Sediment Pore Water Research Articles

Elevated methylmercury production in seasonally inundated sediments: Insights from DOM molecular composition.

Seasonally inundated areas (SIA) within aquatic systems are characterized by elevated methylmercury (MeHg) production. Nevertheless, the response characteristics of dissolved organic matter (DOM) quality in SIA sediments, including its molecular compositions and structure, and their impacts on the MeHg production are not yet fully understood. This research gap has been addressed through field investigations and microcosm experiments conducted in a metal-polluted plateau wetland. The results revealed that DOMSIA had lower levels of chromophoric DOM concentrations, protein-like fractions, molecular complexity, and debris size while exhibiting higher humic-like fractions, molecular weight, COO- groups, and bioavailability than DOM in permanently inundated areas (PIA). Compared with DOMPIA, DOMSIA was more easily biodegraded, and exhibited a higher adsorption capacity while lower binding affinity for Hg(Ⅱ). Moreover, MeHg synthesis by Desulfomicrobium escambiense was 29.6-fold higher in DOMSIA than that in DOMPIA, and DOMSIA amendment also resulted in a higher MeHg production in the sediment. The PLS-PM model demonstrated that DOM compositions positively showed high contributions to MeHg levels in sediment porewater (0.51), while binding affinity had a negative pattern (-0.83), but adsorption capacity had a lower contribution (0.09). These findings provide an updated explanation for the elevated MeHg level in the SIA of aquatic systems, which are closely related to the adaptive response of DOM molecular composition and structure in the sediment.

Array of metabolic pathways in a kleptoplastidic foraminiferan protist supports chemoautotrophy in dark, euxinic seafloor sediments.

Investigations of the metabolic capabilities of anaerobic protists advances our understanding of the evolution of eukaryotic life on Earth and for uncovering analogous extraterrestrial complex microbial life. Certain species of foraminiferan protists live in environments analogous to early Earth conditions when eukaryotes evolved, including sulfidic, anoxic, and hypoxic sediment porewaters. Foraminifera are known to form symbioses as well as to harbor organelles from other eukaryotes (chloroplasts), possibly bolstering the host's independence from oxygen. The full extent of foraminiferal physiological capabilities is not fully understood. To date, evidence for foraminiferal anaerobiosis was gleaned from specimens first subjected to stresses associated with removal from in situ conditions. Here, we report comprehensive gene expression analysis of benthic foraminiferal populations preserved in situ on the euxinic (anoxic and sulfidic) bathyal seafloor, thus avoiding environmental alterations associated with sample recovery, including pressure reduction, sunlight exposure, warming, and oxygenation. Metatranscriptomics, metagenome-assembled genomes, and measurements of substrate uptake were used to study the kleptoplastidic foraminifer Nonionella stella inhabiting sulfur-oxidizing bacterial mats of the Santa Barbara Basin, off California. We show N. stella energy generation under dark euxinia is unusual because it orchestrates complex metabolic pathways for ATP production and carbon fixation through the Calvin cycle. These pathways include extended glycolysis, anaerobic fermentation, sulfide oxidation, and the presence of a membrane-bound inorganic pyrophosphatase, an enzyme that hydrolyzes inorganic pyrophosphate to actively pump protons across the mitochondrial membrane.

Different continuous freshwater contributions to submarine groundwater discharge at a coastal peatland, southern Baltic Sea

ABSTRACT The impact of freshwater sources like surface river runoff and submarine groundwater discharge (SGD) on coastal waters is currently in focus of intense debate and investigation. One of the ongoing challenges in SGD research is the characterization and quantification of the freshwater endmember contributions to the subsurface mixing zone and their influences on element balance and biogeochemical transformations. Long-term investigations of the sediment porewater composition provide characterization and understanding of the physical, hydrological and biogeochemical processes controlling the substance exchanges. In this study, we focus on the hydrochemical and stable isotope (δ 2H, δ 18O) compositions of sediment porewaters along the coastline of a southern Baltic Sea peatland. Coastal surface water and groundwater dynamics were monitored at two coastal sites using 5-m-long stationary lances over a 5-year period. The vertical compositional gradients were used to extrapolate to zero-salinity (ZS) components applying a binary mixing model on the salinity and water isotope composition. The results characterize a subterranean estuary (STE) with three potential mixing endmembers: two fresh groundwaters and the brackish Baltic Sea. Tritium–helium (3H–3He) porewater dating gave ages of more than about 20 years for the freshwater components. The ZS components were compared with other SGD sites along the southern Baltic Sea and North Sea and highlight the importance of local SGD studies for a proper groundwater endmember characterization as basis to understand hydrological and biogeochemical developments at the land–ocean continuum in times of current climate change.

