Abstract
Hydrogen peroxide (H 2 O 2 ) plays key roles in aquatic systems, including metal redox cycling and degradation of organic matter into bioavailable forms. Abiotic photoproduction, via photo‐oxidation of chromophoric dissolved organic matter, has generally been assumed to be the most important source of H 2 O 2 to freshwater systems in the daytime. However, the significance of other H 2 O 2 sources has not been previously examined in situ. In this study, isotopically labeled H 2 O 2 (H 2 18 O 2 ) was added to in‐stream mesocosms exposed to light and dark periods. By measuring total H 2 O 2 and H 2 18 O 2 in tandem, we inferred absolute rates of H 2 O 2 production and decay, which occurred simultaneously. Abiotic photoproduction rates were measured independently by exposing filtered samples to sunlight. Even in these shallow systems, total production rates greatly exceeded rates of abiotic photoproduction, and other sources of H 2 O 2 , most likely biological production, were the dominant control on the H 2 O 2 budget. Thus, H 2 O 2 and its precursor superoxide () may play a greater role in biogeochemical processes, especially in the absence of light, than previously thought.