Impact of light on the properties of aquatic natural organic matter

Natural organic matter (NOM) has the property to absorb light in the UV and visible range. Derived from the stratospheric ozone depletion, there is a reasonable concern about the influence of lower wavelength irradiation on the role of NOM in aquatic systems. Experiments were performed under defined conditions simulating sunlight with increased UV-B content for irradiation of NOM in aerated and deaerated aqueous solutions. The results show that there is a general bleaching effect in the irradiated samples and a change in the molecular size distribution of NOM. irradiation causes a decrease in the higher molecular range and an increase of smaller molecules, along with an increase of the assimilable amount of organic matter. The toxicity for Daphnia magna is higher in irradiated samples, especially in deaerated ones. A decrease in the resolution of the lifetime distribution and a shift of the maximum lifetime to longer values are found for time-resolved fluorescence. Steady-state fluorescence shows an increase of the relative intensities with decreasing wavelength in case of the aerated samples, and the reverse tendency for the deaerated ones. (C) 1998 Elsevier Science Ltd.