Evaluating drinking water treatment residuals as an in-situ capping material for metal-contaminated sediments

This study evaluated drinking water treatment residuals (DWTR) as an in-situ capping material for metal-contaminated sediments using Gust-chamber experiments. Metal release from non-capped and DWTR-capped sediments was measured under increasing shear stress (τ) from 0.05 to 0.4 Pa. Fathead minnow (FHM) juveniles (Pimephales promelas) were exposed to water from these sediments in 96-h bioassays to assess DWTR's efficacy in reducing metal toxicity. Sand was used as an inert capping material for comparison. Diffusive gradients in thin films (DGT) assessed DWTR's impact on vertical metal concentration profiles in sediment pore water and overlying water, with concentrations determined by ICP-MS. Without capping, increasing τ raised metal concentrations in the overlying water from 45 to 95 mg/L for Cd and Zn, 4–10 mg/L for Cu, and 2–4 mg/L for Pb. Sand capping reduced these levels, with Cd and Zn ranging from 4 to 21 mg/L, Cu from 0.26 to 0.63 mg/L, and Pb from 0.051 to 0.23 mg/L. DWTR capping significantly lowered metal concentrations in the overlying water, with Cd ranging from 1 to 8 μg/L, Zn from 30 to 40 μg/L, Cu from 2.5 to 5 μg/L, and Pb from 1 to 2 μg/L. Therefore, beyond the physical barrier effect, the DWTR cap immobilizes metals through other mechanisms such as sorption and precipitation. Bioassays showed that DWTR significantly decreased metal toxicity to FHM, while sand-capped and non-capped sediments caused 100% mortality. DGT confirmed DWTR reduced metal fluxes at the sediment-water interface by up to two orders of magnitude.

Developing a consistent model for predicting equilibration in polymeric passive samplers across various HOC classes in sediment pore water.

The use of polymeric passive samplers (often polydimethylsiloxane, PDMS, or low-density polyethylene, LDPE) to indicate water concentrations of hydrophobic organic compounds under environmental conditions generally requires two key parameters, a compound-specific polymer-water equilibrium partition coefficient (), and the degree of equilibration achieved in a given environmental exposure scenario. Herein, we have developed model to extrapolate equilibration of performance reference compounds between polymer and pore water across different compound classes that is also dependent upon accurate estimates of . We have also developed quantitative structure-activity relationship (QSAR) models, to estimate for PAHs, PCBs, DDx, and dioxin/furans for the two most common polymeric samplers, PDMS and LDPE. The QSAR models developed in this study provide high accuracy consistent estimates for across the different HOC families. The root mean square error of the QSAR for was 0.226 log units based upon measured values for 159 compounds and 0.184 log units for based upon measured values for 131 compounds.

