When streams dry, do important ecological processes still occur?

The decomposition of organic carbon (such as leaves) is a fundamental ecological process that underpins energy transfer throughout the biosphere. This process is particularly important in streams and rivers which process a similar amount of carbon as the entire terrestrial sink for anthropogenic emissions.

Schematic summarising variation in the rates of organic carbon decomposition among intermittent stream environments and the role of groundwater inputs (indicated with dashed blue arrows) in sustaining the saturated hyporheic zone. The size of each leaf is proportional to the degree of organic carbon decomposition in surface patches of gravel bars, riffles, and pools, as well as for the hyporheic zone

However, there are few data on organic carbon decomposition within intermittent streams, which represent approximately 70% of headwater streams globally. Streamflow intermittency is currently increasing due to climate change and groundwater extraction. Resolving the dominant factors driving organic carbon decomposition in intermittent streams has thus emerged as an important challenge to understanding how the global carbon cycle will respond to human stressors.

We measured the rates of organic carbon decomposition in surface and sub-surface environments of intermittent streams throughout eastern Australia.

We found that the sub-surface environment of intermittent streams, called the hyporheic zone, was a ‘hotspot’ of organic carbon decomposition, even when surface flows cease. Organic carbon decomposition remained high in the hyporheic zone because its sediment remained wetter for longer compared to in surface environments (see figure).

Groundwater inputs to intermittent stream channels was a key factor enabling wet hyporheic sediments. Consequently, groundwater extraction, and other human-induced changes that reduce flow, may lead to large reductions in the rates of organic carbon decomposition in many intermittent streams.

Our research highlights the critically important role of the hyporheic zone of intermittent streams for organic carbon processes.

This work appeared in Nature Scientific Reports.

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