Photochemical transformation of dissolved organic matter in lakes

In a survey of photochemical transformation of dissolved organic matter (DO M) in lake water, we found photochemical production of dissolved inorganic carbon (DIC) and low molecular weight carboxylic acids (oxalic, malonic, fo rmic, and acetic acid), upon simulated solar ultraviolet (UV) radiation exp osure of filter-sterilized surface water from 38 investigated lakes. Given the large variability in water chemistry of the studied systems, we suggest that these photoproducts are universally produced in sunlight exposed lake water. The measured production of carboxylic acid carbon corresponded to 3 4.4% (median) of the produced DIC. The production of DIC and carboxylic aci ds during 8 h of mild UV irradiation corresponded to an average of 4.7% (SD 2.9) of the total dissolved organic carbon pool. Hence, photochemical degr adation could be a major sink for DOM in the surface layer of lakes. All ph otoreaction rates were strongly correlated to the total radiation absorbed during UV exposure (r(2) > 0.8), which depends mainly on amount of colored DOM in the water. The variability in DIC production after normalizing for a bsorbed radiation energy could partly be attributed to general water chemis try parameters (e.g., pH, iron concentration, conductivity).