The high primary productivity zone (HPPZ) of estuaries is known for its flourishing fisheries and active interactions with coastal and oceanic ecosystems. However, the spatiotemporal patterns and underlying mechanisms that regulate the HPPZ remain unclear, especially in the face of drastic changes in riverine inputs. Using 40 years of in situ monitoring data along the Yangtze River estuary, the spatiotemporal evolution of the HPPZ regulated by basin-estuarine-offshore flux fluctuations has been reconstructed for the past and conclusions drawn for future developments. Moreover, the biological processes that influence the formation of the HPPZ were explored within the context of estuarine filtration and buffering effects. The obtained dataset includes chlorophyll a (Chl-a) concentrations and multiple environmental factors. The results displayed that the HPPZ is characterized by a high annual average Chl-a concentration of 3.6 ± 2.4 μg/L, which is driven by sufficient light and nutrient availability that promote phytoplankton blooms. In contrast, the inner high turbidity zone exhibits an average annual Chl-a concentration of 1.0 ± 0.7 μg/L, primarily due to limited light availability inhibiting phytoplankton growth. Meanwhile, the outer lower nutrient zone, with an average annual Chl-a concentration of 0.9 ± 1.1 μg/L, results from nutrient deficiencies that limit phytoplankton growth. Notably, the synergistic effect of sediment declines and eutrophication has resulted in a 6.5 μg/L increment of the HPPZ’s annual Chl-a concentration and a 3628 km² expansion of its area extent over 40 years. This significant change is attributed to the increase in water transparency resulting from a reduction in sediment transported from the watershed to the sea, along with an increase in riverine nitrogen and phosphorus discharge. A future projection, based on the historical total suspended matter and nutrients over the past 40 years, suggests that annual Chl-a concentration in the HPPZ will reach 10.5 μg/L, and the area is projected to increase to 7,904 km² by 2050. This study presents the first quantification of Chl-a concentrations and spatial range of the HPPZ in the estuary, focusing on the interaction between riverine and oceanic materials. These findings offer a deeper understanding of managing ecological risks in large estuaries.

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.

The development of storm-induced fluid mud is an important factor to disturb the waterway transportation. Based on the observation data of fluid mud from 2010 to 2016, the basic characteristics and dynamic factors of the storm-induced fluid mud in the North Passage of the Yangtze River Estuary are analyzed. The main conclusions are as follows: (1) The sediment composition of the storm-induced fluid mud in the North Passage has little difference with the suspended sediment, which shows high correlation with the bed sediments in the middle/lower channel and the north beach of the North Passage, but the space difference of which is weak. (2) Large-thickness fluid mud in the North Passage mainly locates in the manual dredged navigation channel, and cannot stay in the steep slope beaches. It manly distributes between IIN-C and Y channel unit where is under the protection of the south and north embankments. (3) The storm-induced fluid mud in the North Passage characterizes as three stages. The primary-stage fluid mud develops during the storm surge, characterizes as low density, blurred upper and lower interfaces. It migrates quickly following the tidal current, and can be easily weaken by the peak tidal velocity. The development-stage fluid mud mainly occurs after the storm surge, characterizes as clear upper interface, "h" type density profile, with good stability and slowly migration. The dissipation-stage fluid mud characterizes as decreasing sediment amount, increasing sediment density, fuzzy lower boundary, "L" type or multi-steps type density profile, high stability and very weak flowability. (4) The cumulative wave energy during storm surge processes is the most important factor to determine the scale of the storm-induced fluid mud in the North Passage. The stronger the cumulative wave energy, the longer duration and the larger scale of the storm-induced fluid mud will develops. In addition, the weaker tidal power during the storm surge processes is favorable to the formation of the storm-induced fluid mud in the North Passage. Stronger tidal force would cause the shorter dissipation period of the storm-induced fluid mud. (5) The mechanism that up layer tidal current disturbs the fluid mud layer that make its sediment tends to dissipation and transport to the downstream and reciprocating following the tidal current, which plays the main role during the local extinction process of the storm-induced fluid mud in the North Passage. (6) The process of the high-sediment concentration gravity flow generates in the steep slope of the beach and near-bed invades to the manual dredged navigation channel during the storm surge process, is the key process mechanism for the rapid accumulation of storm-induced fluid mud in the North Passage.

Due to the impact of the Three Gorges Dam on water and sediment storage, the sediment flux into the Yangtze River Estuary has dropped sharply by 70%, and the suspended sediment concentration in the estuary has responded accordingly. From the comparison of the measured suspended sediment concentration data of the Yangtze River estuary for many years, it is known that the suspended sediment concentration in the South Passage has been reduced by about 60% recently, and that in the middle and upper reaches of the North Channel and the South Channel has been reduced by about 40%. On the other hand, A series of artificial engineering has been completed in the past 20 years, such as the 12.5m Deep-Waterway Regulation Engineering, the Nanhui Shoal Slush-enclosure Engineering, and the Hengsha Shoal Slush-enclosure Engineering, etc. These engineering have significantly changed the original water and sediment transport pattern of the Yangtze River Estuary. It resulted in a significant change of the estuarine turbidity maximum zone and the corresponding river mouth bar topography. This paper intends to discuss the impact of human activities on the dynamic sedimentation process of the maximum turbidity zone in the Yangtze River Estuary based on field measured data. Results are as follows:(1) Compared to two decades ago, the suspended sediment concentration in the North Passage, the South Passage and the North Channel, and the middle and lower reaches of the North Branch is still high, which is related to the existence of the turbidity maximum zone and river mouth bar in these river sections.(2) The implementation of man-made engineering such as the submerged diversion dike between the North Passage and the South Passage and the Nanhui Shoal Slush-enclosure Engineering changed the flow structure in the upper section of the South Passage, leading to the turbidity maximum zone and the corresponding river mouth bar have completely disappeared.(3) Affected by the 12.5m Deep-Waterway Regulation Engineering, the turbidity maximum zone and the corresponding river mouth bar originally located at the upper section of the North Passage have also disappeared.(4) The longitudinal circulation flow structure, salt water wedges, and stagnation points in the middle and lower sections of the North Passage and the South Passage still exist. The positions of the turbidity maximum zone and the corresponding river mouth bar topography are not significantly affected by the engineering. And the core area of ​​the obvious turbidity maximum zone and the river mouth bar (only in the South Passage) still exist. Due to the artificial dredging of the navigation channel in the North Passage, it actually appeared as an invisible river mouth bar that has been dredged by continuous dredging. (5) The drastic reduction of sediment flux from the basin has caused seabed erosion adjacent to the Yangtze River Estuary, and the corresponding eroded sediment has become one of the main sediment budget sources of the turbidity maximum zone.