Cleaner air reveals growing influence of climate on dissolved organic carbon trends in northern headwaters

Surface water browning, the result of increasing concentrations of dissolved organic matter (DOM), has been widespread in northern ecosystems in recent decades. Here, we assess a database of 426 undisturbed headwater lakes and streams in Europe and North America for evidence of trends in DOM between 1990 and 2016. We describe contrasting changes in DOM trends in Europe (decelerating) and North America (accelerating), which are consistent with organic matter solubility responses to declines in sulfate deposition. While earlier trends (1990–2004) were almost entirely related to changes in atmospheric chemistry, climatic and chemical drivers were equally important in explaining recent DOM trends (2002–2016). We estimate that riverine DOM export from northern ecosystems increased by 27% during the study period. Increased summer precipitation strengthened upward dissolved organic carbon trends while warming apparently damped browning. Our results suggest strong but changing influences of air quality and climate on the terrestrial carbon cycle, and on the magnitude of carbon export from land to water.

Impact/Purpose

Dissolved Organic Carbon (DOC) is the primary form of carbon that moves from soils to surface waters. Concentrations of DOC in surface waters of Northern Europe and Eastern North America have been increasing for parts of the past 3 decades. This is of concern for a number of reasons, including increased mobilization of toxic metals (like mercury), and the production of toxic disinfection byproducts when drinking water is treated. Many previous studies have attributed the DOC increase to recovery of sensitive soils from the effects of acidic deposition, and in particular have shown the close correlation between rates of change of Sulfur and Chloride deposition with changing DOC. The current study is a follow up of the largest of these previous studies involving monitoring data from over 425 lakes and streams in Europe and North America. It confirms that changes in deposition are still the dominant drivers of DOC trends, but notes the emerging importance of changes in precipitation and temperature in controlling DOC fluxes.

Citation

de Wit, H., J. Stoddard, D. Monteith, J. Sample, K. Austnes, S. Couture, J. Fölster, S. Higgins, D. Houle, J. Hruška, P. Krám, J. Kopáček, A. Paterson, S. Valinia, H. van Dam, J. Vuorenmaa, AND C. Evans. Cleaner air reveals growing influence of climate on dissolved organic carbon trends in northern headwaters. IOP Publishing LIMITED, Bristol, UK, 16(10):104009, (2021). [DOI: 10.1088/1748-9326/ac2526]

DOI: Cleaner air reveals growing influence of climate on dissolved organic carbon trends in northern headwaters