Abstract:The distribution, production, and decay of hydrogen peroxide (H 2 O 2 ) were studied in 10 boreal lakes of differing physical-chemical characteristics. Diurnal and vertical fluctuations in H 2 O 2 concentration were followed in the lakes by sampling at six depths three times per day. In addition, incubations of water filtered through 0.2-µm mesh were made under artificial irradiation to study the abiotic production and decay of H 2 O 2 . H 2 O 2 concentrations after 8 h of artificial irradiation were significantly correlated with neither absorption coefficients at 320 nm nor with dissolved organic carbon (DOC) concentrations. However, H 2 O 2 concentration increased rapidly with DOC concentration among lakes with DOC concentrations below 10 mg·L -1 . The H 2 O 2 concentration after exposure to ultraviolet radiation was positively related to the half-life of H 2 O 2 , which in turn was negatively correlated with iron and manganese concentrations (r 2 = 0.68 and 0.70, respectively). In situ H 2 O 2 concentrations at the surface of the water column ranged between 30 and 1041 nmol·L -1 and were largely determined by the decay rates of H 2 O 2 and the mixing depth of the water column.