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The three large marine ecosystems (LMEs) bordering China (Yellow Sea/Bohai Sea, East China Sea, and South China Sea) have received excess nitrogen (N) and phosphorus (P) in the past decades with de...

We have examined the possible use of Mn/Ca ratios in benthic foraminifera as a proxy for hypoxia, which has increasingly become a serious environmental issue in many coastal zones. We collected samples from the Yangtze River Estuary, one of the largest and most intense seasonally hypoxic zones in the world. In order to obtain high quality Mn/Ca data, unaffected by contamination, we examined two different cleaning protocols and instrument-based analysis methods. Our results showed that a relatively simple physical cleaning approach coupled to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) provided excellent results on single specimens. Our observations were comparable to ICP analyses of solutions made up from dozens or even hundreds of specimens and involving complicated sample pretreatments. Using the LA methodology, we analyzed Mn/Ca ratios from living Florilus decorus (F. decorus) specimens from stations in the Yangtze River Estuary with varying bottom water dissolved oxygen (DO) concentrations. Our results showed that Mn/Ca ratios in penultimate chambers of living benthic foraminifera are sensitive to bottom water DO concentrations. We also observed significant variations of both Mn/Ca and Mg/Ca ratios from different chambers within foraminiferal shell specimens. These fluctuations were likely a response to changing ambient water DO and temperature during the specimen’s growth history. Combined use of foraminiferal Mn/Ca ratios together with other proxies could provide a powerful tool for historical reconstruction of low oxygen conditions in seasonal hypoxia areas.

The hydro-biogeochemical processes in the world's largest area of green tide occurrence, off the Jiangsu coast in the Yellow Sea, are investigated in the summer, and their implications for Ulva prolifera blooms are discussed. The results show that the offshore transport of coastal water and the inshore upwelling of offshore bottom water both occur off the Jiangsu coast, and the upwelling position is consistent with the 20- to 30-m isobath off the Subei Shoal. The upwelling results in nutrient supplementation off the shoal, where a rapid decrease in the suspended particulate matter content contributes to good light conditions. As a result, a high-value area of phytoplankton is formed within the 20- to 30-m isobath. Eutrophication in the shoal has provided nutrients for the frequent occurrence of Ulva prolifera in recent years, whereas the upwelling area off the shoal has served as a "service station" or "courier station" for floating Ulva prolifera and promoted the species' propagation. The propagation of Ulva prolifera in the upwelling area and its blooms within the Subei Shoal can have a spatially synergistic effect, leading to its large-scale development. Our findings reveal the mechanisms that trigger the world's largest green tides from the perspective of physical-biogeochemical interactions.

The Changjiang (Yangtze) River Estuary (CJE) is one of the largest and most intense seasonal hypoxic zones in the world. Here we examine the possibility that submarine groundwater discharge (SGD) may contribute to the summer hypoxia. Spatial distributions of bottom water 222Rn suggest a hotspot discharge area in the northern section of the CJE. SGD fluxes were estimated based on a 222Rn mass balance model and were found to range from 0.002 ± 0.004 to 0.022 ± 0.011 m3/m2/day. Higher SGD fluxes were observed during summer hypoxia period. The well-developed overlap of the distribution patterns for SGD flux and dissolved oxygen (DO) implies that SGD could be an important contributor to summer hypoxia in the region off the CJE. We suggest that SGD contributes to the seasonal hypoxia either: (1) directly via discharge of anoxic groundwaters together with reducing substances; and/or (2) indirectly by delivering excess nutrients that stimulate primary productivity with consequent consumption of DO during organic matter decomposition.

