Chemical and optical changes in freshwater dissolved organic matter exposed to solar radiation

We studied the chemical and optical changes in the dissolved organic matter (DOM) from two freshwater lakes and a Sphagnum bog after exposure to solar radiation. Stable carbon isotopes and solid-state C-13-NMR spectra of DOM were used together with optical and chemical data to interpret results from experimental exposures of DOM to sunlight and from seasonal observations o f two lakes in northeastern Pennsylvania. Solar photochemical oxidation of humic-rich bog DOM to smaller LMW compounds and to DIC was inferred from lo sses of UV absorbance, optical indices of molecular weight and changes in D OM chemistry. Experimentally, we observed a 1.2 parts per thousand enrichme nt in delta C-13 and a 47% loss in aromatic C functionality in bog DOM samp les exposed to solar UVR. Similar results were observed in the surface wate rs of both lakes. In late summer hypolimnetic water in humic Lake Lacawac, we observed 3 to 4.5 parts per thousand enrichments in delta C-13 and a 30% increase in aromatic C relative to early spring values during spring mixin g. These changes coincided with increases in molecular weight and UV absorb ance. Anaerobic conditions of the hypolimnion in Lake Lacawac suggest that microbial metabolism may be turning over allochthonous C introduced during spring mixing, as well as autochthonous C. This metabolic activity produces HMW DOM during the summer, which is photochemically labile and isotopicall y distinct from allochthonous DOM or autochthonous DOM. These results sugge st both photooxidation of allochthonous DOM in the epilimnion and autotroph ic production of DOM by bacteria in the hypolimnion cause seasonal trends i n the UV absorbance of lakes.