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Abstract-Hydrogen peroxide (H,O,) formation rates (nM h-l), photoproductive capacity (nM W-l h-l m-*), and H202 formation efficiency (bH202) were measured on water samples exposed to a standard light source with a spectral composition similar to natural sunlight. Samples were from lakes and wetlands with varying levels of dissolved organic C (DOC), P, Fe, Ca, and pH. The relationship between H,O, formation rates and DOC was a power function @ItO = 49.65 DOC1-71; r* = 0.94), whereas the relationships between formation rates and DOC fluorescence (DOCFLJ w202 = 118.32 '+ 33.06 DOCm, r* = 0.98) and absorption coefficients at 3 10 (J&,) were linear (H202 = 185.0 + 55.50 Kaslo; r* = 0.9 1). +H202 was independent of DOC (r* = 0.12). Apparent quantum yields decreased with increasing wavelengths (300-400 nm). However, when apparent quantum yields were corrected with solar irradiance data, values were greatest in the UV-A (320-400 nm) region. The patterns observed were consistent for samples from temperate to arctic regions.

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Bacterial utilization of dissolved organic matter (DOM) in surface waters is closely linked to photochemical transformations of DOM. Photochemically produced reactive oxygen species (ROS) play a central role in many photochemical reactions, but the role of ROS for the photochemical facilitation of bacterial utilization of DOM is previously not known. We exposed lake water with high DOM concentrations to simulated sunlight, with and without the addition of ROS scavengers, and quantified the effect on the production of CO2, the loss of DOM absorbance, and bacterial growth. The photodegradation of DOM through microbial-photochemical interactions was dependent on the action of ROS. The use of ROS scavengers in irradiations of the lake water revealed that photobleaching below 300 nm and the production of CO2 are highly dependent on the action of ROS. Photobleaching and CO2 production in irradiated waters decreased significantly with the addition of ROS scavengers, but post-irradiation bacterial growth in the samples containing an ROS scavenger increased significantly above those without. The decrease in ROS activity (CO2 production) likely caused an accumulation of bioavailable DOM and enhanced microbial processes. Rapid degradation of DOM through the action of ROS would be especially important in high DOM systems. The high photochemical ROS activity may counterbalance the positive effects on bacterial activity of DOM photolysis into bioavailable molecules.