Recent research at The University of Montana’s Flathead Lake Biological Station has provided important data about how introductions or invasions of nonnative organisms can lead to major changes in the structure of aquatic ecosystems.

UM Assistant Research Professor Bonnie Ellis and Jack Stanford, director of the station, were among a team of scientists from around the Pacific Northwest who studied how the organisms can affect biological diversity of Flathead Lake, according to a news release from the University of Montana.

The research was published in the Proceedings of the National Academy of Sciences, and the scientists’ study, “Long-term Effects of a Trophic Cascade in a Large Lake Ecosystem,” will be highlighted in an upcoming issue of Nature.

The researchers looked at a 120-year record of the food web structure and dynamics of the lake, the largest freshwater lake in the western United States.

They examined the mechanism of a trophic cascade, which occurs when reciprocal effects of predators on prey alter the abundance, biomass or productivity of a population, community or trophic level across more than one link in the food web.

The data reveal four distinguishable periods: the pre-1920 native period, when native species dominated the fish community (although numerous nonnative fishes had been introduced); the kokanee period from 1920 to 1984, when many nonnative fish species first appeared and nonnative kokanee expanded to a large population size, replacing cutthroat trout as the predominant angler catch; the period from 1985 to 1988, when the population of introduced opossum shrimp, Mysis diluviana, grew rapidly and then declined to less than half the peak density while the kokanee population crashed; and the lake trout period from 1988 to today, when a new community dominated by the roles of shrimp and lake trout seem to have stabilized.

In particular, the team looked at the invasion of the opossum shrimp during the 1980s.

“The population of opossum shrimp in the lake exploded from 1985 to 1988,” Ellis said. “During that time, the population of kokanee in the lake fell and never recovered, bull trout declined and lake trout came to be the dominant top predator. At the same time as the kokanee decline, bald eagle numbers dropped with the collapse of their primary prey.

“But the most important and unexpected finding is that the rate of primary productivity (growth of algae) increased suddenly by 21 percent, exactly coherent with the peak mysid numbers in 1986, and has not decreased since then. Basically, the mysid invasion changed the entire food web and in that way altered the water quality in Flathead Lake by increasing algae growth.”

The study also noted the loss of the kokanee salmon.

Anglers reported kokanee (landlocked sockeye salmon) in Flathead Lake as early as the 1920s. The Flathead Lake stock came from the hatchery at Bonneville, Ore. The kokanee began spawning very successfully in two groundwater upwelling zones on the lake shoreline.

By 1940 kokanee replaced cutthroat trout as the dominant catch of anglers.

From 1979 to 1983, the kokanee population was estimated at 1.6 million to 2.3 million. From 1980 to 1985, high congregations of bald eagles gathered to feed on the kokanee spawning run at the McDonald Creek spawning site in Glacier National Park.

“With the invasion of the opossum shrimp, lake trout that had been introduced 80 years earlier but remained at low densities flourished,” Stanford said. “The shrimp provided a deep-water source of food where little had been available previously. Lake trout, who fed on the shrimp, now dominate the lake fishery, at the expense of the native fishes.”

In addition to the loss of the kokanee population in the lake, native bull and westslope cutthroat trout also are imperiled, he said.

The research shows that recovery of bull and cutthroat trout will be difficult given strong food web control by the expansive lake trout population.

“An important challenge now is to determine the tipping point for what might be the next ecosystem state as the community continues its internally driven dynamics – and how external drivers such as climate change and direct human intervention, such as introduction of yet another exotic species or manipulation of the lake trout population by netting in an attempt to protect the remaining bull trout, affect those dynamics,” Ellis said.