Drivers of Seasonal and Diel Methane Emissions From a Seagrass Ecosystem

AbstractSeagrass meadows are effective sinks of atmospheric carbon dioxide (CO2). However, there is little insight on how methane (CH4) emissions may potentially offset carbon sequestration in seagrass meadows. Here, we resolve diel and seasonal dynamics of CH4 and CO2 water‐air fluxes over a cold‐temperate Zostera marina seagrass meadow using high‐resolution timeseries observations in seawater. CH4 was emitted from the seagrass‐dominated coastal bay year‐round to atmosphere with CH4 fluxes ranging from 0.2 to 2.6 μmol m−2 d−1. These fluxes are at the lower end of earlier estimates based mostly on short‐term (i.e., 1 day) observations. The 13‐fold seasonal fluctuations in CH4 emissions were greater than the 6‐fold diel fluctuation. Radon observations imply that dissolved CH4 was primarily originated from sediment porewater. The main fate of CH4 in the water was outgassing to the atmosphere via wind forcing. Oxygen and temperature partially controlled dissolved CH4 seasonal dynamics. There was an annual average uptake of CO2 from the atmosphere (−0.9 ± 1.5 mmol m−2 d−1) driven by enhanced photosynthesis in the spring and summer. The CO2‐equivalent CH4 outgassing (0.5 ± 0.6 g CO2 eq m−2 yr−1) offsets only 0.8% of the sediment carbon accumulation in this cold‐temperate Z. marina meadows over a 20‐year time horizon. The CO2‐equivalent CH4 flux was 6% of the average annual CO2 uptake. Hence, CH4 emissions from this cold‐temperate seagrass meadow acted as a minor offset to carbon sequestration.

First dating of an early Chibanian (Middle Pleistocene) glacial overdeepening in the Alpine Foreland using the 4He/U-Th method

Abstract The 4He/U-Th dating method can be used to estimate the residence time of pore waters in low-permeable rocks and consolidated sediments, serving as a proxy for sediment deposition time. This residence time is inferred from the accumulation time of radiogenic 4He measured in the pore water being produced by the local decay of U and Th in the sediment matrix. We applied the 4He/U-Th method to date the pore waters of unconsolidated sediments from a glacial overdeepening in the Swiss Plateau (northern Alpine Foreland), where prior studies suggested sediments older than Marine Isotope Stage (MIS) 6 (191–130 ka). We show that compact and fine-grained (glaci)lacustrine sediments provide low-permeability conditions that allow 4He to accumulate in the pore water and be preserved in the pore space. The 4He/U-Th dating indicates that the sediments between 40 m and 140 m are 606 ± 122 ka. The dated infill was deposited in a glacial overdeepening eroded by a foreland glaciation larger than that of the Last Glacial Maximum (LGM). The results reveal extensive foreland glaciations and intense glacial overdeepening erosion during the early part of the Chibanian (i.e., Middle Pleistocene). This work highlights the potential of the 4He/U-Th method for dating sediments in similarly favorable hydrogeological settings.

Delving into nitrogen and phosphorus dynamics in shallow eutrophic lakes: Multi-interface response to freeze-thaw cycles

Assessing the impact of freeze-thaw cycles on nutrient transfer in lakes is crucial for addressing the global eutrophication of freshwater ecosystems in cold and arid regions. However, available information about the dynamics of nitrogen (N) and phosphorus (P) release during intact freeze-thaw cycles, especially in the sediment–porewater–water column continuum of lakes, is limited. This study collected the samples during ice-covered (January) and non-ice-covered (April, July, and October) periods. The changes in total nitrogen (TN) and total phosphorus (TP) were analyzed to estimate their release fluxes from the sediment-water interface. Both redundancy analysis (RDA) and variance partitioning analysis (VPA) were used to explore the effects of environmental variables on N and P. The results indicated that during the ice-covered period (ICP), the surface water content of different forms of N and P was lower than that of the overlying water, whereas the opposite was true during the non-ice-covered period (NICP). The overall trend for the different forms of N was TN > DIN > NH4-N > NO3-N > NO2-N, and for P, it was TP > PP > DTP > DOP>DIP. The vertical profiles of the porewater TN and TP generally demonstrated an increase followed by a decrease from the surface to the bottom, with an inflection point at 15 cm. Sediment TN and TP trends over time matched porewater trends, with both being spatially highest at the lake inlet. ICP sediments acted as sinks for TN and TP, and NICP sediments acted as sources. N and P in the water column were significantly correlated mainly with physicochemical indicators, whereas sediment contributed positively to TN and TP in the porewater. VPA indicated that environmental factors explained 46.45 % of the variation in TN and TP in the porewater. These findings emphasize the importance of freeze-thaw cycling processes in driving N and P nutrient enrichment, source-sink effects, and multi-media coupling in shallow lakes in arid plateau regions.

