Invasive species trigger a cascade of calamities in the Great Lakes. Words :: Leslie Anthony // photos :: Inspired Planet Productions.
I first heard the term “invasional meltdown” from an old Royal Ontario Museum colleague, Dr. Nicholas Mandrak. Downing beers in a musty New Orleans bar with other former labmates at a 2006 conference, our conversation wandered the scientific map. When, as always, invasive species came up, Mandrak—then an aquatic ecologist at the Department of Fisheries and Oceans and these days a University of Toronto prof—took over. Setting down his glass, he described with great animation the phenomenon in which two or more invasive species combine to have a greater effect than either would on its own. Though the concept was new to me, I was intrigued enough to eventually pen a book on the subject. At the time, however, I was simply gobsmacked by his descriptions.
Mandrak’s frightening tableau began with the accidental introduction to the Great Lakes of the zebra mussel (Dreissena polymorpha). Flushed from the ballast tanks of transoceanic cargo ships, this native of Eastern Europe and the Ponto-Caspian was first reported in 1988 from Lake St. Clair (the basin briefly interrupting the river that connects lakes Erie and Huron). Only two years later it was found in all five lakes. A reproductive dynamo whose females produce 40,000 eggs per breeding cycle and up to a million each year, the fingernail-size mussel rapidly multiplied out of control, wreaking havoc on ecosystems and infrastructure in the Great Lakes; by 1993 the itinerant mollusk had made its way down the Mississippi to where we now sat discussing its dubious success.
Meanwhile, a deeper-water relative, the quagga mussel (D. bugensis), had also tumbled into the lakes from ballast alongside a small Ponto-Caspian fish, the round goby (Neogobius melanostomus), for whom young dreissenid mussels were a favoured snack back home. With a fast-growing preferred food source paving its new environment, the equally prolific goby (females lay some 5,000 eggs several times each season) also took off, outcompeting native bottom-dwelling fish and scarfing not only mussels, but other fishes’ eggs. While it first seemed the consumptive relationship between goby and mussel might be a fortuitous brake on the latter, in some areas it simply lapsed into a classic predator-prey cycle of boom and bust. But as with anything in ecology—a science noted for revealing hidden complexities and context-dependent relationships—it wasn’t quite that simple, said Mandrak, winding up for the finale.
Not only were dreissenid mussels prolific breeders, but also prolific filter-feeders, each processing a litre or more of water a day (we’ll come back to this), a proclivity with a momentarily welcome effect: Notoriously turbid water bodies became preternaturally aquamarine, an illusion of health recalling the spooky-but-appealing clarity of acid-rain-affected lakes in the 1970s. As the lakes cleared, however, increased light penetration supported invasion by an exotic aquatic plant, Eurasian watermilfoil (Myriophyllum spicatum), sending nearshore ecosystems back in the direction of the weed-choked 1960s. In some areas, the newfound limpidity combined with agri-runoff and climate change to facilitate massive blooms of cyanobacteria (aka blue-green algae) which produced toxic microcystins, chemicals harmful to animals—including humans—if ingested. When the algal blooms died off, their decomposition burned up oxygen, encouraging the growth of anaerobic bacteria like Clostridium botulinum, the source of deadly botulinum toxin.
Concentrated through bioaccumulation in the food chain, avian botulism caused unprecedented bird die-offs around the lakes. At the same time, the mussels’ tendency to anchor on any hard surface or substrate was putting native bivalves out of business—up to 10,000 mussels could coat the shell of a single indigenous freshwater clam. Another Ponto-Caspian transplant, the hydroid Cordylophora caspia, fed on plankton like free-swimming mussel larvae—or veligers—using mussel shells as an anchorage. Given such facilitation, invasive hydroid populations expanded dramatically after extensive beds of mussels formed in the lakes, but they were eating more than just mussel larvae. Indeed, the entire invasive-mussel filtering machine and its symbiotic cabal was now starving out the lakes’ plankton-and-bacteria-eating macroinvertebrates, food source of baitfish like alewife. Though alewife was itself a no-love-lost invader, its now rapid disappearance was impacting the lakes’ top predators like salmon and trout.
That, Mandrak had said, catching his breath, was invasional meltdown. World-weary biologists all, well-studied in myriad tipping-point ecological cascades, we sat in stony, contemplative silence. At last someone spoke. “Another round?”
The entire invasive-mussel filtering machine and its symbiotic cabal was now starving out the lakes’ plankton-and-bacteria-eating macroinvertebrates, food source of baitfish…
Only in hindsight did I register that query’s potential double-entendre—an allusion to either more beer or the introduction of another invasive species. Actual meaning mattered little, however, since by that point the entire Great Lakes ecosystem was tipping as quickly into befuddlement as we were. And no one has been more aware of the ensuing ecological chaos than those who fish the lakes.
With a $7 billion recreational and commercial fishery focused on top-of-food-chain species, an 80 per cent decrease of preyfish biomass in the past 30 years is nothing short of cataclysmic. Reasons initially pointed to a combination of impacts from introduced Pacific salmonids and the compounding effects of sequential explosive expansions of dreissenid mussels and other invasives. The only bright spot was the Lake Superior preyfish community, where, despite some biomass loss, the proportion of native species in the mix rose, supporting recovery of wild native lake trout (with water chemistry less conducive to shell formation, Superior isn’t as badly plagued by mussels). A 2015 chart of the preyfish symphony, however, revealed one particularly sour note: Commercially important lake whitefish were dropping in lock-step with preyfish like bloater, alewife and rainbow smelt. Why?
