Recent Spatio-Temporal Variations of Suspended Sediment Concentrations in the Yangtze Estuary

Human Intervention–Induced Changes in the Characteristics of the Turbidity Maximum Zone and Associated Mouth Bars in the Yangtze Estuary

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated.

Utility of radium quartet for evaluating porewater-derived carbon to a saltmarsh nearshore water: Implications for blue carbon export

Saltmarshes are global hotspots of carbon sequestration and storage and are known as effective blue carbon ecosystems. However, the role of porewater exchange in saltmarshes as a source of carbon to the nearshore waters is still poorly constrained. Herein, we examined the radium quartet, dissolved inorganic (DIC) and organic (DOC) carbon in the porewater and nearshore surface water of Chongming Dongtan saltmarsh, China. Multiple methods based on the radium quartet were applied to estimate the porewater exchange, including the three-endmember model, mass balance model and time series observation. All methods revealed that the porewater exchange rate in Chongming Dongtan saltmarsh equaled 3.37 ± 1.23 cm d^-1. The porewater-derived DIC and DOC fluxes were then estimated to be (1.51 ± 0.64) × 10⁷ and (9.97 ± 6.96) × 10⁵ mol d^-1, respectively, which correspondingly made up 64.6% and 35.6%, of the total inputs into the Chongming Dongtan saltmarsh nearshore water. Considering the intertidal area covered by saltmarsh vegetation, carbon export through the porewater exchange was 3.87 ± 1.55 g C m^-2 d^-1, and was 1.2-fold greater than the carbon burial rate, accounting for approximately 29% of carbon outwelling in Chongming Dongtan saltmarsh. This study highlights the significance of porewater exchange for evaluating carbon sequestration capacity, and suggests that porewater exchange should not be overlooked in blue carbon assessments of saltmarshes.