Tropical rivers face severe oxygen stress caused by heat and organic matter: new evidence by stable isotopes.

Many rivers worldwide exhibit declining dissolved oxygen (DO) concentrations due to changes in hydroclimatic conditions and land use. Our study shows a typical tropical river in Sri Lanka (the Deduru Oya) with DO concentrations between 1.5 and 9.7 mg L-1. In this dataset, almost 80% of all collected water samples were undersaturated. In addition, rice paddies and extensive surface runoff during the monsoon season added organic matter, enhancing respiration and DO consumption. DO concentrations and associated stable isotopes (expressed as δ18ODO in ‰) provided new insights into how temperature, respiration, and photosynthesis interact and control the aqueous DO cycle. Values of δ18ODO varied from + 13.2 to + 27.7 ‰. Most headwater streams were within the range for atmospheric equilibrium (+ 24.6 ‰ ± 0.4 ‰). Further, downstream isotope values and DO saturations confirmed that the river became dominated by respiration. Only connected reservoirs and their nearby downstream sections showed influences by photosynthesis with typical isotope values lower than + 24.6 ‰. The new dataset is representative for tropical rivers and outlines critical zones and time periods that offer improvement of water management strategies and related ecosystem functioning.