risk level. Using ASSETS, we established a framework for red tide risk assessment of the Pearl River Estuary. We analysed the external and internal factors causing the red tide based on meteorological data and seawater monitoring data in the PRE. The results show that the temperature was higher than the annual monthly average temperature of 1.265 °C, and east and north winds at velocities of 3-4 m/s could result in the formation of red tides. However, precipitation inhibits the formation of the red tide in PRE.

Occurrences and potential lipid-disrupting effects of phthalate metabolites in humpback dolphins from the South China Sea

Phthalate esters (PAEs) are ubiquitous environmental contaminants, arising growing public concern. Nevertheless, information on the exposure and risks of PAEs in wildlife remains limited. Here, we conducted the first investigation of the occurrences, spatiotemporal trends, and potential risks of twelve metabolites of PAEs (mPAEs) in 74 humpback dolphins from the northern South China Sea during 2005-2020. All twelve mPAEs (∑₁₂mPAEs: 9.6-810.7 ng g^-1 wet weight) were detected in the dolphin liver, and seven major mPAEs showed increasing trends during the study period, indicating high PAE contamination in the coastal environment of South China. Monoethylhexyl phthalate accounted for over half of the ∑₁₂mPAE concentrations. The accumulation of mPAEs in the dolphins was neither age-dependent nor sex-specific. Compared to parent PAEs, mPAEs generally induced higher agonistic effects on the dolphin peroxisome proliferator-activated receptor alpha/gamma (PPARA/G) as master regulators of lipid homeostasis. Although short-term in vitro assays revealed no significant activation of dolphin PPARA/G by tissue-relevant doses of mPAEs, long-term in vivo evidence (i.e., correlations between hepatic mPAEs and blubber fatty acids) suggested that chronic exposure to mPAEs might have impacted lipid metabolism in the dolphin. This study highlighted the potential health risks of PAE exposure on marine mammals.

Impact of climate change on coastal water quality and its interaction with pollution prevention efforts

The impact of climate change on nearshore coastal water quality and its interaction with pollution prevention efforts (e.g., the development of green and gray water infrastructure) still lack systematic investigation. This study performed a holistic analysis of the impact of climate change on the salinity and concentrations of total nitrogen (TN), total phosphorus (TP) and chlorophyll a (Chl.a) in Shenzhen Bay between Shenzhen and Hong Kong, the two most developed megacities in South China, based on three-dimensional hydrodynamic and water quality modeling. The major study findings were as follows. First, Chl.a was the most sensitive parameter, and its bay-wide average concentration in 2100 was predicted to be approximately 13% and 46% higher than those in 2015 under mild and rapid climate change scenarios, respectively. Second, sea level rise was found to be a major driver of all four water quality parameters, while temperature and radiation mainly influenced Chl.a and precipitation mainly influenced nutrients. Third, water quality responses to climate change were highly heterogeneous over the bay. Even under a mild climate change scenario, the highest location-specific changes (2100 vs. 2015) in salinity and TN, TP and Chl.a concentrations were projected to be approximately 21%, 19%, 25%, and 65%, respectively. Fourth, changes in seasonal variation due to climate change may lead to an enhanced ecological risk of algal blooms. Finally, the effect of reducing TN and TP concentrations by proposed water infrastructure development was found to be significantly weakened (nearly 40% and 20% for TN and TP, respectively, under a mild climate change scenario), while the negative effect (i.e., increase in the Chl.a concentration) was notably accelerated. Regional cooperation is critical for protecting the water quality of the bay, particularly under climate change. The insights obtained in this study are applicable to other coastal water zones around the world with similar socioeconomic backgrounds and climatic conditions.

Multilocus DNA metabarcoding diet analyses of small cetaceans: a case study on highly vulnerable humpback dolphins and finless porpoises from the Pearl River Estuary, China

Accurate diet identification of top predators is crucial to fully understand their ecological roles. Compared to terrestrial animals, gathering dietary information from cetaceans is notoriously difficult. Here, we applied a multilocus metabarcoding approach to investigate the diet of vulnerable Indo-Pacific humpback dolphins and Indo-Pacific finless porpoises from the Pearl River Estuary (PRE), China. Our analyses identified 21 prey fish species from the 42 humpback dolphin stomachs, as well as 10 species of fish and 1 species of cephalopod from the 13 finless porpoise stomachs. All of the taxa were assigned to the species level, highlighting that the multimarker approach could facilitate species identification. Most of the prey species were small- and medium-sized fishes that primarily fed on zooplankton. The calculated similarity index revealed a moderated dietary overlap between the 2 cetaceans, presumably due to the feeding of the 2 predators in association with fishing vessels in the PRE. A more diverse diet was observed in humpback dolphins in the closed fishing season compared to the fishing season, implying the influence on the dolphin diet due to the availability of commercial fishery resources. However, according to the results of species rarefaction curves, our findings on the feeding habits of the 2 cetaceans are still limited by insufficient sample size and therefore should be interpreted with caution. This study represents a first attempt to apply the multilocus DNA metabarcoding technique in the diet analysis of small cetaceans, although more efforts are needed to improve this type of analysis.

Spatial distribution and influencing mechanism of CO2, N2O and CH4 in the Pearl River Estuary in summer

Estuaries, considered as the important carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) sources to the atmosphere, are increasingly affected by near-bottom hypoxia. However, the impact of estuarine hypoxic zone development on GHGs production and discharge remains poorly understood due to the seasonal and spatially distributed heterogeneity of estuarine hypoxia occurrence and the lack of simultaneous monitoring of the distribution of bottom hypoxic waters and the vertical distribution of GHGs. Here, we conducted high spatial resolution vertical stratification sampling and analysis of water column GHGs in the Pearl River Estuary (PRE), a large estuary with frequent hypoxia in recent years. Our results showed that Pearl River runoff is the main source of GHGs in the PRE. Strong nitrification is an important N₂O production mechanism in the PRE. In situ generation of water and resuspension of surface sediments were the main sources of CH₄ in bottom water, while massive organic matter (OM) mineralization is the main driver of CO₂ in bottom water. The development of a hypoxic zone in the PRE significantly increased the concentration of N₂O and CH₄ in the bottom water and thus increased air-water fluxes. The air-water fluxes of N₂O, CH₄ and CO₂ of PRE in summer were 31.9 ± 7.5 μmol m^-2 d^-1, 192.5 ± 229.4 μmol m^-2 d^-1 and 51.9 ± 14.1 mmol m^-2 d^-1, respectively. This study reveals that GHGs fluxes from estuarine waters to the atmosphere will increase significantly with increasing eutrophication caused by human activities and the expansion of hypoxic zones in estuarine waters.

