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Systematic surveys to examine seasonal variation, degradation, and bioavailability of dissolved organic matter (DOM) in the Changjiang Estuary and the adjacent East China Sea were conducted in July 2016 and February 2017. Concentrations of dissolved organic carbon (DOC) and total hydrolyzable amino acids (THAA) were higher in July than in February. THAA and chlorophyll a (Chl‐a) were positively correlated in July, but not in February. The carbon normalized yields of THAA (THAA‐C%) in surface waters in July and February were not significantly different. However, degradation index (DI) values in the surface water were higher in February than in July. Compared with outer estuary, the inner estuary had lower THAA‐C% and DI values in both surveys. Solar radiation experiments showed that THAA and THAA‐C% values increased with time at station A6‐11 in the oceanic water but declined at C3 in the freshwater, possibly due to the different origins, chemical compositions, or initial degradation states of the DOM at the time of collection. Microbial incubation experiments showed that accumulated DOC and DON in surface waters were bioavailable to the microbial community of the surface layer, but recalcitrant to the microbial fauna from the bottom layer. Leucine (Leu) was selectively consumed, while glycine (Gly), threonine, and alanine appeared to be recalcitrant in summer (July) microbial incubations; and histidine, Gly, and methionine were preferentially consumed, while aspartic acid, serine, phenylalanine, and Leu were recalcitrant in winter (February) incubations. Plain Language Summary: As one of the most abundant components of labile dissolved organic matter (DOM), amino acids are an important reservoir of organic carbon and nitrogen in the ocean, and amino acid cycling is closely associated with phytoplankton and bacteria. Photochemical reactions and microbial consumption are crucial parts of the transformation and degradation pathways of DOM in marine environments. Analysis of amino acids can be used to evaluate the quality and degradation state of organic matter. In this study, we investigated the temporal and spatial distributions and compositions of total hydrolyzable amino acids (THAA), evaluated the bioavailability of DOM based on amino acids, and investigated the removal mechanisms of DOM via seawater photochemical and microbial incubation experiments. This study indicated that phytoplankton and terrestrial runoff played the important roles in controlling dissolved organic carbon and THAA concentrations. The bioavailability of DOM was higher at offshore sites than the inner Changjiang Estuary sites. We suspect that accumulated DOM in surface waters were removed via bacterial consumption and then transported to the bottom layer. Individual amino acids were selectively utilized by heterotrophic bacteria. Key Points: The concentrations of dissolved organic carbon (DOC) and total hydrolyzable amino acids and the bioavailability of dissolved organic matter showed significant spatial and seasonal variationsAccumulated DOC and dissolved organic nitrogen in surface waters were removed via bacterial consumption and then transported to the bottom layerIndividual amino acids were selectively utilized by heterotrophic bacteria [ABSTRACT FROM AUTHOR]

The Changjiang Estuary and its adjacent sea make up a highly dynamic ecosystem where the processes influencing the sources and degradation of organic matter (OM) in sediments are still not well understood. In this study, we analyzed the total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and total hydrolysable amino acids (THAA) to investigate the distribution and degradation of sedimentary organic matter (SOM). TOC and TN contents ranged from 0.13% to 0.93% and 0.03% to 0.15%, respectively, with elevated values occurring in the muddy areas near the estuary. The δ13C of SOM became increasingly enriched from inshore (−23.5‰) to offshore (−20.5‰), and the TOC/TN molar ratios (C/N ratios) decreased from 8.16 to 5.84, indicating increasing contributions of marine OM. As the labile fraction of bulk OM, THAA concentrations ranged from 1.36 to 11.76 μmol/g and the distribution of THAA was influenced by terrestrial inputs along with the complex hydrography. The δ13C values and C/N ratios indicated that the SOM in the study area was mainly from marine and terrestrial origins (soil derived). The relatively low degradation index (DI) of SOM (mean: −0.57 ± 0.36) indicated that the OM had already undergone extensive degradation. Consistent with previous findings, our results indicated that the degraded OM was adsorbed by fine‐grained particles and selectively transported southward along the coast. The fresh SOM in the sandy area was mainly from in situ production. Additionally, diatoms appeared to be the primary source of the fresh OM buried in the sediments of the study area. Key Points: The source and degradation of sedimentary organic matter were investigated using amino acids parametersThe relatively low degradation index values of the samples indicated that the sedimentary organic matter were highly degradedPrimary production of diatoms was the main source of fresh organic matter in the surface sediments of the study area [ABSTRACT FROM AUTHOR]

