A comparison of in situ and laboratory optical measurements of dissolved organic matter (DOM) in the Yukon River, Alaska

In situ measurements of chromophoric dissolved organic matter (CDOM) have become increasingly important as proxies for dissolved organic carbon (DOC) concentrations in studies where frequent discrete sampling of DOC concentrations is logistically difficult. Many studies employing this approach have found statistically significant positive correlations between in situ CDOM measurements and DOC concentrations. However, almost all of these studies were conducted in systems with relatively low suspended sediment concentrations (less than 35 mg/L), or employed in-line filtration prior to CDOM measurements. Here we present results of daily in situ and laboratory DOM measurements from the Yukon River at Pilot Station, Alaska, during May 2009. CDOM data were obtained using an in situ fluorometer (excitation wavelength: 370 nm; emission wavelength: 460 nm) without in-line filtration, whereas DOC concentration, UV-Vis absorbance and fluorescence excitation-emission (EEM) data were measured in the laboratory on filtered water samples. The data suggest that an adequate in situ CDOM/DOC relation can be established at this site, and that suspended sediment concentrations as high as 1000 mg/L were not an issue for deriving accurate DOC concentrations from in situ CDOM data obtained without benefit of in-line filtration. A comparison of in situ CDOM measurements with laboratory EEM measurements on filtered water appears to support the conclusion that useful CDOM measurements may be made on unfiltered water in high sediment rivers.