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Suspended particles from the lower Changjiang were collected monthly from 2003 to 2011, which corresponds to the three construction periods of the Three Gorges Dam. Organic carbon (%OC), organic carbon to total nitrogen molar ratio, stable carbon isotope, and terrestrial biomarkers were examined. Rating curve studies were applied for the temporal trend analysis. The composition of particulate lignin phenols exhibited clear annual and periodic variations but only minor seasonal changes. Lignin phenol ratios (vanillyl/syringyl and cinnamyl/vanillyl) indicated that the terrigenous organic matter (OM) was primarily composed of woody and nonwoody tissue derived from angiosperm plants. The low-lignin phenol yields (Λ8) in combination with higher acid to aldehyde ratios reflected a substantial contribution from soil OM to the particle samples or modifications during river transport. The temporal shift of the lignin phenol vegetation index with the sediment load during the flood seasons revealed particulate organic matter (POM) erosion from soils and the impact of hydrodynamic processes. The dam operations affected the seasonal variability of terrigenous OM fluxes, although the covariation of lignin and sediment loads with discharged water implies that unseasonal extreme conditions and climate change most likely had larger influences, because decreases in the sediment load and lignin flux alter the structure and composition of particulate OM (POM) on interannual time scales, indicating that they may be driven by climate variability. The modification of the composition and structure of POM will have significant impacts on regional carbon cycles and marine ecosystems.

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Mangrove ecosystems with high sediment deposition and active carbon cycling are a source of methane (CH 4 ) to the coastal atmosphere. We investigated diurnal and seasonal variations in CH 4 emissions from a subtropical mangrove estuary in southern Texas, northwest Gulf of Mexico. Tidal processes, amplitude (spring vs. neap tides) and topographic characteristics are crucial factors controlling CH 4 cycling in mangrove creeks. Higher CH 4 concentrations were observed during the ebb in spring tides due to the combination of processive export of CH 4 along the creeks during ebb tides and the addition of porewater CH 4 in upper intertidal sediment under water inundation in spring tides. The annual CH 4 emissions offset approximately 0.15% of the carbon stock in normal years, indicating that these mangrove creeks are a weak CH 4 source. However, significantly elevated CH 4 emissions were observed from mangrove dieback after the extreme cold-freezing event in February 2021. The average CH 4 flux from the mangrove creeks (126.1 AE 128.3 μmol [m 2 Ád] À1 ) increased 45% in 3 months after mangrove die-off in comparison with the overall average in normal years (87.0 AE 64.4 μmol [m 2 Ád] À1 ). It is obvious that the previous small CH 4 offset of the healthy mangrove forest was enlarged by the dieback event.Because the mangrove forests in this study live close to the limit of their survival range, our study highlights the important management considerations for blue carbon projects in vulnerable areas.