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 (a₄₄₀, 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 R² 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 a₄₄₀ in UGLS lakes. Although both DOC and Fe_diss were significant variables in linear and log-log regressions on a₄₄₀, DOC was the stronger predictor; adding Fe_diss to the linear a₄₄₀-DOC model improved the R² only from 0.90 to 0.93. Furthermore, experimental additions of Fe^III to colored lake waters had only small effects on a₄₄₀ (average increase of 0.242 m^-1 per 100 μg/L of added Fe^III). For 136 visibly stained waters (with a₄₄₀ > 3.0 m^-1), where allochthonous DOM predominates, DOM accounted for 92.3 ± 5.0% of the measured a₄₄₀ values, and Fe_diss accounted for the remainder. In 75% of the lakes, Fe_diss accounted for < 10% of a₄₄₀, 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 (SUVA₂₅₄) generally were also low. Although Fe_diss accounted for 5-10% of measured SUVA₂₅₄ in a few samples, on average, 98.1% of the SUVA₂₅₄ signal was attributable to DOM and only 1.9% to Fe_diss. DOC predictions from measured a₄₄₀ were nearly identical to those from a₄₄₀ 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 a₄₄₀ and SUVA₂₅₄, and negligible effects on the accuracy of DOC estimated from a₄₄₀, data for which can be obtained at broad regional scales by remote sensing methods.