Changjiang River Estuary Research Articles

Seasonal impacts of Kuroshio intrusion on pico-phytoplankton dynamics near the Changjiang River estuary

The strong western Pacific boundary current of the Kuroshio significantly affects the oceanographic and ecological processes of the East China Sea through its branches. To understand the seasonal variation of Kuroshio intrusion and its ecological effects, 10 cruises were conducted from February 2015 to January 2016 in the waters adjacent to the Changjiang River estuary to collect hydrological data and abundances of phytoplankton assemblages of Prochlorococcus (Pro), Synechococcus (Syn) and picoeukaryotes. High salinity bottom water representing the Nearshore Kuroshio Branch Current (NKBC) appeared in the spring, peaked in the summer, and then almost disappeared at the end of the autumn. During this period, the seasonal variations of Pro and Syn subgroup with lower orange fluorescence (dim type Syn) abundances correlated and synchronized with the intensity of the NKBC intrusion. Water masses analysis further illustrated that the spring was the critical season for NKBC to influence and transport phytoplankton along the pathway to coastal waters. NKBC will further impact the phytoplankton dynamics and ecological processes in this sea area.

Dominant host phase of mercury within the sediments of the East China Sea inner shelf: Implications for mercury inputs

Mercury (Hg) concentrations normalized to their dominant hosts have been widely used to identify geological events such as massive volcanic eruptions. However, the modern Hg cycle has significantly changed, and the implications of host-normalized Hg concentrations for the Hg dynamics in contemporary coastal environments (e.g., sources, delivery mechanisms, and Hg fluxes) remain poorly understood. In this study, 66 surface sediment samples from the continental shelf of the East China Sea (ECS) were analyzed, and the total organic carbon (TOC), aluminum (Al), total sulfur (TS), and Hg concentrations were determined. Our results support that organic matter predominantly hosts Hg in these sediments. Factors such as the Hg source and abundances of organic fractions influence Hg distributions, with notable enrichment in the Changjiang River estuary and the inner shelf of the ECS, especially near the mud depocenter located off the Oujiang River estuary. The oxic to suboxic redox state and the lack of significant sulfide accumulation in surface sediments suggest that sulfides are not the primary Hg hosts. The Hg/TOC ratios, ranging from 3.0 to 169.6 (ppb/%), are particularly high in the Changjiang River estuary and exhibit an exponential correlation with the Hg accumulation rates (Hg-AR). These findings suggest that the host-normalized Hg concentration could serve as a valuable tool for assessing Hg dynamics where the accumulation rates are uncertain.

Proxies of hypoxia and submarine groundwater discharge in the coastal ocean: Foraminiferal shell chemical perspectives

Coastal marine settings are important in terms of geochemical cycles and biological productivity. Climate change is predicted to affect coastal environment via hypoxia and Submarine Groundwater Discharge (SGD). Appropriate proxies could help to better understand oxygenation history and the role of SGD in regulating hypoxia. This would also benefit prediction of potential outcomes of future environmental changes. The sensitivity of benthic foraminiferal shell chemistry to environmental conditions opens the possibility to use them as proxies of coastal hypoxia and SGD. We report here that the average Mn/Ca ratios in tests of living benthic foraminiferal shells in the Changjiang River Estuary (CJE) is 2.3 times higher during hypoxia periods than under well-mixed conditions. In addition, Ba/Ca ratios in living benthic foraminiferal shells co-varied well with radon-inferred SGD signals. Fluctuations of Mn/Ca and Ba/Ca ratios in tests of a single foraminiferal shell along successive chambers corresponds well with seasonal-scale variations of hypoxia and SGD. We suggest that Mn/Ca and Ba/Ca ratios within intra-tests of benthic foraminifer can provide a reliable proxy for past hypoxia and SGD trends.

