‘The zebra was in heaven’: How the Rideau was lost
They came. They didn’t see much — mussels have no eyes. But they sure conquered.
Zebra mussels overran the Rideau River during the last three decades, and now a study from the Canadian Museum of Nature says they lucked into a place with a full buffet, perfect water and no enemies.
Then they reproduced like mad: la revanche du berceau all over again.
André Martel saw the first arrivals in our region in the fall of 1990. He had been a PhD student when the mussels invaded Lake Erie in the late 1980s, spilled from the ballast of a ship from the Black Sea area, where they are native.
When it reached the Rideau, “The zebra was in heaven,” said Martel, a biologist at the Canadian Museum of Nature.
In 1990, “I knew this was going to unfold in various places, so I started to search, and the Rideau River was next to the museum, and I searched, and it was right there.”
Since then he has monitored their explosive domination of the river, producing 26 years of data published Wednesday.
“I was expecting it (the fast spread), knowing that the river has the right kind of chemistry, geology, limestone bedrock all across.
“That means calcium carbonate is available,” and calcium from limestone is what mussels use to build shells, he said.
“Things were in place, an ideal spot for the zebras.”
The little mussels have almost no natural enemies here. At home in Eurasia, there are aquatic animals that eat them. Here, Martel said, the odd fish takes a nibble, as do diving ducks. “But it’s like licking a large wedding cake,” he said. “You won’t eat it that way.”
By 1995, mussel shells were forming a crust a couple of centimetres thick in parts of the Rideau — hundreds of thousands of tiny mussels per square metre of rock.
But not all parts of the Rideau system are equally vulnerable. The downstream part of the river (north of Manotick) has the greatest number of mussels.
“Upstream it’s not as bad, although you see a lot of ecological impact,” Martel said.
Fast currents and rapids are harder for the mussel to colonize.
Meanwhile there is a second invasive mussel, nicknamed the quagga because of its stripes, which has also come from Eurasia and is out-competing the zebra in parts of the Great Lakes and St. Lawrence River. Somehow, the quagga has yet to muscle its way into the Rideau.
People ask Martel: “But at least the zebra will eat up all that dangerous blue-green algae, right?” No.
“They’re clever beasts,” Martel said. “They like diatoms (plankton), they like the good algae (the regular green stuff.)
“For the blue-green algae — it doesn’t like it, so it doesn’t really eat the thing.”
By eating up harmless green algae “on a number of occasions, they are helping blue-green algae to explode.”
The study is published in the Canadian Journal of Zoology.
Martel’s biggest surprise: “How one species can be so successful filling in an ecological niche that was free.” The zebra’s larvae swim with the current and spread everywhere, faster than native clams can reproduce, covering and killing the native species.
But there’s one bit of good news: There are still pockets of resistance where native clams still dominate — deep areas where they can burrow into the sand and smother zebras that attach to them.
Martel said this is “very promising. There are positives out there. The river is not dead.”