Climate change: temperature and oxygen impacts on the photooxidation of dissolved organic carbon (DOC)

Over the past 20 years, the Amazon has experienced extreme floods, droughts and warmer temperatures due to climate change. Water temperature in the Rio Negro, a major tributary of the Amazon River, reached it highest October value during the 2023 drought. In Lake Tefé, connected to the Rio Solimões (another major tributary of the Amazon river), water temperatures reached 39°C. Increasing temperatures, and associated decreases in oxygen, will plague the Amazon and other regions, altering and accelerating links in the carbon cycle, such as photooxidation of dissolved organic carbon (DOC). We determined the response of DOC photooxidation rate in the Rio Negro (black-water) and Rio Solimões (white-water) to increases in water temperature between 20°C and 40°C and oxygen concentration between 0.01 mg O₂.l^-1 and 8 mg O₂.l^-1. The temperature coefficient (Q10) averaged 1.165, indicative of the dominance of diffusive processes, presumably of reactive oxygen species involved in photooxidation. Direct kinetic release of CO₂ was 15% to 21% of normoxic CO₂ production, and did not respond to temperature. The activation energy (Ea) of photooxidation was 13.14 kJ.mol^-1 in the Rio Solimões and14.09 kJ.mol^-1 in the Rio Negro. The Eas were not significantly different, suggesting no differences in the cost of photooxidation between the two rivers. They align with UVB Eas. Photooxidative production of CO₂ only became oxygen limited between 0.5 mgO₂.l^-1 – 0.8 mgO₂.l^-1 (1.2 kPa – 1.9 kPa, 23°C). Thus, near-surface levels of oxygen are unlikely to directly depress CO₂ production of DOC as temperatures rise.