In order to examine the mixing behavior of dissolved uranium (U) in estuaries under different suspended particulate matter (SPM) regimes, three laboratory-based experiments were conducted by mixing seawater with river water containing different concentrations of SPM. Comparing this study with other field and laboratory-based experiments, dissolved U behaved differently depending upon the concentration of SPM. When SPM concentrations are > 0.8 g/L in the Yellow River, desorption/dissolution of U from SPM becomes predominant and dissolved U is enriched relative to the theoretical mixing line. However, when SPM concentrations are

Biogenic Silica (BSi) plays an essential role in availability of dissolved silicate (DSi) in aquatic ecosystems , but its role in silica cycling is still poorly characterized in the river and estuary systems. This study addressed the riverine BSi composition and reactive silica (RSi) export in the Yellow River, and estimated the contribution of terrestrial BSi dissolution to the estuary silica pool. Our results indicated that phytoliths comprised 1.3%–87% of the riverine BSi, while diatoms accounted for 13%–99% of the riverine BSi. The Yellow River transported 24 Gg yr −1 of BSi and 26 Gg yr−1 of DSi loadings, respectively; 63% of the riverine BSi and 54% of the riverine DSi fluxes were transported during the period from April to July. In total 46% of annual BSi loading was contributed by phytolith BSi in the Yellow River. The terrestrial BSi dissolution in the estuary would represent 15%–63% of riverine DSi loading on the basis of dissolution test, and low salinity could play an important role in enhancement of terrestrial BSi solubility, especially in the first few days during the estuarine mixing process. Although the terrestrial BSi represents a significant DSi contribution to the silica biogeochemical cycle in coastal waters, it tends to be refractory with the increasing of salinity.

The study was carried out to address a method for separation of terrestrial and marine biogenic silica (BSi) in estuaries based on BSi compositions and δ 13 C values in BSi associated organic matter (δ 13 C BSi ). We used two world-class major rivers - the Changjiang (Yangtze) and Huanghe (Yellow) Rivers as examples to illustrate our approach. Our results for these rivers indicate that riverine BSi is comprised mainly of phytoliths and diatoms. River BSi concentrations vary with terrestrial inputs and in-stream primary production. Although the fluvial BSi sources are complex, the terrestrial δ 13 C BSi signals are quite unique (− 24.7 ± 0.8), significantly lower than the marine δ 13 C BSi values (− 21.3 ± 0.07, central Yellow Sea) ( p 13 C within BSi organic matter can provide terrestrial source information on the biogeochemistry of silicon in estuaries and the adjacent shelf. The δ 13 C BSi combination could potentially act as an efficient tool to study environmental change in coastal areas on decadal time-scales since this index may respond to variable terrestrial fluxes from land, as well as to changed phytoplankton assemblages in the coastal ocean.

One of the largest “green tide” (Ulva prolifera) outbreaks in the world has occurred every year from 2007 to present in the Southern Yellow Sea, China. Currently, the coastal area around Jiangsu Province (Subei Shoal region) is thought to be the origination point of these giant green tide blooms. The combination of high nutrient demand but low river discharge and other inputs suggests that there is a significant flux of submarine groundwater discharge (SGD) in this area. By using a radium mass balance model, we estimated the SGD flux in the area to be (0.7–1.4) × 109 m3 d−1 (6.1–12 cm d−1), at the high end of SGD fluxes worldwide. Geographically, Subei Shoal is less than 5% of the entire Southern Yellow Sea area, while our calculated SGD flux just for the shoal area is ~3 times larger than previously documented for the whole Southern Yellow Sea. Therefore, Subei Shoal may be considered a SGD hotspot that plays an important role in SGD associated material fluxes. Compared to inputs from local rivers, atmospheric deposition, and anthropogenic activities, SGD-derived nutrients are the main source term that can support the growth of macroalgae. We specifically highlight that this type of areas that are shallow, intensively mixed, anthropogenically polluted, sandy or muddy with heavy bio-irrigation, may have a higher risk of suffering harmful ecological problems, even with limited terrestrial runoff.

Radium isotopes are known to be excellent geochemical tracers for study of oceanographic processes. We show here that radium isotopes can also be used to assess adsorption/desorption and transport processes in rivers. The Yellow River (Huanghe), one of the longest, most turbid and heavily regulated rivers in the world, is used as an example. We first investigated the temporal and spatial behavior of radium isotopes (224Ra and 226Ra) in the lower reaches of the river, and found that this zone displayed some of the highest known riverine radium concentrations and fluxes in the world. Suspended particulate matter (SPM) is shown to be the dominant factor controlling radium activities. Laboratory simulation experiments showed that radium desorption from SPM obeys an exponential relationship in fresh water (S = 0). When salinities are >10, the increase in radium concentration follows a linear increase with respect to the amounts of SPM added. Significantly higher radium concentrations (3–5 times), especially for short-lived 224Ra, were observed during the “Water-Sediment Regulation Scheme” (WSRS), an annual management event when ∼15%–55% of the annual water discharge and ∼30%–75% of the annual sediment load are released from a reservoir to control sedimentation in the Yellow River. The radium fluxes during WSRS periods (∼2 weeks long) accounted for more than half of the entire annual load during the periods studied. Sediment erosion and pore water release are also thought to be important processes supplying radium to the river. After a WSRS, Ra desorption from SPM increases and becomes the prevailing process.

