Spatial variation and mechanism of the dissolved/particulate organic carbon partitioning of seawater adjacent to the mangrove in Zhangjiang Estuary

Mangrove is one of the most carbon-rich ecosystems in the world. Nevertheless, the understanding of organic carbon partitioning between dissolution and particle in the processes of mangrove outwelling is rather limited, flood–ebb samplings were conducted in January 2021 with 8 sampling sites covering the upper tidal boundary to the mouth in the Zhangjiang Estuary. Water samples were collected to analyze physicochemical properties, dissolved organic carbon (DOC) and particulate organic carbon (POC) contents, and their phase portioning were investigated to analyze the spatial variation between the two phases (dissolve and particle), and their influencing factors. The results showed: (1) DOC was the dominant content with the wide range of 0.48–3.29 mg·L⁻¹, accounting for 77.6% on average; while POC contents were 0.17–1.86 mg·L⁻¹, accounting for 22.4% on average. (2) Silt was dominant component in estuarine suspended sediment, accounting for 68.0% on average; clay accounted for 17.5% of the suspend sediment, and was mainly located in mangrove forest area; sand proportion was the lowest. (3) Both the closed Beijiang sluice and ponds effluents could significantly increase the water organic carbon content, but only ponds effluents changed the phase distribution of organic carbon. In natural processes, a significant positive correlation between particulate and dissolved organic carbon and dissolved oxygen (DO) in the upper estuary (r > 0.9), and a significant negative correlation between particulate and dissolved organic carbon and salinity in the lower estuary (r < −0.7) were observed. The study indicated that DOC was the dominant content, the phase partition coefficients were significantly increased due to the DOC content from pond effluents. In the natural processes, the autotrophic activities by microalgae in the upper estuary and the seawater mixed dilution followed with heterotrophic activities are the important influencing factors.