Carbon and Nitrogen Turnover as Key Drivers of Dissolved Oxygen Depletion in Rivers: A Focus on Seasonal Variability

The water temperature and salinity are typically important factors that significantly affect the solubility of dissolved oxygen (DO), playing a crucial role in the occurrence of low oxygen levels. In addition to these factors, studies have indicated that the turnover processes of carbon and nitrogen in rivers are highly coupled with the consumption of oxygen, becoming important limiting factors for DO depletion in river systems [6-8]. During the conversion of organic carbon to inorganic carbon and the transformation of low-valence ammonia nitrogen to high-valence nitrate nitrogen, oxygen is utilized as an electron acceptor in oxidation reactions, resulting in the depletion of dissolved oxygen in water. The stoichiometric relationship between aerobic respiration of organic carbon and chemical reactions such as ammonia oxidation and nitrite oxidation suggests that 1 mole of carbon requires 1 mole of oxygen for oxygen-consuming degradation, while 1 mole of ammonia nitrogen and nitrite nitrogen oxidation consumes 1.5 moles and 0.5 moles of oxygen, respectively [9]. Furthermore, the products of nitrification may serve as electron acceptors for carbon turnover, affecting the competition between aerobic and anaerobic degradation of carbon, thus forming a coupling relationship between carbon, nitrogen, and oxygen. In summary, the carbon and nitrogen turnover processes complicate the mechanism of oxygen depletion and create conditions favorable for the formation of oxygen-depleted and hypoxic water bodies in river basins, playing a crucial role in the utilization of dissolved oxygen. Therefore, it is important to investigate the reactive transport of carbon and nitrogen in current large rivers and determine the extent of DO depletion caused by their processes, as well as the differential driving influences during wet and dry seasons.