Hidden Threats of Dissolved Inorganic Carbon to River Ecosystem Metabolism

The river is one of the most important freshwater ecosystems serving as a hub for water, gas, and energy exchange among land, ocean, and atmosphere. River ecosystem metabolism vibrates with external stresses such as solar radiation, flow stability, and temperature. We discovered that the internal factor dissolved inorganic carbon (DIC) plays a hidden role in amplifying riverine metabolic sensitivity, which, however, is rarely concerned. In this study, machine learning is used to reproduce global riverine DIC datasets, and then the global rivers are divided into high-DIC and low-DIC rivers. Apparent oxygen utilization (AOU) is used as an indicator of ecosystem metabolism intensity. High-DIC rivers exhibit intensified ecosystem variability, which is more obvious in colder climate zones. Facilitated gross primary production (GPP) by DIC boosts ecosystem respiration, which increases the risk of hypoxia and biological stress in high-DIC rivers. A modified Michaelis-Menten equation-based model is developed and well simulates the historical variation of global mean annual AOU. The model was further applied to project DIC’s role in long-term river ecosystem variation under different future climate scenarios. The model results demonstrate that high-DIC rivers have a higher risk of oxygen depletion in all scenarios with different applications of fertilizer situations compared with low-DIC rivers.