Effects of phosphorus limitation on sinking velocities of phytoplankton during summer in the Changjiang River Estuary

Effects of phosphorus limitation on sinking velocities of phytoplankton during summer in the Changjiang River Estuary

Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang River Estuary in summer 2020

Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang River Estuary in summer 2020

Inter-annual variations of dissolved oxygen and hypoxia off the northern Changjiang River (Yangtze River) Estuary in summer from 1997 to 2014

Inter-annual variations of dissolved oxygen and hypoxia off the northern Changjiang River (Yangtze River) Estuary in summer from 1997 to 2014

Spatial and Temporal Characteristics and Mechanisms of Marine Heatwaves in the Changjiang River Estuary and Its Surrounding Coastal Regions

Marine heatwave (MHW) events have significant consequences for marine ecosystems and human society. This paper investigates a MHW’s spatial–temporal characteristics in the Changjiang River Estuary and its surrounding coastal regions (CRESs), as well as analyzes the drivers, using satellite and reanalysis data spanning from 1982–2021. The findings show that, during the last 40 years, all four of the MHW indicators have increased. The summer MHW is more severe than other seasons, showing a rising pattern from southeast to northwest. The rise of MHWs can be attributed to the increase in sea surface heat flux, weak wind speed, and powerful El Niño events. Additionally, two special MHW events were detected during the entire study period: Event A lasted for 191 days from 9 October 2006 to 17 April 2007; Event B had an average intensity of 4.93 °C from 5 July 1994 to 1 August 1994. For locations so close to each other, the characteristics of MHWs can also vary, and the mechanisms behind them are highly complex.

Emission of CO2 and its related carbonate system dynamics in a hotspot area during winter and summer: The Changjiang River estuary

The carbonate chemistry in river-dominated marginal seas is highly heterogeneous, and there is ongoing debate regarding the definition of atmospheric CO2 source or sink. On this basis, we investigated the carbonate chemistry and air-sea CO2 fluxes in a hotspot estuarine area: the Changjiang Estuary during winter and summer. The spatial characteristics of the carbonate system were influenced by water mixing of three end-members in winter, including the Changjiang freshwater with low total alkalinity (TA) concentration, the less saline Yellow Sea Surface Water with high TA, and the saline East China Sea (ECS) offshore water with moderate TA. While in summer with increased river discharge, the carbonate system was regulated by simplified two end-member mixing between the Changjiang freshwater and the ECS offshore water. By performing the end-member mixing model on DIC variations in the river plume region, significant biological addition of DIC was found in winter with an estimation of −120 ± 113 μmol kg−1 caused by wintertime organic matter remineralization from terrestrial source. While this biological addition of DIC shifted to DIC removal due to biological production in summer supported by the increased nutrient loading from Changjiang River. The pCO2 dynamics in the river plume and the ECS offshore were both subjected to physical mixing of freshwater and seawater, whether in winter and summer. In the inner estuary without horizontal mixing, the pCO2 dynamics were mainly influenced by biological uptake in winter and temperature in summer. The inner estuary, the river plume, and the ECS offshore were sources of atmospheric CO2, with their contributions varying seasonally. The Changjiang runoff enhanced the inner estuary's role as a CO2 source in summer, while intensive biological uptake reduced the river plume's contribution.

Salinity mixing in a tidal multi-branched estuary with huge and variable runoff

The study of estuarine hydrodynamics within an isohaline framework has been well established in recent years. However, most studies of estuarine mixing and exchange flow using salinity coordinates have been conducted for idealized estuaries or realistic estuaries with simple bathymetry and constant runoff. In this study, a realistic numerical model is applied to the Changjiang River estuary, a complex multi-branched estuary with huge and variable runoff. The destruction of salinity variance is used to quantify salinity mixing, in which numerical mixing accounts for about one third of the total mixing. The results of total mixing per salinity class mS indicate that the universal law of estuarine mixing applies to realistic simulations under huge and variable runoff when averaged over two spring-neap cycles (28 days). During the flood season, volume integrated salinity mixing is approximately twice that of the dry season due to the double runoff. Salinity mixing at different channels displays distinct characteristics in both horizontal and vertical distributions. The patchy distribution of effective vertical diahaline velocity implies that the volume flux across an isohaline surface can exhibit significant spatial variability. The most vigorous salinity mixing occurs in the North Passage, where about 58 % of the freshwater enters the sea. We found that the estuarine mixing within different branches is linearly correlated with the corresponding water transport. Dikes fringing the navigational channel on both sides downstream of the North Passage significantly increase the length of saltwater intrusion in the North Passage, alter the distribution of horizontal and vertical flow velocities, and further cause significant changes in local mixing, especially near the mouth of the dikes.

