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River damming reduces sediment load in rivers, leading to instability and erosion of coastal deltas; this is a global issue in many large-river delta-front estuaries (LDEs). The total organic carbon, stable carbon isotope, total nitrogen, lignin phenols in suspended particulate matter, and three sediment cores collected from the Changjiang LDE were analyzed to study the effects of river damming and delta erosion on the redistribution and burial of organic carbon (OC) in the coastal margin. The OC budgets that include the delta erosion process were established to better constrain the burial fluxes of OC in the Changjiang River system before and after river damming. We used a three-end-member mixing model with Monte Carlo simulation and found that the ratio of deltaic OC to riverine OC significantly increased in estuarine SPM and sediment cores in the Changjiang LDE after the operation of the Three Gorges Dam (TGD), reflective of reduced sediment load and enhanced delta erosion. Our budget showed that, compared with the pre-dam period (1953-1985), the burial of terrestrial OC in the Changjiang LDE sediments had been reduced by ~97.3%, on the basis of 72.7% and 82.7% reductions in sediment load and particulate OC flux, respectively. Although trapping of terrestrial OC in reservoirs is the key driver in these reductions, re-exposure and decomposition of OC from eroding delta sediments also contribute significantly to OC losses in coastal sediments. On the contrary, the ratio of marine OC to riverine OC increased in sediment cores after TGD building, due in part to more inputs of marine OC because of serious eutrophication in recent decades. As continued dam construction and sea-level rise change the spatial and temporal dynamics of carbon sequestration in the coastal zones of LDEs, further consideration of alterations in these carbon burial "hotspots" is needed in global carbon models., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Anaerobic ammonium oxidation (anammox) is an important process in marine nitrogen cycle. In this study, diverse anammox bacteria were identified in the sediments of the Changjiang (Yangtze) Estuary and its adjacent area. Specifically, the community characters of anammox bacteria in the studied area were studied by quantitative polymerase chain reaction (qPCR), as well as 16S rRNA gene- and functional gene (hzo)-based Roche 454 sequencing. The abundance of denitrifying bacteria detected by the nirS gene was greater than that of anammox bacteria. 16S rRNA and hzo gene fragments affiliating with known anammox bacterial lineages were recovered, and the two major phylotypes belonged to the Candidatus Scalindua (Ca. Scalindua) genus, with >90% sequence similarity. A phylogenetic analysis detected the Scalindua and Brocadia genera together with some anammox-like bacterial clusters, which suggested a higher diversity in the studied ecosystem than in open ocean environment, where only Scalindua genus was detected. A redundancy analysis (RDA) showed that total organic carbon (TOC) and total nitrogen (TN) content in sediments significantly influenced anammox bacterial abundance of. Spearman correlation analyses confirmed that the spatial variation in anammox bacterial abundance was highly correlated with TOC (P<0.01) and TN (P<0.01) contents in sediments., (Copyright © 2019 Elsevier GmbH. All rights reserved.)

Suspended particulate matter (SPM) samples were collected along a salinity gradient in the Changjiang Estuary in June 2011. A custom-built water elutriation apparatus was used to separate the suspended sediments into five size fractions. The results indicated that Cr and Pb originated from natural weathering processes, whereas Cu, Zn, and Cd originated from other sources. The distribution of most trace metals in different particle sizes increased with decreasing particle size. The contents of Fe/Mn and organic matter were confirmed to play an important role in increasing the level of heavy metal contents. The Cu, Pb, Zn, and Cd contents varied significantly with increasing salinity in the medium-low salinity region, thus indicating the release of Cu, Pb, Zn, and Cd particles. Thus, the transfer of polluted fine particles into the open sea is probably accompanied by release of pollutants into the dissolved compartment, thereby amplifying the potential harmful effects to marine organisms., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