Water samples were collected from the East China Sea (ECS) in October 2015 to investigate the distribution, degradation and bioavailability of organic matter. Concentrations of dissolved organic carbon (DOC), total hydrolyzable amino acids (THAAs, including dissolved free, DFAA and combined fraction, DCAA), particulate amino acids (PAAs), and total dissolved carbohydrates (TDCHO, including monosaccharides, MCHO and polysaccharides, PCHO) were measured. DOC and TCHO concentrations exhibited similar distribution patterns with high values occurring at nearshore stations, revealing the effects of terrestrial input and similar source and removal pathways of DOC and TCHO. The distributions of THAA, DCAA, and PAA displayed declining trends from the north to south of the ECS. Elevated THAA values simultaneously occurred in the center of the transect. The onboard incubation experiments with surface seawater from one station showed that the values of degradation index based on amino acids decreased with the increase of dissolved inorganic nitrogen (DIN) concentrations, indicating the mineralization of THAA to DIN during degradation process. TCHO-C% and THAA-C% are defined as the percentages of carbohydrates and amino acids in DOC, respectively. There were 21 stations suffering P limitation, implying that PO43−-P content was the key factor limiting the growth of phytoplankton. High TCHO-C% values were found at P-limited stations, indicating that phytoplankton preferentially produced carbohydrates when experiencing nutrient limitation. The difference of dissolved organic matter bioavailability between surface and bottom water were probably due to water stratification. Overall, the present study may have implications for the source, removal and bioavailability of organic mater in the ECS. [ABSTRACT FROM AUTHOR]

Total organic carbon (TOC), total N (TN) and amino acids (AAs, including the bacterial biomarker D-AAs) were determined in sediment cores from the mud belt of the East China Sea (ECS). The concentration of total hydrolyzable AAs (THAAs) ranged from 3.35 to 13.44 µmol/g dry wt (dw) of the sedimentary organic matter (SOM), exhibiting a decreasing trend downcore. Major constituents of the THAAs were glycine (Gly), L-serine (L-Ser), L-alanine (L-Ala), L-glutamic acid (L-Glu) and L-aspartic acid (L-Asp), whereas the D-aspartic acid (D-Asp), D-glutamic acid (D-Glu), D-serine (D-Ser) and D-alanine (D-Ala), along with non-protein AAs (γ-aminobutyric acid and β-alanine) together accounted for ca. 7% of the THAA pool. Given the C/N ratio values, the OM in three cores was predominantly of marine origin. Based on D-Ala yield, bacterial OM represented on average 18% of TOC and ca. 30% of TN. C-normalized THAAss (THAA-C%) was a sensitive indicator of SOM diagenetic alteration in the upper 40 cm of sediment. However, the degradation index (DI) and reactivity index (RI) values for the cores did not exhibit any definite trend, which may indicate that these two indicators were not sensitive in the early stage of sediment degradation. Negative correlation between the D/L ratio of Ala and THAA-C% suggested a close coupling between the extent of degradation and the accumulation and/or selective preservation of bacterial material in the sediments. In addition, correlation between clay content and THAA-C% suggested that fine grained particles played an important role in affecting the quality of SOM. [ABSTRACT FROM AUTHOR]

The occurrence of 17 target PFAA analytes was determined in surface sediments ( n = 37) of the East China Sea and potential influencing factors were examined. ΣPFAAs ranged from 0.41 ng/g dw to 3.06 ng/g dw, with perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) as the most abundant perfluorocarboxylic acid and perfluoroalkyl sulfonate, respectively. PFAAs in the sediments were strongly influenced by terrigenous input. Analysis of the relationship between dynamic influence factors and PFAA concentrations showed that the characteristics of PFAA distribution were rather complex. ΣPFAA concentrations and TOC were positively correlated ( p < 0.0001). Circumfluence also influenced the whole PFAA distribution and seasonal variation. In addition, correlation analysis suggested that log K oc values increased with increasing perfluoroalkyl chain length. Given the rapid economic development of eastern coastal cities of China, the environmental hazards of land source pollution cannot be ignored. [ABSTRACT FROM AUTHOR]

Dissolved sulfide, iron (Fe), and phosphorus (P) concentrations in sediments of the East China Sea were simultaneously measured in situ by diffusive gradients in thin films (DGT) technique. The results, by combination with solid-phase Fe speciation, were used to characterize the interplays of Fe, S and P. Diverse distributions of dissolved sulfide among the sites are attributable to highly dynamic diagenetic regimes and varying availability of labile organic carbon (OC). The DGT technique provided high-resolution evidence for coexistence of microbial iron reduction (MIR) and sulfate reduction in localized zones, and for Fe-coupled P mobilization. Measured Fe 2 + /P ratios suggest that Fe 2 + reoxidatiion at the oxic zones can serve as an efficient scavenger of P. Empirical estimation indicates that MIR plays an important role in anaerobic OC mineralization in the sediments, which is a combined result of low availability of labile OC, high reactive Fe content, and unsteady diagenetic regimes. [ABSTRACT FROM AUTHOR]