Recognizing microbial manganese reduction in lacustrine carbonate and its linkage to terrestrial biogeochemical processes

Carbonate authigenesis, the in situ precipitation of carbonate minerals within the sediment porewaters, is a key pathway for carbonate deposition and plays a crucial role in global biogeochemical cycles. Heterotrophic microorganisms are essential in regulating authigenic carbonate formation, primarily through the consumption of organic matter. Although bacterial manganese reduction is known to influence the formation of rhodochrosite and dolomite, its role in limestone deposition is unclear. Here, we present a systematic investigation of mixed calcareous siliciclastic rocks from a Paleogene freshwater lake to identify the formation of authigenic carbonate, decode the role of microbial Mn reduction, and understand the microbial response to ancient lacustrine environmental changes. The positive correlation between carbonate fraction in bulk samples (Carb%) and Mn content in carbonate minerals (Mncarb) suggests that carbonate precipitation is stimulated by Mn2+ enrichment. The dissimilarity between Mncarb and Fecarb, along with the synergic variations of Mncarb and diagenetic indicators, support an authigenic rather than a hydrogenetic origin for the carbonates. Using a one-dimensional diffusion–advection-reaction model, we quantify the impact of Mn reduction on promoting carbonate precipitation. Furthermore, correlations between Pcarb and other values–positive with the chemical index alteration (CIA), negative with Mncarb, and none with TOC–suggest that nitrogen availability, regulated by continental weathering, is likely the primary factor limiting both the primary productivity and the bacterial reduction intensity at the study site. Overall, this study uncovers the role of microbial Mn reduction in stimulating authigenic carbonate precipitation, and reveals the modulation mechanism of Mn-reducing microorganisms in an ancient lake. These findings shed new light on the authigenic limestone formation mechanisms and provide a new perspective on interpreting the authigenic impacts on carbonate chemistry.

EARLY DIAGENESIS GEOCHEMISTRY OF BOTTOM SEDIMENTS IN THE PLEISTOCENE CORE OF LAKE KOTOKEL (Eastern Baikal Region)

Chemical composition of bottom sediments and pore waters of organic-mineral sediments (sapropel) of Lake Kotokel (Eastern Baikal region) has been studied, based on long drilling cores, 14.5 and 16.5 m. A reduction type of diagenesis has been established, during which destruction of organic matter, transformation of the chemical composition of pore waters and the formation of authigenic minerals occur. Even in the uppermost intervals of sapropel, organic matter is being profoundly transformed and differs significantly in composition from that of bioproducers (plankton). The major role in diagenetic transformations of organic matter belongs to different physiological groups of microorganisms, primarily heterotrophic, amonifying and sulfate-reducing bacteria. During diagenesis, the basic chemical composition of pore waters (HCO3–, SO42–, Cl–, Ca2+, Mg2+, K+, Na+) changes, trace elements (Fe, Mn, Sr, Ba, Pb, As, Co, Ni) redistribute, concentrations of HCO3–, NH4+, PO43− and Si increase; this is caused by destruction of organic matter. In the process of bacterial sulfate reduction in pore waters, the concentration of SO42– decreases along the depth of the section, and in the sediment the proportion of reduced forms of sulfur increases and the isotopic composition of sulphur δ34S changes. Transformation of chemical composition of pore waters and the activity of microorganisms leads to the formation of authigenic pyrite, rhodochrosite, and barite.