Oysters for legacy and emerging per- and polyfluoroalkyl substances (PFASs) monitoring in estuarine and coastal waters: Phase distribution and bioconcentration profile

Per- and polyfluoroalkyl substances (PFASs) are a diverse group of widely used anthropogenic chemicals that are environmentally persistent and bioaccumulative, especially in aquatic ecosystem. The heavily industrialized and urbanized Greater Bay Area in China represents a notable contamination source for PFASs, which may potentially influence the health of local oysters as a keystone species in local ecosystems and a popular seafood. In this study, samples of oysters and their surrounding waters were collected from the littoral zones of the Pearl River Estuary (PRE), China during winter 2020, where 44 PFASs, including 19 perfluoroalkyl acids (PFAAs), 8 emerging PFASs, and 17 PFAA precursors (or intermediates), were analyzed. Total PFAS concentrations ranged 13.8-58.8 ng/L in the dissolved phase, 3.60-11.2 ng/g dry weight (dw) in the suspended particulate matter (SPM), and 0.969-1.98 ng/g dw in the oysters. Most short-chain PFASs were present in the dissolved phase (>95%), while long-chain PFASs generally showed higher concentrations in the SPM. Log field-based bioconcentration factors (BCFs) of long-chain PFASs increased linearly (r = 0.95, p < 0.01) with increasing estimated log membrane-water (D_mw) and protein-water (D_pw) distribution coefficients. Perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA) exhibited higher measured BCFs than those estimated by their D_mw and D_pw. Considering the widespread occurrence of their precursors, the contribution of precursor transformation was likely to be a significant source of PFHxA and PFHpA. Oysters from the PRE littoral zones posed low risks to human health associated with PFAS consumption, which might be underestimated due to limited toxicity data available for PFAA precursors and emerging PFASs. This study sheds light on the practicality of applying oysters as biomonitors for timely PFAS monitoring in coastal environments.

Widespread occurrence of emerging E-waste contaminants - Liquid crystal monomers in sediments of the Pearl River Estuary, China

Liquid crystal monomers (LCMs), commonly used in screens of electronic devices, have recently been identified as a group of emerging chemicals of concern associated with e-waste. They are potentially persistent, bioaccumulative, and toxic substances, and may pose a threat to the marine ecosystem. The Pearl River Estuary (PRE) receives organic contaminants discharged from the Pearl River Delta region, where primitive handling of e-waste is widespread. However, information on the pollution status of LCMs in the PRE is absent. Herein, a rapid and robust analytical method was established using ultrasonic extraction, solid phase extraction cleanup, and GC-Orbitrap-MS analysis. The spatial distribution of 39 target LCMs was investigated in 45 surface sediment samples from the PRE. Ten LCMs were detected, with ΣLCMs ranged from 0.9 to 31.1 ng/g dry weight. Our results demonstrated a widespread occurrence of LCMs in the sediments of the PRE, and a gradient of their contamination from inshore to offshore regions, indicating land-based origins. Our reported ΣLCMs concentrations were relatively higher compared to many other legacy and emerging pollutants found in the same investigated area. Preliminary risk assessment showed 3VbcH, Pe3bcH and tFMeO-3bcHP might be the top 3 risk contributors in the PRE. Further investigation on the ecological impact of LCMs on marine benthic ecosystems, as well as identification of their sources and control measures are warranted.

Source Apportionment, Hydrodynamic Influence, and Environmental Stress of Pharmaceuticals in a Microtidal Estuary with Multiple Outlets in South China

Pharmaceutical residues in the environment are of great concern as ubiquitous emerging contaminants. This study investigated the presence of 40 pharmaceuticals in water and sediment of the Pearl River Estuary (PRE) in the wet season of 2020. Among psychiatric drugs, only diazepam was found in water samples while six of them were detected in the sediment. The Σantibiotics levels ranged from 6.18 to 35.9 ng/L and 2.63 to 140 ng/g dry weight in water and sediment samples, respectively. Fluoroquinolones and tetracyclines were found well settling in the outlet sediment, while sulfonamides could be released from disturbed sediment under stronger tidal wash-out conditions. After entering the marine waters, pharmaceuticals tended to deposit at the PRE mouth by the influence of the plume bulge and onshore invasion of deep shelf waters. Low ecological risks to the aquatic organisms and of causing antimicrobial resistance were identified. Likewise, hydrological modeling results revealed insignificant risks: erythromycin-H₂O and sulfamethoxazole discharged through the outlets constituted 30.8% and 6.74% of their environmental capacity, respectively. Source apportionment revealed that pharmaceutical discharges through the Humen and Yamen outlets were predominantly of animal origin. Overall, our findings provide strategic insights on environmental regulations to further minimize the environmental stress of pharmaceuticals in the PRE.

Diel change in inorganic nitrogenous nutrient dynamics and associated oxygen stoichiometry along the Pearl River Estuary

The reactive nitrogen (N) emitted from continents significantly perturbs the pristine N cycle around the land-ocean boundary resulting in eutrophication and hypoxia. As nutrients are transported downstream through an estuary, various types of biological processes co-occur to modulate nitrogen speciation to influence the biogeochemical habitats for downstream microorganisms. We surveyed the Pearl River Estuary to examine the N transfer dynamics among nitrogen species with considering process-specific oxygen production and consumption. By using ¹⁵N pulse-tracing techniques, we measured ammonia oxidation and uptakes of ammonium, nitrite, and nitrate simultaneously under dark and light conditions in parallel. Light strongly inhibited nitrification but enhanced N uptake, and such light effect was further considered in the calculation for nitrogen transformation rates over a diel cycle. We found both oxidation and uptake of ammonium decreased seaward as substrate decreased. The nitrifier and phytoplankton work in antiphase to draw down incoming ammonium rapidly. Contrary to ammonium uptake, uptake of nitrite and nitrate showed a seaward increasing pattern. Such an inverse spatial pattern implies a shift in N preference for phytoplankton. Such high ammonium preference inhibits nitrate/nitrite uptake allowing them to behave conservatively in the estuary and to travel farther to outer estuary. By integrating oxygen consumption and production induced by N transformation processes over the diel cycle, oxygen was produced although allochthonous ammonium input is high (∼250 μM). For most stations, ammonium was completely consumed within 2 days, some stations even less than 0.5 days, implying that although the water residence time is short (2-15 days), tremendous input of ammonium N from upstream was transformed into particulate organic or nitrate forms during traveling to modulate the biogeochemical niche, including substrate, organics and oxygen, of coastal microbes in water column and sediments.

