The objectives of this research were to characterize dissolved organic matter (DOM) by excitation-emission matrix (EEM) fluorescence spectroscopy and to apply humification index (HIX) to differentiate the origin and extent of humification of DOMs in watersheds of Lake Biwa and Lake Kasumigaura, Japan. EEM and HIX techniques were applied to lacustrine, riverine, and effluent organic matters. EEM spectra suggest that algal activity and the thermocline formation are critical to composition and distribution of DOM in Lake Biwa. Lake Kasumigaura DOM is evenly distributed over the lake. The effects of allochthonous sources are more conclusive for Lake Kasumigaura DOM than for Lake Biwa DOM. Seasonal and vertical variations in HIX were observed in Lake Biwa similar to results by EEM approach. Upstream DOMs were mostly humified fulvic acid originating from forest soil. DOM from a paddy field and municipal wastewater had the lowest HIX value and consisted mainly of less humified, fresh organic materials. Lacustrine and effluent DOMs demonstrated the same extent of humification in watersheds of Lake Biwa and Lake Kasumigaura. Allochthonous sources such as elution from a paddy field and municipal wastewater are more responsible for the composition of Lake Kasumigaura DOM compared to that of Lake Biwa DOM.

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