| Summary: | The samples were collected from different areas (from natural, planted, and deforested mangrove forests) to study the impact of anthropogenic activity on biodegradable and seasonal variation of organic carbon (OC) in Xuan Thuy mangrove forest, North Vietnam. We observed a positive relationship between TSS and both POC and DOC (<i>r</i><sup>2</sup> = 0.993; <i>p</i> < 0.0001 và <i>r</i><sup>2</sup> = 0.942; <i>p</i> < 0.0001, respectively), suggesting that erosion and resuspension increase OC delivery at the mangroves. Mean DOC and POC concentrations in the rainy season (4.8 ± 3.0 mgCL<sup>−1</sup> and 6.9 ± 1.6 mgCL<sup>−1,</sup> respectively) were significantly higher than those in the dry season (3.1 ± 0.9 mgCL<sup>−1</sup> and 2.9 ± 0.4 mgCL<sup>−1</sup>, respectively). The mean percentage of biodegradable for 40 days was about 47 ± 15% of BTOC, 45 ± 17% of BDOC, and 48 ± 18% of BPOC. The rainy season indicated that there was more biodegradable OM in the mangroves compared to the dry season. The molar ratio of OC and total nitrogen was from 0.4 to 25.9, showing a hybrid of terrestrial organic matter and aquatic. Lower C/N and great variation of POC/Chla in the deforested mangroves was due to augmented OM input from aquaculture activities in this area. The results of this study indicate that deforested mangrove and an increase in aquaculture activities will contribute to anthropogenic OM reaching the sea.
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