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The Changjiang (Yangtze River) Estuary (CE) and adjacent inner East China Sea (ECS) shelf are among the most well‐known hypoxic aquatic environments. However, the coupled effects of coastal upwelling and the river plume on hypoxic dynamics in this region are rarely examined. Based on observations off the CE and Zhejiang coast in the summer of 2018, the dynamic mechanisms were revealed by highlighting the combined role of the river plume and upwelling in regulating hypoxia. The bottom hypoxic water formed in the steep slope on the inner ECS shelf expands upward and seaward. Coastal upwelling and the river plume combine to regulate the primary production regime and hypoxia. The frontal regions in the upper layers are prone to phytoplankton blooms, while the convergent slope generally acts as a hot spot of bottom hypoxic center along cross‐shelf sections. The regional selectivity of hypoxia is responsible for the decoupling between locations of bottom‐water hypoxia and upper‐water phytoplankton patches. The upwelling transports oxygen‐deficient water to the upper layers, resulting in shoreward shoaling of the oxycline. Meanwhile, mainly forced by the seaward expansion of low‐oxygen water protruding from the bottom hypoxia in the sloping region, a pronounced midlayer hypoxic plume forms. Together, upwelling and offshore spreading potentially expand the spatial volume of the hypoxic zone. Vertical secondary circulation and subsequent physical–biogeochemical processes leading to hypoxia are indicated off the CE and Zhejiang coast. Our results demonstrate that the combination of upwelling and the river plume plays an important role in regulating the spatial distribution of coastal and estuarine hypoxia. Plain Language Summary: The Changjiang Estuary (CE) has become one of the well‐known eutrophic hypoxic systems in summer. Exploring the mechanisms of hypoxia formation is of great significance for evaluating and predicting its effects on estuarine ecosystems. Here, specific attention was given to the hypoxic dynamics by highlighting the combined role of the river plume and coastal upwelling in the CE and adjacent inner East China Sea. We show that bottom hypoxic water spreads upward and seaward on the inner shelf. A pronounced intermediate‐layer hypoxic plume was observed, expanding offshore from the bank slope area. Vertical secondary circulation and associated physical–biogeochemical processes regulate hypoxia within the coupled river plume–upwelling region. Our results offer new insight into the spatial variations and mechanisms of hypoxia in estuarine ecosystems under coastal upwelling and river plume. Key Points: Hypoxic water spreads upward and seaward under convergent upwelling on the inner East China Sea shelfA pronounced intermediate‐layer hypoxic plume was observed to expand offshore from the bank slope areaSecondary circulation and associated physical‐biogeochemical processes are suggested to regulate hypoxia in the coupled upwelling‐river plume region [ABSTRACT FROM AUTHOR]

A gravity core collected from the East China Sea (ECS) inner shelf was analyzed for elemental and stable isotopic composition, lignin-phenols, and sedimentary pigments to investigate changes of organic carbon (OC) inputs during the past two centuries. In particular, we examined the linkages between terrestrial and marine OC inputs with climate variability and anthropogenic activities. The decrease of terrestrial OC contribution (from 41% to 28%) and increasing diagenetic indices of lignin-phenols (P/(S + V): from 0.12 to 0.22; 3,5-Bd/V: from 0.03 to 0.09) after the 1970s were possibly attributed to intensified deforestation, dam construction, and channel erosion. Lignin content (Λ8) ranged from 0.35 mg/100 mg OC to 6.92 mg/100 mg OC, with lower values corresponding to the worst flooding events in the Changjiang watershed and weaker East Asian Winter Monsoon (EAWM), while higher Λ8 was more correlated to the strengthening of EAWM. This indicates that terrestrial inputs to Zhe-Min Coast are different from those in Changjiang Estuary during flooding events and strongly linked with regional climate variability. The total contents of sedimentary chloropigments (i.e. pheophorbide-a, pheophytin-a, pyropheophytin-a, sterol chlorin esters, and carotenol chlorin esters) ranged from 663.4 to 74.9 nmol g−1 OC, and decreased exponentially downwards. Sedimentary chloropigments that were used to document historical change of phytoplankton biomass were decoupled with historical changes of Changjiang riverine nutrient inputs but corresponded well to the fluctuation of regional climate variability. Higher phytoplankton biomasses usually were observed during positive phases of Pacific Decadal Oscillation (PDO) and/or warm El Niño-Southern Oscillation (ENSO) events, and lower algal biomass usually corresponded to the negative phase of PDO and/or cold ENSO events. [ABSTRACT FROM AUTHOR]