Saltwater intrusion affecting NO2- accumulation in demersal fishery species by bacterially mediated N-cycling

To investigate the underlying effects of saltwater intrusion (SWI) on bottom aquatic ecosystems, a set of environmental parameters and the bacterial community were determined and analyzed by sampling bottom water and surface sediments at the Modaomen waterway of the Pearl River Estuary. Biodiversity of fishery species and their relationship with the environment variables were analyzed together. NO₃^- and NO₂^- concentration down-regulation and NH₄⁺ concentration up-regulation in water and sediment were observed along the resulting salinity gradient, indicating that SWI affected N-cycling. Further investigation via 16 s sequencing revealed that taxonomic and functional composition of the bacterial community in the sediment displayed greater discretization than in water, implying that SWI exerted a greater impact on the sedimentary bacterial community. Metagenomic sequencing showed that the sedimentary bacterial community was associated with NO₃^-, NO₂^-, and NH₄⁺ transformation under SWI, and that this was driven by salinity and conductivity. Nitrogen metabolism and denitrification related genes were expressed at higher levels in high salinity than in low salinity, consistent with the increased enzymatic activities of NiR and NR. The NO₂^- concentration in the muscle of six selected fishery species was significantly decreased by 11.15-65.74% (P < 0.05) along the salinity gradient, indicating that SWI reduced NO₂^- accumulation. The results suggest that SWI alleviates NO₂^- accumulation in demersal fishery species via bacterial mediation of N-cycling.

Microplastic contamination in marine-cultured fish from the Pearl River Estuary, South China

Microplastics are ubiquitous in the aquatic environment. However, the occurrence of microplastics in farmed fish is under-researched. Herein, microplastic abundance in the stomach and intestine of marine-cultured hybrid groupers (Epinephelus fuscoguttatus × Epinephelus lanceolatus) in the Pearl River Estuary (PRE) was examined. Microplastics were detected in all fish samples, with an average abundance of 35.36 n/individual or 0.62 n/g. The fish intestine contained more microplastics (23.91 n/individual, i.e., 1.10 n/g) than the stomach (12.80 n/individual, i.e., 0.37 n/g). In addition, the ingested microplastics were predominantly fibre-shaped (70.1%), and nearly 70% were smaller than 1 mm in diameter. Potential factors affecting the ingestion of microplastics by farmed hybrid groupers include fish diet and the availability of microplastics in their feeding habitat.

Source-specific ecological risk assessment and quantitative source apportionment of heavy metals in surface sediments of Pearl River Estuary, China

In this study, surface sediments of the Pearl River Estuary were collected from 29 stations and investigated the spatial distribution, pollution level, quantitative source apportionment, and source-specific ecological risk of 10 heavy metals. The mean concentrations followed the order of Mn > Zn > Cr > Cu > Ni > Pb > As > Co > Cd > Hg. In terms of spatial distribution, it showed that the heavy metals were enriched in the inner Pearl River Estuary with 'extremely high' level of Hg, whereas, Cd and Zn posed 'moderate to high' contamination potential. We apportioned four main sources using positive matrix factorization model, in which natural geogenic and industrial manufacturing sources accounted for 36.84% and 27.11% of the total, respectively. However, the source-specific risk assessment suggested that mixed anthropogenic sources were the main contributors, and ecological risks were strongly affected by anthropogenic imports from the surrounding cities.

[Population characteristics of Collichthys lucidus in the Pearl River Estuary during 2017 and 2020]

To understand the population characteristics of Collichthys lucidus, an important economic fish in the Pearl River Estuary, the biological characteristics and resource density distribution characteristics of C. lucidus were preliminarily analyzed using bottom trawling by cruises conducted in each spring and autumn during 2017 and 2020. The results showed that the body length and weight of C. lucidus ranged between 22-168 mm and 0.23-103.11 g, respectively. Female individuals were larger than the male ones. The length of sexually mature individuals intensively ranged between 90 mm and 140 mm. Neither of them evidenced the earlier of sexually maturity nor the minimizer of dominant group. The population of C. lucidus in Pearl River estuary still developed in safe status in all, but its habitat downgraded than in 1988, as indicated by the fact that the allometric growth factor (b=2.9057) of the body length to body weight had no significant annual variations, but the conditional factor (a=3.029×10^-5) was drama-tically decreased than in 1988. The population was at a state of overexploitation due to the estimated exploitation rate of 0.67. The resource density averaged 77.73 kg·km^-2, showing a pattern of higher in the middle and west than in the east and relatively uniform of latitudinal distribution. The four high densities of sampling zones suggested that the zone around Nansha Port was probably the core of spawning ground of C. lucidus. Considering the annual average resource density in 2017-2020 sharply decreased by 93.5% than in 1980 to 1982, it was pressing to establish the protection zone in spawning ground in spring to protect the recruiting and spawning stocks of C. lucidus population.

Evaluating salinity variation and origin in coastal aquifer systems with integrated geophysical and hydrochemical approaches

Public concerns have been dramatically increased over potential saltwater intrusion resulting in freshwater resources shortage in coastal aquifers in the past decades. Investigating the distribution and origin of saline water is a key factor to understand coastal groundwater evolution and further assist its management. Here we evaluate the horizontal and vertical spatial variability of the coastal groundwater salinity and its potential key influencing factors based on integrated hydrochemical and geoelectrical approaches in the Pearl River Estuary (PRE), South China. The electrical resistivity tomography and geochemical data show a decrease of salinity from the coast to the inland with a water type varying from Cl-Na to HCO₃-Ca at the regional scale. Points with higher/lower salinity values than those in the surrounding environment occasionally exist in the study region. In the cross section, the zone 2-90 m below the surface has low resistivity values, which correspond to high-salinity values in the subsurface. The moderate resistivity values at 0-2 m depth illustrate an infiltration of freshwater. The complex salinity distribution pattern is mainly controlled by the heterogeneity of formation and distribution of primary flow paths, while the coastal groundwater salinity evolution is shaped by the joint influence of paleo-seawater intrusion, the ion mixing processes, and the water-rock interaction.

