Dissolved organic matter (DOM) is an important component of aquatic ecosystems, and it influences a range of physical, chemical, and biological properties. Reactions induced by solar radiation may oxidize DOM to inorganic C or break large molecules into smaller ones. Therefore, photodegradable DOM is removed with exposure to light, and the remaining DOM pool might become less photodegradable as photorecalcitrant DOM accumulates. This possibility has led to speculation that previous light exposure might influence the susceptibility of DOM to photodegradation and that forested low-light streams might have highly photodegradable DOM. To assess this possibility, we measured: 1) the susceptibility of stream DOM to photoreactions and compared our results to studies in other aquatic ecosystems, 2) the relative importance of the ultraviolet (UV) portion of the solar spectrum to DOM photoreactions, and 3) the photoreactivity of DOM collected from streams with and without upstream lakes. We measured DOM properties of stream water exposed for ∼56 h to 1 of 3 treatments: full sunlight, sunlight with the UV portion of the spectrum (<400 nm) removed, and a dark control. Exposure to light reduced the UV light-absorbing ability of DOM and, to a lesser extent, its concentration. Most alterations of DOM properties could be attributed to the UV portion of the solar spectrum. We found no evidence that previous light exposure significantly influenced photodegradability of stream DOM. Our results suggest that other DOM-processing agents, such as heterotrophic uptake, can obscure the effect of upstream photoexposure on downstream DOM photodegradability.
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