The Water–Sediment Regulation Scheme (WSRS) is an important water conservancy project in the Yellow River basin, which is usually operated annually from June to July to control water and sediment release from the Xiaolangdi Reservoir in the middle reaches. As a greatly concentrated period of delivering terrigenous materials from the Yellow River to the sea, the WSRS can serve as a natural laboratory to examine the geochemical behavior of elements during their transport along the river. Uranium isotopes (234U and 238U) were measured in Yellow River waters at stations Xiaolangdi (located in the middle reaches of the Yellow River) and Lijin (the last hydrologic station near the Yellow River estuary) during the WSRS 2012. Compared with station Xiaolangdi, dissolved uranium concentration at station Lijin was markedly higher, showing a significant impact from the WSRS. Budget calculation for dissolved uranium during the WSRS indicated that two major sources of new added dissolved uranium in the section of the Yellow River between Xiaolangdi and Lijin: suspended particles (46%) and porewater of bottom sediment (45%). The flux of dissolved uranium from the Yellow River to the sea was estimated to be 2.40 × 107 g during the WSRS 2012.

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.

Submarine groundwater discharge (SGD) in estuaries brings important influences to coastal ecosystems. In this study, we observed significant SGD in the Yellow River estuary, including a fresh component, during the Water–Sediment Regulation Scheme (WSRS) period. We used the 222Rn and 220Rn isotope pair to locate sites of significant SGD within the study area. Three apparent SGD locations were found during a non-WSRS period, one of which became much more pronounced, according to the remarkably elevated radon levels, during the WSRS. Increased river discharge (from 245 m3 s−1 to 3560 m3 s−1) and the elevated river water level (from 11 m to 13 m) during the WSRS led to a higher hydraulic head, enhancing groundwater discharge in the estuary. Our results suggest that high river discharge (>3000 m3 s−1) might be necessary for elevated fresh submarine groundwater discharging (FSGD). Vertical profiles of salinity, DO and turbidity anomalies along the benthic boundary layer also indicated significant FSGD in the estuary during the WSRS. Nutrient concentrations had positive correlations with 222Rn during a 24-h observation, which indicates that SGD is a dominant nutrient pathway in this area.

Phosphorus (P) speciation, burial, and transformation are poorly constrained under low-oxygen conditions. Sequential chemical extraction techniques, in-situ incubation, and laboratory incubation were employed to explore P cycling in the low-oxygen area of coastal Rushan. The study determined that the total P concentrations in the coastal area of Rushan Bay were higher than those of other China shelf seas, and largely affected by anthropogenic activities. The phosphate (DRP) fluxes in the study area calculated using an incubation method (0–1960 μmol m − 2 day − 1 ) and measured based on pore water gradients (1.5–50.4 μmol m − 2 day − 1 ) were both highly correlated with oxygen conditions. Sediment incubations showed that DRP diffusion from the sediment mainly originates from Fe–P and Auth-P dissolution and that Org-P recycling contributed only a small portion of the total released P pool. The benthic phosphate flux can be 60 times higher under low bottom-water oxygen levels of 63–150 μmol L − 1 than under oxygen levels exceeding 150 μmol L − 1 in the study area. The P accumulation rates and burial efficiencies in this study area ranged from 16.5–33.3 μmol cm − 2 year − 1 and 81.1–83.4%, respectively, and were regulated by the oxygen level and diffusive DRP flux. This study indicates that low oxygen levels between 63 and 150 μmol significantly govern P transformation and preservation in the sediment and P pools in the water column.