On a mild September morning on the aft deck of the research vessel Blue Heron, Donn Branstrator sniffed the contents of a sampling bottle that some graduate students had just hauled up from the depths of Lake Superior. “That’s why fish smell the way they do,” said Branstrator, an ecologist at the University of Minnesota in Duluth. The fishy odor came from planktonic crustaceans just a few millimeters long, a primary food source for all the fish in the lake.
The most important are Daphnia—a genus that comprises some 100 freshwater species. These tiny animals are critical to lake health: Besides providing food for fish, they graze on floating algae, beating their legs constantly to create microcurrents that pull the algae toward their miniature maws. By keeping algae in check, Daphnia help keep the system in balance.
But Daphnia are declining in Lake Superior and nearly every other body of water in the Great Lakes region. Their numbers have been decimated by a fierce invasive predator, the spiny water flea.
Bythotrephes longimanus is a crustacean several times larger than Daphnia—about half an inch long, making it a titan of the plankton world. It’s a visual predator, with a single black eyespot, prominent mandibles, and a barbed tail that makes up about 70 percent of its length. Native to Lake Ladoga, near the Baltic Sea in Russia, it arrived in Lake Ontario in the early 1980s after ships from European ports discharged ballast water into the St. Lawrence River. By 1987 it had reached Lake Superior. It’s now established in dozens of smaller lakes across the entire region, where it feeds on Daphnia and other zooplankton, ripping them apart with its mandibles.
Recent studies have found that populations of native plankton in some of Minnesota’s lakes have fallen by as much as 60 percent since the arrival of the spiny water flea. The plankton die-off in turn has affected yellow perch, walleye, and other native game fish. Fish in lakes invaded by the spiny water flea grow more slowly during their first year of life, which makes them more vulnerable to predators. “The spiny water flea is really a voracious predator of plankton,” said Branstrator. “So it’s a direct hit on the energy and nutrition that support fish. All young fish feed on plankton.”
Inside a cramped lab in the Blue Heron’s forecastle, Megan Corum, one of Branstrator’s grad students, used a microscope hooked to a widescreen monitor to show me a few of these creatures, captured in a drop of Lake Superior water. There were no spiny water fleas in view, but Corum pointed out Daphnia, with their bristly antennae and black compound eyes. She focused on one of them for a few moments. Through its glassy shell we could see its delicate, reddish-brown, tubular heart, gently quivering.
One pest among many
The plankton-shredding flea is just one of many intruders into the Great Lakes, which host more invasive species—more than 180— than any other freshwater system on the planet. Lampreys navigated from the Atlantic through newly built shipping canals, reaching Lake Ontario in the mid-1800s. These long-established predators latch onto trout with their toothy, disc-shaped mouths and drain the trout’s bodily fluids. Barriers, poisons, and traps have successfully prevented lampreys from destroying the Great Lakes’ multi-billion-dollar fishery.
Other invaders arrived by the same route as the spiny water flea, in the ballast tanks of oceangoing freighters. Quagga mussels, another Baltic interloper, have completely transformed the ecology of Lakes Michigan and Huron over the past 30 years. Now numbering in the hundreds of trillions in Lake Michigan alone, the mussels filter about half the lake’s water every few days, sucking up microscopic algae. Unlike Daphnia and other native grazers, the mussels strain nearly everything from the water column.
“Lake Michigan now almost looks like open Caribbean water,” said Hugh MacIsaac, an invasive species biologist from the University of Windsor in Ontario. As recently as two decades ago, its waters were brownish and chock-full of plankton. Today the lake’s limpid waters look inviting, but the clarity is a symptom of lifelessness—the plankton population crash has rippled across the food web.
The state now stocks the lake with fewer salmon—a deliberately introduced non-native species that has become a prime driver of a $7 billion sport fishery—because the salmon’s algae-eating prey fish have declined. “Some of the main prey species for salmon are growing at half the rate they did before mussels,” said Edward Rutherford, a fishery biologist at the Great Lakes Environmental Research Laboratory in Ann Arbor, Michigan.
The good news on the invasive species front, such as it is, is that new ballast water laws have closed what had been a wide-open door. Since 2008, all ships entering the St. Lawrence River from the Atlantic have been required to flush their ballast-water tanks with salt water, which kills any freshwater creatures that might have hitched a ride. And in 2017, a United Nations regulation mandated that all new ships be equipped with ballast-water treatment systems. The results have been dramatic.
“We went from almost two invasive species per year [in ballast water] to zero in the last decade,” said Rochelle Sturtevant, an ecologist with the National Oceanographic and Atmospheric Administration.
But in many cases the damage has already been done. There are no effective ways to control invasive mussels or spiny water fleas. Fish don’t eat them: The flea’s barbed tail gets caught in the throats of fish, so fish spit them out. Spiny water flea populations can become so dense that they even plague fishermen, clogging lines with gooey blobs that prevent the line from passing through the eyelets on a fishing rod.
Humble creatures like Daphnia fall beneath the notice of most of us. Yet the wholesale alteration of plankton communities in freshwater lakes may have more consequential effects on biodiversity than the extinction of more iconic endangered species. It’s an unintended ecological experiment gone haywire, unfolding before our eyes, with unknown outcomes. While it’s unlikely that the spiny water flea will completely eliminate Daphnia and other plankton from the Great Lakes, the tiny invader has already permanently altered the base of the food web on which all the lakes’ fish depend. What that means for the future of the Great Lakes remains unclear.
“We like elephants, we like giraffes,” said MacIsaac. “We don’t want rhinos to go extinct. But what happens if we start losing our native species of plankton? Most people don’t care.” One recent study found that the spiny water flea wiped out three species of plankton in a single lake in northern Ontario.
“Think about it,” said MacIsaac. “If we have 150 lakes colonized by spiny water fleas—it might be a lot more than that—multiplied by three species per lake, that’s 450 populations of zooplankton that have been exterminated by one invader.”