Land-margin ecosystems surrounding river mouths are hydro-biogeochemical hotspots where water, carbon, and nutrients are exchanged between land and ocean. The land-margin ecosystems have recently experienced significant variations in surface water extent (Fw) due to increasing intensity of climate and environmental changes. The variations of the Fw at the river mouth areas are closely linked with the changes in biogeochemical cycles, including greenhouse gas emissions, ocean chlorophyll production and nutrient exports from land-margin ecosystems. Multi-source environmental remote sensing data records were used to investigate how changes in Fw affect hydroclimates, including precipitation, surface soil moisture, and root-zone soil moisture, and biogeochemical fluxes associated with heterotrophic respiration, atmospheric CH4, and terrigenous dissolved organic matter (tDOM). The study focused on 253 major river mouth sites, identified within the boundary of each river mouth using the MERIT-Hydro map derived from a digital elevation model. The long-term (2003–2022) satellite-derived Fw data showed a strong increasing trend in the mean annual Fw over global land areas and major river mouths. However, the Fw trends varied across aridity zones in response to climate and environmental changes —likely due to the changes in surface dryness and permafrost melting dynamics —with 46 % of river mouths showing a decreasing Fw trend, indicating lower surface wetness conditions. Fw generally showed positive correlations with heterotrophic respiration in the area surrounding river mouths. Its relationship with atmospheric CH4 concentration was also positive in river mouth areas located in semi-arid and sub-humid zones. Particularly, in arid regions, the increasing Fw led to enhance heterotrophic respiration, but significantly reduced atmosphere CH4 concentrations. The deceased flux of tDOM exported from land to water may be linked to the reduced runoffs from river mouth areas as indicated by the Fw decreases. The decreased Fw lowered tDOM exports to coastal waters in 61 % of the studied river mouth areas. The results highlight that long-term satellite-derived Fw observations, alongside multi-source remote sensing data, are critical for monitoring surface wetness in land-margin ecosystems and assessing its impact on hydro-biogeochemical fluxes in near-coastal environments.
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