Photo of Yellow Bay on Flathead Lake by Jack Stanford
Study reveals fundamental changes to Flathead Lake food web
In a protected cove on Flathead Lake’s east shore, UM operates the oldest active biological station in the United States.
Only six years younger than the 118-year-old University itself, Flathead Lake Biological Station has collected data about the lake since 1899. The state of Montana was just 10 years old when Morton J. Elrod, UM’s first biology professor, established the first station near Bigfork and began recording information about the lake and its fishery.
More than a century later at Yellow Bay, where the station moved in 1908, scientists such as Bonnie Ellis continue and expand on Elrod’s work. They use those archives started by Elrod to produce important research.
The National Academy of Sciences earlier this year published an article by Ellis and others demonstrating how the introduction or invasion of non- native organisms, even in a lake as large as Flathead, can lead to significant – and sometimes surprisingly rapid – changes in an aquatic ecosystem.
“This archive, which has been very detailed since the late 1970s, shows how invaders have altered the lake’s food web and how important nutrients are to the food web,” says Ellis, a research assistant professor in limnology at UM. “We didn’t think to go back more than 100 years until we had first looked at the past 30.”
That led to an article titled “Long-term effects of a trophic cascade in a large lake ecosystem” written by Ellis, biological station Director Jack Stanford and more than a half-dozen other scientists and researchers.
Their findings also were featured in Nature, an international weekly journal of science, earlier this year.
A trophic cascade, as Ellis describes it, is “the theory that when you alter predator and/or prey in an ecosystem, you can alter the abundance, biomass or productivity of a population, community or trophic level across more than one link in that food web.”
Enter opossum shrimp – Mysis diluviana – into Flathead Lake.
UM scientists have long led the way in documenting the effects of the establishment of the mysid shrimp in Flathead Lake in the 1980s. It brought about the collapse of the lake’s kokanee salmon fishery, but the exact mechanisms for the demise of the kokanee were not well understood.
There’s an interesting back story to all that, but a more important breaking story up front. Because what this new research suggests is that the introduction of shrimp also can be tied to the explosion in the non-native lake trout population and indirect effects that cascaded through the food web.
For their article Ellis and her nine co-authors – including biological station research scientist James A. Craft – divided the lake’s history into four distinct periods.
“Put it all together and you get the entire picture of how the food web in the lake has changed,” Ellis says.
The first, the native period, existed before 1920, when only 10 species of native fish are known to have lived in Flathead despite the introduction of 14 non-native fish from 1890 to 1920. Then the kokanee period ran from 1920 to 1984. Early in that period anglers began to report the non-native lake whitefish and kokanee, and by 1940 kokanee replaced cutthroat trout as the dominant catch of anglers. Toward the end of this period, native cutthroat remained at low densities, non-native lake whitefish continued to expand and non-native lake trout remained at low densities.
Interestingly, the next period lasted but four years.
“The population of opossum shrimp in the lake exploded from 1985 to 1988,” Ellis says. “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 crashed, bald eagles that concentrated in large numbers in Glacier National Park to feed on the spawning salmon dispersed to other regions where prey was more abundant.”
This also is the time when primary productivity – the production of organic compounds via photosynthesis – shot up by 21 percent. While the opossum shrimp numbers quickly dropped to about a third of what they were at their peak in 1986, primary productivity has not gone down.
The time from 1989 to the present has been labeled the “mysid-lake trout period.” It’s two decades where today’s latest Flathead ecosystem has settled somewhat into its routines.
For all intents and purposes, Ellis says, we’ve had “two different lakes and two different food webs – before Mysis and after Mysis.”
Ellis witnessed the changes that can come quickly in a body of water as a youngster. Growing up in Orange, Texas, across the river from Louisiana, she and her brothers would bait the string on their bamboo poles with bacon and fish the local bayous for blue crab.
“We lost our favorite site to discharge from a pulp mill,” she says.
But that loss sparked her interests, and Ellis came to UM and the biological station in 1977 to study the limnology of Flathead Lake. She never left and eventually earned her doctorate.
So she was there when the opossum shrimp planted by Montana Fish, Wildlife and Parks in lakes to the north – ironically, in an effort to increase kokanee populations – made their way into Flathead.
“They came from Waterton Lake, where they’re native,” Ellis says, “and they were put in Kootenay Lake to try and increase the size and numbers of rainbow trout.”
Instead, in Kootenay the shrimp increased the size and numbers of the kokanee salmon.
“Fish managers figured, ‘Great, we’ll add it to a lot of lakes to stimulate kokanee populations,’” Ellis says.
But Kootenay was unique, she says. The contours of the bottom of the lake and its currents pulled the Mysis shrimp off the bottom and into shallow bays where the kokanee could feed on them during the day.
In other lakes – and most certainly in Flathead – the shrimp could stay on the bottom of the lake away from the sight-feeding kokanee during the day and come up at night to feed. Because both Mysis and kokanee prefer the same zooplankton, it was believed early on that competition for the same prey caused the decline in kokanee. But the deep-water lake trout, which had been introduced 80 years earlier but had never gotten much of a foothold in Flathead, suddenly had an abundant new food source, Mysis, on the lake bottoms where little previously was available. Lake trout flourished, and recent research shows the voracious fish decimated the kokanee fishery and concerns grow that native fishes may be in peril. The change in the fishery promoted a rapid shift in community structure, resulting in a trophic cascade affecting bald eagles, fish, zooplankton and algae.
“Understanding trophic cascades requires that long-term data sets be formalized by robust models because of the extreme complexity of interactions,” Ellis and her co-authors wrote in their article. “One important challenge is to determine the tipping point for what might be the next ecosystem state as the community continues on its internally driven dynamics, and as external drivers such as climate change and direct human intervention (a lake trout reduction program is under way, for example) further force the system.”
The findings by Ellis and her counterparts are very important to Flathead Lake but have serious implications for other bodies of water around the globe as well.
And the science? It’s backed by more than a century of data collected by researchers at UM’s Flathead Lake Biological Station.
— By Vince Devlin
Scientist Bonnie Ellis, shown here in the Freshwater Research Lab at the Flathead Lake Biological Station, has unveiled important research about the lake’s changing food web. Research scientist Tyler Tappenbeck can be seen in the background studying lake zooplankton samples.
An unintended consequence: The planned introduction of opossum shrimp caused an explosion of lake trout numbers during the 1980s.
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