Do Indus Delta mangroves and Indus River contribute to organic carbon in deltaic creeks and coastal waters (Northwest Indian Ocean, Pakistan)?

Abstract The biogeochemistry of organic carbon (OC) is of great significance to regional and global biogeochemical cycles. Studies on dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations and their fluxes from the Indus River, Indus Delta and coastal waters of the shelf area off the Indus Delta are limited. Major creeks of the Indus Delta (Waddi Khuddi Creek (WK), Dabbo Creek (DC), Hajamro Creek (HC) and Khobar Creek (KC)), along with the Indus River and the adjacent offshore area of the Arabian Sea, were sampled in the winter, summer, and post and pre-monsoon periods to investigate the fate, transformation, and source of OC. In this study, a high concentration (3.1 mg L-1) of POC was found in KC (the main river channel), which is directly influenced by runoff. The POC concentration varied seasonally in the creeks (WK, HC and DC). The POC% decreased as the total suspended matter (TSM) increased, and this correlation strengthens during the southwest monsoon (SW monsoon). The concentrations of DOC and POC have decreased (by 19.3 and 22.7%, respectively) relative to values reported in previous studies from the Indus River. The source of the DOC and POC was mainly the mangrove litter and benthic microalgae. Stable carbon (δ13C) was used as a proxy to trace the autochthonous and allochthonous sources of DOC and POC in the creeks. The contribution of POC to TOC in WK, DC, HC, and KC was 19.2 ± 7.5%, 26.5 ± 10.7%, 27.1 ± 8.7%, and 61.3%, respectively, increasing from west to east in the creeks; the contribution of POC to TOC in the offshore region was 22.3 ± 17.2%. There was clear evidence through the isotopic studies that the mangroves contribute significantly to the DOC and POC in the Indus Deltaic creeks. Towards to the Arabian Sea, high fluxes of TSM (5.43 × 1013 kg year-1), were recorded during the SW monsoon period when there were rains and floods. In comparison with the previous studies, POC and DOC flux is reduced from 1.30 × 109 to 2.42 × 105 kg year-1 and 3.83 × 109 to 2.89 × 105 kg year-1, respectively. In comparison with the Himalayan rivers (Brahmaputra and Ganges), the reduction in POC and DOC flux is also observed due to the low discharge of Indus River. There is still need of the study for the discharge and fluxes (TSM, DOC and POC) of the creeks of eastern and western side of Indus River for the complete transfer of OC through the creeks of Indus Delta to the world oceans.