The Yangtze River is the longest in Asia, with a length of approximately 6,400 kilometers ( Figure 1A ). The Yangtze River basin included 264 large reservoirs (capacity exceeding 100 million m³) in 2020 ( Figure 1B ), with a total capacity equivalent to 27.78% of the average annual runoff of the Yangtze River (Liu et al., 2022). The Yangtze River estuary (121°E–124.5°E, 29.5°N–33°N), adjacent to the East China Sea ( Figure 1C ), is the third largest estuary in the world. Due to the nutrients carried by the significant runoff and sediment from the Yangtze River into the sea, the complex flow system, and the distinctive topography, a zone of high primary productivity has been formed in the estuary.

To explore the effect of nutrients and sediment flux into the sea on the HPPZ of estuarine, field cruise in situ survey data from the Yangtze River estuary for the period 1980–2020 were analyzed. The dataset consists of a total of 649 station series from 13 summers ( Figure 1C ; Supplementary Table S1 ), including Chl-a concentration (indicative of the HPPZ) and five environmental factors: DIN, DIP, DSi, SSS, and TSM. Besides, two apparent optical properties of a water body were calculated in the study; light attenuation coefficient (K_d ) and light attenuation (I_m ). The detailed testing and calculation methods for these indicators are outlined in Supplementary Text S1 .

The Datong hydrological station is the final runoff control station located in the lower reaches of the Yangtze River ( Figure 1A ), approximately 640 kilometers away from the entrance to the sea. The upstream watershed monitored by this station encompasses an area of 170,500 km², representing around 95% of the total Yangtze River watershed area (Zuo et al., 2022). The annual flux data into the sea, including Suspended Sediment Concentration (SSC), DIN, DIP, and DSi, from 1960 to 2020 were collected from the Datong hydrometric station (http://www.cjw.gov.cn/zwzc/zjgb/), the State Key Laboratory of Estuarine and Coastal Research, and related research papers (Gao et al., 2012; Song et al., 2023; Wu et al., 2018).

The spatial distribution pattern of the HPPZ was investigated by analyzing cumulative Chl-a data obtained from field investigations conducted over the last four decades. The results indicate that the Chl-a concentration in the Yangtze River estuary during the summer exhibits a characteristic low-high-low gradient distribution. The Chl-a concentration increases progressively from the inner estuary towards the sea, reaching a peak in the middle region of the estuary before subsequently declining ( Figure 2A ). Analysis of the fitted curve relating Chl-a concentration to salinity reveals two distinct inflection points when Chl-a concentration reaches 1.0 µg/L. The first inflection point occurs at 122.5°E, corresponding to a salinity of approximately 10 PSU, which aligns closely with the ebb frontier of brackish water in the Yangtze River estuary. The second inflection point is found at 123.5°E, where salinity is around 30 PSU, indicative of plume fronter of brackish water ( Figures 2A, B ; Li et al., 2021). These inflection points (boundaries) reflect variations in salinity, nutrient concentration, and hydrodynamic characteristics of the water body resulting from the interaction between freshwater and seawater in the Yangtze River. Consequently, by integrating the distribution characteristics of Chl-a concentration along with variations in other environmental factors such as turbidity, salinity, and nutrient concentrations, the Yangtze River Estuary was delineated into three distinct ecological functional zones: the inner high turbidity zone (HTZ), the central high primary productivity zone (HPPZ), and the outer low-nutrient zone (LNZ).

HPPZ is the most biologically productive region for phytoplankton within the Yangtze River Estuary. The two boundaries of the HPPZ correspond to the boundary of brackish water at the ebb front and the boundary of brackish water at the Yangtze River plume front. It is situated between 122.5°E and 123.5°E, exhibiting a salinity range of 10 to 30 PSU ( Figure 2 ). The average annual Chl-a concentrations in the inner HTZ (<10 PSU) and outer LNZ (>30 PSU) were 1.0 ± 0.7 μg/L and 0.9 ± 1.1 μg/L, respectively, while the HPPZ exhibited an average annual Chl-a concentration of 3.6 ± 2.4 μg/L ( Figure 2 ; Table 1 ). This information provides critical evidence for a more detailed examination of the mechanisms underlying phytoplankton productivity characteristics.

