Effect of short-term sunlight irradiation on absorbance spectra of chromophoric organic matter dissolved in coastal and riverine water

Samples of riverine and coastal, filtered (filter pore size 0.2 μm) water were exposed to short-term sunlight irradiation which reduced their absorbance in the UV and visible regions. Absorbance losses in coastal chromophoric dissolved organic matter (CDOM) were up to 10-fold smaller than those in riverine CDOM. Accompanying changes of absorbance spectra shapes (increased slope parameter) were, probably, a result of decrease of the mean molecular size of light absorbing organic matter. The potential of coastal CDOM to photodegradation was smaller and was exhausted during the course of a day-long experiment. A distinctive feature of spectral changes after sunlight exposure was a maximum absorbance decrease which appeared at 300 nm in riverine and at 280 nm in coastal water. That selective absorbance loss has been ascribed in both cases to the disappearance of chromophores of terrestrial origin which, in coastal water, had a lower mean molecular size (due to flocculation and/or prior photodegradation) but, nevertheless, retained their molecular properties.