Effects of natural light and depth on rates of photo-oxidation of dissolved organic carbon in a major black-water river, the Rio Negro, Brazil

Systems rich in terrigenous dissolved organic carbon (DOC), like the Rio Negro, can contribute significant amounts of carbon dioxide back to the atmosphere and support important microbial communities. We investigated photo-oxidation in the Rio Negro: (1) the depth to which light causes complete photo-oxidation to CO₂ and changes in DOC structure, (2) the daily rate of change of absorbance indices, (3) the relationship between sub-surface rates of photo-oxidation to CO₂ and light exposure, (4) the areal rates of photo-oxidation, and (5) the stability of fluorophore signals. Experiments were run in an outdoor pool of Rio Negro water, under natural sunlight during the dry seasons of 2015 and 2018. In 2018, rates of complete photo-oxidation and changes in absorbance indices decayed exponentially, approaching their asymptotes between 9 and 15 cm depth. In 2015, direct absorbance indices ceased changing at 14 cm depth. Fluorescence of humic acid-like moieties continued to decrease, sometimes to 35–43 cm depth. This indicates that partial photo-oxidation of DOC, and thus interaction with the microbial community, occurs to greater depths than previously expected. Areal rates of CO₂ production were 28.8 and 39.3 mg C m⁻² d⁻¹ (two experiments, October 2018). Sub-surface (1.1 cm) rates were strongly related to light levels, reaching a maximum of 0.68 mg C l⁻¹ d⁻¹ in September. Complete photo-oxidation ceased below 29.6 mW cm⁻² d⁻¹ UV radiation, providing a daily baseline for observable production of CO₂. Absorbance indices changed by 9 to 14% d⁻¹ at high light levels, except for R₂₅₄/₃₆₅ (4.4% d⁻¹). Fluorophore emission ranges were stable between 2014 and 2018, indicating that emissions can be compared across time and space. This study contributes to better estimates and understanding of photo-oxidation in tropical, black-water rivers, which will be useful for carbon modelling.