The temporal evolution of HPPZ demonstrated a significant expansion over the past 40 years. The annual average concentration of Chl-a increased by 6.5 μg/L between 1981 and 2020 (P<0.001; Figure 2B ). In addition, based on Equation 1 , the area covered by Chl-a > 1 μg/L in the HPPZ was 1486, 2165, and 5114 km² in years 1981, 2004 and 2020, respectively. This represents an increase of 679 km² from 1981 to 2004, and 3,628 km² from 1981 to 2020 (P<0.001; Figure 2C ).

The spatial distribution of TSM, and nutrient concentrations in the 1980s and 2010s exhibited decreasing levels with increasing salinity from inner to the outer estuary ( Figures 3A–K ). The average annual TSM concentrations in the HTZ, HPPZ, and LNZ were 184.6 ± 125.6, 78.5 ± 67.2, and 20.5 ± 13.7 mg/L, respectively ( Table 1 ). Influenced by TSM and Chl-a concentrations, the underwater light attenuation coefficient K_d showed an increasing trend and then slightly decreased from inner to the outer estuary ( Figures 3C, H ; Table 1 ). The highest nutrient concentrations were found in the HTZ, where the concentrations of DIN, DIP, and DSi were 105.1 ± 29.5, 1.3 ± 0.4, and 110.7 ± 16.1 μμ, respectively ( Table 1 ). The HPPZ’s nutrient levels were moderate, with DIN, DIP, and DSi concentrations of 137.3 ± 22.1, 0.5 ± 0.3, and 44.9 ± 29.8 μM, respectively ( Table 1 ). Conversely, the lowest nutrient concentrations were observed in LNZ, where salinity exceeded 30 PSU, with DIN, DIP, and DSi levels of 5.5 ± 4.1, 0.2 ± 0.1, and 10.9 ± 5.5 μM, respectively ( Figures 3D–F ; Table 1 ). Nutrient limitation criteria for phytoplankton growth were defined based on previous research (Justić et al., 1995; Harrison et al., 1977): absolute nitrogen limitation occurs when DIN<1 μM, absolute phosphorus limitation when DIP<0.1 μM, and absolute silicon limitation when DSi<2 μM. Relative nitrogen limitation is indicated by DIN/DIP<10 and DIN/DSi<1; relative phosphorus limitation by DIN/DIP>22 and DSi/DIP>22; and relative silicon limitation by DSi/DIN<1 and DSi/DIP<10. In this study, the ratios of DIN, DIP, and DSi were calculated at each sampling point to assess nutrient limitations in the surface seawater of the LNZ from 1980 to 2020 based on the above thresholds. The results showed that 10% of the sampling points exhibited absolute nitrogen limitation, 50% exhibited absolute phosphorus limitation, and 16% exhibited absolute silicon limitation. Additionally, 80% of sites showed relative phosphorus limitation, while 10% exhibited relative silicon limitation.

The analysis of 40 years of field data on TSM and nutrients in the estuary was used to reconstruct the historical variations over the past half-century. The findings indicate a significant decrease in TSM concentrations, particularly inside HTZ and HPPZ, by 300 mg/L and 50 mg/L, respectively ( Figure 3G ). The reduction in TSM has led to an increase in underwater illuminance levels, with the most significant rise of 65 μmol photons m^-2 s^-1 occurring in the HPPZ ( Figure 3H ). Similar to TSM, concentrations of DSi also showed a decreasing trend over the past 40 years, with a maximum decrease of approximately 13.3 μM inside HTZ and 7.1 μM in HPPZ, respectively ( Figure 3K ). However, DIN and DIP exhibited a consistent upward trend, particularly in HTZ (increases of 60 μM for DIN and 1.3 μM for DIP) and HPPZ (increases of 42 μM for DIN and 0.65 μM for DIP) ( Figures 3I, J ).

To comprehensively investigate the influence of the watershed environment on the development of estuarine HPPZ, nutrients and SSC discharged from the Yangtze River basin over the last six decades were systematically collected and assessed for their impact on TSM and nutrient levels in the estuary. The results show the annual SSC and DSi concentrations in the Yangtze River basin have decreased by 85% and 67% over the past 60 years ( Figures 4A, D ), and the average TSM and DSi levels in the Yangtze River estuary have decreased by 45% ( Figure 4A ) and 22% ( Figure 4D ), respectively. Moreover, concentrations of DIN and DIP at the Yangtze River basin (Datong Station) have increased by approximately 600% and 500% over the past 60 years. Similarly, an increase of such nutrients is occurring in the Yangtze River estuary with overall increases of 59% ( Figure 4B ) and 56% ( Figure 4C ).

