LIGHT-INDUCED ALTERATION OF THE PHOTOPHYSICAL PROPERTIES OF DISSOLVED ORGANIC-MATTER IN SEAWATER .1. PHOTOREVERSIBLE PROPERTIES OF NATURAL-WATER FLUORESCENCE

Fluorescence of humic substances changes when these substances are exposed to an ultraviolet monochromatic light source. Fluorescence decreased with time of exposure when the irradiation took place at wavelengths of 334.1, 366 and 404.5 nm and increased with time at wavelengths of 265.2, 280.4, 296.7 and 313 nm. In addition, the process involved appeared to be partially photoreversible. The fluorescence decay resulting from irradiation at one of the longer wavelength bands could recover when followed by an irradiation interval at one of the shorter wavelength bands. In contrast, with polychromatic sunlight irradiation, only decreases in fluorescence were observed. Although the actual nature of the chromophores responsible for these fluorescence processes is still unknown, the results of this work suggest that the chromophoric unit responsible for the observed changes exists in both marine and terrestrial materials.