“The lake definitely is feeling the impacts of a changing climate,” said Bonnie Ellis, a research professor working from the shores of Yellow Bay. That's where the University of Montana has for a century housed the Flathead Lake Biological Station, a scientific research base recognized worldwide for its work on freshwater lake and river systems.
If you walk out of Ellis' office, down through the forest of scattered larch and fir to water's edge, you'll discover a lake that, from the surface, looks much as it has for millennia.
Science calls it the thermocline, the place where temperature changes extremely fast, a full degree per meter or more.
That line separates the epilimnion - warmer waters up top - from the hypolimnion - colder waters beneath.
In winter months, Ellis said, when the lake is consistently cold top to bottom, the thermocline disappears and waters mix easily in one big brew. A relatively lazy breeze can churn the entire water column, blending bottom waters with surface waters.
And when water from the “light zone” flows down into the “dark zone,” it takes tiny plankton with it, away from the light that feeds them.
But in the heat of summer, Ellis said, the density difference between cold waters below and warm waters above creates a thermal barrier, and nothing blends. The lake stratifies into distinct and isolated layers.
“And what we've seen related to climate change is that this thermal stratification is starting earlier and lasting longer,” Ellis said.
Cold-water runoff is bleeding dry sooner each year, she said, and fall's cold rainstorms are arriving later. The season of heat is stretching, and so is the season of a layered lake, unable to mix itself top to bottom.
“And that changes things,” she said.
Plankton don't flow to the bottom anymore. Instead, they stay up top, in the sunlight zone, where they continue to grow.
Also, the zooplankton find in that warm upper layer a refuge from mysis shrimp, which usually gorge on zooplankton, but generally can't tolerate water warmer than about 60 degrees.
And so they not only grow bigger and faster, trapped as they are in that warm layer of light, but they also don't get eaten, escaping predation by sticking to warmer waters.
In the last few years, the size of this “thermal refuge” for zooplankton has grown a full 20 percent, as Flathead Lake's warm-water volume has increased along with temperatures.
“So they multiply,” Ellis said. “And they keep on growing.”
That, in turn, changes the size of phytoplankton in the water column, because different sizes of zooplankton eat different sizes of phytoplankton. And that, in turn, changes what sort of food is available to everyone else up the food chain, which ultimately changes the whole fishery.
Many native fish, including several species of trout, like their waters cool, Ellis said, which means that as the lake warms - and stays warm longer - available trout habitat shrinks.
Most trout don't do well in waters warmer than about 67 degrees. But shallow bays and Flathead Lake shoreline can hit 80 degrees during long stretches of heat, such as western Montana has experienced throughout the last month.
That pushes trout to places they normally wouldn't be, Ellis said, exposing them to both prey and predators they otherwise wouldn't encounter. The effects cascade through the food chain.
Water quality, too, is affected by all these changes, Ellis said, as all those various plankton explode in the upper water column and turn it green with growth.
The plankton biomass skyrockets, algae blooms and the lake gets, well, green and slimy. Then all that biomass begins to die, rains down through the water column to the bottom, and forms the substrate for bacterial growth.
That bacteria then blossoms, sucking up all the oxygen in the depths. Without new water mixing from the surface, the bottom of Flathead Lake begins to suffocate.
“It all starts with the higher temperatures,” Ellis said, “and then the impacts ripple out.”
In a lake the size of Flathead, tracking temperatures is tricky business. Trouble is, a good wind can stir things up and cause dramatic water temperature changes in no time at all.
In four short hours on a spring day in June 2004, lake temperature dropped from 50 degrees to 40 beneath a stiff breeze. Throughout the year, the changes are even more remarkable - 34 degrees in mid-winter, 50 by spring, 80 at summer's peak, down to the mid-50s by Halloween.
So rather than try to consistently plot temperature in lots of places over lots of time, one way to track temperature trends is to monitor how soon the lake stratifies in the summer, and how late the layers break down in the fall. The longer the season, the higher the overall temperatures.
That's exactly what Ellis has been doing, and the results are both compelling and obvious. The trend, she said, is toward a much longer layering season, which means more time for algae to grow, more time for food web disruptions, more stress on fish, and much less oxygen available on the bottom.
“All of which is driven first and foremost by temperature,” Ellis said.
That certainly jibes with data collected by the Flathead Basin Commission, a multi-agency organization charged with monitoring and safeguarding water quality throughout the Flathead.
“What's happening this summer is absolutely unprecedented,” FBC spokesman Mark Holsten said. “Waters throughout the region are hotter than ever.”
Thousands of fish are dying in shallow lakes, he said, especially places such as Rogers Lake and Ashley Lake. But those lakes don't experience the stratification common on Flathead Lake. Instead, they get hot from top to bottom, and fish can find no refuge in deeper waters.
The heat, as in Flathead Lake, increases the ability of the waters to produce and support algae, which turns shallow waters green and leads, eventually, to oxygen loss from the bottom up.
Not surprisingly, the highest water temperatures recorded by the Flathead Basin Commission occurred in the hottest summers - fiery ones such as 1994, 1998, 2000, 2003 and 2007. All of the hottest years on record have come in the past decade, and so all of the hottest water years have been recent as well.
The difference is dramatic. Brian Thornton, who works with Holsten at FBC, notes that in “normal” years Flathead Lake waters peak at perhaps 68 degrees, a comfortable zone for the life evolved to live there. But in the hottest summers the temperature has pushed 80 degrees, resulting in abrupt habitat changes.