Correlations were analyzed between SSC, DIN, DIP, and DSi concentrations in the watershed over the past decades and the corresponding TSM, DIN, DIP, and DSi concentrations in the estuary for the same years. The results indicate a positive and consistent response (R² = 0.51–0.73) of estuarine ecosystems to changes in the watershed environment ( Figures 4E–H ). The slopes of the regression equations ranged from 0.2 to 0.7, suggesting that the decreases in estuarine TSM and DSi were only 0.2 to 0.7 times the declines observed in the basin. Similarly, estuarine DIN and DIP increases were only 0.2 and 0.3 times the increases recorded in the basin ( Figures 4E–H ).

Based on the Equations 2–5 and database from 1981 to 2002, a regression equation of Chl-a concentration and environmental factors in the HPPZ was established as follows.

Similarly, the regression correlation between 2004 and 2020 was inferred as follows,

Based on the above results, it can be inferred that the degree of regulation of Chl-a concentration by the same environmental factor was varied before and after 2003. Assuming that TSM and nutrient concentrations in the estuary, along with their effects on Chl-a concentration in the HPPZ, continue to fluctuate over the next 30 years in a manner consistent with the patterns observed over the past 40 years, we can analyze these dynamics quantitatively. By calculating the difference in the coefficients for the same environmental factors from Equations 6, 7, we can determine the annual rate of change by dividing this difference by 20. This rate is then multiplied by 30 to project the potential relationship between these factors and Chl-a over the period from 2020 to 2050. This relationship can be expressed mathematically as follows (Equation 8).

Assuming that environmental factors continue to exhibit the same fluctuations observed over the past 40 years, we can project changes in DIP, DIN, DSi, TSM, and I_m over the next 30 years based on the incremental TSM and nutrients in the HPPZ at section A before and after 2003 ( Supplementary Figure S1 ). The projected changes are as follows: DIP and DIN are expected to increase by 26.9 and 0.65 μM, DSi is projected to decrease by 15.9 µM, TSM is likely to decline by 53.6 mg/L, and I_m will increase by 19.3 μmol photons m^-2 s^-1. Using Equation 8, we anticipate that the average Chl-a concentration in the HPPZ at transect A of the Yangtze River Estuary will peak at 8.18 μg/L by the year 2050. Furthermore, based on the relationship between the average Chl-a concentration at transect A and the HPPZ over the past 40 years, as well as the predicted Chl-a concentration at transect A in 2050, the mean Chl-a concentration in the HPPZ at the Yangtze River Estuary is projected to peak at 10.5 μg/L in 2050, according to the following equation:

Where C _{(predicted HPPZ Chl-a)} represents the predicted mean Chl-a concentration in the HPPZ at the Yangtze River Estuary in 2050. C _{(HPPZ Chl-a)} refers to the average Chl-a concentration at the HPPZ over the past 40 years, which is 3.60 μg/L ( Figure 2A ). C _{(transect A Chl-a)} is the average Chl-a concentration at transect A over the past 40 years, which is 2.79 μg/L ( Supplementary Figure S1 ), while C _{(predicted transect A Chl-a)} represents predicted average Chl-a concentration at transect A in 2050, which is 8.18 μg/L.

Likewise, the HPPZ exhibited an expansion trend, although this change has not been linear over the past 40 years. To facilitate the analysis of this complex dynamic process, the variation in HPPZ area over extended time scales in this study was approximated as a linear change. The annual variations observed in the 40-year time series data provide a more accurate representation of the gradual changes induced by large-scale, sustained reservoir construction on estuarine HPPZ. Thus, it can be concluded that the HPPZ, has increased by an average of 93 km² per year over the last four decades ( Figure 2C ). If this trend continues, the HPPZ is projected to expand to approximately 7904 km² by 2050. It is worth mentioning that while PCR can simplify the model by reducing the dimensionality of the variables, it may overlook complex interactions or nonlinear relationships, making it difficult to uncover the true causal relationships between variables, and instead only capturing their correlations.

The spatial distribution of TSM and nutrients showed a decreasing trend from the inner HTZ to the outer LNZ, with intermediate nutrient concentrations in the HPPZ ( Figures 2B, C ). This spatial pattern is primarily attributed to the estuarine filter and buffer effect, where various biogeochemical processes, including sedimentation, flocculation, resuspension, seawater dilution, and biological uptake, progressively reduce the concentrations of TSM and nutrients from the inner to the outer estuary (Asmala et al., 2014; Dias et al., 2016; Kitheka et al., 2016; Deng et al., 2023; Wan et al., 2023; Zhao L. et al., 2024). In contrast, the spatial distribution of Chl-a exhibited a distinct pattern characterized by low-high-low concentrations across the HTZ, HPPZ, and LNZ ( Figure 3B ). This distribution is similarly shaped by the estuarine filtration and buffering effects, with distinct Chl-a zones established through various filtering and buffering processes (Becherer et al., 2016; Restrepo et al., 2018; Xiao et al., 2018).

