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Dam construction in river systems affects the biogeochemistry of nitrogen (N), yet most studies on N cycling in reservoirs do not consider the transformations and retention of the different N species. This study addresses the N inputs, transport, transformations, and retention in the Three Gorges Reservoir (TGR) in the Changjiang River, the world largest water storage project, during its filling phase in 2004-2007. The results indicate that dissolved inorganic nitrogen (DIN) was the most abundant N form in the TGR and particulate N was only 2% to the total N flow and retention. About 34% of the dissolved organic nitrogen (DON), 64% of nitrite, and 44% of ammonia were retained by transformation within the TGR in 2007. Time-series data show that about 8% of inflowing total dissolved N was lost by denitrification and sedimentation. Transformations of DON, nitrite and ammonia caused an increase of the nitrate load by 6%; and at the same time the DIN flux increased by 4% while the water moved through the TGR, which is the balance between formation and retention of DIN. The large contribution of biogeochemical transformations to nitrate loading in the TGR indicates the importance of internal processing for the river N species composition. [ABSTRACT FROM AUTHOR]

The Changjiang Diluted Water (CDW) significantly influences the chemical, biological, and sedimentary processes in the Yellow and East China Seas. Based on in situ observations during the summers of 2006 and 2008 and associated satellite-derived data, the offshore detachment of the CDW plume and its mechanisms are investigated, and the related ecological impacts associated with the detached CDW are examined. We show that the detached low-salinity water from the CDW plume can partially reach the seabed, with its volume gradually diminishing from the surface to the seabed, and with a horizontal distribution that initially shifts eastward and then southward. The double-upwelling system, combined with the prevailing southerly wind and the anticyclonic eddy off the Changjiang Estuary, favors the detachment of the CDW plume. In particular, the anticyclonic eddy provides a habitat or venue for the formation and maintenance of the detached low-salinity water, and is responsible for the local presence of low-salinity water along the seabed. Data analysis indicates that this detachment can induce phytoplankton blooms and that enhanced chlorophyll a (Chl- a) contents were significantly associated with moderate nutrient concentrations and good light conditions in the offshore low-salinity water. This study also demonstrates that the variability in the vertical distribution of Chl- a off the Changjiang Estuary is related to the offshore detached CDW, and that the local deepening of the DCM (depth of Chl- a maximum) and the peak primary production occur within the offshore CDW. More importantly, we find that high Chl- a concentrations in the bottom water can be induced by the anticyclonic-eddy-featuring offshore CDW. Our results may facilitate a better understanding on the role of the detached CDW in local marine biogeochemical processes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Surface water and suspended particulate matter (SPM) samples were collected along a salinity gradient in the Changjiang Estuary in June, 2011 to examine the distribution, mixing behavior and transformation of dissolved and particulate phosphorus (P) species. Particulate P was fractionated into five size fractions using a water elutriation method, followed by a sequential extraction procedure (SEDEX). Dissolved inorganic P (DIP) concentrations ranged from 0.53 to 3.02 μmol L − 1 with a mean of 1.44 μmol L − 1 , while total particulate P (TPP) concentrations ranged from 20.56 to 26.90 μmol L − 1 (24.21 μmol L − 1 ). The variation of DIP and P species in both bulk and size fractionated particles along the salinity gradient indicated a significant transformation between different P phases, especially for labile P such as exchangeable P (Ex-P), organic P (Or-P) and Fe-bound P (Fe-P) in a particulate P pool. Among all P forms, detrital P (De-P) and Or-P in SPM were the most abundant, accounting for 43.9% and 26.1% of TPP in bulk particles. De-P increased with increasing particle grain size, with an average concentration of 8.92 ± 2.20 μmol g − 1 in the clay fraction (< 8 μm) and 12.6 ± 1.49 μmol g − 1 in the sand fraction (> 63 μm), whereas Or-P and other P forms decreased gradually with increasing grain size. The average concentration of Or-P in clay fraction was 6.93 ± 1.92 μmol g − 1 and it decreased to 1.59 ± 0.61 μmol g − 1 in the sand fraction. The flux of bioavailable P (BAP, sum of Ex-P, Or-P and Fe-P) in clay fraction (< 8 μm), defined as “truly” suspended particles, was estimated to be 0.21 × 10 4 t P yr − 1 and accounted for 29% of the bulk BAP flux. The particulate P to specific surface area ratios (P/SSA) was lower in fine suspended particles than in coarse particles, indicating lower preservation of P in these particles. When compared with the bed sediments of the Changjiang Estuary, the P/SSA loadings in fractionated suspended particles were maintained at the same level for De-P and Fe-P, slightly higher for authigenic P (Au-P) and refractory P (Re-P) and obviously higher for labile P such as Ex-P and Or-P, further indicating the differential behavior of P forms during particle settlement in this dynamic 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]

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]