Effects of marine produced organic matter on the potential estuarine capacity of NOx- removal

Dissolved inorganic nitrogen (DIN) is very high in the Pearl River Estuary (PRE) and nitrate (NO_x^-) removal processes such as denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) are important for determining export of DIN to coastal waters. However, fluxes of NO_x^- removal and influencing factors in the PRE are still unclear. We conducted 4 cruises at 11 sites in the PRE to investigate potential NO_x^- removal rates, their contributions, and corresponding gene abundances, and controlling factors in surface sediments (0-5 cm). The results showed that the potential rates of denitrification, anammox, and DNRA as well as their contributions varied spatially and seasonally. Denitrification (1.98 ± 1.7 μg N g^-1 d^-1) was the major NO_x^- removal processes (68.43 ± 14.61%) while DNRA (0.45 ± 0.28 μg N g^-1 d^-1) contributed 22.61 ± 14.89% in NO_x^- removal. The NO_x^- removal processes and corresponding gene abundances were correlated with the chlorophyll concentrations in both overlying water and sediment, indicating that marine-produced organic matter was the major driver for benthic NO_x^- removal processes. In addition, water column turbidity had important effects on primary production, which affects benthic N processes. Our study provides evidences for that the turbidity-regulated primary production in overlying water is the primary driver for benthic NO_x^- removal processes. The contribution of sediment NO_x^- removal fluxes to water column NO_x^- concentration was low in the upper estuary and increased in the lower estuary where marine produced chlorophyll a was higher. However, daily fluxes of NO_x^- removal were estimated to account for only 0.18-7.22% (mean 1.85 ± 1.62%) of NO_x^- in the whole overlying water column. This suggests that most riverine NO_x^- was exported out into the adjacent coastal waters.

Seasonal and inter-annual variability of bacterioplankton communities in the subtropical Pearl River Estuary, China

It is widely recognized that environmental factors substantially influence on the seasonal and inter-annual variability of bacterioplankton communities, yet little is known about the seasonality of bacterioplankton communities in subtropical estuaries at longer-term time scales. Here, the bacterioplankton communities from the eight major outlets of the subtropical Pearl River Estuary were investigated across 3 years (2017-2019) using full-length 16S rRNA gene sequencing. Significant seasonal and inter-annual variation was observed in bacterioplankton community compositions across the 3 years (p < 0.05). In addition, the inferred functional composition of the communities varied with seasons, although not significantly, suggesting that functional redundancy existed among communities and across seasons that could help to cope with environmental changes. Five evaluated environmental parameters (temperature, salinity, pH, total dissolved solids (TDS), total phosphorus (TP)) were significantly correlated with community composition variation, while only three environmental parameters (temperature, pH, and TDS) were correlated with variation in inferred functional composition. Moreover, community composition tracked the seasonal temperature gradients, indicating that temperature was a key environmental factor that affected bacterioplankton community's variation along with seasonal succession patterns. Gammaproteobacteria and Alphaproteobacteria were the most dominant classes in the surface waters of Pearl River Estuary, and their members exhibited divergent responses to temperature changes, while several taxa within these group could be indicators of low and high temperatures that are associated with seasonal changes. These results strengthen our understanding of bacterioplankton community variation in association with temperature-dependent seasonal changes in subtropical estuarine ecosystems.

Characteristics of expanded polystyrene microplastics on island beaches in the Pearl River Estuary: abundance, size, surface texture and their metals-carrying capacity

While expanded polystyrene (EPS) microplastics have been widely recognized as one of the most important components of plastic litter in the intertidal zones of the global ocean, our understanding of their environmental fate on island beaches is insufficient. In this study, we intended to reveal that the latest EPS microplastic pollution status on 5 island beaches in the Pearl River Estuary, China, by comprehensively assessing the abundance, distribution, size, surface texture and carrying capacity of heavy metals (Cd, As, Cr, Ni, Cu, Pb, Mn, Fe, Al). High level of EPS microplastic abundance ranged from 328 to 82,276 particles m^-2 was found, with the highest abundance at Guishan Island and the lowest at Dong'ao Island. Spatial distribution of EPS microplastic abundance was significantly different among different islands. EPS microplastics in the size range of 1-2 mm were the most abundant. The content of heavy metals in EPS microplastics collected on the beaches was greater than that in the new EPS products. The average concentrations of heavy metals in EPS microplastics from 5 islands are Cd (0.27 ± 0.19 μg g^-1), As (5.50 ± 3.84 μg g^-1), Cr (14.9 ± 8.25 μg g^-1), Cu (15.0 ± 7.66 μg g^-1), Ni (17.2 ± 17.6 μg g^-1), Pb (24.8 ± 7.39 μg g^-1), Mn (730 ± 797 μg g^-1), Fe (8340 ± 4760 μg g^-1), and Al (9624 ± 6187 μg g^-1), respectively. The correlation between heavy metals in EPS microplastics and sediments was better than that between heavy metals in EPS microplastics and seawater. The study results indicated that EPS microplastics could act as a carrier for the transport of heavy metals, which might pose a threat to biological and human health.

Measurement, quantification, and potential risk of microplastics in the mainstream of the Pearl River (Xijiang River) and its estuary, Southern China

The goals of this study were to investigate the distribution profiles, sources, and inventory of microplastics in the surface water of the mainstream of the Pearl River (Xijiang River) and its estuary, China, and to assess the potential ecological risks of the microplastics in this subtropical riverine habitat. The results showed that the microplastic abundances of the Humen (HUM, 16.33 ± 0.88 items/L), Zhaoqing (ZQ, 15.33 ± 0.67 items/L), and Hutiaomen (HTM, 14.67 ± 1.33 items/L) sites were significantly higher than those of the other sampling sites, indicating that the microplastics in Xijiang River and its estuary exhibited an unbalanced spatial distribution. The most common microplastic color was transparent and the major shape of the microplastics was fragments. Microplastics with a size of 0.01-0.1 mm were dominant throughout the 16 sampling sites. Polyolefin elastomer (POE, 33.33%), polyethylene terephthalate (PET, 23.81%), and polyurethane acrylate (PUA, 14.29%) were the dominant microplastic polymers. The microplastic abundance co-varied with most of the selected socio-economic indicators, including the population density, urban land area, gross domestic product (GDP), freshwater aquatic products, and freshwater cultivated area, but without significant differences, indicating that there are additional factors affecting the microplastic abundance. The potential risk (i.e., the potential ecological risk, RI; polymer risk index, H; and pollution load index, PLI) posed by the microplastics was higher in HUM, followed by ZQ and HTM, and it was lower for the other sites, suggesting that the main outlets and the biggest city on the Xijiang River contributed more to the high risks of microplastic pollution. Consistent with microplastic abundance distribution profiles, the average potential risk index values of the western river outlets were higher than those of the eastern river outlets and the Xijiang River, implying that the western river outlets had non-negligible potential ecological risks. Our findings deepen the understanding of the risks posed by microplastics and further contribute to microplastic risk management of riverine ecosystems.

