Accelerated melting of permafrost due to global warming is increasing the release of organic matter into Arctic and subarctic lakes and ponds, causing them to darken, according to new research. The “browning” effect alters the visual appearance of the landscape, but more importantly, it is highly disruptive to ecosystems.
The study was carried out by researchers from Québec, Denmark, Finland, and Sweden and was published recently in Limnology and Oceanography Letters.
According to the study, some of the largest deposits of organic carbon in the world are contained in frozen tundra soils. As the climate warms and permafrost thaws, there is a bigger risk that carbon which has been trapped in soil for thousands of years will be released into the atmosphere as greenhouse gases, especially in the form of methane produced by aquatic ecosystems in the Arctic and subarctic regions.
Until now, however, there has been little research into how the release of carbon affects dissolved organic matter in surrounding bodies of water, including freshwater lakes and ponds.
Between 2002 and 2016, the team gathered over 350 samples from 256 ponds in the circumpolar North, covering a wide geographic area stretching from Alaska to Russia. The 14 different regions from which samples were collected had a wide range of mean annual temperatures, vegetation types and permafrost cover.
An analysis of the samples revealed that thawing of permafrost is increasing concentrations of organic matter from catchment soil in high-altitude freshwater bodies. The results suggest there is “a strong terrestrial imprint on freshwater ecosystems” in areas where permafrost is melting, according to the report.
“Land-derived organic carbon is having a growing influence on Arctic and subarctic ponds, which carries over into the food web,” the authors of the study said in a press release published by the University of Québec’s National Institute of Scientific Research (INRS).
This type of carbon leads to darkening and stratification of the bodies of water. Because the organic matter absorbs sunlight at high rates, the increased presence of carbon in the ponds raises the temperature, and therefore the permafrost-melting rate, thereby creating a positive feedback loop.
The effects, which the authors collectively refer to as “browning,” can cause significant disruption to ecosystems.
“The browning of these systems leads to oxygen depletion and cooler water at the bottom of the ponds, which can have a major impact on the microbial activity responsible for the production and consumption of greenhouse gases,” according to the study’s authors. They noted that “the production of methane, a powerful greenhouse gas,” is particularly affected.
The authors warn the phenomenon of “northern browning” could be worsened by extreme weather events and increased rainfall in the future, resulting in more greenhouse gases being produced across the circumpolar North.