The spatial distributions of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), DMSP-lyase activity (DLA) and their controlling factors including nutrients, phytoplankton community and bacterial abundance were investigated in the East China Sea (ECS) during fall from October 19 to November 2, 2015. Diatoms and dinoflagellates dominated the phytoplankton community, while other taxonomic groups were rare and mainly found in the oligotrophic open sea. Affected by the high nutrients concentrations, Chl a, DMS, DMSP and DLA showed high values in eutrophic inshore waters, and decreased from the costal zones to the open sea. Statistical analysis suggested that diatoms and dinoflagellates were the main controlling factors of DMS, DMSP, and DLA in ECS. For size-fractionated samples, a reduced contribution of the microplankton from inshore stations to offshore stations affected by the trophic conditions was noted. Meanwhile, this decrease in microplankton led to an increase in the ratio of DLA contributed by picoplankton and free-living bacteria from the estuary area to the offshore region. The DMS sea-to-air flux was calculated using the equation of Nightingale et al. (Glob Biogeochem Cy 14(1):373-387, 2000), and approximately 2.88 × 10 Tg of sulfur was transferred from the sea into the atmosphere in the form of DMS in ECS during fall. [ABSTRACT FROM AUTHOR]

Concentrations of amino acids in dissolved and particulate pools in the East China Sea (ECS) were determined during two cruises (October–November 2013 and May–June 2014). The spatial distributions of total hydrolyzed amino acids (THAA), dissolved combined amino acids (DCAA), dissolved free amino acids (DFAA), and particulate amino acids (PAA) in the study area were considerably influenced by phytoplankton biomass and circulations. In addition, in situ phytoplankton production and terrestrial input are the major sources of amino acids in the ECS. The %C-PAA and %N-PAA positively correlated with chlorophyll a (Chl- a ), thus reflecting their phytoplankton sources. The concentrations of amino acids were higher in coastal waters than in open sea, revealing the effects of terrestrial input on amino acid concentrations. Moreover, the vertical distributions of THAA, DCAA, DFAA, and PAA exhibited similar patterns, i.e., decreasing trends from the surface water to the bottom water. Our study indicated that the deposition of phytoplankton biodetritus may affect the vertical distributions of amino acids. The major constituents of THAA were glycine, serine, and glutamic acid, whereas the major constituents of PAA are glycine, glutamic acid, and alanine. The diel variation of THAA presented a certain hysteresis, following the variation of Chl- a . Particle sorption had a contribution in describing the particulate pool and dissolved pool in the Changjiang River. [ABSTRACT FROM AUTHOR]

Concentrations of dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and its major compound classes—total hydrolysable amino acids (THAA) were measured at 4 cross-shelf transects of the East China Sea in July 2011. Surface concentrations of DOC, DIN, DON and THAA at the nearshore stations were mostly in excess of those found at the offshore sites, indicating either substantial autochthonous production or allochthonous inputs from the Changjiang River. The vertical distributions of DOC, DON and THAA showed similar trends with higher values in the surface layer, whereas the elevated concentrations of DIN were observed in the bottom layer. Major constituents of THAA presented in the study area were glycine, serine, alanine, glutamic acid, aspartic acid and valine. The mole percentages of neutral amino acids increased from surface water to bottom water, whereas acidic and hydroxy amino acids decreased with the water depth. Concentrations of DOC and THAA were negatively correlated to the ΔDIN values (the difference between the real concentration and theoretical concentration), respectively, indicating the coupling relation between dissolved organic matter (DOM) remineralization and nutrient regeneration in the water column. The C/N ratios in the water column exhibited different characteristics with elevated values appearing in the surface and bottom layers. Box and whisker plots showed that both degradation index (DI) values and THAA yields displayed a decreasing trend from the surface layer to the bottom layer, implying increasing degradation with the water depth. Our data revealed that glycine and alanine increased in relative abundance with decreasing DI, while tyrosine, valine, phenylalanine and isoleucine increased with increasing DI. [ABSTRACT FROM AUTHOR]

