Interaction of extrinsic chemical factors affecting photodegradation of dissolved organic matter in aquatic ecosystems.

Photochemical degradation of dissolved organic matter (DOM) plays an important role in the carbon cycle. Irradiation experiments were conducted to evaluate the influence of chemical factors, specifically those expected to be altered in natural waters by atmospheric acid deposition, on photodegradation of DOM. These included pH, nitrate, iron and calcium. The experiments were carried out using stream and lake water samples with a wide range of dissolved organic carbon (DOC) concentration. Decreasing DOC concentration along with decreasing absorbance was observed during three-week exposures to natural solar radiation as well as during laboratory experiments with artificial solar radiation. The pH of the samples significantly affected degradation rates of DOM especially with elevated iron, while no influence of nitrate or calcium concentration was observed. Addition of Fe(III) did not significantly affect photodegradation and photobleaching rate constants in samples at circumneutral pH. Acid pH increased photodegradation rates. The results suggest that photodegradation rates of DOM will decrease during recovery from acidification. Hence, lower photodegradation rates may be responsible for increases in DOM observed in some regions of North America and Europe.