Integrated transcriptomics and proteomics revealed the distinct toxicological effects of multi-metal contamination on oysters

The Pearl River Estuary (PRE) is the largest estuary in southern China and under high metal stress. In the present study, we employed an integrated method of transcriptomics and proteomics to investigate the ecotoxicological effects of trace metals on the Hong Kong oyster Crassostrea hongkongensis. Three oyster populations with distinct spatial distributions of metals were sampled, including the Control (Station QA, the lowest metal levels), the High Cd (Station JZ, the highest Cd), and the High Zn-Cu-Cr-Ni (Station LFS, with the highest levels of zinc, copper, chromium, and nickel). Dominant metals in oysters were differentiated by principal component analysis (PCA), and theirgene and protein profiles were studied using RNA-seq and iTRAQ techniques. Of the 2250 proteins identified at both protein and RNA levels, 70 proteins exhibited differential expressions in response to metal stress in oysters from the two contaminated stations. There were 8 proteins altered at both stations, with the potential effects on mitochondria and endoplasmic reticulum by Ag. The genotoxicity, including impaired DNA replication and transcription, was specifically observed in the High Cd oysters with the dominating influence of Cd. The structural components (cytoskeleton and chromosome-associated proteins) were impaired by the over-accumulated Cu, Zn, Cr, and Ni at Station LFS. However, enhanced tRNA biogenesis and exosome activity might help the oysters to alleviate the toxicities resulting from their exposure to these metals. Our study provided comprehensive information on the molecular changes in oysters at both protein and RNA levels in responding to multi-levels of trace metal stress.

Photochemistry of ozone pollution in autumn in Pearl River Estuary, South China

To explore the photochemical O₃ pollution over the Pearl River Estuary (PRE), intensive measurements of O₃ and its precursors, including trace gases and volatile organic compounds (VOCs), were simultaneously conducted at a suburban site on the east bank of PRE (Tung Chung, TC) in Hong Kong and a rural site on the west bank (Qi'ao, QA) in Zhuhai, Guangdong in autumn 2016. Throughout the sampling period, 3 days with high O₃ levels (maximum hourly O₃ > 100 ppbv) were captured at both sites (pattern 1) and 13 days with O₃ episodes occurred only at QA (pattern 2). It was found that O₃ formation at TC was VOC-limited in both patterns because of the large local NO_x emissions. However, the O₃ formation at QA was co-limited by VOCs and NO_x in pattern 1, but VOC-limited in pattern 2. In both patterns, isoprene, formaldehyde, xylenes and trimethylbenzenes were the top 4 VOCs that modulated local O₃ formation at QA, while they were isoprene, formaldehyde, xylenes and toluene at TC. In pattern 1, the net O₃ production rate at QA (13.1 ± 1.6 ppbv h^-1) was high, and comparable (p = 0.40) to that at TC (12.1 ± 1.5 ppbv h^-1), so was the hydroxyl radical (i.e., OH), implying high atmospheric oxidative capacity over PRE. In contrast, the net O₃ production rate was significantly higher (p < 0.05) at QA (16.3 ± 0.4 ppbv h^-1) than that at TC (4.7 ± 0.2 ppbv h^-1) in pattern 2, and the OH concentration and cycling rate were also higher, indicating much stronger photochemical reactions at QA. These findings enhanced our understanding of O₃ photochemistry in the Pearl River estuary, which could be extended to other estuaries.

High levels of microplastic pollution in aquaculture water of fish ponds in the Pearl River Estuary of Guangzhou, China

Microplastics (MPs) have caused great concern worldwide recently due to their ubiquity in the aquatic environment. The current knowledge on the occurrence of MPs in aquaculture fish ponds in a typical estuary system remains meagre. This study investigated the abundance and characteristics (shape, color, size and composition) of MPs in aquaculture water and pond influents in the Pearl River Estuary of Guangzhou, China, using an improved separation method. The bulk sampling and improved separation method by the combination of ethanol and polyaluminum chloride (PAC) significantly increased the MP separation efficiency, especially for particles with size less than <333 μm. The investigation results showed that MPs were detected in all water samples of fish ponds at two experimental stations with abundances of 10.3-60.5 particles/L (S1) and 33.0-87.5 particles/L (S2), respectively. Moreover, the average abundance of MPs in aquaculture water (42.1 particles/L) exhibited higher value than that in pond influents (32.1 particles/L). Most of MPs were colored and fibrous in appearance. MPs with the size range of <1000 μm (56.3-87.7%) prevailed in aquaculture water. MPs with size <333 μm that usually ignored in most studies were detected with percentage of 43.7% at S1station and 33.2% at S2 station, respectively. The small-sized MPs (<100 μm) in aquaculture water (23.7% at S1 station and 14.6% at S2 station) were more abundant than those in pond influents (7.2% at S1 station and 2.5% at S2 station). The main composition of MPs was polypropylene (PP) and polyethylene (PE). These findings indicated a high level of MP pollution in aquaculture fish ponds. The MPs originated from the Pearl River Estuary were accumulated in aquaculture fish ponds. This study provides an insight into MP pollution in aquaculture fish ponds at a typical estuarine system and highlights the load of MPs in the pond influents.

Characteristics of chlorinated and brominated polycyclic aromatic hydrocarbons in the Pearl River Estuary

The spatial distribution of halogenated polycyclic aromatic hydrocarbons (H-PAHs) (8 chlorinated PAHs (Cl-PAHs) and 9 brominated PAHs (Br-PAHs)), in the Pearl River Estuary (PRE) was investigated. The total concentrations of 17H-PAHs in sediment samples were ranged from 1.4 ng g^-1 to 40.9 ng g^-1 with an average concentration of 14.7 ng g^-1. The average H-PAH concentration in the suspended particulate matters (SPMs) (475.6 ng g^-1) was 30 times higher than that in the sediments (14.7 ng g^-1), and H-PAHs were rarely detected in the aqueous phase. The dominant H-PAHs in sediments were 9,10-dichloroanthracene (31.2%) and 9-chlorophenanthrene (32.0%); 2/9-chlorofluorene (27.4%) was prominent in surface SPMs, while no significant dominant congeners were found in bottom SPMs. Principal component analysis showed that the congener profiles of H-PAHs in sediments were different from those of SPMs especially for surface SPM samples which might be caused by point source discharges of H-PAHs from the coast of the PRE. In general, riverine inputs were the major source of H-PAHs in the PRE, and SPMs were important carriers for transporting H-PAHs. Diagnostic analyses showed that the H-PAHs might originate from waste incineration and automobile emissions in the Pearl River Delta (PRD).

