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.
Science Moab: Can you explain the process by which a large reservoir or lake emits greenhouse gases?
Deemer: When people think of methane, they think of cows. The 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 the sources of greenhouse gases are and reservoirs don’t tend to come to mind. But 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: Just knowing how many lakes and reservoirs and ponds there are is a complex challenge. With satellite reflectance products, we see some ponds are in the middle of a forest or temporary. To get the earliest estimates of reservoir greenhouse gas flux, they averaged all these different estimates of emission together to get an average flux. Then they took an estimated [water] surface area and multiplied those together to try to see how big that emission could be. Over 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. 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 to where all that came from. Those models don’t line up perfectly, so there are 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. 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 are 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 [at Lake Powell]. 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. In Lake Powell, the timeframe of exposure and re-inundation is a lot longer than in wetter environments.
Science Moab: Are there any downstream effects of these 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 coming from the bottom of the reservoir, where decomposition has happened. The oxygen from the surface can’t mix down fast enough. One interesting thing that’s happening is 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 gases, 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 folks’ attention. The next step is efforts to include reservoirs on 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 are 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.
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