Dynamics of trace metals with different size species in the Pearl River Estuary, Southern China

Microbial pterin biomolecules facilitate algal blooms in response to nutrient pressure in estuarine and coastal continuum

Pterins are ubiquitous biomolecules synthesized by diverse phytoplankton, serve as pigments, cofactors, precursors, and redox sensors, playing crucial roles in ocean carbon fixation and nutrient cycling. However, the mechanisms driving their production, distribution, and fate in marine ecosystems are not well understood. This study examines the generation and behavior of microbial pterins in the Jiulong River Estuary, a system affected by nutrient-rich discharges. Results reveal distinct patterns: pterin concentrations remain uniform across water columns during flood periods but vary significantly during dry periods. During flooding, microbial pterins rank as isoxanthopterin > neopterin > dihydroneopterin > biopterin, while biopterin and isoxanthopterin dominate during dry conditions. Elevated pterin levels in the upper estuary during flooding suggest rapid production in response to nutrient influx, which drives algal proliferation. Positive correlations between pterins and chlorophyll-a (chl-a) highlight photoautotrophic microbes as primary contributors. Notably, cellular biopterin peaks during exponential algal growth, indicating its preparatory role in bloom proliferation. As dual biomarkers with chl-a, microbial pterins enhance the specificity of bloom detection and offer insights into bloom dynamics and nutrient-driven changes. These findings underscore the ecological significance of pterins in nutrient cycling and their potential as bioindicators, warranting further research into their broader environmental implications.

Coastal redox shifts over the past 167 years and preservation of total organic carbon and total nitrogen

This study reconstructs the environmental history of Xincun Lagoon over the past 167 years using sediment core XCW, employing Cu/Zn as a proxy for redox changes. Time-series analysis of Cu/Zn ratios reveals a significant decline (linear regression slope = -0.00082, p < 0.001; Mann-Kendall τ = -0.601, p < 0.001), indicating a steady reduction in redox potential and oxygen levels throughout the study period. This trend is attributed to anthropogenic activities, eutrophication, and climate-induced changes. Additionally, correlation analysis highlights strong linear relationships between Cu/Zn ratios and both total organic carbon (TOC) and total nitrogen (TN), emphasizing the role of redox conditions in the preservation of TOC and TN in lagoon sediments. The study also identifies a shift in organic matter sources from predominantly marine to a mix of marine and terrestrial inputs after 1990. These findings offer new insights into the interplay between redox dynamics and sedimentary processes, advancing our understanding of the lagoon's environmental evolution.

Hidden threat in turbid waters: Quantifying and modeling the bioaccumulation and risks of particulate metals to clams

A major proportion of metal contaminants in aquatic environments is bound to suspended particulate matter (SPM), yet environmental monitoring typically focuses on dissolved metals, with the filtration step removing SPM. This step may inadvertently hide the potential risks posed by particulate metals. In this study, we used stable isotope tracers to quantify the contributions of SPM-bound metals to the bioaccumulation of nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) in Ruditapes philippinarum, a widely distributed clam crucial to global aquaculture. A microporous hollow-fiber filter system was employed to maintain consistent dissolved metal concentrations across treatments, enabling precise assessment of elevated bioaccumulation due to particle ingestion. Our results demonstrated that particulate metals contributed negligible Cd, but accounted for 21.1% of Ni, 67.3% of Zn, 69.5% of Pb, and 73.6% of Cu bioaccumulation in the presence of 10-68 mg L-1 SPM. Assimilation efficiencies varied among metals, with Pb at 1.5%, Ni at 14.7%, Zn at 48.4%, and Cu at 85.8%. Toxicokinetic modeling further revealed that the bio-uptake of particulate metals can surpass the dissolved metal, challenging the assumption that particulate metals are less bioavailable. Field sampling validated the model's predictive capacity for metal bioaccumulation. These findings underscore the need to reconsider environmental monitoring protocols and revise water quality criteria, especially in turbid coastal waters where aquaculture is prevalent.

Adsorption and distribution of heavy metals in aquatic environments: The role of colloids and effects of environmental factors

The study investigated the distributions of heavy metals (Cd, Cr, Cu, Mn, and Pb) between dissolved fraction (<0.7 µm) and particles (>0.7 µm) during the adsorption process. The dissolved fraction was further separated into truly dissolved (<3 kDa) and colloidal (3 kDa-0.7 µm) fractions. Significant metal adsorption occurred on the colloids, resulting in their aggregation into particles, which in turn influenced the particle adsorption kinetics. Colloids could either accelerate or inhibit the transformation of metal ions into particulates, depending on their stability. Competitive metals for colloids (Pb and Cr) were more susceptible to the effects of colloids than other elements. DOM was the predominant environmental factor influencing colloid behavior. The XDLVO theory showed that DOM enhanced the negative charge of colloids and made the colloid surface more hydrophilic, inhibiting the aggregation of colloids. DOM resulted in substantial increases in the concentrations of colloidal Pb and Cr from 0.31 μg/L and 4.58 μg/L to 20.52 μg/L and 43.51 μg/L, respectively, whereas the increment for less competitive metals (Cd and Mn) was smaller. These findings suggest that the distribution of heavy metals is influenced not only by adsorption from particles and ions but also by the complex dynamics of colloids.

