Colored dissolved organic matter (CDOM), a major component of the dissolved organic carbon (DOC) pool in many lakes, is an important controlling factor in lake ecosystem functioning. Absorption coefficients at 440 nm (a 440 , m -1 ), a common measure of CDOM, exhibited strong associations with dissolved iron (Fe diss ) and DOC in 280 lakes of the Upper Great Lakes States (UGLS: Minnesota, Wisconsin, and Michigan), as has been found in Scandinavia and elsewhere. Linear regressions between the three variables on UGLS lake data typically yielded R 2 values of 0.6-0.9, suggesting that some underlying common processes influence organic matter and Fe diss . Statistical and experimental evidence, however, supports only a minor role for iron contributions to a 440 in UGLS lakes. Although both DOC and Fe diss were significant variables in linear and log-log regressions on a 440 , DOC was the stronger predictor; adding Fe diss to the linear a 440 -DOC model improved the R 2 only from 0.90 to 0.93. Furthermore, experimental additions of Fe III to colored lake waters had only small effects on a 440 (average increase of 0.242 m -1 per 100 μg/L of added Fe III ). For 136 visibly stained waters (with a 440 > 3.0 m -1 ), where allochthonous DOM predominates, DOM accounted for 92.3 ± 5.0% of the measured a 440 values, and Fe diss accounted for the remainder. In 75% of the lakes, Fe diss accounted for < 10% of a 440 , but contributions of 15-30% were observed for 7 river-influenced lakes. Contributions of Fe diss in UGLS lakes to specific UV absorbance at 254 nm (SUVA 254 ) generally were also low. Although Fe diss accounted for 5-10% of measured SUVA 254 in a few samples, on average, 98.1% of the SUVA 254 signal was attributable to DOM and only 1.9% to Fe diss . DOC predictions from measured a 440 were nearly identical to those from a 440 corrected to remove Fe diss contributions. Overall, variations in Fe diss in most UGLS lakes have very small effects on CDOM optical properties, such as a 440 and SUVA 254 , and negligible effects on the accuracy of DOC estimated from a 440 , data for which can be obtained at broad regional scales by remote sensing methods.
Bibliographical noteFunding Information:
We gratefully acknowledge support from the National Science Foundation, the Minnesota Environmental and Natural Resources Trust fund, as recommended by the Legislative-Citizen Commission on Minnesota Resources, and Univ. of Minnesota's Office of the VP for Research and Retirees Association, U-Spatial Program, Sea Grant Program, and Agricultural Experiment Station. We thank numerous collaborators and student workers for assistance in sample collection and analysis. The senior author gratefully acknowledges mentoring early in his career by G.F. Lee, himself an iron researcher.