Lakes may not sound like particularly complicated places, but or quagga mussels, the complexity of Lake Powell has dictated the magnitude of their impact on the ecosystem. This week, we speak with Nathan St. Andre, who recently completed his thesis in biology at Brigham Young University and has been studying the effects of the quagga mussel on fish populations in Lake Powell.
Science Moab: What are quagga mussels?
St. Andre: Quagga mussels look like a white-and-brown to black-striped mussel with these little threads that allow them to attach onto just about any hard surface, and even soft surfaces like mud. Most mussels in the United States don't have that ability.
Science Moab: How did they make their way to Lake Powell?
St. Andre: Quagga mussels were first detected in Lake Powell in 2012. A boater at one point had picked up water from an infected reservoir outside the state and had it either in the ballast at the bottom of the boat, or in the live wells where they keep fish. When they got to Lake Powell, somehow that contaminated water got dumped into the lake. That led to a full colonization of quagga mussels.
Science Moab: How do quagga mussels change lake environments?
St. Andre: Quagga mussels do filter feeds: they attach themselves to walls, and feed on microscopic phytoplankton, tiny microplankton, and zooplankton. Along the way, they defecate — like all animals, they have to poop. But unlike other animals, quagga mussels will get sediment and organic materials they can't eat. They spit that stuff out, which we call “pseudofeces.”
So quagga mussels tend to remove nutrients from the pelagic part of a lake, which is open water. Then they deposit the nutrients on the side of the lake. Animals that live along the shorelines suddenly have this boost of nutrients, and the animals living in the open lake suddenly lose all their food sources.
Science Moab: What did your research examine?
St. Andre: We were trying to figure out if quagga mussels were impacting Lake Powell. Lake Powell is super complicated; it's long, it's narrow. It has sections that are 700 feet deep, sections that are fairly shallow, side channels. So, do the fish respond the same way as at other lakes?
We investigated this question by looking at the lake’s food chain and the trophic structure, which refers to who eats whom, and how energy moves around. We use something called stable isotopes. If you know how to read stable isotopes, they essentially write the world. Different animals will have different isotope signatures based on where they are in the food web and where they eat. We used nitrogen and carbon isotope signatures from fish.
We use that data to build a picture. You can build a food web and ask how the food web is getting shuffled around, and start asking why.
Science Moab: What impacts did you find?
St. Andre: We found that fish in the southern part of the lake seem to be affected by quagga mussels. But fish in the northern part don't seem to be bothered much by the mussels. A handful of fish in the southern part of the lake decreased in nitrogen levels, so instead of being the top dog, they were slowly knocking their way down the food chain. The fish in the northern part of the lake either returned to their pre-quagga mussel invasion level or went up. They actually became the bigger, badder top dog of the lake.
Science Moab: Why that differentiation?
St. Andre: In the paper, we made a couple guesses. The best one is that the silt entering the northern part of Lake Powell essentially chokes out the quagga mussels. Some studies found that sediment-heavy water affects the way quagga mussels feed, because if they're trying to filter feed and they're catching more dirt than zooplankton, they're spending a lot of energy to get nothing out of it.
Essentially, the high sediment load that comes into Lake Powell chokes out the quagga mussels, and so they don't do nearly as well in the upper parts of the lake. So that's why we're seeing this divergence within the system where we see southern fish with decreasing trophic levels, while the northern fish seem to be either increasing or not changing at all.
Science Moab is a nonprofit dedicated to engaging community members and visitors with the science happening in Southeast Utah and the Colorado Plateau. To learn more and listen to the rest of Nathan St. Andre’s interview, visit www.sciencemoab.org/radio. This interview has been edited for clarity.