As a significant source of nutrients and other components into the coastal ocean, submarine groundwater discharge (SGD) is a combination of multiple spatial‐temporal scale processes, including fresh terrestrial groundwater (FSGD), recirculating seawater (RSGD), and porewater exchange (PEX). Quantifying the different types of SGD is extremely important for understanding the coastal biogeochemical cycles of various materials. The green tide bloom in the southern Yellow Sea (China) has attracted increasing attention during the past decade. Haizhou Bay is thought to be an important green tide proliferating area, but the source of an apparent high supply of nutrients has not been identified yet. We report here on our investigations of the distribution patterns of Ra and Rn isotopes in groundwater and seawater in Haizhou Bay. By solving a combined mass balance for 224Ra, 223Ra, 226Ra, and 222Rn, we estimated that the bay's water residence time is 28.8 days, FSGD is highest at 3.6 cm d−1, RSGD is 2.7 cm d−1, and PEX is lowest at 0.6 cm d−1. The total SGD into Haizhou Bay is estimated at 9.40 × 107 m3 d−1 (6.9 cm d−1), about 12 times that of the local river discharge into the bay. SGD‐derived nutrients are shown to play an important role when considered among all known nutrient sources. Our results suggest that dissolved inorganic nitrogen (DIN) and silicate (DSi) are transported by SGD mainly via FSGD. We also note that the phosphorus (DIP) budget is heavily influenced by RSGD. Plain Language Summary: Submarine groundwater discharge (SGD) has been drawing attention as an important benthic boundary exchange pathway in the global marginal seas. As any water flow entering the sea, SGD may happen on small scales (centimeters, minutes to hours) as porewater exchange (PEX), and/or large scales (>meters, days to seasons) as fresh groundwater discharge (FSGD) and seawater recirculation (RSGD) in the subterranean estuary. To better understand its significance, it is necessary to interpret multi‐scale SGD processes and quantify associated nutrients fluxes accordingly. In this study, different types of SGD and associated nutrient fluxes were quantified based on multiple radium and radon isotopes in Haizhou Bay, China, which is one of the world's largest green tide proliferating areas. The FSGD and RSGD were found to be the main processes, accounting for more than 90% of the total SGD fluxes. SGD was recognized as a key nutrient source in Haizhou Bay, which is more than one order of magnitude higher than that of the local river discharge. This study highlights the crucial role of benthic boundary exchange for the health of marine ecosystem and sustainability of the future ocean. Key Points: Multi‐scale submarine groundwater discharge processes were differentiated and quantified via a multi‐isotope mass balance approachThe flux of fresh groundwater was about 30% higher than recirculated saline groundwater, and about 6 times higher than porewater exchangeFresh groundwater is a main nutrient source to Haizhou Bay, but recirculated saline groundwater makes a larger contribution to DIP inputs [ABSTRACT FROM AUTHOR]

Radium (Ra) isotopes are useful for tracing water mass transport and examining estuarine hydrological dynamics. In this study, several hydrological parameters, nutrients, chlorophyll- a (chl- a), suspended particulate matter (SPM) and Ra isotopes (Ra, Ra and 226Ra) of surface waters of the Zhujiang (Pearl) River estuary (ZRE) were measured. This was done for both winter (December) and summer (July) seasons, to quantitatively understand the seasonal characteristics of river plume flow rate and trajectories, as well as the ecological response. The results show that Ra concentrations in summer were higher than in winter, especially Ra (about 2-5 times higher). The spatial distribution of three Ra isotopes and relative Ra water ages indicated that river water mainly flushed out of ZRE through the western side in winter, where the water transport was about 5 days faster than in the eastern zone. In summer, diluted river water expended to the east side, resulting in fairly similar water ages for both sides of the river mouth. Although nutrients were higher during the summer season, lower chl- a concentrations indicated that reduced primary production might be caused by high SPM (low light penetration). The results obtained from this study will provide knowledge needed for effectively developing and managing the ZRE. [ABSTRACT FROM AUTHOR]

Based on literature data and shipboard observations, this study investigated the main environmental characteristics of the seafloor topography, current field, front, and upwelling that are closely related to hypoxia occurrence off the Changjiang estuary. The physical processes of the plume front and upwelling off the Changjiang estuary in summer were coupled. The vertical distribution pattern of the plume front was closely related to the upwelling. By reviewing and analyzing the historical summer hypoxia events off the Changjiang estuary, we statistically demonstrated the spatial structure of the horizontal distribution of the hypoxic zone and investigated the location of occurrence zone of the hypoxia. We found that the dissolved oxygen (DO) concentration on the inner continental shelf off the estuary showed a 'V' shape in relation to station depth. Therefore, we noted that the hypoxic water on the inner continental shelf mostly occurred on the slopes with steep seafloor topography. Base on the current understanding of the hypoxic mechanisms off the Changjiang estuary, we analyzed the biogeochemical mechanisms that could cause the steep terrain off the Changjiang estuary to become the main areas prone to summer hypoxia and explained the internal relations between the location of the hypoxic zone on the slopes and the plume front and upwelling. The plume front and upwelling off the Changjiang estuary and their coupling were important driving forces of summer hypoxia. The continuous supply of nutrients affected by the interaction of the plume front extension of the Changjiang Diluted Water (CDW) and upwelling and the favorable light conditions were important mechanisms causing the phytoplankton blooms and benthic hypoxia off the Changjiang estuary in summer. By analyzing oxygen utilization, organic carbon mineralization, and nutrient regeneration in the hypoxic zone, we observed that the significant oxygen utilization process off the Changjiang estuary in summer also mainly occurred near the steep slopes with front and upwelling features and confirmed the apparent nutrient loss in the benthic hypoxic zone. Meanwhile, our analysis revealed that the sediment resuspension in the benthic boundary layer in the mud areas off the Changjiang estuary could also affect the oxygen utilization and mineralization of organic carbon and nutrient recycling and regeneration. This study also demonstrated that the steep terrain off the Changjiang estuary was the main location for summer acidification, and the coupling between the plume front and upwelling on the steep slopes was an important physical driving force inducing summer benthic acidification. Finally, we discussed issues to address in future studies of the hypoxic zone and water acidification off the Changjiang estuary. [ABSTRACT FROM AUTHOR]

