ABSTRACT

The temporal and vertical distributions of dissolved organic matter in two Chinese eutrophic lakes (Lake Hongfeng and Lake Baihua) were investigated by measuring dissolved organic carbon (DOC), chlorophyll a (Chl a) concentrations and excitation emission matrix (EEM). DOC concentrations were almost similar in those two lakes, but higher (169-330 μM C, average 223 ± 36 μM C) in the epilimnion (0-8 m), and lower (151-303 μM C, 193 ± 25 μM C) in the hypolimnion (10-25 m). Five fluorescence peaks (peaks M and A of fulvic acid-like; peak C of fulvic acid-like or autochthonous DOM-like; peaks T and T_UV of tryptophan-like components, respectively) were observed in the lake waters, indicating various sources of fluorescent dissolved organic matter in lakes. The fluorescence intensities of those peaks decreased (3-61%) in the epilimnion during the summer stratification period compared to hypolimnion. Chl a concentrations were 2.7-65.5 μg l^-1 in the epilimnion and 1.4-41.0 μg l^-1 in hypolimnion with the maximum level in July when water temperature (WT) was the highest of the year. DOC concentrations were significantly correlated with Chl a and WT. Chl a was well correlated with WT in the epilimnion of Lake Hongfeng, suggesting that biological activities might be a major DOC source in the lake waters. DOC concentrations were weakly correlated or not correlated with fulvic acid-like fluorescence in the epilimnion, but they were significantly correlated with tryptophan-like fluorescence, indicating that the tryptophan-like fluorescent material was from new DOM in the lakes during the summer period. But these correlations were often insignificant or absent in the waters of Lake Baihua except peaks T and T_UV, which may be caused by the input of organic contaminants from the industrial activities near Lake Baihua. This study may assist to better understand the sources of lake DOM and the effects of organic matter pollution on the biogeochemical cycling of DOM in lake environments.