Changjiang Estuary, the largest estuary in China, encompasses a wide range of nutrient loading and trophic levels from the rivers to the sea, providing an ideal natural environment to explore relationships between functional diversity, physical/chemical complexity, and ecosystem function. In this study, molecular biological techniques were used to analyze the community structure and diversity of ammonia-oxidizing bacteria (AOB) in the sediments of Changjiang Estuary and its adjacent waters in East China Sea. The amoA gene (encoding ammonia monooxygenase subunit A) libraries analysis revealed extensive diversity within the β-Proteobacteria group of AOB, which were grouped into Nitrosospira-like and Nitrosomonas-like lineages. The majority of amoA gene sequences fell within Nitrosospira-like clade, and only a few sequences were clustered with the Nitrosomonas-like clade, indicating that Nitrosospira-like lineage may be more adaptable than Nitrosomonas-like lineage in this area. Multivariate statistical analysis indicated that the spatial distribution of the sedimentary β-Proteobacterial amoA genotype assemblages correlated significantly with nitrate, nitrite, and salinity. The vertical profile of amoA gene copies in gravity cores showed that intense sediment resuspension led to a deeper mixing layer. The horizontal distribution pattern of amoA gene copies was nearly correlated with the clayey mud belt in Changjiang Estuary and its adjacent area in East China Sea, where higher β-Proteobacteria phylogenetic diversity was observed. Meanwhile, those areas with high amoA copies in the surface sediments nearly matched those with low concentrations of dissolved oxygen and ammonium in the bottom water. [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]

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]

We investigated the remineralization of sedimentary organic carbon (SOC) at 12 sites in East China Sea mobile-muds (ECSMMs) and South Yellow Sea central mud deposits (SYSMDs) - using a time-sequence sediment incubation experiment. We examined pore-water dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), fluorescent dissolved organic matter (FDOM), dissolved inorganic nitrogen (DIN) nutrients (NH 4 + , NO 3 − , and NO 2 − ), redox sensitive elements (Fe 2+ and Mn 2+ ), and major anions (SO 4 2− and Cl − ) in incubated sediments, to better constrain controlling mechanisms of SOC remineralization under different sedimentary regimes. Lower DIC production rates in SYSMDs (2.36–3.13 mmol m −2  d −1 ) than those in ECSMMs (2.94–13.5 mmol m −2  d −1 ), were mainly attributed to cold bottom water masses and a relatively stable sedimentary environment in SYS. Higher DIC production rates were observed mostly at offshore sites of ECSMMs that had relatively enriched 13 C of SOC - which indicated preferential degradation of labile SOC of marine origin. When compared with tropical mobile-muds, higher bottom-water temperatures, thicker mobile-muds, and large inputs of reactive terrestrial OC resulted in more intense remineralization of SOC in Amazon mobile-muds than in ECSMMs. Lower ratios of DOC/DIC production rates in ECSMMs (0.11–0.72) were likely indicative of efficient transformation of OC, and largely due to sulfate reduction. A rapid increase in marine protein-like FDOM components during the incubation indicated that less stable marine SOC was preferentially converted to DOC - and then to DIC. Our SOC budget indicates that 16.8% of SOC was decomposed in sediments of ECSMMs, but only about 5.4% of SOC was decomposed in SYSMDs, suggesting lower SOC preservation efficiency in mobile-muds than distal muds. [ABSTRACT FROM AUTHOR]