Abstract: Provenance discrimination of marine sediments is an important topic in marine geology. Although the catchments of the Changjiang and Huanghe Rivers are regarded as the main sources of sediments on the shelves of the China Sea, identification of detailed sediment provenance is necessary for understanding sediment transport patterns and the paleoenvironment. Magnetic properties of both bulk sediment samples and different-sized particles from the Changjiang and Huanghe Estuaries are studied to discuss the origin of the sediments. The results show that the sediments are dominated by ferrimagnetic magnetite with a minor content of anti-ferromagnetic minerals such as hematite, and a higher concentration of ferrimagnetic minerals is observed for the Changjiang Estuary sediments. The crystal grains of ferrimagnetic minerals in the Changjiang Estuary are coarser than those in the Huanghe Estuary, whereas superparamagnetic grains significantly contribute to the Huanghe sediments. Magnetic parameters of χ SIRM, SOFT and χ ARM from different-sized particles present more obvious differences than these parameters from bulk particles, and S−100 and S−300 for both bulk samples and different-sized particles show distinct differences between the sediments from the two estuaries. Therefore the latter parameters are more useful to distinguish sediment origins. A comparison of the magnetic properties of sediments in these two estuaries with those on the shelves of the adjacent China Sea also suggests that the analysis of sediment magnetic properties is effective to identify sediment provenances. [Copyright &y& Elsevier]

Reactive iron (Fe R) plays an important role in the preservation of organic carbon (OC) in coastal sediments, yet changes in the OC bound to Fe R (OC-Fe R), during transport and deposition, remain poorly understood. The main goal of this work is to investigate the variation of the age and composition of OC-Fe R from estuarine suspended particulate matter (SPM) to coastal sediments, to further understand the role of Fe R in the preservation of terrestrial OC exported from large rivers into marginal seas. We examined OC and its carbon isotopic composition (Δ14C bulk , δ13C bulk), specific surface area (SSA), grain size composition, lignin phenols, Fe R , Mössbauer spectroscopy, and isotopic signatures of OC-Fe R (Δ14C OC-FeR , δ13C OC-FeR) in SPM and surface sediments of the Changjiang Estuary. Particulate OC (POC) and Fe R concentrations in SPM are significantly higher than in surface sediments, with no significant differences between surface- and bottom-water SPM. This indicates that loss of OC and Fe R largely occurs at the sediment–water interface due in part, to rapid Fe cycling. The percentage of OC-Fe R (f OC-FeR) in SPM (6.6 ± 1.9%) is similar to that in mobile-mud sediment (8.8 ± 1.8%). There are no significant differences in OC-Fe R content (p > 0.05) from SPM to mobile-mud sediments, but non-OC-Fe R largely decreases, suggesting that terrestrial OC-Fe R has greater stability compared to terrestrial non-OC-Fe R. Both δ13C OC-FeR and Δ14C OC-FeR are lower than bulk OC, indicating that Fe R is mainly associated with pre-aged soil OC of terrestrial plant origin, especially in estuarine SPM and mobile-mud sediments. Taken together, binding with Fe R is a potential long-term protection mechanism for terrestrial OC. Both Δ14C bulk and Δ14C OC-FeR decrease with an increase in the ratio of hematite to (super)paramagnetic Fe3+, indicating that high-crystallinity iron oxide is largely associated with pre-aged terrestrial OC, and there is a potential joint maturation mechanism between Fe R and its associated OC. Based on literature comparisons of soils, estuarine SPM, and marine sediments, OC-Fe R associations are controlled mainly by sedimentary regimes, Fe R compositions, and OC sources. This work supports the notion that Fe R plays an important role in the stabilization and transport of river-derived terrestrial OC. [ABSTRACT FROM AUTHOR]