As noted in the results, in contrast to previous studies (Li et al., 2021), these three zones were defined by two natural inflection points on the fitted line ( Figure 2B ) representing the relationship between Chl-a concentration and salinity in this study. The inflection points correspond to the saline brackish ebb flow (salinity of approximately 10 PSU) and the boundary of brackish water at the Yangtze river plume front (which has a salinity of roughly 30 PSU). The inner HTZ, located between the estuary gate and the frontier of brackish ebb flow, is characterized by strong sedimentation, resuspension, and flocculation processes (Festa and Hansen, 1978; Hudson et al., 2017; Li and Zhang, 1998). These filtration and buffering processes led to a high TSM concentration (184.6 ± 125.6 mg/L) and a low Chl-a concentration (1.0 ± 0.7 μg/L) in this zone ( Table 1 ), as the TSM significantly exceeds the light limitation threshold of 37 mg/L (Cloern, 1987). This suggests that turbidity-induced low light conditions are the primary limiting factor for phytoplankton growth, resulting in low Chl-a concentrations despite abundant nutrients in the HTZ (Dijkstra et al., 2019; Gameiro et al., 2011).

The middle HPPZ is situated between the frontiers of brackish ebb flow and the boundary of brackish water at the Yangtze River plume front, with strong brackish water drifting and biological uptake processes (Ge et al., 2020). These processes contribute to a reduction in TSM levels within the zone, which, in turn, enhances light availability in the HPPZ compared to HTZ ( Table 1 ; Figures 3G–K ). The favorable light conditions in brackish water, driven by the filtering and buffering effects of the HTZ and HPPZ, along with the availability of inorganic nutrients, promote an increase in Chl-a concentrations in the HPPZ through phytoplankton blooms (Thompson, 1999).

The outer LNZ is situated at the outer frontier of brackish drifting flow, where ocean water biogeochemical processes predominate (Largier, 1993). These processes are influenced by estuarine filtration and buffering. Specifically, the absorption of nutrients by phytoplankton in the HPPZ and the convergence around the plume front limits the transport of nutrients to the shelf water, leading to low nutrient levels in the LNZ ( Table 1 ). The low TSM and high I_m in the LNZ ( Figures 3G–H ) suggest that the surface waters are well-lighted, with limited nutrients ( Figures 3I–K ) playing a key role in regulating Chl-a levels in this zone. The scarcity of essential nutrients restricts the growth and reproduction of phytoplankton (Justić et al., 1995; Harrison et al., 1977), leading to diminished Chl-a levels in LNZ, which is different from the cause of low Chl-a concentrations in HTZ.

Overall, the formation of the HPPZ in the Yangtze River estuary is controlled by the availability of nutrients and light irradiance. Low Chl-a concentrations observed in HTZ and LNZ are attributed to light and nutrient limitations, respectively. The processes of laminarization and flocculation at suspended sediment fronts within the estuary causes the TSM concentration to decrease rapidly near the sediment front, with light limitation primarily confined to the HTZ. This dynamic effect leads to a balanced supply of light and nutrients in the SSS zone of 10–30 PSU, thus creating the estuarine HPPZ. Notably, the results of this study are based on summer data, while seawater stratification, driven by seasonal temperature variations, affects nutrient distribution and may lead to seasonal differences in phytoplankton growth (Liu et al., 2016).