Migration and transformation mechanisms of iron and manganese during river infiltration affected by the changes in riverbed sediment thickness

Riverbed scouring and siltation, affected by variations in river flow and sediment concentration, can cause changes in the sediment thickness, leading to the complex feedback between riverbed permeability and nutrient–reactive transport processes. Currently, the migration and transformation of iron and manganese under changed riverbed sediment thickness have not been taken into account. Based on indoor one-dimensional soil column simulation experiments, along with in-situ monitoring, Rhizon pore water sampling, and 16 s rRNA high-throughput sequencing technologies, this study revealed the migration and transformation patterns of iron and manganese in the river infiltration zone and their contribution rates under different sediment thickness conditions. The results demonstrated that as the riverbed sediment thickness increased, the river infiltration rate and sediment permeability coefficient demonstrated a significant decrease over time. For instance, a 5 cm sediment thickness can decrease the sediment permeability coefficient by 30 % within 32 d of infiltration, expand the disconnection zone to 25 cm, narrow the oxidation–reduction zone, and cause the iron and manganese reduction zone to evolve from a single peak to a double peak pattern. Additionally, as the sediment thickness increased, the contribution of organic matter bound iron-manganese oxidation and iron-manganese oxide reduction to Mn2+ and Fe2+ concentrations in sediment pore water decreased, while the contribution of adsorption and complexation-precipitation increased. This study holds great significance for ensuring the safety of drinking water supply and promoting the sustainable utilization of groundwater resources.

Two-dimensional determination of dissolved manganese in sediment porewaters

We present a new method for imaging dissolved manganese at millimeter scale by coupling DET (diffusive equilibrium in thin film) and colorimetric techniques. The method is an adaptation of the porphyrin approach for the measurement of dissolved Mn by substitution of Mn(II) and Mn(III) to Cd in the Cd(II)–POR complex. Optimization of the Cd-POR concentrations was required for transposition to 2D-DET. A commercial flatbed scanner and a hyperspectral camera were used for imaging. Using the hyperspectral camera, detection limit is about 5 μM and measuring range is up to 520 μM. The method was applied on the field in a tidal mudflat of the French Atlantic coast and in sediments inhabited by polychaetes. These first images allowed to precisely describe two-dimensional millimeter features such as burrows and highlighted the role of bioirrigation in benthic Mn fluxes. This new technique offers the possibility to investigate the reactivity of microenvironments towards dissolved Mn in two dimensions in a wide range of laboratory and in situ studies using a non-destructive tool.

Enhanced phosphorus removal from anoxic water using oxygen-carrying iron-rich biochar: Combined roles of adsorption and keystone taxa

Anthropogenic enrichment of phosphorus (P) in water environment can cause eutrophication, harmful algal blooms, and water quality deterioration. Adsorbents are often used for the removal and recovery of P from water, however, P is highly susceptible to re-release in anoxic benthic environments. As a response, this study prepared oxygen-carrying iron-rich biochar (O-Fe-BC) as an effective oxygen micro-nanobubble carrier (Q = 8.7024 cm³/g STP at 1.5 MPa) and P adsorbent (qm = 16.7097 mg P/g, q0.1 = 3.1974 mg P/g). Over the 90-day experimental period with O-Fe-BC, dissolved oxygen (DO) levels in the overlying water could maintain at ∼4 mg/L (peaking at ∼9.5 mg/L), and total phosphorus (TP) and soluble reactive phosphorus (SRP) levels decreased by over 96 %. The higher inorganic phosphorus content in the surface sediment-biochar mixture, along with the lower labile P and Fe concentration in the sediment pore water in the O-Fe-BC group compared to other groups, suggested the enhanced P immobilization. Further mechanism exploration revealed the combined roles of adsorption and microbial response, in which O-Fe-BC achieved efficient phosphate adsorption primarily through inner-sphere complexation via ligand exchange and keystone taxa (particularly Candidatus Electronema) played a crucial role in driving water chemistry divergence. Specially, these cable bacteria could provide large pools of Fe oxides in the surface sediment, binding with P to prevent its release, as supported by significant correlations between Ca. Electronema abundance and oxidation–reduction potential (ORP), TP, SRP, and sediment Fe-P variations. Additionally, a pot experiment with mung bean seedlings showed that the recovered O-Fe-BC significantly promoted the seed germination and growth, indicating its potential as a novel material for removing and recovering P from eutrophic waters. Taken together, our work provided a promising strategy for sustainable anoxia and P pollution mitigation, and also highlighted the indispensable roles of inner-sphere adsorption in P recovery and microbial keystone taxa in P cycling regulation.

