Increased summer rainfall alters the carbon cycle in the Arctic
14 Jul 2021 | Source: Helmholtz-Centre Potsdam, German Research Centre for Geosciences - GFZ
Arctic rivers are increasingly fed by rainfall in addition to snowmelt. This leads to a stronger interaction of the water bodies with the surrounding plants and soil - and thus to a stronger input of nutrients, carbon, and other organic and inorganic materials. This is shown by researchers led by Joanne Heslop from the German Research Centre for Geosciences Potsdam (GFZ) and Casey Beel from Queens University, Kingston (Canada) in a study published in Nature Communications. They analysed more than ten years of comprehensive data on weather events, streams and their biogeochemical composition. Potential impacts concern water quality, the food chain, and the release of climate gases such as CO2 and methane. Thus, the data are important for the improvement of biogeochemical and climate models.
Importance of Arctic waters for the global carbon cycle
Arctic inland waters are important components of the global carbon cycle. They contain carbon which can be partially converted to CO2 or methane by microbial processes and released to the atmosphere as greenhouse gasses.
Joanne Heslop, a PostDoc at GFZ, and her Canadian colleagues including Casey Beel from Queens University, Kingston have investigated how the input of carbon and other organic and inorganic substances into the waters of the high Arctic changes with climate change itself at the Cape Bounty Arctic Watershed Observatory on Melville Island, Nunavut, Canada.
Not just snowmelt: more frequent rainfall in late summer
The high Arctic is the area above 70 degrees north latitude. These areas far away from any civilisation are poorly accessible and have therefore not been intensively studied. Until now, it was assumed that the waters there are mainly fed by snowmelt water. In the course of the summer, the rivers carry less and less water, and some dry up completely by fall. Accordingly, most of the measurement campaigns have taken place during the snowmelt season.
However, Joanne Heslop and Casey Beel have observed over the years that it rains more and more frequently in the Arctic, especially in the later summer season. Rivers are therefore no longer fed predominantly by snowmelt water in spring and early summer, but still carry water from rainfall later in the summer. Up to 40 percent of annual river flows are now observed later in the year. Many rivers no longer dry up in the fall, as other studies have shown.
More terrestrial material in the runoff water
"This fundamentally changes the biogeochemical composition of water bodies," says Heslop. Snowmelt runoff has limited and very superficial interaction with the landscape before it enters the river because of the snowpack, channelisation and frozen ground.
The precipitation events occur mainly later in the summer when it gets warmer. Then the permafrost has thawed to depths of up to a few dozen centimetres, microbial life awakens and plants grow. There is much more opportunity for the runoff water, which is carried broadly and then drains away, to interact with the surrounding landscape before it enters the larger rivers or lakes. This changes the amount and type of organic or inorganic terrestrial material carried along.
Analysis of large data sets over 14 years
"Until now, this has been little studied and underestimated as an effect," says Heslop. For her study, the biogeochemist and her colleagues analysed hydrometeorological and biogeochemical data from 2003 to 2017 from the high Arctic around the Cape Bounty Arctic Watershed Observatory. They included temperature and precipitation data, runoff volumes, flow energy, concentrations of sediments and nutrients including carbon, and optical characteristics of dissolved organic matter.
In years with higher rainfall, the researchers found larger amounts of terrestrial material in the rivers than in comparatively dry years. When it rained less, it was mainly dissolved matter that was washed away. The heavier the rain, the more flow energy in the water streams, so that heavier particles that would otherwise settle on the riverbed are also carried along and transported downstream.
How the type and amount of material transported in the water changes has significant consequences in two areas: Firstly, an increase in organic substances and nutrients causes changes in water quality that can affect the food chain - from microorganisms to fish and other marine animals, and thus also for their habitat. Secondly - and this is even more important according to Heslop - these substances also influence the formation of greenhouse gases such as CO2 and methane: "If there is fresher carbon and more runoff in warmer periods, there is a possibility that microorganisms will increasingly process this carbon into greenhouse gases."
Summary and outlook
Thus, the team's research has put attention on a new seasonal window later in the summer. "Our results will improve the understanding of how hydrological and landscape characteristics influence greenhouse gas production in the Canadian High Arctic. This will provide us with important new insights into the carbon cycle in polar permafrost regions," Heslop sums up. This is particularly important against the backdrop of ongoing climate change, as a result of which the strength, extent and frequency of summer rains in the Arctic will continue to increase. In future, the focus must shift from the larger river courses to the smaller tributaries upstream, where most of the interaction between water and earth takes place and the effects are more measurable. For all this, new methods and new research approaches are also needed, says Heslop.
Beel, C., Heslop, J., Orwin, J., Pope, M., Schevers, A., Hung, J., Lafreniere, M., Lamoureux, S.: Emerging dominance of summer rainfall driving High Arctic terrestrial-aquatic connectivity. Nature Communications, 12, 1448 (2021). DOI: 10.1038/s41467-021-21759-3
Here, you find a short video showing increased stream power in a High Arctic stream as the result of a fall 'shoulder season' rainfall event. These rainfall events are important yet under-sampled and under-studied mechanisms for supplying Arctic rivers with dissolved and particulate terrestrial material. (Video: Scott Lamoureux, Queen’s University)