Factors Controlling Carbon Metabolism in Riverine Systems: A Review of Hydrological, Biogeochemical, Biological and Anthropogenic Controls

Abstract

Riverine ecosystems are important constituents of the global carbon cycle in which carbon is transported, transformed, stored and exchanged among terrestrial, aquatic and atmospheric sections. Riverine carbon breakdown is commonly defined through the balance between gross primary production (GPP) and ecosystem respiration (ER), but this balance is regulated by relating hydrological, physicochemical, biological and human induced factors. Hydrology regulates residence time, sediment transport, groundwater exchange and gas transfer, while light and temperature strongly influence photosynthesis and respiration. Nutrient accessibility can promote primary production and enhance microbial respiration. The nature and origin of dissolved organic carbon (DOC) and particulate organic carbon (POC) determine substrate availability and therefore the intensity of heterotrophic processing. Sediments play as an important role of carbon storage and transformation, including anaerobic pathways that may produce methane. Catchment land use, wastewater discharge, agriculture, urbanization, dams and channel modification can substantially alter natural carbon pathways. Climate change further modifies these controls through warming, floods, drought and altered terrestrial carbon delivery. The Ganga River illustrates the interaction of these processes under strong monsoonal hydrology, high sediment transport and intense anthropogenic pressure. An integrated understanding of carbon metabolism is essential for interpreting river water quality, greenhouse-gas exchange, aquatic habitat and ecosystem functioning.