<p>Quantifying carbon and nitrogen dynamics is essential for understanding the role of coastal ecosystems in climate change mitigation and habitat connectivity. We assessed organic carbon (OC) and total nitrogen (TN) stocks, accumulation rates, and isotopic composition (δ¹³C and&#xa0;δ¹⁵N) in sediment across diverse seascapes in northeastern Brazil, including high- and low-salinity mangroves, a tidal flat, a freshwater marsh, and an inundated forest. OC stocks in the upper meter of sediment were highest in the freshwater marsh (283 ± 64 Mg OC ha⁻¹) and tidal flat (266 ± 27 Mg OC ha⁻¹), and were comparable to mangroves (239 ± 11 to 263 ± 44 Mg OC ha⁻¹), while the inundated forest stored less (138 Mg OC ha⁻¹). Accumulation rates were highest in the marsh (202 ± 19 g OC m⁻² yr⁻¹) and in the high-salinity mangroves (187 ± 20 g OC m⁻² yr⁻¹), with lower values in tidal flats (108 ± 3 g OC m⁻² yr⁻¹), inundated forests (119 g OC m⁻² yr⁻¹), and low-salinity mangroves (105 ± 11 g OC m^−2&#xa0;yr⁻¹). TN stocks ranged from 7.7 Mg N ha⁻¹ (inundated forest) to 18.9 Mg N ha⁻¹ (marsh), with intermediate values in mangroves (8.3–11.6 Mg N ha⁻¹) and the tidal flat (13.5 Mg N ha⁻¹). TN accumulation rates mirrored those of OC, being highest in the marsh (12.4 ± 3.9 g N m⁻² yr⁻¹) and lowest in low-salinity mangroves (3.6 ± 1.4 g N m⁻² yr⁻¹). Isotopic signatures reflected the dominance of C₃ vegetation, but also highlighted the limited discriminatory power of stable isotopes alone, emphasizing the need for complementary tracers (e.g., fatty acids, pyrolysis, or eDNA) to resolve OM sources across seascapes. Overall, substantial OC and N storage in both vegetated and unvegetated habitats highlights the need to broaden Blue Carbon policies to include tidal flats and inland wetlands within Blue Carbon frameworks and management strategies.</p>