Anaerobic processes fuel carbon dioxide production in Tonle Sap Lake

Newswise — In recent years, research has increasingly shown that the sharp lines thought to separate aquatic and terrestrial ecosystems are more blurred than previously believed, leaving unanswered questions as to where one stops and the other begins.

Lakes and rivers contain high amounts of dissolved carbon dioxide fueled by water (i.e., runoff and rain) as it travels over the landscape. The eventual flow of carbon into the ocean along a network of lakes, rivers and streams is sometimes referred to as a “passive pipe.” Researchers are now finding that this “pipe” is more actively cycling carbon on the way to the ocean, leading to widespread carbon dioxide emissions from these freshwater systems.

Much of this research has been conducted on aerobic microbial transformation of carbon in temperate freshwater systems, whereas little research has been done to measure the scale and significance of anaerobic microbial transformation in the world’s tropical freshwater rivers. 

A new study led by the University of Washington found that anaerobic processes occurring on floodplains of the Tonle Sap, the largest lake in Southeast Asia, are important contributors of the carbon dioxide that is dissolved in surface waters. The findings were published Feb. 14 in the journal Proceedings of the National Academy of Sciences.

“Others have shown that the large amount of atmospheric carbon dioxide that we assume is fixed by the terrestrial landscape in the form of plant biomass, like trees, is actually transferred to freshwaters, where it eventually diffuses back into the atmosphere,” said lead author Benjamin Miller, a postdoctoral researcher at the UW School of Environmental and Forest Sciences. “But, a lot is happening along the way, between this fixation and diffusion.”

Until recently, researchers accepted that because anaerobic respiration pathways are relatively inefficient and occur at much lower rates compared to aerobic respiration, they didn't really contribute to the carbon and carbon dioxide fluxes observed in these freshwater systems.

Tropical rivers like the Mekong, which hydrologically influences Tonle Sap Lake, uniquely overflow their banks and flood for much of the year. These regular flood events create the chemical preconditions needed for methanogenesis to occur in waterlogged soils, the anaerobic process Miller is measuring and the final step in the decay of organic matter. 

“All decay processes produce carbon dioxide. The production of methane and its subsequent conversion to carbon dioxide by microbes on flooded land has not been fully appreciated as a significant contributor to carbon dioxide dissolved in tropical freshwaters,” said Miller.

Miller explained that this anaerobic process is the result of the unique hydrology and flood regimes of large tropical rivers, and additional work is needed to determine if it is more widespread and at what rate it is occurring in different watersheds.

Globally, tropical rivers are responsible for 30% of all the water that drains from land and into the oceans every year. The significance of these findings is that they clearly indicate that a large portion of the carbon dioxide that is dissolved in these rivers and returns to the atmosphere actually originates from methanogenesis.

With these new findings, researchers can more accurately quantify how systems naturally cycle carbon and determine a baseline for how future changes might impact these processes.

Carbon forms the base of every food web. The lower Mekong River Basin produces more than 2 million tons of fish annually at the top of this food web, making it the largest freshwater fishery and one of the most productive freshwater systems in the world. The continued health of this extensive system is vital for the health and prosperity of the region's people.  

Based on this work and the work by others at the University of Washington, the introduction of large-scale hydropower development in the Mekong (and, more broadly, the impacts of climate change) can potentially alter the anaerobic processes measured by Miller and his colleagues. The magnitude and the duration of flooding, which is critical in the transformation of methane into carbon dioxide, would be significantly altered as a result of flows being held back upstream.

Co-authors are Gordon Holtgrieve of the UW School of Aquatic and Fishery Sciences; Mauricio Arias of the University of South Florida; Sophorn Uy of the Cambodian Inland Fisheries Research and Development Institute and the Royal University of Agriculture; and Phen Chheng of the Cambodian Fisheries Administration. 

This research was funded by the National Science Foundation and Margaret A. Cargill Foundation.