Coastal wetlands, marine ecosystems, and deltas depend critically on suspended sediments in the coastal ocean. Recent assessments have highlighted a decline in sediment supply to many deltas due to dam construction, raising concerns about wetland loss. However, the relationship between reduced river sediment supply and coastal suspended sediment concentration (SSC) is complex and not fully understood. While some studies suggest a decline in SSC linked to dam construction, others show increased coastal sediment deposition despite dam building, suggesting that coastal hydrodynamics and sediment transport processes also play significant roles. This study addresses this knowledge gap by analyzing long-term (2000-2020) global coastal SSC changes near major river deltas using satellite data. The main research questions are: (i) What are the spatial and temporal patterns of coastal SSC near river deltas worldwide? (ii) How does SSC respond to river sediment supply changes? (iii) What other factors influence coastal SSC near river deltas?

Previous research on global coastal SSC changes using satellite imagery has revealed decreases in SSC in many areas, but these studies often covered limited time periods and lacked a comprehensive understanding of the underlying drivers. Studies on individual deltas, such as the Yangtze and Mekong, have shown decreases in SSC attributed to dam construction and reduced sediment supply. However, the global extent of these declines and their causes remain unclear. Existing research also highlights the complex interactions between coastal hydrodynamics (river flow, tides, waves) and sediment transport processes (suspension, erosion, deposition, movement) in shaping coastal SSCs. A comprehensive, long-term global analysis is necessary to fully understand these complex dynamics and their implications for coastal ecosystems.

This study used monthly records of coastal SSC near 349 major river deltas from 2000 to 2020 derived from 500-m resolution MODIS Terra and Aqua 8-day surface reflectance (SR) products. A coastal SSC retrieval algorithm (root mean square error = 24.9%) was used, calibrated and validated using six field SSC datasets (Pearl River estuary, Yangtze River estuary, Yellow and East China Seas, SeaSWIR, CoastColour Round Robin, and AquaSat) and compared against Sentinel-3 OLCI products (4 km resolution). The algorithm uses blue, green, red, and NIR reflectance to estimate SSC. In addition to SSC, river sediment plume area (RPA) was analyzed to capture the spatial extent of coastal sediment plumes. Data on river water discharge, annual river sediment flux (Qriver), monthly tidal sediment flux (Qtide), daily wave sediment flux (Qwave), and monthly salinity were gathered from various sources (references 5 and 6). A 70% frequency threshold of SSC above a certain value was used to define the study area for each delta. Long-term trends were analyzed using Sen's slope and the Mann-Kendall test. Relationships between SSC and Qriver, Qtide, Qwave, and salinity were investigated using correlation analysis and multiple general linear model (GLM) regression to quantify the contributions of these drivers to SSC changes. Wetland area changes were assessed using a global tidal wetland change dataset (reference 61) to compare with SSC changes.

The study revealed a substantial spatial heterogeneity in long-term mean surface SSCs among the 349 deltas, ranging from 2.8 to 379.7 mg/L. Globally, coastal SSC increased by +0.46% yr⁻¹ (+0.23 mg/L yr⁻¹, p<0.05) from 2000 to 2020, with 59% of deltas showing increases. RPA also increased globally at +0.48% yr⁻¹ (+0.64 km² yr⁻¹, p<0.05). Increases in SSC and RPA were widespread across continents except Asia, where decreases were observed, likely due to dam construction and sediment extraction. Arctic deltas showed particularly pronounced increases in both SSC and RPA. The relationship between river sediment supply and coastal SSCs varied significantly among deltas, with 45.2% showing opposing trends. Analysis revealed significant correlations between SSC and Qriver, Qtide, Qwave, and salinity (Fig. 4). GLM regression indicated that Qriver, Qtide, Qwave, and salinity significantly contributed to SSC changes in 36%, 13%, 9%, and 7% of 139 deltas respectively; collectively explaining 84.3 ±14.2% of SSC variation. A comparison of wetland area changes with coastal SSC changes in 180 matched deltas showed that nearly one-third of wetlands experienced area loss despite increased coastal SSC, indicating the complex interplay of factors influencing wetland dynamics.

The findings challenge the assumption that decreased river sediment supply inevitably leads to coastal SSC decline and wetland loss. The observed global increase in coastal SSC over the past two decades suggests that coastal hydrodynamics, particularly tidal and wave processes, and salinity play crucial roles in shaping coastal SSCs. The inconsistent relationship between river sediment supply and coastal SSC highlights the limitations of focusing solely on river sediment flux when assessing coastal wetland vulnerability. The influence of tides, waves, salinity, and delta morphology complicates the prediction of coastal SSC changes and wetland response, even in the face of altered river sediment discharge. Although increased coastal SSC can enhance wetland resilience to sea-level rise, the observed wetland area loss in some deltas despite increased SSC underscores the importance of considering other factors, such as human activities (coastal development, land conversion) and natural processes (land subsidence, accelerated sea-level rise).

This study provides a comprehensive global assessment of coastal SSC and RPA near river deltas over the past two decades, revealing a surprising global increase despite concerns about declining river sediment supply. The results emphasize the importance of considering the complex interplay of multiple factors, including hydrodynamics and salinity, in predicting coastal SSC changes and wetland response. Further research is needed to refine our understanding of these complex interactions and their implications for coastal ecosystem management and restoration. Future work could focus on improving the accuracy of remotely sensed data in various hydrodynamic settings, expanding the analysis to include a wider range of environmental factors, and developing more sophisticated models to better predict the future dynamics of coastal SSC and their impact on deltaic wetlands.

The study primarily focuses on surface SSC and RPA, and deeper water column dynamics may differ. The accuracy of the SSC retrieval algorithm may be affected by atmospheric correction methods and variations in water clarity. Data limitations, particularly for Qwave and salinity, restricted the scope of certain analyses. The use of remotely sensed data also introduces uncertainties related to spatial resolution and temporal sampling frequency. The analysis of wetland area changes was limited by the availability of matched data and the temporal resolution of the wetland dataset.