The influences of Yellow River input and nutrient dynamics on colloidal Fe migration in the Bohai Sea, China

The coupling relationship between the <1 kDa, 1-3 kDa, 3-10 kDa, 10-100 kDa, and 100 kDa-0.45 μm Fe fractions and the environmental factors in the Bohai Sea (BS) was investigated. The 1-100 kDa Fe in the surface water exhibited a non-conservative phenomenon during the river-sea mixing process, which was related to the removal of colloidal Fe via flocculation during this process. For the bottom water, the ligands released by the sediments may form additions to the <100 kDa Fe. The COC and DOC were mainly closely related to the behavior of the Fe in the bottom water. The <1 and 3-10 kDa Fe was mainly significantly positively correlated with the DOC, while the <100 kDa-0.45 μm Fe was significantly negatively correlated with the DOC. <100 kDa LMW colloidal Fe exhibited more synergistic behavior with easily absorbed ammonium salts.

Turbid Waters and Clearer Standards: Refining Water Quality Criteria for Coastal Environments by Encompassing Metal Bioavailability from Suspended Particles

Suspended particulate matter (SPM) carries a major fraction of metals in turbid coastal waters, markedly influencing metal bioaccumulation and posing risks to marine life. However, its effects are often overlooked in current water quality criteria for metals, primarily due to challenges in quantifying SPM's contribution. This contribution depends on the SPM concentration, metal distribution coefficients (Kd), and the bioavailability of SPM-bound metals (assimilation efficiency, AE), which can collectively be integrated as a modifying factor (MF). Accordingly, we developed a new stable isotope method to measure metal AE by individual organisms from SPM, employing the widely distributed filter-feeding clam Ruditapes philippinarum as a representative species. Assessing SPM from 23 coastal sites in China, we found average AEs of 42% for Zn, 26% for Cd, 20% for Cu, 8% for Ni, and 6% for Pb. Moreover, using stable isotope methods, we determined metal Kd of SPM from these sites, which can be well predicted by the total organic carbon and iron content (R2 = 0.977). We calculated MFs using a Monte Carlo method. The calculated MFs are in the range 9.9-43 for Pb, 8.5-37 for Zn, 2.9-9.7 for Cu, 1.4-2.7 for Ni, and 1.1-1.6 for Cd, suggesting that dissolved-metal-based criteria values should be divided by MFs to provide adequate protection to aquatic life. This study provides foundational guidelines to refine water quality criteria in turbid waters and protect coastal ecosystems.

Climate and Environmental Variables Drive Stream Biofilm Bacterial and Fungal Diversity on Tropical Mountainsides

High mountain freshwater systems are particularly sensitive to the impacts of global warming and relevant environmental changes. Microorganisms contribute substantially to biogeochemical processes, yet their distribution patterns and driving mechanism in alpine streams remain understudied. Here, we examined the bacterial and fungal community compositions in stream biofilm along the elevational gradient of 745-1874 m on Mt. Kilimanjaro and explored their alpha and beta diversity patterns and the underlying environmental drivers. We found that the species richness and evenness monotonically increased towards higher elevations for bacteria, while were non-significant for fungi. However, both bacterial and fungal communities showed consistent elevational distance-decay relationships, i.e., the dissimilarity of assemblage composition increased with greater elevational differences. Bacterial alpha diversity patterns were mainly affected by chemical variables such as total nitrogen and phosphorus, while fungi were affected by physical variables such as riparian shading and stream width. Notably, climatic variables such as mean annual temperature strongly affected the elevational succession of bacterial and fungal community compositions. Our study is the first exploration of microbial biodiversity and their underlying driving mechanisms for stream ecosystems in tropical alpine regions. Our findings provide insights on the response patterns of tropical aquatic microbial community composition and diversity under climate change.

