An iconic Wisconsin lake goes murky, and a tiny invader is at fault

A tiny, plankton-eating critter of concern in many Minnesota lakes – including Mille Lacs, where it is typically listed among the many sources of trouble for walleye and the anglers who prize them – is the subject of a fascinating new research paper from the University of Wisconsin.

Published earlier this week by the National Academy of Sciences, the paper finds that in the first five years after they were discovered in Madison’s prized Lake Mendota, spiny water fleas of the species Bythotrephes longimanus managed to reduce water clarity by nearly 3 feet.

Converting that .9 meter change to social math, the paper’s lead author, Jake Walsh, explained to the Washington Post that folks who used to be able to see their toes as they waded in a waist-high portion of Mendota’s waters can now only see about as far as mid-thigh.

The lake’s growing murkiness has been a matter of considerable concern to those who use and love it, but at the moment there appears to be little that could be done to reverse the changes. Here’s how they happened:

Back in the 1980s, probably, the spiny water flea arrived in the Great Lakes in ballast water that large ships took on in Europe – Russia is the pest’s native homeland – then carried across the Atlantic Ocean, down the St. Lawrence Seaway, and through one or more of the lakes until the crew decided to dump it.

Although the environmental risks of dumping untreated ballast water were well understood, shipping companies refused to treat their water voluntarily and fought mightily against efforts to force treatment because, of course, it would cost some money.

(There were other arguments, too, including a spirited if dubious case for impracticality. A dozen years ago I toured the deep, dank and dark ballast holds of a Great Lakes freighter, thinking all the while about recent episodes of “The X Files,” with an industry rep who hoped to impress upon a gaggle of journalists that such portions of a vessel’s anatomy simply couldn’t be cleaned. Which, of course, wasn’t really the issue at hand.)

In 2012, not quite 25 years after zebra mussels were first detected in the ballast of an oceangoing freighter in Lake St. Clair, and six years after Congress gave it the authority to do so, the Coast Guard began to regulate ballast water discharges, followed in turn by the U.S. Environmental Protection Agency.

Ballast, then bass boats

But by then perhaps 180 noxious invading species had found new homes in the Great Lakes – and soon afterward, thanks chiefly to recreational boaters, in inland lakes like Mendota. A look at the Minnesota Department of Natural Resources listing of waters where aquatic invaders are a concern shows that Bythotrephes longimanus has been found in more than 60 lakes and streams.

When they were found in Lake Mendota in 2009, their concentrations had already reached some of the highest levels ever found in open water. This set off alarm bells because, as the paper notes, the flea is a “voracious” consumer of the tiny plants and animals known as plankton, which occupy a critical tier in the food web of lakes.

Indeed, the paper states, Bythotrephes “has the capacity to consume more zoo plankton than fish and other invertebrate plantivores combined.” And one of its favorites is a type of Daphnia – you recall these ubiquitous little beauties from high school or college biology, perhaps – which in turn feeds voraciously on the algae whose rising and falling populations have much to do with whether lakes like Mendota get murky or stay clear.

Because Lake Mendota made the wise decision to locate next to a major research university, far more its known about its health and inner workings than even, say, Mille Lacs.

So the researchers can say with confidence that the lake’s plummeting population of Daphnia pulicaria – identified as “the focal point of Lake Mendota’s food web management” well before spiny water flea made the scene – is clearly a consequence of its being the preferred prey of Bythotrephes.

Meanwhile, another form of Daphnia – D. Mendotae – is undergoing something of a population surge as its competitor’s declines. But it’s not as efficient at eating algae, and so the murkiness continues.

So what’s a lake-loving community to do?

Willingness to pay

The good news, I guess, is that other research has established that Madison households are willing to pay $640 apiece, on average, to restore Lake Mendota’s water clarity. The researchers figure that theoretically could yield $140 million for a cleanup plan, and somewhat voluntarily.

Unfortunately, there’s no good way to get the water fleas back out of Mendota without poisoning the whole lake, which would wipe out lots of beneficial organisms in the process.

Perhaps the most plausible alternative would be to deprive the algae of a principal nutrient – phosphorus – while leaving their primary predator in place.

The current sources of phosphorus inputs are well understood – most of it arrives in runoff from agricultural sources – and so the 71 percent reduction the paper calculates as a plausible necessity to re-establish pre-Bythotrephes clarity can also be priced. Depending on certain variables, the cost would come to somewhere between $80 million and $163.

These are of course ballpark numbers, and the paper’s goal wasn’t to reach a set of policy options but rather to use this remarkably clear example of invasives-driven environmental degradation to get at the matter of how best to monetize such harm – an undertaking that remains even murkier than Lake Mendota.

As Walsh said in a UW News Bureau piece announcing the study, the cost to fix Lake Mendota “is a big number, but it gives us a clearer understanding of the ‘true cost’ of invasive species.”

While only a fraction of the 180 species that have entered the Great Lakes have become nuisance enough to be labeled “invasive,” their collective impact is staggering, says Walsh. “There are hundreds of billions of dollars’ worth of damages we can account for right now. If you add in invasive species’ impact on ecosystem services and look at secondary invasions, then that number is likely to be trillions,” he says.

In spite of the staggering sticker shock, Walsh sees a silver lining. Many invasive species eradication or control efforts have been deemed too expensive to entertain researching or implementing them. But, Walsh says, if the price of not fighting these invasions is as high as his study indicates, “maybe we have a much bigger budget than we thought we did.”

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The full paper, “Invasive species triggers a massive loss of ecosystem services through a trophic cascade,” can be read here, but access isn’t free unless you’re a subscriber to the Proceedings of the National Academy of Sciences.