Photodegradation of natural organic matter exposed to fluctuating levels of solar radiation

Irradiation of natural water samples with natural or artificial UVR typical ly results in a progressive loss of color and decreased absorbance; a proce ss often referred to as photobleaching. In a typical photobleaching experim ent, samples are exposed to a relatively constant level of artificial or na tural UVR. However, under most natural situations, the vertical mixing of t he water within the upper mixed layer results in strong and periodic fluctu ations in UV irradiance. In this paper, we present the results of an experi ment in which natural lake water was exposed to solar radiation in quartz t ubes that were incubated either at fixed depths or rotating within the wate r column. We found differences between rotating and fixed samples in (i) ph otobleaching, (ii) nutrient release, and (iii) subsequent use by algae and bacteria. The evidence presented in this study demonstrated that photochemi cal processes might be affected by vertical water motion. The reasons for s uch differences remain largely unknown. Although we offer a potential expla nation for such differences, our proposed mechanism is based on a post-hoc analysis of the data and should be taken solely as a working hypothesis for future research. (C) 2001 Elsevier Science BY. All rights reserved.