A landscape limnology approach to assessing controls on soluble reactive phosphorus in sediment porewater and internal loading risk

Sediment nutrients can be mobilized to overlying water via internal loading, which can be important to aquatic productivity. Using data from 143 Canadian lakes, we show high (~2400-fold) variation of soluble reactive phosphorus (SRP) concentrations in surficial sediment porewater, with results suggesting internal phosphorus loading (IPL) is also likely to vary widely. Consistent with past work at smaller scales, we show that lake depth, pH, trophic status, and bulk sediment Al:P and Fe:P influence porewater SRP, and IPL. Median porewater SRP concentration in lakes with high Al:P (molar ratios >10) were 4.8-fold smaller than in lakes with lower Al:P. In lakes where bulk sedimentary Fe:P molar ratios were >10 porewater SRP was 3.9-fold lower than in lakes with lower Fe:P. High pH (>7.8), along with hyper-eutrophic lakes were associated with higher porewater SRP. Finally, shallow lakes (<4 m depth) had median porewater SRP concentration 6-fold higher than deep lakes (>9 m depth). Important regional differences emerged, linked to regional variation in pH, soils, lake depth and trophic status, and paralleling areas of poor water quality. For example, median porewater SRP in lakes from the Boreal Plains and Prairies ecozones (dominated by Chernozems/Mollisols) was 64-fold and 44-fold higher than in the Boreal Shield (dominated by Podzols/Spodosols) (respectively), although we note that IPL risk is likely important across many ecozones. Using national data, we found in-lake measurements (particularly pH, and salinity) showed strong capacity in predicting porewater SRP (explaining 60–72 % of the variance in the data). Importantly, watershed predictors showed good predictive power, explaining ~50 % of variance in porewater SRP using variables including soil types, and % agriculture. Porewater SRP can be predicted with reasonable accuracy using easily measured variables, as can estimates of internal phosphorus loading, suggesting that landscape limnology holds strong potential in helping to inform lake management by informing understanding of in-lake nutrient sources.

Development of an Underwater Adaptive Penetration System for In Situ Monitoring of Marine Engineering Geology.

In recent years, offshore wind farms have frequently encountered engineering geological disasters such as seabed liquefaction and scouring. Consequently, in situ monitoring has become essential for the safe siting, construction, and operation of these installations. Current technologies are hampered by limitations in single-parameter monitoring and insufficient probe-penetration depth, hindering comprehensive multi-parameter dynamic monitoring of seabed sediments. To address these challenges, we propose a foldable multi-sensor probe and establish an underwater adaptive continuous penetration system capable of concurrently measuring seabed elevation changes and sediment pore water pressure profiles. The reliability of the equipment design is confirmed through static analysis of the frame structure and sealed cabin. Furthermore, laboratory tests validate the stability and accuracy of the electrical and mechanical sensor measurements. Preliminary tests conducted in a harbor environment demonstrate the system's effectiveness.

The impact of organic carbon mineralization on pollution and toxicity of toxic metal in sediments: Yellow Sea and East China Sea study

Organic carbon mineralization is the main driving force of metal migration and transformation in sediments, greatly influencing the distribution, pollution degree, and toxicity of toxic metals. However, relevant research on this subject is still limited. In this study, the concentration of toxic metals (Cr, Cd, Cu, Pb, Zn, Co, Fe, Mn, Ni, As) in the solid and liquid phase (porewater) of sediments were measured, toxic metal pollution degree and toxicity of the Yellow Sea (YS) and the East China Sea (ECS) were assessed. Combined with the rate of organic carbon mineralization, the impact of organic carbon mineralization was analyzed. The results showed that Ni was slightly enriched and posed a certain ecological risk, and As was moderately enriched in the studied area, Pb was at a moderate pollution level in the studied area. Zn, Co, Mn, and Fe were at a moderate pollution level in the mud area of SYS and the west coastal area of ECS. Additionally, the total organic carbon mineralization rate (TCMR) in the ECS (5.12–18.04 mmol C m−2 d−1) was slightly higher than that in the YS (3.29–14.46 mmol C m−2 d−1) during spring. Moreover, organic carbon mineralization promotes metal enrichment, and the TCMR was significantly correlated with the pollution load index. Thus, TCMR can be used as an indicator to predict the degree of metal pollution. Furthermore, organic carbon mineralization promotes the mobilization of Cu from the solid phase to the liquid phase, while facilitating the transfer of Cr, Pb, Co, Ni, and Fe from the liquid phase to the solid phase. This process increases the potential risks of Cu and reduces the toxicity of Cr, Pb, Co, Ni, and Fe. Therefore, the impact of organic carbon mineralization should be considered in future assessments and predictions of toxic metal pollution and toxicity.