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

The research delved into the occurrence and dynamics of dissolved metals, specifically manganese (Mn) and copper (Cu), within the Jiulong River Estuary, South China, a medium-sized subtropical estuary. Our findings unveiled a nuanced seasonal and spatial variability of dissolved metals throughout the entire estuarine system. Notably, dissolved Mn concentrations peaked (~ 3.5 μM) in the upper estuary, diminishing sharply along the salinity gradient, with a modest rise in the middle estuary and outer Xiamen Bay. In the upper estuary, heightened concentrations 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 from increased freshwater discharges and particle scavenging. In contrast, dissolved Cu exhibited differently, with elevated concentrations (29.2-37.5 nM) in the upper and middle estuaries, driven by reductive dissolution of Mn particles and chloride-induced ion exchanges, respectively. Concurrently, heightened inputs of nutrients and metals correlated with elevated phytoplankton productivity (indicated by chlorophyll a) in the upper and outer estuary regions. Our analysis underscored the sensitivity of dissolved metals to environmental parameters, including temperature, pH, and dissolved oxygen. The integration of compiled historical data underscored the dynamic nature of dissolved metals, particularly Cu, in response to geochemical processes.The elevated ion levels indicated intensified ion releases from particles and sediments, attributable to increased anthropogenic perturbation and climatic changes (e. g. ocean warming).

Key factors influencing pollution of heavy metals and phenolic compounds in mangrove sediments, South China

Concentrations of heavy metals (HMs) and phenolic compounds with factors which potentially affected their spatial distribution were investigated in mangrove sediments, South China. Compared to Qi'ao, Futian sediments exhibited higher levels of Pb and nonylphenol (NP), but lower levels of Co and Ni. Seasonal variation showed higher concentrations of Pb, Cr, Co, NP and bisphenol A (BPA), while lower concentration of methylparaben (MP) in wet than dry season. Contaminant levels in sediments collected at different tidal heights showed insignificant variations, except for Zn and NP. MP was found negatively correlated with nearly all HMs and BPA, whereas the latter exhibited positive correlations with each other. Sedimentary total carbon, total nitrogen, C/N and N/P ratios were screened as the most influential factors affecting the distribution of these contaminants. Additionally, both salinity and total phosphate exhibited positive, while both pH and sedimentary particle size registered negative correlation, with one or more contaminants.

Terrestrial nanoparticle contaminants and geospatial optics using the Sentinel-3B OLCI satellite in the Tinto River estuary region of the Iberian Peninsula

The Tinto River is known globally for having a reddish color due to the high concentration of dissolved metals in its waters. The general objective of this study is to analyze the dispersion of nanoparticles (NPs) and ultra-fine particles in terrestrial and geospatial suspended sediments (SSs) using Sentinel-3B OLCI (Ocean Land Color Instrument) satellite images; by examining water turbidity levels (TSM_NN), suspended pollution potential (ADG_443_NN) and presence of chlorophyll-a (CHL_NN). The images were collected in the estuary of the Tinto River, in the city of Nerva, Spanish province of Huelva, between 2019 and 2021. The following hazardous elements were identified in nanoparticles and ultra-fine particles by FE-SEM/EDS: As, Cd, Ni, V, Se, Mo, Pb, Sb and Sn. Sentinel-3B OLCI satellite images detected a 2019 TSM_NN of 23.47 g^-3, and a 2021 reading of 16.38 g^-3.

Higher Risks of Copper Toxicity in Turbid Waters: Quantifying the Bioavailability of Particle-Bound Metals to Set Site-Specific Water Quality Criteria

In coastal waters, particulate metals constitute a substantial fraction of the total metals; however, the prevalent water quality criteria are primarily based on dissolved metals, seemingly neglecting the contribution of particulate metals. Here we developed a method to quantify the toxicity risk of particulate metals, and proposed a way to calculate modifying factors (MFs) for setting site-specific criteria in turbid waters. Specifically, we used a side-by-side experimental design to study copper (Cu) bioaccumulation and toxicity in an estuarine clam, Potamocorbula laevis, under the exposure to "dissolved only" and "dissolved + particulate" ⁶⁵Cu. A toxicokinetic-toxicodynamic model (TK-TD) was used to quantify the processes of Cu uptake, ingestion, assimilation, egestion, and elimination, and to relate mortality risk to tissue Cu. We find that particulate Cu contributes 40-67% of the Cu bioaccumulation when the suspended particulate matter (SPM) ranges from 12 to 229 mg L^-1. The Cu-bearing SPM also increases the sensitivity of organisms to internalized Cu by decreasing the internal threshold concentration (C_IT) from 141 to 76.8 μg g^-1. MFs were derived based on the TK-TD model to consider the contribution of particulate Cu (in the studied SPM range) for increasing Cu bioaccumulation (MF = 1.3-2.2) and toxicity (MF = 2.3-3.9). Water quality criteria derived from dissolved metal exposure need to be lowered by dividing by an MF to provide adequate protection. Overall, the method we developed provides a scientifically sound framework to manage the risks of metals in turbid waters.