A coldwater species found throughout the Great Lakes, lake whitefish live 30 years and grow to six kilos, their previous abundance (~100 million) helping shape a freshwater ecosystem on which humans depended for millennia. Adult whitefish traditionally preyed heavily on amphipods of the genus Diporeia, which, along with opossum shrimp (Mysis relicta), formed the basis of a deep-water food web in the vast proglacial lakes spawned by retreating Pleistocene ice sheets that eventually became the Great Lakes. But Diporeia suddenly disappeared in the early 2000s as preyfish and whitefish began their simultaneous decline; some posited whitefish switched to eating quagga mussels, shifting schools away from areas where commercial fishermen and researchers originally found—and counted—them. Again, however, things proved not so simple. Because ecology.
Fish movements had little heuristic value in explaining the crashing numbers: While many First Nations and family-owned fisheries shuttered in Lake Michigan and Lake Huron, those remaining had a front-row seat to a more than tenfold decline in annual catch—from 1,000,000 lbs. to less than 100,000. Because whitefish were never overharvested, and breeding seemed successful enough to sustain stocks, this suggested a recruitment problem—getting from baby fish to adult. Yet even in an upended ecosystem, with smoking guns on all sides, precise reasons remained opaque. A glimpse of the extent of this enigma can be seen in the abstract to a comprehensive 2021 Great Lakes Fisheries Commission white paper: “A mechanistic understanding of factors important to Lake Whitefish recruitment has been elusive, likely owing to the dynamic interactions of biological and physical processes and the modifying effects of climate change and invasive species. Despite a century of research on… whitefish life history and recruitment, many fruitful research questions remain unanswered or even identified.”
In other words, the whitefish life stages scientists knew least about seemed most vulnerable to the depredations of invasional—and climactic—meltdown, leaving the species “struggling to survive in a world that has been utterly transformed by a single invasive species—quagga mussels,” as a new documentary from Tobermory-based Inspired Planet Productions avers. In “The Last Whitefish,” the first episode of an upcoming three-part TVO series All Too Clear: Beneath the Surface of the Great Lakes, stunning underwater footage gets to the bottom (sorry) of what scientists—who largely work from the surface, sifting through dredged and netted samples—have struggled to understand.
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Sophisticated underwater drones allowed filmmakers Zach Melnick and Yvonne Drebert to be James Cameron at a fraction of the cost, capturing in high definition the story of how mussels gained control of nutrient cycling in the Great Lakes. The tale picks up in the new millennium as quaggas begin expanding into deeper waters; though they predictably covered most hard objects, unlike their zebra cousins, quaggas also colonized sediments. Today, beginning 50 metres down, the soft bottom of Lake Huron supports beds of up to 20,000 mussels/m2 and 30,000-40,000/m2 on attachable surfaces like the 19th-century steamship Africa the filmmakers discovered at 80 m; with barely a centimetre of exposed wood, the ship is perfectly outlined in a mussel crust.
With water now three times clearer than pre-mussel days, even without lights the Africa footage is phantasmagorical, which brings us back to the idea of mussels conservatively filtering a litre of water each day. With quadrillions carpeting the Great Lakes, says Dr. Ashley Elgin of the U.S. National Oceanic and Atmospheric Administration in the film, every drop of water in Lake Michigan could pass through a mussel in a week’s time. With that would go many of the lake’s nutrients, diabolically concentrating these in mussel beds and leaving little in the water column for microbial, planktonic and macroinvertebrate food chains that support baitfish and the young of larger species like whitefish.
As dramatically noted by biologist Jason Smith of the Sault Tribe of Chippewa in Michigan’s Upper Peninsula, zooplankton density has fallen from roughly 700 per litre of lakewater in the 1970s to a ghostly 1-3 per litre today; larval fish that encountered a meal every 2-5 centimetres a century ago now swim thousands of body lengths for the same bite. Indeed, growth rates for larval whitefish are now 50 per cent of historical measurements.
My long-ago conversation with Mandrak taught me there’s always more, and so, as we learn in “The Last Whitefish,” there is. With nutrient drawdown (e.g., phosphorus levels have now dropped below baseline targets set by the surprisingly successful Great Lakes Clean Water Act), spring algal blooms that previously fed zooplankton, which, in turn, fed whitefish hatching in synchrony with the bloom, are on the rocks. Indeed some 40 per cent of zooplankton are now mussel veligers, with nowhere near the nutritional value. And of course, it gets worse: Clearer water may also be facilitating the death of larval whitefish via exposure to UV radiation, which now penetrates deeper and more strongly.
I won’t play spoiler and tell you everything, but there are ecological slivers of hope lodged amidst this messy poster-child of invasional meltdown. Invasive mussels may have changed the lakes’ physical, chemical and food environments, but they haven’t changed nature’s ability to heal itself. You’ll want to tune in to find out how.
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