Evolution of river–aquifer disconnections and the migration and transformation of iron and manganese under non-time-varying/time-varying riverbed permeability

Changes in the composition, structure, and thickness of riverbed sediments caused by riverbed clogging strongly affect the hydraulic connection, migration and transformation of nutrients between river water and groundwater in groundwater source areas. However, previous studies have not extensively investigated the mechanisms of river–aquifer disconnection and the migration and transformation processes of iron and manganese under non-time-varying and time-varying conditions of riverbed permeability. This study developed a model using the COMSOL Multiphysics platform to characterize the riverbed clogging–groundwater exploitation–disconnection process, considering microbial growth and related biogeochemical processes, and investigated feedbacks between the reactive migration of iron and manganese and physical clogging-groundwater exploitation processes or bioclogging processes. The research findings showed that under non-time-varying conditions of riverbed permeability, the evolution of river–aquifer disconnection was strongly affected by the thickness and permeability coefficient of riverbed sediments. The dissolved oxygen attenuation rate in the disconnection zone decreased by up to 88.8 %. Additionally, the Mn2+ and Fe2+ generation rates in sediment pore water decreased by 65.8 % and 62.7 %, respectively. In contrast, during the riverbed bioclogging process, as the biofilms on the surface of the riverbed sediments developed, the sediment pores gradually clogged, leading to a significant reduction in the porosity and permeability coefficient. Consequently, the hydraulic connection between the river and aquifer transitioned from a saturated connection to a disconnection. However, reduced permeability due to riverbed bioclogging primarily controlled the release of Fe and Mn. When the river–aquifer was in complete disconnection, compared to the saturated connection state, the Mn2+ and Fe2+ generation rates increased by up to 5.8 and 3.8 times, respectively. This study deepens our understanding of the biogeochemical cycling mechanisms of Fe and Mn under riverbed clogging conditions in groundwater source areas and contributes to ensuring a secure and stable water supply in these areas.

Insights into response of seagrass (Zostera marina) to sulfide exposure at morphological, physiochemical and molecular levels in context of coastal eutrophication and warming.

Sulfide in sediment porewaters, is toxic to rooted macrophytes in both marine and freshwater environments. Current research on sulfide stress in seagrasses primarily focuses on morphological and physiological aspects, with little known about the molecular response and resistance mechanisms. This study first investigated the damage caused by sulfide to eelgrass (Zostera marina L.) using transcriptomic, metabolomic, and other physiological and biochemical indicators and explored the potential resistance of eelgrass at molecular level through laboratory simulated and in-situ sulfide stress experiments. Comprehensive results showed that sulfide stress severely inhibited the growth, photosynthesis, and antioxidant enzyme activities of eelgrass. Importantly, transcriptome analysis revealed significant activation of pathways related to carbohydrate and sulfur metabolism. This activation served a dual purpose: providing an energy source for eelgrass stress response and achieving detoxification through accelerated sulfur metabolism-a potential resistance mechanism. The toxicity of sulfide increased with rising temperature as evidenced by a decrease in EC50. Results from recovery experiments indicated that when Fv/Fm reduced to about 0 under sulfide stress, the growth and photosynthesis of eelgrass recovered to normal level after timely removal of sulfide. However, prolonged exposure to sulfide resulted in failure to recover, leading ultimately to plant death. This study not only enhances our understanding of the molecular-level impacts of sulfide on seagrasses but also provides guidance for the management and ecological restoration of seagrass meadows under sulfide stress.

