Tracking the Transport and Processing of Dissolved Inorganic Carbon, Particulate Organic Carbon and Particulate Nitrogen in Engineered Rivers and Canals

With a major fraction of global rivers experiencing flow intermittency and pollution, it becomes imperative to understand the changes occurring in the in-stream biogeochemical processes with increasing human footprint. This is particularly relevant for the rivers of the Indian subcontinent, which are experiencing severe engineering modifications owing to the high population density and consequent freshwater demands. In this context, the present study explores the dynamics of dissolved inorganic carbon (DIC), particulate organic carbon (POC), and particulate nitrogen (PN) in different segments of a highly perturbed river (Sabarmati), a relatively free-flowing river (Mahi), and an engineered canal (Narmada Canal) situated in western India, during both high and low flow conditions. Results suggested that water stagnancy leads to an increase in biomass with signatures of enhanced primary production and N₂ fixation/bacterial growth. Wastewater input resulting in a ~70 km section of anoxia in the downstream reaches of the Sabarmati was accompanied by high biomass and elevated DIC with low isotopic composition, indicating high organic matter supply and subsequent degradation. Graphical mixing models were used to estimate the source isotopic signatures and it was observed that, in the absence of allochthonous inputs due to lining along the banks, organic matter degradation was the major contributor of DIC in the engineered segments of the river. Giving a detailed insight into the complex biogeochemistry of urban rivers, the present study highlights the need to incorporate biogeochemical aspects in river development projects and policy making, and the potential role of these rivers in regulating the biogeochemistry of coastal ecosystems.