Rapid variations of phytoplankton blooms and their dynamics off the Changjiang River Estuary

Phytoplankton frequently blooms in estuaries and coastal seas. Numerous dynamic processes affect these regions, generating complex hydrodynamics that induce intense phytoplankton variability over multiple time scales. Especially, the variability over time scales of 100-101 days (event-scale) is a strong signal that is fundamental to coastal aquatic environments and ecosystems. Based on the historical monitoring of harmful algal bloom events and a fully coupled hydrodynamics-sediment-ecosystem numerical model, this study explored horizontal distribution patterns of the phytoplankton maximum off the Changjiang River Estuary over multiple time scales. Our results showed that the bloom events typically lasted less than a week and horizontal distribution of the horizontal chlorophyll maximum varied over the time scale of days. Tidal forcing was shown to dominate the periodic phytoplankton variability. The variations of river runoff and wind forcing also modulated this variability and added more disturbances. Increased runoff and enhanced summer monsoon wind caused the horizontal chlorophyll maximum to physically extend further offshore, while they also biologically stimulated phytoplankton blooms. The analysis of the time scale showed that the regulation of horizontal chlorophyll maximum responds faster to physical effects than in biological ones. At the same time, during neap tides, the adjustment of phytoplankton to the disturbances associated with the hydrodynamic processes was stably salient. Such adjustment was based on the adaptation to light availability and nutrient supply. This study contributes to the understanding of phytoplankton variability in estuaries affected by multiple physical-biological processes over the time scale of days and benefits to the management of environmental conservation.

Devosia aquimaris sp. nov., isolated from seawater of the Changjiang River estuary of China.

A gram-stain-negative, aerobic, rod-shaped bacterium strain CJK-A8-3T was isolated from a polyamine-enriched seawater sample collected from the Changjiang River estuary of China. The colonies were white and circular. Strain CJK-A8-3T grew optimally at 35°C, pH 7.0 and 1.5% NaCl. Its polar lipids contained phosphatidylglycerol, phosphatidic acid, unidentified glycolipids, and a combination of phospholipids and glycolipids. The respiratory quinone was ubiquinone-10, and its main fatty acids were C16:0, 11-methyl C18:1ω7c and Summed Feature 8 (including C18:1ω7c/C18:1ω6c). The phylogenetic tree based on 16S rRNA genes placed strain CJK-A8-3T in a new linage within the genus Devosia. 16S rRNA gene sequence of strain CJK-A8-3T showed identities of 98.50% with Devosia beringensis S02T, 98.15% with D. oryziradicis, and 98.01% with D. submarina JCM 18935T. The genome size of strain CJK-A8-3T was 3.81Mb with the DNA G + C content 63.9%, higher than those of the reference strains (60.4-63.8%). The genome contained genes functional in the metabolism of terrigenous aromatic compounds, alkylphosphonate and organic nitrogen, potentially beneficial for nutrient acquirement and environmental remediation. It also harbored genes functional in antibiotics resistance and balance of osmotic pressure, enhancing their adaptation to estuarine environments. Both genomic investigation and experimental verification showed that strain CJK-A8-3T could be versatile and efficient to use diverse organic nitrogen compounds as carbon and nitrogen sources. Based on phenotypic, chemotaxonomic, phylogenetic and genomic characteristics, strain CJK-A8-3T was identified as a novel Devosia species, named as Devosia aquimaris sp. nov. The type strain is CJK-A8-3T (= MCCC 1K06953T = KCTC 92162T).

Saltwater Intrusion in the Changjiang River Estuary in Response to the East Route of the South-to-North Water Transfer Project in the New Period after 2003

The continuous operation of the Three Gorges Reservoir since 2003 has altered the annual runoff into the Changjiang River Estuary, significantly affecting patterns of saltwater intrusion. This has become more pronounced with the development of the East Route of the South-to-North Water Transfer Project, which has changed the runoff distribution and saltwater dynamics once again. Recognizing the critical need to understand these changes, this study employs numerical simulations to investigate the impact of the East Route of the South-to-North Water Transfer Project’s water abstraction on saltwater intrusion in the Changjiang River Estuary post-2003. It assesses intrusion distances, freshwater availability, and periods when water intake might be compromised due to high salinity. Our findings indicate that the East Route of the South-to-North Water Transfer Project markedly influences intrusion patterns. By modeling various runoff scenarios, the study delineates the correlation between average monthly runoff at the Datong Hydrological Survey Station and estuary salinity. It then suggests optimal ecological discharge levels to manage saltwater intrusion effectively. This research provides insights which are necessary for informed water management and ecological protection in the region.