Strontium Isotopic Composition as Tracers for Identifying Groundwater Recharge Sources in the Choushui River Alluvial Plain, Western Taiwan

Groundwater is a vital resource in the Chuoshui River alluvial plain (CSAP), a key agricultural area in Taiwan. Understanding groundwater recharge is crucial for sustainable water management amidst changing climatic conditions and increasing water demand. This study investigates the major ion composition, solute Sr concentrations, and 87Sr/86Sr ratios in groundwater and stream water from the Choushui River (CSR) to trace groundwater recharge sources. The Piper diagram reveals that most groundwater samples are of the freshwater Ca–HCO3 type, aligning with the total dissolved solids (TDS) classification. TDS and major ion compositions indicate that groundwater near Baguashan Terrace (BGT) and Douliu Hill (DLH) primarily derives from stream water and rainwater. Na+ and Cl− enrichment in some aquifers of BGT and DLH is attributed to the dissolution of paleo-sea salt and mixing with paleo-seawater from sedimentary porewater. Elevated dissolved Sr concentrations and lower 87Sr/86Sr ratios in these aquifers further support the intrusion of paleo-seawater. Groundwater in the proximal fan shows high TDS due to intensive weathering, complicating the use of TDS as a tracer. Sr isotopic compositions and solute Sr2+ concentrations effectively distinguish recharge sources, revealing that the CSR mainstream primarily recharges the proximal fan and BGT region, while CSR tributaries and rainwater mainly recharge the DLH region. This study concludes that Sr isotopic compositions and solute Sr2+ concentrations are more reliable than TDS and major ion compositions in identifying groundwater recharge sources, enhancing our understanding of groundwater origins and the processes affecting water quality.

Sediment porewaters serve as a transient organic carbon pool at the land-ocean interface

The organic carbon (OC) cycle at the land-ocean interface is an important component of the global carbon budget, yet the processes that control the transfer, transformation, and burial of OC in these regions remain poorly understood. In this work, we examined sedimentary OC (SOC) in short core sediments, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM), as well as other solutes in sediment porewaters of the Changjiang Estuary and adjacent East China Sea (ECS) shelf. The main goal of this work is to investigate the variation of the sources and composition of different forms of carbon in estuarine sediments associated with different sedimentary regimes, to further understand the role of sediment porewater in carbon sequestration at the land-ocean interface. Concentrations of Fe2+ and Mn2+ in porewaters of the muddy sediments are much higher than those in the sandy sediments, and SO42− decreases with depth in the deep sediment layer, indicating the degradation of SOC in mobile muds is mainly driven by suboxic and/or anoxic diagenetic processes (e.g., iron-manganese reduction). The accumulation of DIC in the muddy sediment is higher compared to the sandy sediment, indicating relatively complete SOC remineralization. The DOC in porewaters of the muddy areas is mainly composed of highly degraded and low molecular weight humic-like substances (C1), whereas in the sandy area, porewater DOC is mainly composed of less degraded and high molecular weight protein-like substances (C2 and C3). The average DOC stock (28.5 t/km2) in the upper 30 cm sediment porewaters is significantly higher than that of DIC (12.5 t/km2) in sandy area, but less in muddy areas (17.0 t/km2 of DOC vs. 25.4 t/km2 of DIC). The total DOC stock in sediment porewaters of the sandy area accounted for ∼61 % of DOC stock in water column of the ECS, indicating that the porewater is an important DOC pool in the ECS. However, this DOC pool is rather transient due to its high reactivity and mobility, especially in sandy area. Nevertheless, compared with other marine environments, the carbon stock of DOC (average of 43.8 t/km2) in porewaters of stable sedimentary environments is much higher than that of DIC (average of 21.7 t/km2). This work further supports the notion that sedimentary regime plays an important role in OC cycling at the land-ocean interface and highlights the significance of sediment porewaters as a vast carbon pool in marine ecosystems.