Abstract: Spatial distributions of biogenic sulfur compounds including dimethylsulfide (DMS), dissolved and particulate dimethylsulfoniopropionate (DMSPd and DMSPp) were investigated in the South Yellow Sea (SYS) and the East China Sea (ECS) in July 2011. The concentrations of DMS and DMSPp were significantly correlated with the levels of chlorophyll a in the surface water. Simultaneously, relatively high ratio values of DMSP/chlorophyll a and DMS/chlorophyll a occurred in the areas where the phytoplankton community was dominated by dinoflagellates. The DMSPp and chlorophyll a size-fractionation showed that larger nanoplankton (5–20μm) was the most important producer of DMSPp in the study area. The vertical profiles of DMS and DMSP were characterized by a maximum at the upper layer and the bottom concentrations were also relatively higher compared with the overlying layer of the bottom. In addition, a positive linear correlation was observed between dissolved dimethylsulfoxide (DMSOd) and DMS concentrations in the surface waters. The sea-to-air fluxes of DMS in the study area were estimated to be from 0.03 to 102.35μmolm−2 d−1 with a mean of 16.73μmolm−2 d−1 and the contribution of biogenic non-sea-salt SO4 2− (nss-SO4 2−) to the measured total nss-SO4 2− in the atmospheric aerosol over the study area varied from 1.42% to 30.98%, with an average of 8.2%. [Copyright &y& Elsevier]

In the eutrophic coastal ocean, quick formation of iron (Fe) sulfide is environmentally important to effectively prevent accumulation of dissolved sulfide and its detrimental effects on the benthic ecosystem. In this study, 0.5 N HCl-extractable labile Fe (LFe), acid volatile sulfide, and pyrite in the East China Sea inner shelf sediments were examined to investigate the mechanisms of quick sequestration of dissolved sulfide and potential impacts of frequent algal blooms on the capacity of quick sulfide-buffering in eutrophic coastal areas of the large-sized continental shelf subject to massive terrestrial input. The results indicate that sulfate reduction has been competitively suppressed by dissimilatory Fe reduction due to limited availability of labile organic matter. Dissolved sulfide can be quickly buffered by reaction with LFe and, therefore, is difficult to accumulate to a high level. The quick sulfide-buffering capacity has not become exhausted partly because of the formation of un-sulfidized LFe(II) via dissimilatory reduction of less reactive Fe oxides. It is expected that dissolved sulfide will not pose detrimental effects on the benthic ecosystem in the near future if the current biogeochemical/ecological state remains. [ABSTRACT FROM AUTHOR]

Reactive Fe(III) oxides in gravity-core sediments collected from the East China Sea inner shelf were quantified by using three selective extractions (acidic hydroxylamine, acidic oxalate, bicarbonate-citrate buffered sodium dithionite). Also the reactivity of Fe(III) oxides in the sediments was characterized by kinetic dissolution using ascorbic acid as reductant at pH 3.0 and 7.5 in combination with the reactive continuum model. Three parameters derived from the kinetic method: m0 (theoretical initial amount of ascorbate-reducible Fe(III) oxides), k′ (rate constant) and γ (heterogeneity of reactivity), enable a quantitative characterization of Fe(III) oxide reactivity in a standardized way. Amorphous Fe(III) oxides quantified by acidic hydroxylamine extraction were quickly consumed in the uppermost layer during early diagenesis but were not depleted over the upper 100 cm depth. The total amounts of amorphous and poorly crystalline Fe(III) oxides are highly available for efficient buffering of dissolved sulfide. As indicated by the m0, k′ and γ, the surface sediments always have the maximum content, reactivity and heterogeneity of reactive Fe(III) oxides, while the three parameters simultaneously downcore decrease, much more quickly in the upper layer than at depth. Albeit being within a small range (within one order of magnitude) of the initial rates among sediments at different depths, incongruent dissolution could result in huge discrepancies of the later dissolution rates due to differentiating heterogeneity, which cannot be revealed by selective extraction. A strong linear correlation of the m0 at pH 3.0 with the dithionite-extractable Fe(III) suggests that the m0 may represent Fe(III) oxide assemblages spanning amorphous and crystalline Fe(III) oxides. Maximum microbially available Fe(III) predicted by the m0 at pH 7.5 may include both amorphous and a fraction of other less reactive Fe(III) phases. [ABSTRACT FROM AUTHOR]

Halocarbons including chloroform (CHCl3), trichloroethylene (C2HCl3), tetrachloroethylene (C2Cl4), chlorodibromomethane (CHBr2Cl) and bromoform (CHBr3) were measured in the Yellow Sea (YS) and the East China Sea (ECS) during spring 2011. The influences of chlorophyll a, salinity and nutrients on the distributions of these gases were examined. Elevated levels of these gases in the coastal waters were attributed to anthropogenic inputs and biological release by phytoplankton. The vertical distributions of these gases in the water column were controlled by different source strengths and water masses. Using atmospheric concentrations measured in spring 2012 and seawater concentrations obtained from this study, the sea-to-air fluxes of these gases were estimated. Our results showed that the emissions of C2HCl3, C2Cl4, CHBr2Cl, and CHBr3 from the study area could account for 16.5%, 10.5%, 14.6%, and 3.5% of global oceanic emissions, respectively, indicating that the coastal shelf may contribute significantly to the global oceanic emissions of these gases. [Copyright &y& Elsevier]

