Water stagnancy and wastewater input enhance primary productivity in an engineered river system

Rivers play a crucial role in the global carbon cycle, acting as active conduits of CO2 and CH4 and efficient burial sites for carbon. The behaviour of rivers as source or sink of CO2 depends on the dominance of respiration vs primary production which is ultimately governed by the supply of organic matter and nutrients. Under warming conditions and with increasing human perturbations, rivers across the globe are facing drastic shifts in their hydrological regime resulting in fragmentation and disconnection from the catchment. Subsequently, a dependency on in situ primary productivity as the source of organic matter increases and warrants detailed investigation of the nature of primary production in urbanised river systems. In this study, primary productivity was estimated at multiple locations along the continuum of an engineered (Sabarmati) and a free flowing (Mahi) river system using 13C tracer incubation method. Enhanced primary productivity in the riverfront (engineered construction along Sabarmati that holds water supplied by the Narmada Canal) and polluted downstream of Sabarmati was observed. It was observed that water stagnancy, temperature, and nutrient availability are the key factors regulating the rates of primary productivity in rivers.