Molecular size variations of chromophoric dissolved organic matter (CDOM) along a salinity gradient in the Changjiang River estuary

Molecular size is a critical parameter for characterizing dissolved organic matter in aquatic systems. Four cruises were conducted in October 2018, March 2019, July 2019 (with a normal flood), and August 2020 (with an extreme flood), along a transect from the Changjiang (Yangtze) River freshwater to the shelf area with salinities near or > 30. During each cruise surface water samples at 5–6 stations were collected along the salinity gradient, and absorption spectra (to quantify and qualify chromophoric dissolved organic matter (CDOM)) were measured. In addition, ultrafiltration was conducted so that the size partitions including four factions of <1 kDa, 1–3 kDa, 3–10 kDa and 10 kDa–0.2 μm were obtained. In the different seasons, CDOM molecular sizes always showed decreasing trends from land to sea, which was consistent to most estuaries worldwide. In the freshwaters, those with sizes >1 kDa generally contributed 20%–40% of the bulk CDOM in different seasons, but these large colloidal molecules always disappeared at the offshore stations. The extreme flood in August 2020 brought more waters with lower salinity and higher CDOM concentration into the transects, and the extra CDOM materials were generally of larger molecular sizes. In addition to suspended particulate matter (SPM) resuspension, the flocculation of organic matter could also be another important reason for localised increased CDOM molecular size near the river mouth.

Combined Effects of Fronts, Upwelling and the Biological Pump on Organophosphate Esters in the Changjiang (Yangtze) River Estuary During Summer

AbstractEstuarine and coastal environments are important transport pathways and regional sinks for anthropogenic pollutants. In this study, the occurrence and transport of the continuously released organophosphate esters (OPEs) was investigated together with physical and biochemical parameters throughout the water column in the Changjiang (Yangtze) River estuary during the summer. Total dissolved and particulate OPEs showed great spatial heterogeneity, with mean concentrations of 550 ± 280 ng/L in the estuary, 110 ± 270 ng/L in the front/upwelling zone, and 410 ± 450 ng/L in the continental shelf. OPE concentrations in the estuarine bottom waters were high due to massive terrestrial/sediment inputs. In contrast, the “surface enrichment and depth depletion” of OPEs in the continental shelf was closely related to seasonal stratification. Reduced OPE concentrations were observed in the frontal/upwelling zone due to isopycnal heaving. Frontal activity and upwelling induced phytoplankton blooms in the coastal regions, which jointly contributed to elevated OPEs beneath surface water with high phytoplankton aggregation. The OPEs mainly originated from wastewater treatment plant discharges, industrial pollution and consumer products. These OPEs generally posed a low ecological risk to aquatic lives, but their long‐term effects cannot be ignored due to their continuous high production, usage and release.

Tidal Variability of Phytoplankton Distribution in the Highly Turbid Changjiang River Estuary: Mechanisms and Implications

AbstractPhytoplankton distribution in macrotidal estuaries varies strongly due to highly dynamic conditions, contributing to the complexity of aquatic environments and marine ecosystems. Here the Changjiang River Estuary was analyzed as an example to explore this issue, based on field data collected through multiple cruises (including one covering a complete spring‐neap tidal cycle) and a fully coupled hydrodynamics‐sediment‐ecosystem numerical model. The composite analysis of all cruise data clearly showed that the chlorophyll concentrations were relatively low on lunar days 14–17 and 28–2, which corresponded to spring tides. Mechanistic analysis indicated that during spring tides, the river plume front retracts upstream with enhanced vertical mixing. This process restricts the transport of fluvial nutrients while increasing sediment resuspension, and therefore inhibiting net phytoplankton growth. During non‐spring tides, sediment resuspension was reduced but the river plume was further extended with slowed tidal currents, which enhanced the onshore light availability and offshore nutrient conditions and hence stimulated phytoplankton blooms in a larger area. The onshore shifting trend of the chlorophyll maximum region could last till the following neap tide with improved onshore light conditions and activate more conservative nutrients. The non‐conservation of nutrients augments the uncertainty of biomass variations during non‐spring periods. Besides in the spring‐neap tidal cycle, the phytoplankton variability between the perigean and apogean spring tides was also revealed. This study contributes to an improved understanding of intertidal variations of phytoplankton bloom and underlying physical‐sedimentological‐biological coupling mechanisms in tidal estuaries.