Similar to the temporal increase in Chl-a concentrations observed in the HPPZ at the Yangtze River estuary, comparable rises have also been documented along the coasts of East Asia, South Asia, East Africa, South America, and Europe (Elsworth et al., 2020). Elevated Chl-a levels in the HPPZ reflect fluctuation in the physicochemical environmental factors in estuarine waters (Brown and Ozretich, 2009; Ho et al., 2015; Mathew et al., 2021; Shikata et al., 2008). Specifically, water temperature can influence phytoplankton metabolic processes, affecting Chl-a levels, which are observed during seasonal fluctuations (Seguro et al., 2015). This study specifically examines the interannual variability of summer phytoplankton, without incorporating seasonal comparisons. Additionally, the East China Sea’s surface water temperature exhibits a relatively modest change rate, increasing at approximately 0.09°C per decade (Bao and Ren, 2014). Given this minimal fluctuation, the study primarily emphasizes the impacts of eutrophication and sand reduction on phytoplankton dynamics at an interannual scale, while the role of water temperature is not explicitly considered. Furthermore, upwelling, the process of transporting nutrient-rich bottom water to the surface seawater to supply phytoplankton growth, essentially still contributes to the formation of the HPPZ by altering the concentration of nutrients (Brown and Ozretich, 2009). Compared to temperature and upwelling, light and nutrients are more critical factors for Chl-a synthesis (Barrera-Alba et al., 2019). This is because phytoplankton photosynthesis is primarily reliant on light energy, while the production of Chl-a and enzymes related to photosynthesis necessitates nutrients, notably nitrogen, phosphorus, and silicon (Ge et al., 2020; Kessouri et al., 2021; Yu et al., 2019).

The significant reductions of 85% in TSM and 60% in DSi in annual flux into the estuary over the past few decades ( Figures 4A, D ) can be attributed to the construction of dams and reservoirs in the Yangtze River Basin, where these reservoirs retain materials that would otherwise flow from rivers to the sea (Treguer and de la Rocha, 2013; Song et al., 2023; Zhao B. et al., 2024). The number of large reservoirs in the Yangtze River Basin has increased from 2 in 1960 to 268 in 2020, with a reservoir capacity of 500 million m³ to 27.2 billion m³, and an area of 30 km² to 13.6×10³ km² ( Figure 1B ) (Song et al., 2023). Such abrupt decline of sediment flux into the sea occurred in many other basins such as the Mississippi, Danube, Mekong, and Yellow Rivers due to the dam and reservoir constructions (Dethier et al., 2022; Humborg et al., 1997; Maavara et al., 2020). Additionally, the significant increases of DIN (600%) and DIP (500%) in annual flux into the estuary ( Figures 4B, C ) can be attributed to human activities in the basin, including urbanization, industrial operations, and agricultural practices. It is estimated that more than 6,000 large outfalls in the basin (each with annual discharges exceeding 100,000 tons) discharge about 34.41 billion tons of wastewater, of which 25.81 billion tons enter the river in 2020 (https://www.mee.gov.cn/hjzl/sthjzk/sthjtjnb/). The wastewater, including domestic sewage, industrial effluent, and agricultural runoff, carries nitrogen and phosphorus, exceeding the self-purification capacity of the runoff into the river (http://www.cjw.gov.cn/zwzc/zjgb/), ultimately leading to an eutrophic state (Song et al., 2023). Significant reductions in sediment and major increases in N and P nutrient fluxes from the watershed led to yearly rises in light and nutrient levels in the estuarine HPPZ water, fostering algal growth. Therefore, the increase in intensity and expansion of the estuarine HPPZ over the past 60 years has been influenced by the synergistic effects of reduced sediment supply and eutrophication in the Yangtze River basin, primarily driven by dam construction and pollutant discharges. It is important to note that the Chl-a, DIN, and DIP concentrations discharged to the sea, have exhibited a slightly fluctuating downward trend in recent years ( Figures 2 and 4B ), this observation does not contradict the conclusions of this study. This study primarily examines the long-term effects of dam construction and anthropogenic discharges on water quality and sediment since the 1960s. Over this extended period, the overall DIN, DIP, and Chl-a concentrations have demonstrated increasing trends.

However, the amplitude of changes in the estuarine environmental factors has been only 0.2 to 0.7 times that of the changes in the watershed over the past 40 years. Specifically, the concentrations of DIN and DIP in the estuary have increased by only 0.2 and 0.3 times, respectively, compared to those in the watershed ( Figures 4E–H ). This suggests that the estuarine region has a strong regulatory capacity, as the amplitude of nutrient concentration changes is significantly smaller than that of the input from the watershed. At the same time, the Chl-a concentration and the HPPZ region showed an increasing trend over the same period ( Figures 2B, C ). This indicates that the expansion of the HPPZ has played an important buffering role in the incoming nutrients in the estuarine region over the past few decades. Indeed, the combination of abundant light availability and nutrients has promoted algal growth, which, in turn, has facilitated nutrient uptake by the algae, thereby mitigating the rise in nutrient concentrations. Consequently, nutrient uptake by algae not only supports the increase in Chl-a concentrations and the area of the HPPZ but also buffers the heightened nutrient amplitude within the HPPZ.