What is the zebra mussel doing to Lake Michigan?

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Dear Cecil: On a recent visit to Chicago, I noticed that Lake Michigan, close to shore, looks more like the waters of the Caribbean (crystal clear, light aqua color). When I grew up in the city years ago the lake was just a bucket of sludge. I’m told the change in clarity is due to zebra mussels flushed out of the bilges of tankers visiting from Asia. These mussels, I understand, consume the sludge and act like filters, creating the crystal clear waters we see today. Is this true, and will we soon see palm trees on Lake Shore Drive? Chris, New Hope, Pennsylvania


Illustration by Slug Signorino

Cecil replies:

Palm trees, no. There’s this thing in Chicago called winter that the zebra mussels haven’t been able to do much about. But you heard right — zebra mussels are filtering Lake Michigan’s once-turbid water. The accompanying color change is due to an increase in Cladophora, a type of green algae that thrives on the sunlight that now penetrates farther into the lake’s depths. Before you start planning any scenic snorkeling trips, though, let me tell you the bad news about what’s happening to the lakes.

The zebra mussel is a freshwater bivalve that originated in Russia (it was reported in the Caspian Sea in 1769) and spread through much of northern Europe by the 20th century. The mussels were first spotted in the United States in 1988, allegedly having made their way here in the ballast water of one or more transoceanic ships. When such water was mistakenly dumped into Lake St. Clair (between lakes Erie and Huron), the aquatic life within — including, apparently, zebra mussels — was released to the environment. Since then the mussels have spread throughout the Great Lakes, much of the Mississippi River system, and even to freshwater ponds and rock quarries.

While zebra mussels arguably have made the lakes prettier, in other respects they’re a pest. A female can lay more than 100,000 eggs in a year; having few natural predators, the mussels accumulate quickly, reaching densities in the tens of thousands per square meter. With their small size (two inches tops) and tenacious grip they can easily foul equipment on ships, at water treatment facilities, etc — an engineer I know reports seeing them piled up six feet thick in a power plant water intake. Though not ranking with the giant squid as one of the terrors of the deep, zebra mussels lean on other aquatic wildlife pretty hard, too. Clams and crustaceans can end up with so many mussels attached that they’re essentially smothered under the weight.

A zebra mussel is a living water filter, capable of processing about a quart per day. Levels of diatoms — certain single-celled alga — decreased between 82 and 91 percent in Lake Erie after zebra mussels were introduced; the Hudson River saw an 85 percent decrease in plankton after a similar invasion. Lake Michigan, which was formerly cloudy, has been cleared up noticeably by the hardworking little mollusks.

Cleaning up the lakes doesn’t sound like a bad thing, but it’s no unalloyed boon. By removing all that plankton, etc, from the water, zebra mussels steal food from native aquatic life, such as other invertebrates and fish. Then there’s the cladophora. Always present in Lake Michigan and other Great Lakes, it was a problem years ago when farms and industry dumped high levels of phosphorus into the lake, leading to algae “blooms.” Thanks to water-quality regulations, cladophora for a time was fairly well controlled. Then the zebra mussels showed up and began thinning out the lakes’ protective murk. The algae went nuts. It’s been shown that zebra mussel feces are a good food source for algae, making the problem worse.

A little algae may improve the lake’s aesthetics, but a lot doesn’t. Cladophora blooms result in large mats of the stuff washing up onto the beaches. It’s not toxic to humans, but too much looks and smells gross. Worse, seagulls, ducks, and other birds feed on zebra mussels and such hiding in the clumps, resulting in higher levels of bird feces and E. coli and forcing more frequent beach closures. Desperate officials in Chicago this summer were reduced to signing up border collies to patrol city beaches and chase the birds away.

The long-term environmental impact of zebra mussels is uncertain, but since the chances of losing them are slim, we’ve little choice but to adapt. In that vein, a question I’ve been asked lately is whether zebra mussels are edible. The answer is a guarded yes. They’re small and a bit tough, and even devotees of Ohio River delicacies usually won’t eat them. After a lifetime of filtration, the mussels accumulate a lot of nasty stuff such as pesticides and heavy metals. Eh, you’re thinking, maybe I’ll stick to the catfish. OK, but skip any from the Kalamazoo River, which enters Lake Michigan’s crystal clear waters near the resort town of Saugatuck. The EPA says they’re toxic due to pollution with PCBs.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.