Abstract: The speciation, potential bioavailability and preservation of phosphorus (P) in surface sediments of the Changjiang (Yangtze River) Estuary and adjacent East China Sea (ECS) inner shelf were investigated through the analyses of P fractions and sediment bulk properties. A sequential extraction method (SEDEX) was used to separate and quantify the following six sedimentary P reservoirs: exchangeable P (Ex–P), authigenic P (Au–P), detrital P (De–P), organic P (Or–P), refractory P (Re–P) and Fe-bound P (Fe–P). Total P (TP) in surface sediments ranged from 15.0 to 21.4 μmol g−1 and was highest near the Changjiang river mouth. The average contribution of each form of P to TP was 55.6% (De–P), 17.8% (Re–P), 16.1% (Or–P), 5.5% (Au–P), 2.5% (Ex–P) and 2.5% (Fe–P), respectively. De–P showed relatively higher concentrations in the river mouth and the ECS shelf region, off the Changjiang Estuary. High concentrations of Or–P were found mainly in mud areas showing a similar distribution pattern with silt, sediment surface area (SSA), and total organic carbon (TOC). Re–P was mainly distributed near the estuarine area and the Zhe–Min coast. Bioavailable P (BAP), accounted for 9.5–32.0% of TP (with a mean of 21.2%) and showed a similar distribution pattern to that of Or–P. De–P/SSA and TOC/SSA loadings both decreased with increasing of SSA, while Or–P/SSA loadings varied little with SSA, indicating that Or–P may have reached an adsorption–desorption equilibrium on mineral surfaces. TOC to total organic P (TOP; sum of Re–P and Or–P) ratios less than the Redfield ratio (84 in average) may have indicated efficient remineralization of organic matter in mobile muds of the Changjiang Estuary and adjacent ECS inner shelf. Furthermore, the relatively high TOC/Or–P ratios (72–422 with a mean of 188) likely suggest a higher degree of preferential regeneration of labile Or–P over TOC in sediments. [Copyright &y& Elsevier]

Large-river delta-front estuaries (LDEs) and their adjacent shelf margins are sites of dynamic diagenetic processes that play a significant role in coastal biogeochemical cycling. In this study, we used dissolved inorganic carbon (DIC), redox sensitive elements (Fe 2 + and Mn 2 + ), dissolved inorganic nitrogen (DIN) nutrients (NH 4 + , NO 3 − , and NO 2 − ), major cations and anions (K + , Ca 2 + , Mg 2 + , SO 4 2 − , and Cl − ) in bottom-water and sediment pore-waters, to investigate the early chemical diagenesis and authigenic mineral formation in mobile-mud deposits of the Changjiang Estuary and adjacent inner shelf of the East China Sea (ECS). Vertical profiles of DIC and NH 4 + in pore-waters had similar trends at most sites, showing a significant increase with depth near the Changjiang Estuary and being relatively constant at offshore sites. Higher pore-water DIC and NH 4 + concentrations were observed in nearshore sites in winter, which were likely attributed to exposure of deeper deposits by winter coastal erosion. Nitrification was observed at most sites, and AOB (ammonia-oxidizing bacteria) played a leading role in ammonia oxidation in the study areas. The nitrification-denitrification was likely important in contributing to the loss of DIN in offshore sites during summer. Large inputs of organic carbon (OC) and terrestrial materials from Changjiang River resulted in intense sulfate reduction and Fe and Mn reduction in nearshore sites. Lower C/N and C/S ratios coupled with an apparent decrease in pore-water Ca 2 + and Mg 2+ concentrations with depth near the Changjiang Estuary, which indicated that authigenic carbonate formation occurs in these sediments. Decreases in K + and Mg 2 + with depth reflected that reverse weathering was an important process of authigenic mineral formation in these sediments. We conclude that adsorption process, seasonal erosion-redeposition, and summer hypoxic conditions of bottom-waters may play an important role in early diagenesis processes and remineralization of SOC in the Changjiang LDE. [ABSTRACT FROM AUTHOR]