Improvement of water quality in the Pearl River Estuary, China: a long-term (2008-2017) case study of temporal-spatial variation, source identification and ecological risk of heavy metals in surface water of Guangzhou

A series of environmental protective policies have been taken recently in the Pearl River Estuary (PRE) to alleviate water pollution; however, their influence on the reduction of heavy metals in estuarine water has not been known. This study selected Guangzhou as a representative city in the PRE and collected estuarine water monthly from 2008 to 2017 to track the variation of As, Hg, Pb, Cd, Cu, Zn, and Se. During the last decade, the high time-resolved record showed that the concentration of Hg, Pb, Cd, Cu, and Zn in estuarine water reduced by 39.5%, 91.0%, 86.2%, 74.6%, and 97.3%, respectively. However, the concentration of As kept in a stable range (1.89-2.69 μg L^-1) and Se (0.17-0.65 μg L^-1) increased slightly. The principal component analysis (PCA) and absolute principal component scores-multiple linear regression (APCS-MLR) results suggested that the upstream industrial effluents were major sources for Hg (45.5-92.7%), Pb (47.3-100%), Cd (42.0-90.6%), Cu (85.5-100%), and Zn (100%) and the geogenic source was major origin for As (84.6-98.3%) and Se (0-67.5%). The risk quotient of Hg, Pb, Cd, Cu, and Zn to aquatic organisms largely decreased from 0.03, 0.59, 0.03, 2.06, and 0.26 in 2008 to 0.02, 0.05, 0.006, 0.52, and 0.007 in 2017, respectively. The effective control of heavy metal pollution in the study area can be primarily due to the relocation of hundreds of polluting factories during the last decade.

Toxic effects and mechanisms of Prymnesium parvum (Haptophyta) isolated from the Pearl River Estuary, China

Prymnesium (Haptophyta) species form toxic blooms throughout the world resulting in large fish-kills and economic losses. A culture of Prymnesium parvum JX12 was isolated and established from the Pearl River Estuary of China. The toxic characteristics of JX12 to aquatic animals were investigated, considering the effects of temperature, salinity, and nutrients. Cultures of JX12 exhibited significant lethal toxicity to the fish, Mugil sp., M. soiuy, M. cephalus, Ctenogobius sp. and Chrysiptera parasema, and the brine shrimp, Artemia salina, but no significant effects on the shellfish Ruditapes variegatus and the shrimp Penacus orientalis. The toxicity of JX12 to A. salina increased with growth of the culture. The decline phase displayed the strongest potency, followed by the stationary phase, exponential phase and lag phase. Also, there was a significant difference in toxicity among culture fractions of JX12 compared to whole-cell culture. For example, the toxicity of a cell-free culture filtrate on brine shrimp was reduced by 50% by the end of the experiment, while the sonicated and re-suspended cultures caused near 100% mortality. The toxicity of JX12 was also regulated by environmental factors such as temperature, salinity and nutrients. The strongest toxicities were at moderate water temperature (20-25 °C) and salinity (35 psu). The toxicity was reduced significantly at higher or lower temperatures and salinities. Furthermore, the toxicity of JX12 was greatly enhanced when ambient nutrients were deficient. Due to the potent toxicity of JX12 to multiple aquatic animals, especially fish and zooplankton, this species threatens the food web and trophic dynamics of coastal ecosystems. This study suggested that the ability to produce and release toxin(s) is an effective competition strategy for P. parvum to suppress predators and sympatric competitors and thus form blooms.

Atmospheric microplastic over the South China Sea and East Indian Ocean: abundance, distribution and source

At present, microplastic (MP) is pervasive globally and has a regional difference. Recent studies have identified MP in the terrestrial atmospheric environment. However, the connection between terrigenous atmospheric MP emissions and impacts over the ocean is not well known. Here, we present the distribution of atmospheric MP abundance over the ocean based on a transoceanic survey conducted across 21 sampling transects from the Pearl River Estuary (PRE) to the South China Sea (SCS) and then to the East Indian Ocean (EIO). The abundance of atmospheric MP over the PRE (4.2 ± 2.5 items/100 m³) was significantly higher than that over the EIO (0.4 ± 0.6 items/100 m³). However, the abundance of atmospheric MP in the SCS (0.8 ± 1.3 items/100 m³) was not significantly different from the EIO and PRE. This result revealed that MP undergoes long-range transport, more than 1000 km away, through the atmosphere, but atmospheric MP transmission as the main source of oceanic MP based on transoceanic studies is not a plausible assumption. Furthermore, backward trajectory model analysis of 21 sampling transects preliminary showed the potential sources of atmospheric MP over the PRE, SCS, and EIO.

Occurrence, composition and biological risk of organophosphate esters (OPEs) in water of the Pearl River Estuary, South China

Since the production of brominated flame retardants has been gradually phased out, organophosphate esters (OPEs) are increasingly used as the substitutes. Given their toxicity and water solubility, OPEs may jeopardize the aquatic environment and organisms. Here, we examined the concentration, composition, and biological risk of OPEs in the water collected from the eight major waterways in the Pearl River Delta, a highly industrialized region in China. We found a widespread occurrence of OPEs in this region (∑₉OPEs: 134 to 442 ng L^-1), dominated by TCPP, TCEP, and TnBP. Halogenated OPEs were dominant over alkyl and aromatic OPEs. The biological risk of OPEs, mainly contributed by TPhP and TnBP, was low (RQ < 0.1). The contamination level of OPEs in the Pearl River Delta was likely associated with the degree of industrial activities. Although OPEs posed low risk to aquatic organisms, more attention should be paid to some OPEs in the future, such as TnBP, due to the high usage and toxicity. Considering the concentrations of OPEs worldwide and their usage, OPEs may become the emerging pollutants of global concern in the next decade.

