The effects of iron on water color in lakes and rivers: inferences from experimental and observational studies

A laboratory experiment with spectral light absorption as function of factorial combinations of dissolved organic carbon (DOC) and iron (Fe) concentrations showed only weak interactions between Fe and DOC at all wavelengths, such that the effects of DOC and Fe were independent and non-synergistic. The mass-specific absorption coefficient for DOC was 11 times higher than for Fe when averaged across the PAR region (400-700 nm), and 4.7 times higher at the Hazen color wavelength (410 nm). The simple additive model from the gradient mixing experiment predicts well the absorption of colored dissolved organic matter at 410 nm (aCDOM(410)) in 998 Norwegian lakes, and that the Fe contribution could be less than 1% of the total. Partitioning of aCDOM(410) variance across 7 published studies showed that the variance fraction uniquely attributable to DOC was always far higher that corresponding fraction for Fe, while the fraction shared between Fe and DOC generally was highest, reflecting the deep biogeochemical interconnectedness between these two constituents.