IMPACT OF LIGHT ON THE PROPERTIES OF AQUATIC NATURAL ORGANIC-MATTER

Natural organic matter (NOM) has the property to absorb light in the U V and visible range. Derived from the stratospheric ozone depletion, t here is a reasonable concern about the influence of lower wavelength i rradiation on the role of NOM in aquatic systems. Experiments were per formed under defined conditions simulating sunlight with increased UV- B content for irradiation of NOM in aerated and deaerated aqueous solu tions. The results show that there is a general bleaching effect in th e irradiated samples and a change in the molecular size distribution o f NOM. irradiation causes a decrease in the higher molecular range and an increase of smaller molecules, along with an increase of the assim ilable amount of organic matter. The toxicity for Daphnia magna is hig her in irradiated samples, especially in deaerated ones. A decrease in the resolution of the lifetime distribution and a shift of the maximu m lifetime to longer values are found for time-resolved fluorescence. Steady-state fluorescence shows an increase of the relative intensitie s with decreasing wavelength in case of the aerated samples, and the r everse tendency for the deaerated ones. (C) 1998 Elsevier Science Ltd.