Field analysis of PAHs in surface sediments of the Pearl River Estuary and their environmental impacts

Based on a monthly field survey in 2011 of the Pearl River Estuary, the dynamics of polycyclic aromatic hydrocarbons (PAHs) in surface sediments (depth < 5 cm) were explored. The seasonal variations in PAH level and composition were investigated in this study, as well as their environmental behaviors, the role of particles, and source apportionment. The concentration of the sum of 16 priority PAHs (defined as ∑₁₆PAHs) ranged from 0.32 to 1.10 μg/g, while that of the sum of 62 PAHs (defined as ∑_tPAHs) varied from 0.83 to 2.75 μg/g. The levels of both the ∑₁₆PAHs and ∑_tPAHs peaked in February, although the minimum levels appeared in different months-December and August, respectively. The seasonal difference in the ∑_tPAHs was significant (flood season, 7.69 μg/g; dry season, 10.51 μg/g). The 5-ring PAH compound (e.g., perylene) was the most abundant and was responsible for 35% of the total, which implied a terrestrial input source via the Pearl River. Sediment particles were predominantly composed of clayed sand, and sediment PAHs showed a greater tendency to be adsorbed onto the large-sized particles rather than the fine fractions. Total organic carbon (TOC) could considerably facilitate the sediment PAHs. Principal component analysis revealed that vehicle emission sources, petroleum sources, and combustion sources were the major anthropogenic contamination sources. The diagnostic ratios of various individual PAHs were also explored. These findings are particularly useful for understanding the geochemistry of organic pollutants in the complex estuarine environment.

Spectroscopic and molecular-level characteristics of dissolved organic matter in the Pearl River Estuary, South China

Coastal populations are expanding globally, resulting in great anthropogenic impacts on the organic matter in estuaries and regional carbon cycles. However, the molecular-level characteristics of dissolved organic matter (DOM) within highly disturbed estuaries are still not well understood. Here, water samples collected during two seasons (wet and dry) from the subtropical Pearl River Estuary of China were analyzed using absorption and fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to determine the spatiotemporal variations in the DOM characteristics. In the seaward direction, the abundances of chromophoric and fluorescent DOM decreased by greater percentages than the bulk dissolved organic carbon concentration. The spectroscopy and FT-ICR MS analyses collectively indicated seaward declines in the aromaticity of DOM and terrestrial DOM contributions from natural terrestrial markers and anthropogenic synthetic surfactants. In particular, the S content in DOM was much higher here than in previously reported estuaries, suggesting a strong anthropogenic impact on the estuarine DOM. Greater terrestrial and anthropogenic signatures in DOM were observed in the wet season than in the dry season. Importantly, this study implies that the terrestrial and anthropogenic contributions to DOM were strongly driven by season in the anthropogenically disturbed subtropical estuary.

Ecological characteristics and teratogenic retinal determination of Cochlodinium geminatum blooms in Pearl River Estuary, South China

Since 2006, harmful dinoflagellate blooms of Cochlodinium geminatum have infrequently occurred in the Pearl River Estuary, South China. During late October to early November in 2018, C. geminatum blooms occurred again in the region. To investigate the blooming mechanism in certain temporal conditions, we analysed the changes in the environmental parameters and phytoplankton community structure during and after the bloom. The results indicated that the water temperature and salinity had large impacts on the bloom. During the C. geminatum bloom, the phytoplankton community structure changed and the number of dominant species decreased. After the bloom, the species number and abundance of diatoms increased, as the species diversity was recovering. Retinal was detected in the field samples and cultured C. geminatum. It has been demonstrated to exist in some algae species (e.g. Cyanophyta, Chlorophyta, Bacillariophyta, and Euglenophyt), and our results indicates that such teratogens also exist in dinoflagellates. The highest concentration of retinal was detected during the bloom. This result indicates that the retinal content may accumulate during a bloom. Retinal has been demonstrated to be a teratogenic agent and may therefore present a potential risk to aquatic organisms during a bloom episode. This research provided more comprehensive information concerning the ecological influences of C. geminatum blooms.

Distribution, sources and ecological risks of organochlorine compounds (DDTs, HCHs and PCBs) in surface sediments from the Pearl River Estuary, China

The Pearl River Estuary is an important sink of organochlorine compounds (OCs), and OC pollution levels in surface sediments remain largely unknown at present. We collected and analysed residual DDTs, HCHs and PCBs of 45 surface sediments from the Pearl River Estuary in 2017. The values of DDTs (1.83 to 6.98 ng·g^-1) and HCHs (0.43 to 2.14 ng·g^-1) were higher in the Humen outlet, and the values of PCBs (4.6 to 187.4 ng·g^-1) were higher in the coastal areas of Shenzhen. The DDTs and HCHs have generally decreased while the PCBs have been rapidly increasing in recent decades. The DDTs might originate from technical DDT and dicofol. The major source of HCHs was lindane. The main potential sources of PCBs were increased industrial products, ship painting, E-waste disassembly, maricultural and agricultural pollution. The total PCBs and DDTs had medium ecological risks according to the sediment quality guidelines.

Changes in the heavy metals and petroleum hydrocarbon contents in seawater and surface sediment in the year following artificial reef construction in the Pearl River Estuary, China

Marine pollution is an important driver of ocean biodiversity loss, which can be mitigated by the construction of artificial reefs (ARs). Many studies have explored how ARs affect marine organisms, but our understanding of the changes in heavy metals and petroleum hydrocarbons after AR construction is limited. In the current study, we assessed the heavy metal and petroleum hydrocarbon contents of the seawater (surface and bottom seawater) and surface sediment before and after AR construction in AR habitat and in nearby non-reef control habitat in the Pearl River Estuary, China. AR construction tended to decrease the contents of Cu, Pb, Cd, and Hg but tended to increase Zn content in seawater and in surface sediment. Petroleum hydrocarbon content changed irregularly in seawater and surface sediment. Effects of AR construction were similar in the nearby non-reef habitat vs. the AR habitat. Seawater heavy metal and petroleum hydrocarbon contents were correlated with the seawater physicochemical properties (mainly temperature, inorganic nitrogen, chemical oxygen demand, available phosphate, and suspended particulate organic matter), and sediment heavy metal content was correlated with sediment organic matter content. Additional studies over longer time periods and at larger spatial scales are needed to clarify how AR construction affects heavy metal and petroleum hydrocarbon contents in marine environments.