Abstract: The concentrations of six volatile halogenated organic compounds (VHOC)–chloroform (CHCl3), trichloroethylene (C2HCl3), tetrachloroethylene (C2Cl4), carbon tetrachloride (CCl4), methylchloroform (CH3CCl3), and bromoform (CHBr3) in the East China Sea (ECS) in November and December 2010 were measured by a purge and trap system coupled to a gas chromatograph with an electron capture detection (ECD). Mean (range) concentrations of CHCl3, C2HCl3, C2Cl4, CH3CCl3, CCl4 and CHBr3 in the surface water were 16.90 (0.40–62.92), 16.27 (2.78–83.33), 2.40 (0.39–9.33), 32.29 (19.72–57.68), 1.70 (0.39–8.73) and 17.11 (4.33–34.46) pM, respectively. With the exception of C2HCl3, the concentrations of other five kinds of VHOC generally exhibited a decreasing trend with distance from the coast, with the low values found in the open sea. The anthropogenic sources contributed to the elevated levels of CCl4 and CH3CCl3, whereas a combination of the anthropogenic and biogenic sources might be responsible for the elevated levels of CHCl3, C2HCl3, C2Cl4 and CHBr3. In the depth profiles, vertical distributions of the six VHOC in the water column were complicated, with the maxima occurring at 0–100m depths. The mean sea-to-air fluxes of CHCl3, C2HCl3, C2Cl4 and CHBr3 were estimated to be 21.08, 29.94, 2.05 and 35.50nmolm−2 d−1, respectively, indicating that the ECS was a source for the four VHOC in the atmosphere. [Copyright &y& Elsevier]

Abstract: The concentrations of particulate and dissolved dimethylsulfoxide (DMSOp, DMSOd) in the surface water of the East China Sea (ECS) in December 2009 were measured for the first time. The concentrations of DMSOp and DMSOd ranged from 2.4 to 80.0nmolL−1 and from 3.2 to 357.0nmolL−1 with average values of 21.3nmolL−1 and 61.9nmolL−1, respectively. The distribution of DMSOd was obviously influenced by the Yangtze River, with high concentrations appearing around the mouth of it. DMSOd represented the main methylated sulfur compound and dominated over dimethylsulfide (DMS) and dissolved dimethylsulfoniopropionate (DMSPd) by 1–2 orders of magnitude. The distribution of DMSOp did not show the same pattern as that of DMSOd. The concentrations of DMSOp increased offshore in the northeast ECS due to the change of phytoplankton species coupled to the increasing temperature, while in the southwest ECS the concentrations of DMSOp decreased offshore due to the poor DMSO-producers in the open sea. A weak positive correlation was observed between DMSOd and DMS in the whole study area. In addition, a significant relationship between DMSOp/Chl-a and temperature in the northeast ECS as well as a negative significant relationship between DMSOp and particulate DMSP in the southwest ECS was found. However, no relationship was observed between DMSOp and the environmental factors such as chlorophyll a, temperature and salinity. The vertical profiles of DMSOd in the study area were characterized by a maximum at depth from 2 to 25m, while the DMSOp concentrations were characterized by a maximum at the bottom. Both DMSOd and DMSOp concentrations exhibited obvious diurnal variations with the maxima 7.4 and 2.4 times higher than the minima, respectively. [Copyright &y& Elsevier]

Abstract: Temporal and spatial distributions of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) were determined in the East China Sea and the Yellow Sea during June–July, 2006 and January–February, 2007. The concentrations of DMS and total DMSP in surface water in the study area were 5.64 (1.79–12.24) and 28.25 (13.98–44.93)nmolL−1 in summer, and were 1.79 (1.02–3.51) and 11.01 (6.90–17.98)nmolL−1 in winter, respectively. The distributions of DMS and DMSP in the study area were obviously influenced by the Yangtze River effluent and the Kuroshio water. Even under highly variable hydrographic conditions, a significant relationship was observed between DMS and chlorophyll a concentrations in summer as well as in winter, suggesting that phytoplankton biomass might play an important role in controlling DMS distribution in the study area. The summer ratios of DMS/chlorophyll a and DMSP/chlorophyll a were approximately twofold higher than winter values, corresponding with the temporal variation in phytoplankton community structure between summer and winter. The sea-to-air fluxes of DMS were estimated to be 5.32 and 11.92μmolm−2 d−1 using the equations of and , respectively. [Copyright &y& Elsevier]

