Dynamics of blue carbon across various estuaries: Key drivers and pathways

Coastal wetlands are vital organic carbon (OC) sinks. This study examined OC dynamics and the implications for trophic status in the salt marsh-dominated Yangtze River Estuary (YRE) in China and four mangrove-dominated estuaries under varying anthropogenic influence within the Rookery Bay National Estuarine Research Reserve (RBNERR) in the USA, including natural (FHE), agricultural (BWE), canal-modified (FUE), and urbanized (HCE) estuaries. The research focused on three thematic areas: total organic carbon (TOC) composition and degradation, key drivers and their cascading effects, and estuarine eutrophication assessment, employing stable isotope tracing (δ¹³C, C/N), machine learning (BRT models), partial least squares path modeling (PLS-PM), and trophic indices (TRIX, TSI). Results showed that FHE had the highest organic carbon concentrations (4-19.82 mg/L). While terrestrial sources contributed >80% of OC across estuaries, composition varied: YRE's OC was derived mainly from freshwater algae and soil organic matter, whereas in the RBNERR estuaries, it was dominated by mangrove-derived, marine phytoplankton, and anthropogenic sources. Anthropogenic organic matter (OM) inputs (e.g., from agriculture and wastewater) and phytoplankton growth promoted r-strategists and enhanced degradation, likely decreasing burial efficiency in YRE, BWE, and HCE compared to FHE and FUE. In the FHE and FUE, TOC was primarily driven by the degradation of mangrove-derived OM (with hydrodynamic influence in FUE). In contrast, TOC in the YRE, BWE, and HCE estuaries was mainly governed by anthropogenic and phytoplankton inputs, which manifested in distinct ways: tidal dilution, light-limited photosynthetic growth, and degradation of phytoplankton-derived and soil organic matter in the YRE; agricultural organic matter degradation in the BWE; and phytoplankton photosynthesis in the HCE. PLS-PM further revealed that meteorological factors indirectly influenced TOC concentrations through salinity, biological environment, hydrodynamics and nutrients. Overall, the driving mechanisms resulted in the YRE and BWE being eutrophic, while the other estuaries were mesotrophic. In summary, these findings demonstrate that local environmental factors and human activities are key drivers of OC dynamics. This understanding of specific mechanisms is essential for refining blue carbon assessments under global change.