Dissolved rare earth elements in the Pearl River Delta: Using Gd as a tracer of anthropogenic activity from river towards the sea

Growing demands for high-tech uses of rare earth elements (REE) result in their releases into the aquatic environment, but local anthropogenic sources and water chemistry reactions may vary in aquatic systems. In this study, we quantified the yttrium and REEs (REY) in the surface waters of the Pearl River Delta (PRD), Southern China, and investigated their sources and processes controlling the dissolved REY distributions from rivers towards the sea. Spatial variations of dissolved REY concentrations (<0.4 μm) were observed in the surface water due to both natural and anthropogenic sources. Salt-induced flocculation removed colloidal REY from water during estuarine mixing, particularly at low salinity. In contrast, a significant increase of dissolved REY concentration with salinity suggested the occurrence of REY desorption from particles during estuarine mixing. The PAAS-normalized REE patterns (i.e., filtrates <0.4 μm and <3 kDa) showed a significant light REE (LREE) depletion relative to heavy REE (HREE) in dissolved fractions, indicating reactive LREE adsorption onto particles. Positive Gd anomalies in most waters revealed significant impacts from human activities, such as effluents from wastewater treatment plants in the PRD. In particular, the anthropogenic Gd contributed 90 % of the total dissolved Gd along the Pearl River and was conservatively mixed with saltwater in the estuary via the Humen outlet. Positive Sm anomalies were also observed in the PRD, indicating other point sources related to industrial activity. With the increase of urbanization and high demand for green technology, the REY anomalies could be a good indicator of human activities on assessing their fluxes and impacts on the aquatic systems.

Risk assessment and spatio-temporal distribution of dissolved trace metals in Swarna, Sharavati and Kali estuaries, South-West Coast of India

Trace metals act as a limiting nutrient and prerequisite for primary productivity in marine environments. The distribution of metals in dissolved phase along the salinity gradients of Swarna, Sharavati and Kali estuaries in southwestern India, during post and pre-monsoon seasons, were studied. We have investigated the behaviour of trace metals in the estuarine environment and their extent of impact on human health and ecosystem. The study revealed, non-conservative behaviour of dissolved Mn, Fe, Ni, Cd and Co in the estuaries. Whereas Cu behaved non-conservatively in post-monsoon and conservatively in pre-monsoon seasons. Risk assessment studies revealed that higher chronic daily intake (CDI) in humans, through dermal pathway, in Swarna and Sharavati estuaries during post-monsoon, whereas it was during pre-monsoon season in the Kali estuary. Hazard Index values for the studied metals in adults and children are below risk thresholds, though children are more prone to health risk through the dermal pathway.

Occurrence, contamination level and ecological risk assessment of dissolved and particulate trace elements in rivers entering the southwestern Mediterranean Sea

Metal pollution in rivers should not be overlooked before their entry into the sea. However, there are few studies for estimating such contamination in rivers entering the Algerian coastal waters. Semimonthly quantification of dissolved and particulate metals, near the mouths of two industrial-tainted rivers, El Harrach and Mazafran rivers, was carried out during a period of one year. All the trace metals analyzed are originating from anthropogenic sources (EF > 1.5), with higher contamination of dissolved Pb, Cd, Zn and Ni and a slight degree of contamination of particulate Cu and Zn (0 < I_geo < 1). Particulate metals show a stable complex with the particulate phase (e.g. 2 < LogK_d < 6). The risk assessment results indicate that particulate Pb and Zn have a 33% likelihood of toxicity for adverse biological effects. A significant toxicity effect (ΣTUi >4) of the combined particulate metals (Pb, Cd, Cu, Zn, Cr, Ni and As) was primarily due to the higher particulate Cd, Zn, and Cr availability.

Bio- and photo-lability of dissolved organic matter in the Pearl River (Zhujiang) estuary

We investigated the bio- and photo-lability of dissolved organic matter (DOM) from the head, mixing zone, and mouth of the Pearl River estuary. At all three sites, bio- and photo-refractory dissolved organic carbon (DOC) and biorefractory chromophoric DOM (CDOM) dominated over the corresponding bio- and photo-labile constituents, while photolabile CDOM dominated over photo-refractory CDOM. Relative to the mixing-zone and mouth waters, the headwater was enriched with bio- and photo-labile DOC and photolabile CDOM and depleted with biolabile CDOM. Biolabile DOC was richer than photolabile DOC in the headwater, while photolabile CDOM was richer than biolabile CDOM at all three sites. Pre-biotransformation inhibited, stimulated, or had little impact on DOM photodegradation, depending on site. Ultra-violet absorption coefficients are indicators of bio- and photo-refractory DOC. The relative proportions of transparent and chromophoric DOM control the turnover of biolabile DOC and the effect of pre-biotransformation on DOM photodegradation.