Coupling between increased lake color and iron in boreal lakes.

Increasing evidence show that lake color has increased more than DOC in several boreal lakes, promoted by enhanced levels of light absorbing iron (Fe). Fe levels show great spatial and temporal variations in northern freshwaters, and processes regulating long-term Fe trends as well as differences among lakes are not fully understood. In a boreal lake district of SE-Norway, the coupling between lake color and Fe were investigated in 24 non-productive lakes during 1983-2017. The lakes showed significant increases in color, total organic carbon (TOC), specific UV absorption and Fe with time. Based on regression models, TOC and Fe together explained 89% of lake color. Fe averagely contributed to 12% of the lake color. The color contribution from Fe was better predicted by the Fe-to-TOC ratio than the absolute Fe concentration. The variability in Fe contribution to color was large - ranging from 0 to 65% based on 429 lake water samples. Variability in Fe levels and color contribution were related to differences in water retention time (WRT), pH and to the terrestrial input of Fe-organic matter complexes. Size fractionation analyses showed that Fe and colored TOC were mainly in a colloidal form (>10 kDa). Along with reduced acid rain, there has been a significant reduction in ionic strength in several boreal lakes since 1980s, which promote the stability of colloids. Negative correlation between the Fe-to-TOC ratio and WRT, might be due to a size selective removal of colloidal Fe complexes related to water flow, which is supported by a higher Fe-to-TOC ratio in wet years compared to dry years. Responses of Fe complexes to increased runoff are relatively higher in lakes with short compared to long WRT. In addition, increases in hydrological extremes, due to climate change, may enhance the Fe variability among lakes even further in a future climate.