Synechococcus bloom in the Pearl River Estuary and adjacent coastal area-With special focus on flooding during wet seasons

Based on the field surveys aimed at understanding the variations of Synechococcus (Syn) abundance in the Pearl River Estuary during different seasons. We found that heavy terrestrial precipitation result in significant riverine runoffs and promote Syn growth, extension and blooms during warm and wet seasons. To understand the ecological role of Syn play in this estuary during wet seasons, we combined flow cytometry and high throughput sequencing (HTS) of 16S rDNA to investigate the phytoplankton distribution patterns and the potential shaping mechanisms during a typical wet season. During the cruise, picophytoplankton, especially Syn, and Nano-eukaryotes contributed importantly to the total phytoplankton biomass of the estuary. Syn can be further divided into phycoerythrin (PE)-rich Syn and phycocyanin (PC)-rich Syn, with PC-rich Syn about 1.5 times higher than PE-rich Syn in abundance. Both PE-rich Syn (60.75 × 10³ cells ml^-1) and PC-rich Syn (604.05 × 10³ cells ml^-1) reach the highest abundance at the lower part of the estuary. Moreover, PE-rich Syn can be divided into two subgroups which showed different salinity preference, with PE1 distributed in the high salinity area (with salinity >25) while PE2 in the middle salinity area (with salinity 7-20). Our results from the 16S rDNA sequencing also indicated abundant diversity and different niche adaptation of Syn with the operational taxonomic units (OTUs) along the estuary. Besides, analysis also indicated a tight correlation between estuarine Syn and active heterotrophic bacteria, especially groups of Rhodobacteria and Actionobacteria.

Levels of NP and BPA in the Pearl River Estuary, China: Fluctuations with Country Policy Changes over the Past 40 Years

Sediment cores were collected from four outlets in the Pearl River Estuary (Guangdong Province, China) and dated using the ²¹⁰Pb method to investigate the pollution history of the area due to its relatively stable sedimentation status and hydrographic conditions in recent decades. The ages of the sediment cores were dated over 40 years (1968–2015). The concentrations at the four outlets ranged from 2.21 to 48.52 ng g⁻¹ dw for nonylphenol and were non-detectable for 23.64 ng g⁻¹ dw for bisphenol A (BPA), which exhibited a decreasing trend from north to south as well as seaward. The fluxes (2.84 to 112.91 ng cm⁻² yr⁻¹ and non-detectable to 59.33 ng cm⁻² yr⁻¹ for nonylphenol and bisphenol A, respectively) stabilized in the 1980s to 1990s due to the construction of sewage treatment systems. The fluxes increased again in the 21st century, which reached a peak ca. 2010 but declined in recent years due to the establishment of regulations and the decreasing number of industrial enterprises. Fluctuations in the pollution composition coincided with industrial development and governmental policies.

Halogenated flame retardants in mangrove sediments from the Pearl River Estuary, South China: Comparison with historical data and correlation with microbial community

Polybrominated diphenyl ethers (PBDEs), decabromodiphenyl ethane (DBDPE), 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE), tetrabromobisphenol A (TBBPA), hexabromocyclododecane (HBCDD) and dechlorane plus (DP) were measured in sediments collected from three mangrove wetlands of the Pearl River Estuary (PRE) in South China. This study aims to investigate the distribution of these halogenated flame retardants (HFRs) and the correlations between HFRs and microbial community structure in mangrove sediments. Concentrations of PBDEs, DBDPE, BTBPE, TBBPA, HBCDD and DP in mangrove sediments ranged from 6.97 to 216.1, 3.70-26.0, 0.02-0.73, 0.02-37.5, 0.44-127.5 and 0.07-2.23 ng/g dry weight, respectively. Higher levels of PBDEs, BTBPE, HBCDD and DP were observed in sediments from Futian mangrove wetland of Shenzhen, the only nature reserve located in the downtown of China. The highest concentration of TBBPA found in mangrove sediments from Guangzhou was proximate to a ferry terminal and a dockyard where TBBPA is widely used in the coatings. PBDEs were the predominant HFRs in mangrove sediments, with an average contribution of 63.0%. Mangrove sediments from Guangzhou and Zhuhai showed an enrichment of (-)-α-HBCDD, (-)-β-HBCDD and (-)-γ-HBCDD. Concentrations of HFRs in mangrove sediments from Guangzhou increased significantly from 2012 to 2015, which was probably due to the establishment and rapid development of Nansha Free Trade Zone of Guangzhou. Redundancy analysis showed that HFRs may cause a shift of microbial community structure in mangrove sediments and the variations were significantly correlated with TBBPA, syn-DP and BTBPE.

[Temporal and Spatial Characteristics of Heavy Metals in Suspended Particulate Matter in Pearl River Estuary and Its Influencing Factors]

Simultaneous sampling and observation were conducted at 16 stations in the Pearl River Estuary, and the temporal and spatial characteristics of the contents of six heavy metals (Cr, Cu, Zn, Pb, As, and Cd) were analyzed. The influencing factors of suspended particulate matter (SPM) were explored through the combined results of our statistical analyses. With the influence of discharge, temperature and particle size, the content of heavy metals in the dry season is higher than that in flood season. Regarding the estuary filter, the content of heavy metals in the estuary segment was lower than that in the far mouth segment, which was principally caused by the difference in salinity and sediment concentration between the two segments during the dry season. Both the sediment quality standard and enrichment factor methods were used to evaluate heavy metals. The results showed that the threat of heavy metals in the dry season was stronger than that in the flood season. As and Zn were more harmful to the ecological risk, while Cd, As, and Zn were the more abundant pollutants in the Pearl River Estuary. Heavy metal pollution was more severe in the far mouth segment. The degree of pollution for Cu, Zn, As, and Cd- which are classified as heavy pollution, severe to extremely heavy pollution, severe heavy pollution, and extremely heavy pollution, respectively-in the Beijiang River were higher compared to the Xijiang River and Dongjiang River. The pollution level of the six heavy metals in the four eastern outlets and four western outlets were similar, and the most serious pollution in Humen may be related to local industrial development. The source of heavy metals in SPM of the Pearl River Estuary is related to geological background, industry, and the mining area.

Effects of mangrove plant species on accumulation of heavy metals in sediment in a heavily polluted mangrove swamp in Pearl River Estuary, China

The present study compared accumulation of heavy metals in a mangrove swamp dominated by Kandelia obovata with that by Sonneratia apetala in Pearl River Estuary, China. The results showed that the concentrations of heavy metals at all sediment depths in the S. apetala site were significantly higher than that in K. obovata. The geo-accumulation index and potential ecological risk index also showed that S. apetala sediment had a higher contamination of heavy metals, especially Cd. S. apetala significantly altered the biogeochemical cycles of Cd, lead (Pb), nickel (Ni) and chromium (Cr). In S. apetala sediment, TOC played an important role in sequestering heavy metals as reflected by its positive correlations with Zn and Pb. This study demonstrated the importance of plant species in altering soil quality and heavy metal accumulation, and S. apetala is more efficiently working as a pollution barrier than K. obovata.