Zebra mussels could be poised to crash on Lake Minnetonka.

Well, the part of Minnetonka where the invasive mussels exploded most rapidly.

This is important, because zebra mussels are one of those feared “aquatic invasive species” that don’t come from these parts. They get into our waters and smother native mussels. They make the water clearer by eating tons of algae. It’s feared they’ll mess up entire lakes, from plankton to fish.

A crash wouldn’t be the first time. Earlier this month, when the ice went out on Pelican Lake in Otter Tail County, residents were surprised to find thick ribbons of dead zebes piled up along portions of the shore like shell beaches on the Florida coast.

Why’d they die? Probably ate up all their food.

Turns out zebra mussels, which have infested at least 112 lakes, rivers and wetlands in Minnesota, don’t just take over a lake bed and own it forever.

In some lakes, they spike and, apparently, crash. In others, they’re thriving unabated. And some lakes, they just don’t dig at all.

Lake Minnetonka is all three in one.

On Thursday, scientists with the Minnehaha Creek Watershed District, which monitors Minnetonka, unloaded what they’ve learned since zebes were discovered in the lake in 2010.

There’s a lot of interesting stuff here, and it’s new knowledge.

Remember that Minnetonka is a flowage.

“Minnetonka is a series of interconnected bays, almost mini lakes,” said Eric Fieldseth, aquatic invasive species program manager for the watershed district. “And we have these three different water quality types. And how (zebes) behave in these different water quality types is different.”

Zebra mussels don’t like green murky water. The western reaches of Minnetonka, such as Halsted Bay, are eutrophic. Think greenish water, human chemical runoff, blue-green algae. Zebra mussels have established themselves here but haven’t expanded much since 2011. Wrong kind of food, Fieldseth said. And since their arrival, water clarity hasn’t changed much.

Zebra mussels love clear blue waters — perhaps too much. The eastern portions of Minnetonka, such as Browns Bay and Wayzata Bay, are mesotrophic — clean, clear water with sand or gravel bottom. Just like Pelican Lake in Otter Tail County. The algae there is the perfect food for zebra mussels, as measured by chlorophyll-a. Zebra mussel densities in mesotrophic Minnetonka exploded from 2012 to 2013, increasing more than tenfold. They held at those high densities — roughly 77,000 mussels per square meter (not a typo) — for a year. They ate so much food that water clarity increased substantially — by as much as 5 feet — and chlorophyll-a and phosphorus levels fell. Then there wasn’t enough food. Now, zebra mussel densities have fallen to less than 50,000 per square meter. Will there be a die-off in these waters, like in Pelican?

“It’s trending that way,” Fieldseth said.

Zebra mussels might be Goldilocks — just right — in moderately clear waters. The central portions of Lake Minnetonka, such as Smithtown Bay and waters around Spray Island — are between the eutrophic and mesotrophic stages. Water quality and clarity are tempered by a steady influx of nutrients from the western bays. The zebra mussel infestation has been more gradual but steady. Water clarity has increased, but green algae — zebe food — hasn’t fallen significantly. Today densities in these waters are believed to be nearly 70,000 mussels per square meter.

“The population is still climbing, and I expect we’ll see it surpass all other densities in the lake,” Fieldsmith said.

In case you missed it, there’s some really important nuance in all that related to water clarity and water quality. Zebes don’t do well in dirty waters, and they can’t clean them up. So that idea that maybe zebra mussels could help clean up pea-soup waters that could use some filtering? Don’t bet on it. They might remain just a background species in such lakes.

On the flip side, zebes explode in waters that are already clear, but they filter so much algae they make them too sterile for themselves — at least for a while. So if zebes have invaded a lake that you already think is too clear to fish in the daytime, get ready for tougher conditions. Until they crash. No one is sure what will happen after such a crash, like whether the water clarity will decrease. Scientists monitoring Pelican don’t think the invasive mussels will disappear.

“We haven’t seen a die-off like this before, so we’re not sure, just speculating,” said Moriya Rufer, the manager of Pelican Lake for the Pelican Group of Lakes Improvement District, a government agency similar to a watershed district. “We might see less of them this year, but we assume they’re still there. We suspect that the population might just go up and down in the future.”

What does all this mean for the overall health of a lake? No one’s sure. It’s important to understand that when it comes to weeds, fish and other life in a lake, scientists can’t say for certain how they’re affected by zebra mussels. “Too many variables for fish,” said Fieldsmith. “Plus, they live longer, so it takes more time for the impacts to show up.”

Zebra mussels live three to five years, so researchers are just beginning to see full generations come and go from infested waters. Fieldsmith and his staff plan to publish their findings in a scientific journal soon.

There’s one impact zebra mussels have had on Minnetonka that Fieldsmith is pretty confident about: They’ve ravaged native mussels.

“It’s tough to find any live native mussels,” he said. “They’re either covered and smothered with zebra mussels or they’re open shells and dead.”