Abstract: Carbon monoxide (CO) concentration distribution and microbial consumption rate constants, along with atmospheric CO mixing ratios, were measured in the Southern Yellow Sea (SYS) and the East China Sea (ECS) in November 2007. The surface water CO concentrations in the investigated area ranged from 0.13 to 2.30nmolL−1, with an average of 0.68nmolL−1. Vertical profiles showed that CO concentrations declined with depth, with the maximum values appearing in the surface water (0–1m). The diurnal variations of seawater CO concentrations were investigated at two anchor stations and the results showed that CO concentrations varied by a factor of 6–8 in the diurnal cycle, with maximum values appearing in the surface water (0–1m). Atmospheric CO mixing ratios varied from 165 to 671ppbv, with an average of 297ppbv; these ratios decreased from 380 to 253ppbv in the seaward direction and increased from 239 to 345ppbv from south to north in the studied areas. Undersaturation was observed at eight of the 55 stations measured (15%) and the saturation factors ranged from 0.58 to 15.42, with an average of 3.34±2.95. The sea-to-air fluxes of CO in the SYS and the ECS ranged from 0.50 to 4.20μmolm−2 d−1, with an average of 1.82±1.43μmolm−2 d−1. In the incubation experiments, the microbial CO consumption rate constants in the surface water (K co ) ranged from 0.15 to 2.14h−1. A negative correlation between K co and salinity was observed in the present study. [ABSTRACT FROM AUTHOR]

Abstract: We determined the distributions and fluxes of methyl chloride and methyl bromide in the East China Sea (ECS) and the Southern Yellow Sea (SYS) in November 2007. Methyl chloride and methyl bromide concentrations in the surface waters ranged from 47.1 to 163pmolL− 1 and from 0.70 to 9.82pmolL−1, with average values of 87.6 and 2.97pmolL−1, respectively. The distributions of the two methyl halides were clearly influenced by the Yangtze (Changjiang) River effluent and Kuroshio water, with high concentrations appearing in the coastal zone and low values occurring in the open waters. A positive linear correlation was observed between methyl chloride and methyl bromide concentration anomalies in the surface waters, suggesting that they may share some origins in this coastal area. However, no correlation was found between the two methyl halide concentration anomalies and chlorophyll a in the surface waters. The vertical profiles of the two methyl halides were characterized by the maxima in the upper mixed layer. Both gases were generally supersaturated in the surface seawater, with mean sea-to-air fluxes of methyl chloride and methyl bromide of 391 and 20.0nmolm− 2 d−1, respectively. [Copyright &y& Elsevier]

The distribution and chemical properties of chromophoric dissolved organic matter (CDOM) in the Changjiang River estuary and the adjacent ECS were determined in February and July 2014. CDOM concentration (a CDOM (355)) decreased along the salinity gradient, indicating that terrestrial input significantly influenced CDOM in the study area. The runoff from the Changjiang River with smaller spectral slope ratios ( S R ) dominated S R variability in our study area in July, and the photodegradation of allochthonous CDOM increased S R in the open sea in February. CDOM excitation/emission matrix spectra subjected to parallel factor analysis identified four components: terrestrial humic-like component (C1), autochthonous protein-like components (C2 and C4), and terrestrial tryptophan-like component (C3). C1 and C3 showed a conservative mixing behavior, whereas C2 and C4 were appeared to be influenced by chlorophyll- a (Chl- a ). Our sunlight exposure experiment showed that a CDOM (355) (42% at station C1 and 16% at station A6-8) decreased and S R (27% at station C1 and 9.2% at station A6-8) increased. These findings suggested that CDOM molecules were photochemically degraded from high-molecular weight dissolved organic matter (DOM) into low-molecular weight organic substances. The fluorescence intensities of terrestrially derived C1 and C3 decreased with prolonged illumination in samples from both sites. However, autochthonous C2 and C4 moderately increased after they were exposed to sunlight. Results of this study demonstrated that terrestrially derived DOM was more susceptible to irradiation than autochthonous DOM. [ABSTRACT FROM AUTHOR]

