Worldwide, there are many more miles of small streams and rivers than large rivers but their contribution to the global carbon cycle is not yet fully understood. A recent review article published in Reviews of Geophysics explored the contribution of headwater streams to carbon storage and release. The editors asked some of the authors to explain more about this aspect of the carbon cycle and suggest where further research efforts are still needed.

What role do streams and rivers play in the global carbon cycle?

Streams and rivers, collectively termed “running waters,” are the most important pathways for the transport of carbon from continents to the oceans which, together with the atmosphere, are the three largest carbon reservoirs. The storage and release of carbon dioxide by different mechanisms leads to carbon losses from these aquatic environments. To date, these losses remain unquantified in the Intergovernmental Panel on Climate Change’s (IPCC) estimates of global carbon cycling. Nonetheless, such quantifications are increasingly important to better predict carbon balances.

What are “headwater streams” and why are they particularly significant globally?

By definition, a headwater stream starts where surface runoff is sufficiently concentrated to scour the catchment surface and thus create distinct flowing waters. Together, small streams comprise more than 96 percent of the total number of streams and rivers worldwide. However, the enormous number of headwater streams exerts so far unknown contributions to basin-wide carbon cycling. For instance, in terms of carbon dioxide release to the atmosphere, cumulative contributions of headwaters are currently underrepresented in the global carbon cycle. Conservative estimates suggest that globally 36 percent of total carbon dioxide release from rivers and streams originates from headwaters.

What factors influence the amount of carbon in headwater streams?

Headwater stream carbon contents directly reflect carbon inputs from surrounding soils and groundwaters. Organic carbon phases (i.e. dissolved organic carbon) originate from the biosphere and mobilize from soils, whereas inorganic carbon phases (i.e. carbon dioxide) derive from decomposition of organic matter in soils and streams, as well as from bedrock weathering and input via groundwater. Whereas relative inputs of carbon dioxide by decomposition and weathering are poorly known and quantified, carbonate rock weathering is able to dominate inorganic carbon inputs even if carbonates only make up a small proportion of the rocks in the catchment.

What instruments and technologies are used to measure carbon levels in streams?

Most stream carbon dioxide data found in the scientific literature was calculated from measures of total alkalinity, pH and temperature. Because of measurement uncertainties of these parameters and organic acids that can contribute to total alkalinity, the use of dissolved inorganic carbon instead of total alkalinity results is recommended. However, preferred methods are direct measurements of stream carbon dioxide concentrations. Such methods include the analysis of headspace samples that have been equilibrated with water (photo right), the application of floating chambers (photo top) and other field-devices. Restrictions such as temporal and spatial resolution as well as flux measurements constitute challenges in the future.

What are the major unresolved questions in this area of research?

Even though carbon dioxide concentrations in headwater streams have been extensively investigated during recent years, sources and relative contributions from groundwaters and soil waters, as well as within stream turnover, remain largely unknown. This hampers up-scaling efforts and the accuracy of global estimates. Furthermore, headwaters contribute large but so far poorly quantified amounts of carbon dioxide to the atmosphere. With this, representative quantification of carbon dioxide release from headwater streams on a global scale is an important future challenge.

Where are additional data or modeling efforts needed?

Better characterization of the role of groundwater inputs to streams and their influence on headwaters are imminent needs. So far, groundwater components are either neglected or approximated via springs in most headwater studies. Future studies would benefit from better instrumentation for groundwater sampling and synchronized runoff gauges, the application of multiscale geophysical imaging approaches, and of integral isotope measurements. Such tools also hold the potential to fill knowledge gaps of high-resolution carbon dynamics on temporal (i.e. diurnal) and small spatial scales.

—Anne Marx, Robert van Geldern, and Johannes Barth, Department of Geography and Geosciences, Friedrich-Alexander University Erlangen-Nuremberg, Germany; email: [email protected]