Abstract: The estuarine-inner shelf region of the East China Sea (ECS) is a major sink of the Yangtze River-derived fine-grained sediments and associated organic materials. In this work, surface sediment samples from a matrix of seventy-three sites that extend from the Yangtze River estuary (YRE) to the southern inner shelf were measured for their elemental, stable isotopic and molecular indices to provide a process-oriented study on the sources, distribution and fate of sedimentary organic matter (SOM) in this region. The results indicated that the re-suspension and alongshore transport of sediments could play a key role on the accumulation of SOM in the area. In addition to the physical reworking, the low C/N ratios and the enriched δ13C values could also be likely related to the presence of microorganism-derived organic matter (OM) and soil-derived OM. The composition and principal component analyses of the n-alkanes indicated that the majority of the riverine terrigenous SOM was primarily restricted within the mud deposits along the coastal ECS. The wide occurrence and southward increasing trend of high molecular weight n-alkanes along the coastal ECS suggest an effective preferential dispersal of the terrigenous organic components. However, the local supply of the marine-derived OM potentially promotes the subsequent degradation of these terrigenous OM, which are likely responsible for the decreasing trend of the carbon preference index (CPI25–33) from the northern YRE to the southern inner shelf. This indicates that the transported SOM from the YRE could become more homogenized as it moved toward the southern inner shelf. The presence of unresolved complex mixtures (UCM), lower Pr/Ph ratios and patterns of more stable geochemical biomarkers (hopanes and steranes) in the nearshore region reveals a petroleum contamination in the coastal environment. [Copyright &y& Elsevier]

In this study, sediment grain size (MGS), specific surface area (SSA), total organic carbon (TOC) contents, C/N molar ratios, stable carbon isotope, and P species in a sediment core, collected from the East China Sea (ECS) inner-shelf were measured to explore the applicability of detrital phosphorus (De-P) as a potential indicator of past flooding events in the Changjiang River Basin (CRB). In particular, we examined the linkages between the evolution of floods with regional climate changes and anthropogenic activities in the CRB. Peaks of De-P concentrations in sediments corresponded well with the worst flooding events of the CRB over the past two centuries (e.g., 1850s, 1860s, 1900s, 1920s, 1950s, 1980s, and 2000s). Moreover, De-P also corresponded well with the extreme hypoxic events in 1981 and 1998 in the Changjiang Estuary as indicated by Mo/Al ratios, indicating potential linkages between De-P as a flooding proxy to flood-induced hypoxia events in this region. In addition, a robust relationship was found among De-P, the floods in 1950s, 1980s, and 2000s of the CRB, the intensive El Niño-Southern Oscillation (ENSO), the abnormally weak East Asian Summer Monsoon (EASM) and the warm phase of Pacific Decadal Oscillation (PDO), suggesting that De-P also provided insights to linkages between regional climate change and flooding events in this region. [ABSTRACT FROM AUTHOR]

Hydrodynamic sorting processes have a significant influence on the dispersal of sedimentary organic carbon (OC) in large-river delta-front estuaries (LDEs). Density fractionation was conducted on 6 sediments from the Changjiang LDE to investigate the sources, distribution, and degradation of OC among density fractions. Mass, elemental composition (C and N) and stable carbon isotope ratios, lignin, and specific surface area (SSA) of density fractions and bulk sediments were determined. Mass was concentrated in high density fractions (2.0–2.5 g cm − 3 and > 2.5 g cm − 3 ) (up to 67.8%) while OC was dominated in 2.0–2.5 g cm − 3 fraction (up to 77.3%) due to extremely low OC in the highest density fraction (%OC < 0.10%). The less dense fractions (< 1.6 g cm − 3 and 1.6–2.0 g cm − 3 ) contained the highest OC (up to 21.4%) and lignin concentrations (up to 3.17 mg g − 1 or 2.85 mg/100 mg OC), and thus were most depleted δ 13 C (down to − 25.2‰). A three end-member mixing model using δ 13 C and lignin content (Λ 8 ) as source markers and Monte-Carlo simulation, to distinguish the relative importance of terrestrial vascular plant, soil, and marine plankton OC sources, showed that less dense fractions were dominated by terrestrial OC (the sum of soil- and vascular plant-derived OC), whereas marine OC was the predominant OC source in high density fractions. Distributions of OC from different sources in density fractions and bulk sediments both indicated a differential transport and deposition of OC along the coast, showing a gradual decreasing of terrestrial OC and addition of marine OC. Ratios of vanillic acid to vanillin (Ad/Al)v, 3,5-Bd to total V phenols (3,5-Bd/V) and total p-hydroxy phenols to sum of vanillyl and syringyl (P/(S + V)) among lignin oxidation products increased with increasing particle density suggesting selective degradation of lignin among the different density fractions. Distinctively low OC/SSA loadings (< 0.40 mg m − 2 ) were observed in the highest density fraction and the bulk sediments, indicating low preservation of OC in these sediments, consistent with the results of lignin decay parameters. [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]