In marine sediments factors that influence iron (Fe) geochemistry and its interactions with other elements are diverse and remain poorly understood. Here we comparatively study Fe speciation and reactive Fe-bound organic carbon (Fe-OC) in surface sediments of the East China Sea (ECS) and the south Yellow Sea (SYS). The objectives are to better understand the potential impacts of geochemically distinct sediment sources and depositional/diagenetic settings on Fe geochemistry and OC preservation by Fe (hydr)oxides in sediments of the two extensive shelf seas around the world. Contents of carbonate- and acid-volatile-sulfide (AVS)-associated Fe(II) (Fe AVS + carb ) and magnetite (Fe mag ) in the ECS sediments are about 5 and 9 times higher, respectively, than in the SYS. This could be ascribed to the ferruginous conditions of the ECS sediments that favor the formation/accumulation of Fe carb and Fe mag , a unique feature of marine unsteady depositional regimes. Much lower total Fe(II) contents in the SYS than in the ECS suggest that lower availability of highly reactive Fe (Fe HR ) and/or weak Fe reduction is a factor limiting Fe(II) formation and accumulation in the SYS sediments. The ratio of Fe HR to total Fe is, on average, markedly higher (2.4 times) in the ECS sediments than in the SYS, which may be a combined result of several factors relevant to different sediment sources and depositional/diagenetic settings. In comparison with many other marine sediments, the percent fractions ( f Fe-OC ) of Fe-OC to total organic carbon (TOC) in the ECS and the SYS are low, which can be ascribed to surface adsorption of OC rather than coprecipitation or organic complexation as the dominant binding mechanisms. Based on the f Fe-OC in this study, total Fe-OC estimated for global continental shelves is equivalent to 38% of the atmospheric CO 2 pool, which indicates the important role of sorptive stabilization of Fe-OC in continental shelf sediments for buffering CO 2 release to the atmosphere. In the SYS, consistently less 13 C-depleted Fe-OC relative to 13 C of non-Fe-bound OC ( 13 C non-Fe-OC ) suggests selective sequestration of labile marine OC in the marine OC-dominated sediments of the central SYS. In the ECS, however, efficient oxidation of OC and frequent redox cycling of Fe in the unsteady depositional regimes may complicate the isotopic compositions of Fe-OC. A combination of our results and literature data demonstrates that Fe-OC contents are strongly dependent on the availability of TOC and reactive Fe, but the f Fe-OC is primarily controlled by the processes of Fe redox cycling in the sediments. [ABSTRACT FROM AUTHOR]

Spatial distribution and sea-to-air flux of isoprene in the East China Sea and the South Yellow Sea in July 2013 were investigated. This study is the first to report the concentrations of isoprene in the China marginal seas. Isoprene concentrations in the surface seawater during summer ranged from 32.46 to 173.5 pM, with an average of 83.62 ± 29.22 pM. Distribution of isoprene in the study area was influenced by the diluted water from the Yangtze River, which stimulated higher in-situ phytoplankton production of isoprene rather than direct freshwater input. Variations in isoprene concentrations were found to be diurnal, with high values observed during daytime. A significant correlation was observed between isoprene and chlorophyll a in the study area. Relatively higher isoprene concentrations were recorded at stations where the phytoplankton biomass was dominated by Chaetoceros , Skeletonema , Pennate-nitzschia , and Thalassiosira . Positive correlation was observed between isoprene and methyl iodide. In addition, sea-to-air fluxes of isoprene approximately ranged from 22.17 nmol m −2 d −1 –537.2 nmol m −2 d −1 , with an average of 161.5 ± 133.3 nmol m −2 d −1 . These results indicate that the coastal and shelf areas may be important sources of atmospheric isoprene. [ABSTRACT FROM AUTHOR]

Carbohydrates are the largest identified fraction of dissolved organic carbon and play an important role in biogeochemical cycling in the ocean. Seawater samples were collected from the East China Sea (ECS) during June and October 2012 to study the spatiotemporal distributions of total dissolved carbohydrates (TCHOs) constituents, including dissolved monosaccharides (MCHOs) and polysaccharides (PCHOs). The concentrations of TCHOs, MCHOs and PCHOs showed significant differences between summer and autumn 2012, and exhibited an evident diurnal variation, with high values occurring in the daytime. Phytoplankton biomass was identified as the primary factor responsible for seasonal and diurnal variations of dissolved carbohydrates in the ECS. The TCHOs, MCHOs and PCHOs distributions in the study area displayed similar distribution patterns, with high concentrations appearing in the coastal water. The influences of chlorophyll- a , salinity and nutrients on the distributions of these carbohydrates were examined. A carbohydrate enrichment in the near-bottom water was found at some stations, implying that there might be an important source of carbohydrate in the deep water or bottom sediment. [ABSTRACT FROM AUTHOR]

Highlights: [•] Reactivity of Fe oxides was quantified by the reactive continuum model. [•] Factors influencing Fe oxide reactivity and its variation were documented. [•] Spatial pattern of reactive Fe oxides is generally controlled by clay fraction. [•] Redox cycles caused a decrease in amount and reactivity of reactive Fe oxides. [Copyright &y& Elsevier]

Highlights: [•] Contents and sulfur isotopes of three organic sulfur (OS) forms are investigated. [•] Hydrolyzable OS is almost biological in origin and diagenetically refractory. [•] Terrigenous OS accounts for 77–79% of HA-S and sulfurization for 41–71% of FA-S. [•] Humic-acid S is generally inert in early diagenesis, but fulvic-acid S is reactive. [•] Low humic-sulfur in the sediment is due to a low extent of sulfurization. [ABSTRACT FROM AUTHOR]

