Special Seminar: How Lake Powell and the southwest mega-drought have fundamentally altered downstream nutrient dynamics

Bridget Deemer, Research Ecologist
Bridget Deemer, Research Ecologist

Bridget Deemer, Research Ecologist , US Geological Survey Southwest Biological Science Center will present a seminar on dams and drought on May 5, 2023 at 10:30 am in 901 South of Hardin Hall.

How Lake Powell and the southwest mega-drought have fundamentally altered downstream nutrient dynamics.

Rivers below dams often experience seasonal or persistent reductions in phosphorus (P) inputs due to phosphorus retention in reservoirs. Lake Powell, a large water storage reservoir on the Colorado River, retains the vast majority (95-99%) of the phosphorus that it receives. This creates phosphorus limiting conditions downstream throughout the year. Long-term data from an adaptive management program show how declines in phosphorus releases from Lake Powell can lead to major collapse in the tailwater rainbow trout population. The bio-availability of phosphorus in this ecosystem is largely controlled by calcium carbonate biogeochemistry, where ecologically meaningful reductions in sediment phosphorus release can occur when river pH increases by one unit (from 7 to 8). As reservoir levels go down, as is happening in the Southwest’s current drought conditions, water is beginning to be withdrawn from Lake Powell’s surface waters, where photosynthesis drives pH up. This, combined with empirical observations of the vertical distribution of soluble reactive phosphorus in the water column, suggests that phosphorus bioavailability downstream of the reservoir will decline as lake levels drop. Thus, declining reservoir water levels may further exacerbate food limitation in the Colorado River below Glen Canyon Dam, with critical implications for fish and invertebrate populations.

Bridget Deemer is a research ecologist at the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center where she leads a long-term water quality monitoring program on Lake Powell. Her research informs the Glen Canyon Dam Adaptive Management Program. Bridget is also passionate about understanding the role of artificial reservoirs in the global carbon balance. A major theme of her work examines how dam and reservoir management affects water quality, ecosystem productivity and greenhouse gas emissions. 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.

Follow Bridget Twitter: @BridgetDeemer | Twitter- @USGSAZ

More details at: https://snr.unl.edu/aboutus/when/seminars.aspx