More Frequent\nRunoff and Connected Sources in Headwaters\nPromote Browning of Northern Freshwaters

Sustained browning\nof northern waters has prompted inquiries\ninto\nthe drivers of increasing concentrations of organic matter. While\nreduced sulfur deposition is a key cause, an increasing role of hydrologic\nmechanisms as a result of cleaner air and progressing climate change\nhas been repeatedly suggested. How these controls act remains however\nunclear. Here we examine over 30 years of organic carbon concentration\nand discharge data from four reference streams located across Norway\nto pinpoint consistent hydrologic changes that may promote water browning.\nStable slopes with changing intercepts of the concentration-discharge\nrelations indicate that the influence of air pollution on soil solution\nchemistry is plausible, supporting available chemical explanations\nfrom a hydrologic perspective. Decreasing ratios of concentration\nto discharge variability, observed in autumn over the years, point\nto less spatial heterogeneity of the sources of organic carbon. A\nclear rise in the frequency of runoff events, which increases the\nopportunities for dissolved organic carbon to transit from soil to\nstreams, also indicates higher hydrologic connectivity and more even\nmobilization of carbon sources. More connected sources and more frequent\nrunoff events, which jointly enhance the likelihood of organic carbon\nreaching rivers, may thus contribute to the observed browning of northern\nwaters.