Abstract: Solid-phase sulfur speciation and stable sulfur isotopic compositions are used to elucidate the formation and burial of pyrite-sulfur (Spy) and organic sulfur (OS) at three selected sites in mud sediments of the East China Sea (ECS) inner shelf, and to infer potential factors influencing the preservation of Spy and OS in the sediments. Our results in combination with previous studies show that the overall reactivity of sedimentary organic matter (OM) is low, while OM at the site impacted by frequent algal-bloom events displays somewhat enhanced reactivity. We observed characteristically low contents of acid volatile sulfide (AVS) and Spy in the sediments, which can be attributed to low sulfate reduction rate due to high redox potential together with limited availability of labile OM. Several geochemical features, for example, persistent occurrence of S0, good coupling among the profiles of AVS, S0 and Spy, and large 34Spy depletion, all suggest that the polysulfide pathway and disproportionation are likely involved in the pyrite formation. Organic sulfur amounts in the sediments are at the lower end of OS contents reported in many other marine sediments around the world. The sources of OS are both biosynthetic and diagenetic, with the biosynthetic OS being the major share (59–73%). In one site studied (C702), enhanced accumulation of OS within the upper layers (14cm) is believed to be associated with frequent algal-bloom events. Net burial fluxes of Spy and OS in the three sites studied range from 0.27 to 0.82mmol/m2/d and from 0.22 to 0.74mmol/m2/d, respectively. Sedimentation rate and algal-bloom events are two important factors influencing the spatial variability of Spy and OS burial fluxes in the whole shelf. [Copyright &y& Elsevier]

Marine isoprene concentrations in the East China Sea were measured during two oceanographic cruises: one from October 10, 2013 to November 21, 2013 and another from May 16, 2014 to June 13, 2014. Mean (range) isoprene concentrations in the surface water in the study area were 32.85 (16.31–108.2) pmol L −1 in late spring and 26.83 (11.22–42.13) pmol L −1 in autumn. The isoprene concentrations showed seasonal variation and were higher in late spring, which could be attributable to terrigenous diluted water and seasonal variations of phytoplankton structure. Isoprene and chlorophyll a (Chl- a ) concentrations were significantly correlated in late spring and autumn even under complex hydrographic conditions. Isoprene and Chl- a concentrations also exhibited similar variations during a 25 h period and remained high during daytime. Analysis of depth profiles indicated that isoprene concentrations in the water column peaked at the surface layer and gradually decreased with increasing depth. The atmospheric concentration of isoprene (average: 53.4 ppt; range: 14.8–141 ppt) was high in the onshore area but low in the offshore area in late spring. The mean sea-to-air fluxes of isoprene in the East China Sea were 36.12 (late spring) and 48.34 (autumn) nmol m −2 d −1 . Results revealed the substantial isoprene emission from surface seawater, which was also a net source of atmospheric isoprene during the study period. [ABSTRACT FROM AUTHOR]

Abstract: Speciation and reactivity characterization of solid-phase Fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity, the diagenetic cycling of Fe and its regulating roles on many other elements in sediments. In this study, a combination of sequential and single-step extractions was used for the determination of seven Fe pools in surface sediments of the East China Sea (ECS) continental shelf: (1) carbonate associated Fe (Fe(II)carb) plus acid volatile sulfide-Fe (Fe(II)AVS), (2) easily reducible amorphous/poorly crystalline Fe oxides (Feox1), (3) reducible crystalline Fe oxides (Feox2), (4) magnetite (Femag), (5) poorly reactive sheet silicate Fe (FePRS), (6) pyrite-Fe (Fepy), and (7) unreactive silicate Fe (FeU). Total Fe (FeT) in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the FeT with that of the Yangtze River and global riverine particulates. The size of FePRS is found to be the largest pool, followed by FeU, Feox2, Femag, Fe(II)AVS+carb, Feox1 and Fepy. The large FePRS may result from neoformation of Fe-rich clay minerals via reverse weathering and subsequent ageing. The small sizes of Fe(II)AVS+carb and Fepy pools is believed to be the result of low SO4 reduction due to generally low labile organic matter together with the oxic/suboxic, dynamic environments of the surface sediments. The occurrence of Feox1, Feox2 and FePRS in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter. Highly reactive Fe(FeHR) in the sediments is comparable to that in global marine sediments, but apparently lower than in the Yangtze River and global riverine particulates due probably to sequestration in the Yangtze Estuary. The ratios of FeHR/FeT, FePR/FeT and FeU/FeT in the ECS surface sediments consistently show more similarity to those in the Yangtze River particulates than in the global continental margin or deep-sea sediments. The surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive Fe(III) oxides (Fe(III)HR) in FeHR. [Copyright &y& Elsevier]