Iron influence on dissolved color in lakes of the Upper Great Lakes States

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.