Wave–Tide Interaction by Typhoon Ampil on Wave and Storm Surge in the Changjiang River Estuary and Its Adjacent Coastal Areas

The study used the SCHISM ocean model combined with the WWM III wind wave model to quantify the interaction between wind waves and tides in the coastal zone of the Changjiang River Estuary and its adjacent areas. The wave and storm surge during Typhoon Ampil, which made landfall in July 2018 in Shanghai, were simulated by using the Climate Forecast System Version 2 (CFSv2) and Medium-Range Weather Forecasts (ECMWF) latest reanalysis (ERA5) wind dataset from 1 July to 31 July. Model results with CFSv2 forcing show better performance in terms of significant wave height and storm surge than those with ERA5 forcing. To investigate the interactions between waves, water levels, currents, and their combined effects on significant wave and surge variations, six numerical sensitivity experiments were designed according to the different coupling methods between SCHISM and WWMIII. The research shows that in coastal areas with water depths of less than 10 m, waves are affected by water levels and currents. The differences in the effect on significant wave height between wave-tide coupling and one-way coupling with water levels and currents are negligible. Wave setup is an important physical term which cannot be ignored during the variations of storm surge caused by Typhoon Ampil. The contributions of wave set up were concentrated in coastal areas with water depths less than 10 m. The peak wave setup occurred in the Changjiang River Estuary, reaching 0.15 m. In Xiangshan Bay and Sanmen Bay, wave radiation stress makes the proportion of wave setup to the total surge reached more than 30%. The consideration of wave-tide interaction can effectively improve the accuracy of numerical wave and storm surge simulations, which can provide more accurate hindcasts of wave and storm surge variations in the Changjiang River Estuary and its adjacent coastal areas.

Mechanisms Controlling Interannual Variability of Seasonal Hypoxia Off the Changjiang River Estuary

AbstractHypoxia has long been a symptom of deteriorating ecosystem that threatens the health of estuarine and coastal waters. Episodic hypoxia events and intraseasonal variation of coastal hypoxia have been amply investigated. However, interannual variability of coastal hypoxia has only been assessed in few regions. Bottom hypoxia forms seasonally off the Changjiang River Estuary in the East China Sea mainly due to the large riverine inputs. Large river discharge and its interactions with ambient water combine to contribute to the development of episodic hypoxia events and the intraseasonal migration of bottom hypoxia. However, little is known about the interannual variation of bottom hypoxia in this region. This study used a well‐evaluated, coupled physical–biogeochemical model to explore the long‐term feature of hypoxia in the East China Sea. The bottom water in the hypoxic zone lost oxygen with a rate of −1.2 mmol/m3/year. Bottom hypoxia showed large interannual variations of geographical location, severity, volume expansion, and sustainment. Large Changjiang River discharge was a prerequisite for hypoxia formation and the associated interannual variation. The interannual variations in the direction and strength of shelf wind controlled long‐term distribution of Changjiang diluted water. The delivery of freshwater fundamentally determined the strength of vertical stratification and the rates of biogeochemical cycles, contributing 73% of the interannual variation of bottom hypoxia.

Impacts of algal blooms on sinking carbon flux and hypoxia off the Changjiang River estuary

Impacts of algal blooms on sinking carbon flux and hypoxia off the Changjiang River estuary

Prokaryotic diversity and community composition in the surface sediments of the Changjiang River Estuary in summer

Prokaryotic diversity and community composition in the surface sediments of the Changjiang River Estuary in summer

Dissolved iodine in the Changjiang River Estuary, China.

The distribution and behavior of total dissolved iodine (TDI) and its species-iodate, iodide, and dissolved organic iodine (DOI) in the Changjiang River Estuary (CJE) surface and subsurface waters were studied along the salinity gradient. Results showed that TDI concentration in the freshwater endmember of CJE was 0.037 μM and existed as iodide. Although the transformation of dissolved iodine forms was active, TDI showed a conservative behavior, ranging from 0.037 μM to 0.42 μM in the estuary. Iodate showed removal behavior (ranging within 0-0.277 μM), iodide showed additive behavior (ranging within 0.037-0.131 μM), whereas DOI showed additive (0 < salinity < 20) (ranging within 0-0.099 μM) and removal (20 < salinity < 33.5) behavior (ranging within 0.099-0.022 μM). The iodine atoms in DOI were supplied primarily by iodide when salinity was <10 but by iodate when salinity was >10. The iodine-carbon ratios in DOI from different sources were more than 10 times different. The annual flux of iodine in the Changjiang River was 4.0 × 109g, accounting for about 4% of the global river iodine flux.