Global carbon cycle has received extensive attention, among which the river-estuary system is one of the important links connecting the carbon cycle between land and ocean. In this paper, the distribution and control factors of particulate organic carbon (POC) were studied by using the data of organic carbon contents and its carbon isotopic composition (δ13C) in the mainstream and estuary of Passur River in the Sundarban area, combined with the hydrological and biological data measured by CTD. The results show that POC content ranged from 0.263 to 9.292 mg/L, and the POC content in the river section (averaged 4.129 mg/L) was significantly higher than that in the estuary area (averaged 0.858 mg/L). Two distinct stages of POC transport from land to sea in the Sundarban area were identified. The first stage occurred in the river section, where POC distribution was mainly controlled by the dynamic process of runoff and the organic carbon was mainly terrestrial source. The second stage occurred during estuarine mixing, where the POC distribution was mainly controlled by the mixing process of seawater and freshwater. The source of POC was predominantly marine and exhibiting vertical differences. The surface and middle layers were primarily influenced by marine sources, while the bottom layer was jointly controlled by terrestrial and marine sources of organic carbon. These findings are of great significance for understanding the carbon cycle in such a large mangrove ecosystem like the Sundarban Mangrove.
Figure 1. Study area and station distribution map (modified from Zou et al., 2022). (a) A large map indicating the location of a large area. (b) A smaller map shows the Sundarban range and the location of the Passur River. (c) Specific sample distribution map. AB is river section and CD is estuary section.The red dot represents the sampling location of the section, and the yellow triangle represents the location of the mooring station.
Figure 2. Distribution of salinity (b-1, c-1, and d-1), chlorophyll (b-2, c-2, and d-2) and turbidity (b-3, c-3, and d-3) in the transect AB (left panel), transect CD (middle panel) and mooring observation station M (right panel). Figure a is a schematic diagram of the position of transects AB and CD, and the mooring observation station M (modified from Zou et al., 2022).
Figure 3. Distribution of δ13C (a-1, b-1, and c-1), POC (a-2, b-2, and c-2) in the transect AB (left panel), transect CD (middle panel) and mooring observation station M (right panel).
Figure 4. Regression analysis between different parameters (POC, POC%, SSC, Chl-a, Salinity, POC/Chl-a, algal-POC%). The data of mooring station is considered estuarine data.
Figure 5. The (a) POC and (b) POC% in surface, middle, and bottom layers of water column, respectively. In the abscissa, red and orange represent the station of section AB and section CD, green represents the time of mooring station.
Figure 6. The (a) POC/Chla and (b) algal-POC% in surface, middle, and bottom layers of water column, respectively.
Figure 7. Changes in (a) salinity, (b) POC%, (c) algal-POC%, and (d) POC/Chla, during the tidal cycle.
Figure 8. The picture shows the river-sea mixing process of the POC and the change of POC source during the process. The brown arrows indicate the direction in which runoff is moving, the blue arrows indicate the direction in which highly saline water is moving. Yellow and purple arrows indicate re-suspension and brackish water mixing, respectively.
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