Sulfate- and sulfite-reducing prokaryotes (SSRP) communities play a vital role in both sulfur and carbon cycles. Community composition and abundance of SSRP were investigated using dissimilatory sulfite reductase β subunit ( dsrB ) gene sequencing in sediments from the Changjiang estuary and its adjacent area in the East China Sea (ECS). Clone libraries were constructed and real-time fluorescence quantitative polymerase chain reaction (qPCR) was applied to understand the community information of SSRP. In addition to sequences affiliated to sulfate-reducing prokaryotes (SRP), those affiliated with sulfite-reducing prokaryotes (SiRP) were also observed. Four phylotypes of SRP in this study showed genetic similarity to Desulfobulbaceae, Syntrophobacteraceae, Desulfobacteraceae and Peptococcaceae, and an unknown group that could not be clearly affiliated with known lineages was found. Salinity, temperature and contents of total organic carbon (TOC) were most closely correlated with the SSRP communities by canonical correspondence analysis (CCA). 210 Pb activities demonstrated the sedimentary environment at S33 was more stable than that at S31. Intense resuspension and reconstruction of sediments made the vertical abundance profile of SSRP fluctuate violently. For surface sediments, the dsrB gene copy numbers near the Changjiang estuary were higher than those in the mouth of Hangzhou Bay and the mud deposits along the Zhejiang coast, and contents of TOC were positively related to the copy numbers of dsrB gene. Our data provided valuable information to achieve a better understanding of the potential role of SSRP in sediments from the Changjiang estuary and adjacent East China Sea. [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]

Abstract: Surface sediments were collected from the inner shelf of the highly dynamic East China Sea (ECS) in December 2009 and November 2010. In between the two cruises, a big flood event occurred in July, 2010. Elemental, stable C, N isotope, radiocarbon isotope (∆14C), black carbon (BC) analysis, and chemical biomarkers were utilized to study the effect of the sources, short-term events and physical processes on the composition, abundance, and age of the surface sedimentary OC in this dynamic system. Bulk N/C ratio of 0.10 to 0.14 and δ13C of −23.1‰ to −20.9‰ in these samples indicated a mixture source of terrestrial, deltaic and marine derived OC during both cruises. The TOC was 1,980 to 5,040yr old based on ∆14C, and BC extracted from two 2009 sediment samples was older: 5,480 and 5,920yr old. While the Changjiang River was a main source of the terrestrial derived OC, refractory BC and reworked marine organic matter seemed to comprise the older, less reactive carbon pool in the surface sediments of the ECS. Monte Carlo simulations indicated that more than 50% of the TOC was derived from the marine end-member, followed by the deltaic OC. The chlorophyll a of the 2010 sediment samples (11.06±10.15nmolg−1 OC, n=7) was significantly lower than that of the 2009 samples (34.58±20.29nmolg−1 OC, n=7). However, zeaxanthin, a biomarker for cyanobacteria, had significantly higher concentration in the 2010 sediment samples (9.26±3.48nmolg−1 OC, n=7) than that of the 2009 samples (5.50±4.62nmolg−1 OC, n=7). The lack of benthic–pelagic coupling of pigment concentrations in surface sediments after the flood of 2010 was likely due to lateral transport of surface sediments to and from this region. While there were no significant linkages between in situ shelf-derived phytoplankton sources of OC and surface sediment pigment concentrations in 2010, other algal inputs from local deltaic lakes may have contributed to the compositional changes in concentration of sediment carotenoids. The flood of summer 2010 likely played a critical role in the observed shift of the locations for the highest terrestrial biomarker concentrations in sediments. Little evidence remained from the 2010 flood event, which suggested that winter wind/wave energy and hydrodynamic sorting had a substantial effect on sediment OC redistribution. [Copyright &y& Elsevier]