Is Lake Champlain Prepared for a Quagga Mussel Invasion?

Lake Champlain is a complex ecosystem with over 90 fish species and many other organisms that interact to form the Lake Champlain food web. Each predator and prey species has the potential to cause a ripple effect throughout the food web with far-reaching consequences, depending on their abundance and whether they are native or invasive.

Invasive species are typically non-native to an area and are able to outcompete or otherwise harm native species’ populations. Lake Champlain currently has 51 known aquatic invasive species which include fish, plants, and other organisms. While this may seem like a lot, the number of invasive species here is actually much fewer than the neighboring Great Lakes ecosystems. The Great Lakes are more connected to outside waterways and much larger than Lake Champlain. Therefore, invasive species prevention and management is more difficult, and species often enter the Great Lakes well before invading Lake Champlain.

In 1993, invasive zebra mussels were first documented in Lake Champlain—about five years after their introduction to the Great Lakes. Quagga mussels, a related species to the zebra mussel and native to the Dneiper River drainage of Ukraine, were introduced in the Great Lakes in 1989 but have yet to be discovered in Lake Champlain.

As quagga mussels loom in nearby waterways, their introduction to Lake Champlain is inevitable. Unlike their zebra mussel relatives, quagga mussels have potential to infiltrate all depths of the lake, posing a more serious threat to aquatic life. They are filter feeders, meaning they eat organic particles in the water, namely phytoplankton—the foundation of the aquatic food web.

“Quagga mussels will likely have the biggest impact on spring and deep water phytoplankton by reducing their biomass, as observed in the Great Lakes,” Ariana Chiapella, lead researcher and lecturer at the University of Vermont Rubenstein School, anticipates. “This has potential consequences for larger organisms in the lake because they rely on a food web supported by these primary producers. But because energy flows in the lake ecosystems are complex, predicting which species will be the most affected by the quagga mussel invasion is challenging.”

Multiple biological, physical, and chemical variables in the lake will impact how quagga mussels interact with the Lake Champlain food web. Researchers at the University of Vermont designed a study to determine the current state of the Lake Champlain food web and to understand how the introduction of quagga mussels might impact it.

The researchers concentrated on how the introduction of quagga mussels would impact low and middle trophic levels in the lake. Trophic levels describe how high or low a species sits within a food web. The lowest trophic levels consist of phytoplankton and other primary producers, then zooplankton and other small invertebrates (primary consumers). Middle trophic level species include organisms that eat a combination of producers, invertebrates, and other primary consumers. Mid trophic levels include planktivorous fish species like alewife and smelt—the fishes sampled in this study—and Mysis, a small, shrimp like crustacean that migrates between mid-depths and the bottom of the lake. The uppermost trophic levels are reserved for Lake Champlain’s top predators like lake trout.

As lake trout populations recover in the lake, the threat of quagga mussels to lower trophic level resources poses more complex problems leading to what scientists call a “trophic squeeze.” This is when there are pressures on both ends of the food web—from both predation and food availability—that impact middle trophic species. In this case, changes cannot be attributed to only one variable, and the interactions among organisms are harder to predict.

To explore this, the researchers sampled three sites in Lake Champlain: a 40-meter-deep site in the Main Lake, a 100-meter-deep site in the Main Lake, and a 40-meter-deep site in the Northeast Arm. At each site, researchers sampled the lower food web biweekly and the full food web monthly from May through November 2019. They collected samples of Mysis, zooplankton, and three fish species; alewife, rainbow smelt, and slimy sculpin.

After collecting samples, researchers separated fish by size: large, medium, and small. They analyzed 20 samples of each size per species. Once back to the lab, the team identified and weighed the contents of the fish stomachs and then analyzed the fishes, invertebrates, and primary producers for stable isotopes, which were used in something called an isotope mixing model. Isotope mixing models use these specific isotopes to estimate the importance of different food resources in an organism’s diet. In lakes, benthic resources – such as benthic invertebrates – live in the sediment at the bottom of the lake. Pelagic resources live in the open water and include phytoplankton, zooplankton, and some fish species.

Findings

Researchers found that most of the fish species in the study relied on zooplankton or a combination of zooplankton and Mysis, while the Slimy sculpin, a fish that prefers the rocky bottom of the lake, relied more on a combination of benthic resources and Mysis.

In this study, Mysis were abundant in the Main Lake and rare in the Northeast Arm. Their diets seemed to primarily consist of zooplankton and detritus. Phytoplankton had little contribution to their diet, even during the spring bloom when phytoplankton biomass is highest.

Zooplankton, on the other hand, relied heavily on the spring algae bloom early in the season, supplemented by suspended phytoplankton in the water column and the deep chlorophyll layer. The deep chlorophyll layer (DCL) contains the most concentrated phytoplankton population in the water column. It occurs just below the top layer of warm water in the summer because phytoplankton can still take advantage of light penetration from above, while accessing the higher nutrient concentrations that occur in the cold water. According to the model, the DCL was an essential resource for zooplankton during the late summer season,  contributing to 90 percent of their energy. In the Northeast Arm, zooplankton relied on a diet of detritus and phytoplankton.

Post-Invasion

From their findings, the researchers posited how susceptible the food web would be to a quagga mussel invasion. The Main Lake food web is likely more at risk to the impacts of quagga mussels than the Northeast Arm, as zooplankton depend more on the spring bloom and the deep chlorophyll layer­­—resources which will be the most threatened by quagga mussels. If zooplankton populations decline, their predators will lose a vital food source. Fishes in the Northeast Arm rely more on benthic resources which will likely be impacted less by quagga mussels, but this part of the lake gets too warm for many cold-water fishes in summer, so its ability to supplement diets of fish from the Main Lake is limited.

The fate of post-invasion food web stability may be in the hands of Mysis. They have a somewhat flexible diet, able to utilize both pelagic and benthic resources—but scientists don’t yet know the relative importance of each. In the Main Lake, Mysis are likely underutilizing their benthic resources. If Mysis can shift their diet in response to a quagga mussel invasion to consist of mostly benthic resources, they (and their predators) may be at less risk.

With that said, this shift could make Mysis’ quality as a prey resource decline for fish like rainbow smelt and spiny sculpin. Additionally, the decline in zooplankton populations will likely increase predation pressure on Mysis, thus decreasing their population. Considering Mysis are such a critical food source for the pelagic fishes in this study, the impacts quagga mussels have on the larger food web will depend on Mysis’ ability to shift their diet based on available resources.

“Luckily, researchers at the Rubenstein Ecosystem Science Lab are actively working to better understand the ecology of Mysis so we have a clearer idea of how this and other incoming invasions may change Lake Champlain’s food web” says Chiapella. “Knowing the potential threats prior to the arrival of invasive species allows for proactive lake management that minimizes socio-ecological impacts.”