Missoulian  (Missoula, Montana, U.S.)
Sunday, August 05 2007

Posted on Aug. 4 (296)
Firefighters chart plan, adjust highway closures for wildfire
threatening Seeley, Placid lakes

Posted on Aug. 4 (105)
Heat wave making life tough for Montana trout (103)
Winds fan flames; Rock Creek evacuations ordered (101)

Posted on August 5 (237)
In hot water: Flathead Lake feeling the effects of scorching summers (181)
Governor flies to Seeley Lake fire as area residents are evacuated

In hot water: Flathead Lake feeling the effects of scorching summers
By MICHAEL JAMISON of the Missoulian

KALISPELL – Look closely into the clean, clear waters of Flathead
Lake and you can actually see the summer heat. It’s green and it’s
growing, and it’s not necessarily good news.

“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.
*
But if you keep walking, submerging beneath the waves and continuing
along the sloping bottom, you’ll soon cross a very important line,
where your toes are cold but your head remains in warmer waters.

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.”

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