Knowledge of the fate of sedimentary organic carbon (SOC) in large-river delta-front estuaries (LDEs) is critical for understanding the global carbon cycle. In this study, remineralization of total organic carbon (TOC) in the Changjiang Estuary and adjacent East China Sea (ECS) shelf were investigated by combining sediment and pore water analyses. Distinctively low TOC to sediment surface area (TOC/SSA) loadings (<0.40 mg m −2 ) were observed mainly in the mud deposits comparable to tropical deltaic mobile muds or delta topset, indicating an inefficient SOC preservation. Anaerobic incubations showed that the net reaction rate of ΣCO 2 in pore waters over the whole core (0–24 cm) of sediments in the Changjiang Estuary was 5.1 mmol m −2 d −1 , and over the upper 15 cm was 9.3 mmol m −2 d −1 . ΣCO 2 in pore waters from incubated sediments decreased with time and the consumption rate in the lower layer of sediments was 4.2 mmol m −2 d −1 , close to the consumption rate of major cations, especially calcium, indicating the precipitation of authigenic carbonates. Overall, remineralization of SOC in mobile-mud belts could play a critical role in the biogeochemical cycling and storage of OC, and other important biogenic elements in this highly dynamic LDE. [ABSTRACT FROM AUTHOR]

• Reservoirs dramatically increased POC trapping in the Changjiang River basin. • The proportion of autochthonous, labile POC increased in the Changjiang POC export. • Due to reduced export and the growing labile fraction, carbon sedimentation in the Changjiang estuary is declining. • Global POC trapping in nine large rivers strongly impacted by dams is 12% of the global riverine POC export. The long-term changes in composition and export of particulate organic carbon (POC) by rivers due to dam construction are poorly known. Based on observations, incubation experiments and modelling, this study analyzed the sources, spatial and temporal distribution and annual export of POC in the Changjiang River during recent decades to explore the POC changes due to dam construction. Changes in POC composition resulted from both increasing POC sequestration and carbon fixation in the river basin. The allochthonous POC (from terrestrial input) export by the Changjiang River to its estuary decreased by ∼90% between 1956 and 2002 and 2013–2019. We estimated that 2.3 ± 0.5 Mt C/yr allochthonous POC has been sequestered in the Changjiang watershed since 2013. The autochthonous (entirely from in-stream riverine primary production) contribution increased from 1% to 55% of total POC export during the same period due to improved light transparency caused by decreased sediment discharge. Global POC trapping in nine large rivers strongly impacted by dams is 12% of the global riverine POC export. The reduced POC export and increase in labile autochthonous POC strongly impact the estuarine carbon cycle. Therefore, with continued dam construction in the future, important changes in the riverine and estuarine carbon cycle can be expected. [ABSTRACT FROM AUTHOR]

Abstract: Eutrophication is one of the most significant ecological problems in large-river delta-front estuaries (LDEs) around the world. We used TOC, TN, δ13C, δ15N and three stable sedimentary chloropigments (pyropheophytin-a [pPHtin-a], sterol chlorin esters [SCEs] and carotenol chlorin esters [CCEs]) as geochemical proxies to examine historical trends of eutrophication over the last few decades in the Changjiang and Mississippi LDEs. Concentrations of sedimentary pPHtin-a, SCEs and CCEs increased from 15, 12 and 120nmolg−1 OC in 1960s to 51, 32 and 256nmolg−1 OC in 1990s on the inner shelf of the East China Sea, respectively, and from 57, 69 and 31nmolg−1 OC in 1950s to 70, 90 and 44nmolg−1 OC in 2000s in the Mississippi Canyon, respectively. Riverine loading of DIN flux to LDE increased from (261±109)×106 kgyr−1 in pre-1980 to (1385±209)×106 kgyr−1 in post-1990 in the Changjiang LDE, while nitrate flux increased from (322±89)×106 kgyr−1 in the 1950s to 1960s to (589±123)×106 kgyr−1 in the 1970s, and DIN flux kept relatively stable in (963±250)×106 kgyr−1 from the 1980s to 2000s in the Mississippi LDE. This work reveals that the stable sedimentary chloropigments in accumulating sediments on the inner shelf are suitable biomarkers for examining past changes in eutrophication in the Changjiang and Mississippi LDE ecosystems. The historical record of riverine nutrient inputs as related to changes in the watershed (e.g., fertilizers and manure) is well-correlated with down-core concentrations of stable pheopigment biomarkers in sediments at both LDEs. These results support the coupling between enhanced phytoplankton abundance and increasing anthropogenic nutrients input to the inner shelves of these LDEs. [Copyright &y& Elsevier]