‘Glen Canyon Dam has fundamentally altered downstream ecosystems in the #ColoradoRiver’ — Bob Berwyn


From the Summit County Citizens Voice (Bob Berwyn):

..an in-depth three-year study done as part of the Glen Canyon Dam Adaptive Management Program offers a stark reminder of the changes wrought by drastically altering the river’s hydrological regime.
The study used food webs to feeding relationships. According to a press release about the study, scientists can predict how plants and animals living in an ecosystem will respond to change by describing the structure of these webs.

“Given the degraded state of the world’s rivers, insight into food webs is essential to conserving endangered animals, improving water quality, and managing productive fisheries,” said study oauthor Dr. Emma Rosi-Marshall, an aquatic ecologist at the Cary Institute of Ecosystem Studies.

“Glen Canyon Dam has transformed the ecology of the Colorado River,” said lead author Dr. Wyatt Cross, of Montana State University. “Immediately downstream, cold, low-sediment waters have favored exotic plants and animals that haven’t co-evolved with native species. We now see reduced biodiversity and novel species interactions that have led to the instability of these river food webs.”

Near Glen Canyon Dam, the researchers found food webs dominated by invasive New Zealand mud snails and non-native rainbow trout, with large mismatches in the food web and only a small percentage of available invertebrates eaten by fish. In contrast, downstream food webs had more native fish species, and fewer invertebrates that were more efficiently consumed by fish, including a federally-listed endangered species, the humpback chub.

In March of 2008, the Department of Interior conducted an experiment that simulated pre-dam flood conditions, providing an opportunity to see how high flows affected food webs with very different characteristics.

“Food web stability increased with distance from Glen Canyon Dam, with downstream sites near tributaries proving the most resistant. At these locations, the flood didn’t cause major changes in the structure of food webs or the productivity of species,” said Rosi-Marshall.

Near the dam, the story was quite different.

“These energy inefficient, simplified food webs experienced a major restructuring following the experimental flood,” said co-author Dr. Colden Baxter, an aquatic ecologist with Idaho State University. “New Zealand mudsnails were drastically reduced. And changes in algal communities led to a rise in midges and blackflies — favored foods of trout — resulting in a near tripling of non-native rainbow trout numbers,” he said.

More Colorado River Basin coverage here and here.

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