Lake Powell Methane
When considering sources of greenhouse gases, lakes and reservoirs may not immediately come to mind, but they are a significant part of the global greenhouse gas budget. Bridget Deemer is a research ecologist at the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center. Her research aims to understand how human activities are affecting the way that energy and nutrients cycle through ecosystems. She also continues to be involved in efforts to understand methane emissions from lakes and reservoirs, including Lake Powell.
Meet the Scientist: Bridget Deemer
Bridget Deemer is a research ecologist at the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center. Her research there informs the Glen Canyon Dam Adaptive Management Program as well as the Lake Powell Water Quality Monitoring Program. She is also passionate about understanding the role of artificial reservoirs in the global carbon balance and practicing the process of scientific writing.
Bridget Deemer received her PhD from the School of the Environment at Washington State University where she studied reservoir nitrogen removal and greenhouse gas production in the Global Change and Watershed Biogeochemistry Lab. There she was involved in a policy-oriented traineeship that developed her passion for work that crosses the science/policy boundary. Broadly, her research aims to understand how human activities are affecting the way that energy and nutrients cycle through ecosystems. Specifically, she is interested in identifying reservoir management win-wins as well as trade-offs, which is critical as the quantity and quality of water becomes increasingly variable under a changing climate.
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Interview Transcript - Lakes and Greenhouse Gases
Science Moab: Can you explain the process by which a large reservoir or lake emits greenhouse gasses?
Deemer: When a lot of people think of methane, they think of cows and the fermentation of material in a cow gut. Those same microbes that exist inside cows also exist in the sediments of lakes and reservoirs. These microbes are decomposing organic matter, anything from trees to little algae that grow in the water. In this process of decomposition, if there’s no oxygen around, one result is the production of methane. It’s this biological activity that’s predominantly responsible for the greenhouse gas emissions in a lot of lakes and reservoirs. I think people have a mental model of what some sources are for greenhouse gasses, and reservoirs don’t tend to come to mind. But in terms of methane, aquatic sources make up approximately half of all the methane sources around the globe.
Science Moab: How do you estimate the total emissions coming out of reservoirs?
Deemer: Knowing how many lakes and reservoirs and ponds there are is a similarly complex challenge to knowing how much greenhouse gas is coming from the systems. With satellite reflectance products, some ponds are in the middle of a forest or temporary. To get the earliest estimates of reservoir greenhouse gas flux, they compiled all the published measurements of emission, and averaged all those different estimates of emission together to get an average flux. Then they took an estimated surface area and multiplied those together to try to see how big that emission could be. Through time, as scientists have realized that reservoirs are a really important part of the global greenhouse gas budget, those calculations have gotten more sophisticated. We have a better understanding of water body surface area from satellite products, and also lake productivity, or how much algal production is happening in the lake, and how those factors influence emission. And so we can take all that information together to get a better idea of the contribution to the budget. What I just described is a bottom-up estimate where we take emission estimates from the water bodies themselves, and then we count how many water bodies there are, and we turn that into an emission estimate. But there are also top-down models that look at how much methane we observe coming into the atmosphere and then try to backtrack where all that nothing came from. Those models don’t line up perfectly, so there’s some open questions still to resolve.
Science Moab: How is Lake Powell different than some of the other lakes you might have looked at?
Deemer: Arid lakes and reservoirs are underrepresented in ecology and limnology research. Reservoirs are particularly important in arid environments, because they represent significant water storage, like Lake Powell does. One of the reasons that the Lake Powell water quality monitoring program started was because of concerns about salinity in the Colorado River Basin. So there’s a salinity control program that funds the monitoring work. It’s not a high salinity system, but it’s higher salinity than many lakes and reservoirs. I was interested in taking some greenhouse gas measurements on Lake Powell because there’s a general conceptual model that saltier systems tend to emit less methane, but there’s not a lot of saline reservoirs with data. The last thing I’ll say, in terms of uniqueness, is the extent of water level decline. That’s made the news and is a big issue in terms of the water storage in the Colorado River Basin. But it’s also really interesting from an ecological and biogeochemical standpoint, just thinking about this area of the shoreline that’s periodically inundated and then dry. A lot of reservoirs have that region, but in Lake Powell, the timeframe of exposure and reinundation is a lot longer than in wetter environments.
Science Moab: Are there any downstream effects of these lake bottom methane events?
Deemer: I love thinking about the connections between Lake Powell and the river below it. At the most basic level, all the decomposition that happens in the lake results in oxygen drawdowns. The water that is withdrawn from Lake Powell and flows out into Glen Canyon is undersaturated with dissolved oxygen because it’s spilling from the bottom waters of the reservoir, where decomposition has happened. The oxygen from the surface can’t mix down fast enough to replace the decomposition. One interesting thing that’s happening is that when a large inflow comes and remobilizes dried sediment, you can get these low dissolved oxygen events in the middle of the Lake Powell water column. Those low dissolved oxygen plumes are the result of both biological activity that would produce greenhouse gasses, and also some degree of chemical oxygen consumption from the sediments themselves. Those plumes can get pulled towards the dam, and can result in lower concentrations of dissolved oxygen coming into Glen Canyon, which is a concern for the rainbow trout fishery there.
Science Moab: What steps do you think should be taken to address lake emissions?
Deemer: I think the first step is just talking about and acknowledging this phenomenon and bringing it to folk’s attention. And the next step is these efforts such as reservoirs now being included in the Intergovernmental Panel on Climate Change’s required inventory reporting, so countries have to estimate how much greenhouse gas emission is coming from reservoirs as part of their flooded lands inventory. The third step is thinking about ways to mitigate or reduce that flux where we can because reservoirs are managed systems. I think there’s a lot of interesting opportunities there, knowing that more nutrient enriched systems tend to emit more, but also knowing what we know about the size of systems. We also know that the depth of the system is important; shallower systems tend to emit more than deeper systems. Keeping those things in mind and a broader planning context is a powerful approach.
