School of Aquatic and Fishery Sciences

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. 

Along the Mekong River in Cambodia, UW researchers are racing to determine how hydropower demand will impact the supplies of rice and fish—and the communities who rely on them. Learn more

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.

For more information, contact Miller at blm8@uw.edu and Gordon Holtgrieve at gholt@uw.edu.

This research constitutes part of Miller’s dissertation on the carbon dynamics on floodplains of the Yangtze and Mekong Rivers. The first chapter of his dissertation, published in the Journal of Geophysical Research: Biogeosciences, quantifies the magnitudes and drivers of methane and carbon dioxide fluxes in riparian environments in Three Gorges Reservoir.

This article originally appeared in UW News

Accidentally trapping sharks, seabirds, marine mammals, sea turtles and other animals in fishing gear is one of the biggest barriers to making fisheries more sustainable around the world. Marine protected areas — sections of the ocean set aside to conserve biodiversity — are used, in part, to reduce the unintentional catch of such animals, among other conservation goals.

Many nations are calling for protection of 30% of the world’s oceans by 2030 from some or all types of exploitation, including fishing. Building off this proposal, a new analysis led by the University of Washington looks at how effective fishing closures are at reducing accidental catch. Researchers found that permanent marine protected areas are a relatively inefficient way to protect marine biodiversity that is accidentally caught in fisheries. Dynamic ocean management — changing the pattern of closures as accidental catch hotspots shift — is much more effective. The results were published Jan. 17 in the Proceedings of the National Academy of Sciences.

“We hope this study will add to the growing movement away from permanently closed areas to encourage more dynamic ocean management,” said senior author Ray Hilborn, a professor at the UW School of Aquatic and Fishery Sciences. “Also, by showing the relative ineffectiveness of static areas, we hope it will make conservation advocates aware that permanent closed areas are much less effective in reducing accidental catch than changes in fishing methods.”

These techniques could include devices that keep sea turtles away from shrimp fishing, or streamer lines on boats to deter seabirds from getting caught in fishing lines.

The international team of researchers looked at 15 fisheries around the world — including Californian swordfish, South African tuna and Alaskan pollock — and modeled what would happen both to the targeted fish and to species caught accidentally, called bycatch, if 30% of fishing grounds were permanently closed, compared with dynamic management. In practice, dynamic management tracks real-time data of bycatch and closes smaller areas that can move year to year based on where species are most affected.

One of the critiques of permanent marine protected areas is that many of the species they are supposed to protect — marine mammals, turtles, seabirds — move around and may leave the protected area altogether. The study found that, on average for all fisheries studied, restricting fishing in 30% of a fixed area did reduce bycatch by about 16%. But in dynamic closed areas, over the same fraction of the ocean, bycatch was reduced by up to 57%.

“We found we can significantly reduce bycatch without decreasing the catch of target species by closing small fishing areas that can move year to year,” said lead author Maite Pons, an independent fisheries consultant based in Uruguay who completed this work as a UW postdoctoral researcher. “This dynamic approach is increasingly valuable as climate change drives species and fisheries into new habitats, altering these interactions.”

The authors acknowledge that goals differ for various marine protected areas, and if the main purpose is to protect a critical habitat, a biodiversity hotspot or unique feature, static closures might be more effective and easier to enforce. In this way, all conservation goals should be broadly considered when determining which types of ocean protections to put in place, they said.

“I hope this study encourages everyone to consider how best to reduce bycatch and protect marine ecosystems,” Hilborn added.

A full list of co-author names and institutions is listed in the paper. No outside funding was used in this research.

For more information, contact Hilborn at rayh@uw.edu and Pons at mpons@uw.edu.