More Frequent Runoff and Connected Sources in Headwaters Promote Browning of Northern Freshwaters

Sustained browning of northern waters has prompted inquiries into the drivers of increasing concentrations of organic matter. While reduced sulfur deposition is a key cause, an increasing role of hydrologic mechanisms as a result of cleaner air and progressing climate change has been repeatedly suggested. How these controls act remains however unclear. Here we examine over 30 years of organic carbon concentration and discharge data from four reference streams located across Norway to pinpoint consistent hydrologic changes that may promote water browning. Stable slopes with changing intercepts of the concentration-discharge relations indicate that the influence of air pollution on soil solution chemistry is plausible, supporting available chemical explanations from a hydrologic perspective. Decreasing ratios of concentration to discharge variability, observed in autumn over the years, point to less spatial heterogeneity of the sources of organic carbon. A clear rise in the frequency of runoff events, which increases the opportunities for dissolved organic carbon to transit from soil to streams, also indicates higher hydrologic connectivity and more even mobilization of carbon sources. More connected sources and more frequent runoff events, which jointly enhance the likelihood of organic carbon reaching rivers, may thus contribute to the observed browning of northern waters.