Photochemical production of reactive triplet states in water samples from mountain lakes, located below or above the tree line
Résumé
The production of excited triplet states of chromophoric dissolved organic matter (CDOM) was measured upon irradiation of water samples, taken from lakes located in a mountain area (NW Italy) between 1450 and 2750 m above sea level. The lakes are located below or above the tree line and surrounded by different vegetation types (trees, alpine meadows or exposed rocks -no vegetation). The most photoactive samples belonged to lakes below the tree line and their fluorescence spectra and CDOM optical features suggested the presence of a relatively elevated amount of humic (allochthonous) material. The lowest (negligible) photoactivity was found for a lake surrounded by exposed rocks. Its CDOM showed an important autochthonous contribution (in-lake productivity) and a considerably lower spectral slope compared to the other samples, suggesting low molecular weight and/or aromaticity. Overall, the CDOM photoactivity (rate of triplet state production under irradiation) decreased with changing vegetation type (trees, meadows, rocks), which would be connected with a decreasing contribution to CDOM from catchment runoff and an increasing contribution from autochthonous processes and possibly precipitation. Interestingly, CDOM in precipitation is poorly photoactive and could contribute to the low photoactivity of samples from lakes located above the tree line, in particular in the presence of exposed rocks.