Characterization of CDOM in the Milwaukee River and its Estuary in Lake Michigan

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The optical and fluorescent characteristics of the colored portion of dissolved organic matter (CDOM) were analyzed in two sample sets along the Milwaukee River from near its source to its estuary in Lake Michigan. CDOM abundance, as determined by the absorption coefficient at 254 nm decreased from river source to outlet and was greater for the sample set that had more recent rainfall and higher water levels. A more dramatic decrease in CDOM abundance was observed as the distance from the outlet into the estuary increased, indicating DOM dilution in the estuary and possible source shifts. Spectral slope at the wavelength interval between 275 nm and 295 nm was used as an analog for bulk DOM molecular weight (MW). No spatial trends in MW were observed, but a reduction in MW of river samples was observed in the more heavily rain influenced sample set. Excitation emission matrix spectroscopy (EEMs) was used to track quality changes in CDOM through peak tracking and fluorescence indices. The Biological, index, humification index, and common peak tracking all indicated terrestrial DOM sources as most important in riverine samples and an increasing importance of autochthonous production in estuary samples parallel factor (PARAFAC) modeling was used to deconstruct the EEMs data set into four major fluorescent DOM components including 3-humic like and 1 protein-like DOM components. The humic-like components dominated fluorescent output for the riverine samples, indicating a dominance of terrestrial DOM sources within the river system. Two of the three humic-like components decreased in importance as distance into the estuary increased. The protein-like component became more prominent in estuarine samples indicating an increasing importance of aquagenic DOM sources from river to lake waters.