“The trend lines show it heating up over time,” Thornton said. “From our data, it looks like the lake is warming up. Over time, it's definitely showing an increase.”
Which, of course, is exactly what Ellis inferred from her observations that stratification is starting earlier and ending later.
“This is the trend,” she said. “And we need to understand that if we're going to understand how a change in climate might cause changes in Flathead Lake, if we're going to continue this cycle of hotter summers, then we can expect a very different kind of lake for future generations.”
Reach reporter Michael Jamison at 1-800-366-7186 or by e-mail at mjamison@missoulian.com
Flathead Lake water quality targets missed
KALISPELL - There are targets, invisible and obscure and sometimes seemingly arcane, floating out there in Flathead Lake.
Trouble is, we keep missing them.
“It's not great news,” said Bonnie Ellis, a research professor at the University of Montana's Flathead Lake Biological Station. “We need to hit those targets.”
The targets are called TMDLs, which is short for total maximum daily load, which is pretty much what it says it is - the total amount of a pollutant the lake can take in without exceeding water quality standards.
Ellis talks a lot about TMDLs in her annual “State of the Lake” report, which she delivered last week.
The TMDLs, Ellis said, were crafted by the multi-agency Flathead Basin Commission, and are targets aimed at protecting - perhaps even enhancing - Flathead Lake water quality.
One measure is carbon, which contributes to the lake's “primary productivity,” or its ability to produce algae. The target is not to exceed 80 grams of carbon per square meter of water per year.
“But we're 24 percent higher than that target,” Ellis said, “which is not good.”
A second measure is dissolved oxygen, levels of which should not be allowed to decrease on the lake's bottom.
“But that is also not very good news, I'm afraid,” Ellis said. “There is a significant decrease in the dissolved oxygen level at the bottom of Flathead Lake.”
Algae grows at the top of the lake, dies and drifts to the bottom, where bacteria eat it and blossom and then eat up all the oxygen. The waters, which should not dip below 90 percent oxygen saturation, were as low as 50 percent in some places, Ellis said.
Then there's the matter of periphyton, the algae that grows green on rocks around the shoreline. The target is to see no more of that than was measured back in 1987.
But Ellis' State of the Lake report showed periphyton growth “is significantly higher than what was measured in 1987. In fact, every year we've measured it, the value has been higher than the TMDL target.”
It is not, however, all bad news on the Flathead. A fourth target is to not see blooms of anabaena flos-aquae or other pollution-caused algae, and in fact, there hasn't been a significant bloom this century, with the exception of one minor patch in 2006. That's after four substantial blooms in the 1990s, “and that's very good,” Ellis said.
And the last target, chlorophyll A, which shouldn't exceed one microgram per liter of lake water, is looking good too. It means the total biomass of algae is not too high.
Ellis figures zooplankton are eating the phytoplankton, keeping the biomass in check. How long they can hold down the fort, however, remains unknown, and is a point of concern.
Algae growth is driven by lots of factors, Ellis said, including heat and light. As summer temperatures climb and warm waters layer atop colder waters, no longer mixing surface to bottom, algae blooms become more likely.
The problem is complicated by nutrients such as nitrogen, which is essentially fertilizer for algae. And nitrogen levels, Ellis said, are increasing steadily in Flathead Lake, pouring in from a confounding brew of sources: car exhaust, fertilizers, upper atmospheric pollutants, septic tanks, residential development.
She's studied the remote headwaters, where nitrogen levels are low, and has studied the urbanized bottomlands, where nitrogen is high, “and we know the increase is dramatic in the human-dominated portion of the watershed.”
But another algae food, phosphorous, has been on the decline since the region banned phosphorous-containing detergents and upgraded sewage treatment plants.
“We've done very well with phosphorous,” Ellis said. “Now we have to turn our attention. Nitrogen is really the next big thing to tackle if we want to keep algae production low in Flathead Lake.”
Ellis concluded her 2007 State of the Lake by noting that “of the 189 largest freshwater lakes of the world, Flathead Lake is one of the cleanest, but there are several water quality concerns that warrant continued monitoring.”
Including a few that aren't even here yet.
“We really need to be aware of the non-native mussels invading the West,” she said.
The zebra mussel, for instance, was long an East Coast problem, but now has colonized waterways in the neighboring Dakotas, and could arrive here any day.
“And that,” Ellis said, “has the potential to alter the lake far more than introduced fish or introduced mysis shrimp or any other non-native invasion.”
A single zebra mussel can filter a liter of lake water every day, she said, stripping out all the suspended particles. It then discharges fecal nutrient, which falls to the bottom and feeds algae growth there.
As the water column is scoured, she said, the light penetrates deeper, and the bottom algae blossom.
“What you end up with,” she said, “is a forest on the bottom and no food in the water up top. And if that doesn't scare you, what should concern you is the young fish need those zooplankton in the water as a food source.”
So anglers beware, and watch out for zebras.
The mussels work like mini-filters, she said, like kidneys for the lake, and over time pollutants and toxins build up in their tissue. Any fish that eat them take up those toxins, making the fish potentially dangerous for humans to eat.
“Personally, I wouldn't eat a mussel,” she said. “They're little time bombs of pollution.”
The good news is, zebra mussels generally like warmer waters than those found in Flathead Lake.
The bad news is, Flathead Lake is getting warmer, and staying warmer longer.
“And the really bad news is the quagga mussel,” Ellis said. It's a cousin of the zebra, and has now made its way West, too.
“The scary thing about the quagga,” she said, “is it likes cold water. Just like the water in Flathead Lake.”
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