Tropical Wetlands: Productive but Leaky Systems?

River systems play an integral role in the global carbon cycle by connecting the terrestrial biosphere, the atmosphere and the oceans. Extensive tropical wetland systems, such as those found in the Amazon region, have been shown to export significant amounts of carbon to river waters as dissolved carbon dioxide (CO₂) that can be transported and emitted hundreds of km downstream. The assessment of both regional and global carbon budgets could therefore be improved by quantifying these lateral carbon fluxes, especially from highly productive temporarily or permanently flooded areas where substantial CO₂ evasion from inland waters can occur.

The Nile is the longest river in the world and the headwaters are located in the extensive Papyrus dominated wetlands in central Africa that are associated with Lake Victoria. From its source the White Nile flows northwards through wetlands in Uganda and Sudan before it joins the Blue Nile. Papyrus wetlands have been shown to be some of the most productive global ecosystems, with recorded rates of aerial net primary productivity of up to 3.09 kg C m^-2 yr^-1. In addition, where anaerobic conditions occur they also accumulate large amounts of carbon in the form of peat, and under these circumstances they represent a significant carbon sink. However, as water moves through these wetlands and is exchanged with surrounding rivers and lakes significant quantities of dissolved organic and inorganic carbon as well as suspended particulate organic matter are exported, which are either released further downstream by degassing, decomposition or deposition.

We describe here our attempts to constrain the carbon cycle of wetlands at the Nile headwaters and provide more information on the regional scale carbon budget.