Photodegradation of autochthonous and allochthonous dissolved organic matter in a natural tropical lake

Dissolved organic carbon (DOC) is one of the most abundant fractions of organic matter in aquatic systems and plays an important role in the dynamics of aquatic environments, controlling both the penetration and the underwater light radiation climate. DOC can be photodegraded by light, thus facilitating biodegradation, especially in regions where the incidence of solar radiation is high, such as higher altitudes and lower latitudes. This study quantified the photodegradation of dissolved organic material in a natural tropical lake surrounded by native forests (Brazilian Atlantic Forest) through two experiments: i) the first experiment exposed concentrated autochthonous, allochthonous, and lake water to in situ solar radiation; ii) this experiment also exposed the same organic material to artificial UV radiation in an incubator under controlled conditions. The quality and quantity of dissolved organic carbon were measured using indices based on carbon absorbance and fluorescence spectrum. In the in situ experiment, it was observed that the DOC degradation profile of the concentrated allochthonous and autochthonous organic material were distinct from each other in the absorbance indices, and the lake water mostly resembled the latter one. On the other hand, we did not see evidence of any significant difference among treatments in the laboratory experiment. An increase in the SR index and a concomitant decrease in the fluorescence of humic compounds and SUVA₂₅₄ over time were observed. In both experiments, the amount of degraded organic material over time was low and some possible explanations are discussed.