Lake Michigan's Vanishing Shrimp
This shrimp is the biggest small thing you've never heard of, and its decline could leave the Great Lakes with a dramatically different cast of characters.
Mar 4, 2008 Jeff Smith
Aquatic researcher Tom Nalepa holds a vial containing 513 diporeia.
Holiday season 2005 had arrived, and just like everybody else, aquatic biologists at the Great Lakes Environmental Research Laboratory in Ann Arbor were ready to party. So on Tuesday, December 6, they gathered in the banquet room of the Ann Arbor Brew Pub. Tuesday — not a hot party night, but that didn't dampen the good cheer as about 60 researchers, support staff and significant others chose between vegetarian lasagna and the beer hall platter and swayed to a juke box reggae beat.
Dr. Science merrymaking also came with the evening's grand challenge. Partiers had to guess how many diporeia — a delicate crustacean, a shrimp, just shy of 1/4 inch long — were floating preserved in a small glass vial. Revelers held the vial to the light, tilted it and watched the diporeia drift in the clear liquid like fat flakes in a snow globe. They wrote their guesses on paper and put them into a hat. The answer was 513.
But the lighthearted game held a dark message. Passing around the vial gave aquatic researcher Tom Nalepa one more opportunity to spread word about a potentially catastrophic change playing out in America's great freshwater seas. Across broad swaths of Great Lakes bottomland, the diporeia, a key piece of the food chain, is disappearing.
Nalepa, who has been sounding the diporeia alarm across the Great Lakes, the nation and the world for the past dozen years, even made sure the winning number held a lesson. When scientists sampled organisms living in the Lake Michigan bottomland — collectively called the benthos — in the 1960's, 70's and 80's, that's roughly how many diporeia they would pull up in a single grab sample. It works out to 10,000 diporeia in a square meter. Today when researchers return to the exact same sites, most of their samples come up with just a few or none.
Nalepa and his colleagues still don't know precisely what the Great Lakes will be like with far fewer diporeia, but early signs are not good. Salmon and lake trout populations are crashing in Lake Huron. Whitefish are scrawny in Lake Michigan. The small fish that big fish eat — alewives and sculpins — weigh less, and are possibly weaker. Nalepa is one of the first researchers to discover the decline. He has devoted thousands of hours sampling lake bottomland to find out why the decline began with the invasion of the zebra mussel and quagga mussel, and what a loss of diporeia will mean to the lakes that make up the shimmering blue heart of the Midwest.
It's autumn 2005, and Tom Nalepa keeps a nervous watch on weather forecasts and the location of the research ship Laurentian. He doesn't like what he sees. Nalepa must make an autumn sampling run on Lake Michigan to complete his research for the year, but the boat, which, like GLERL lab, is operated by the National Oceanic and Atmospheric Administration, eludes him. For weeks, NOAA assigns the boat to an emergency research job studying the lack of oxygen in the middle of Lake Erie. On its way to Lake Michigan, fall winds blow and blow, and the boat sits at a dock in Lake Huron for days. And when it finally reaches the homeport of Muskegon, bad weather locks it in.
The autumn clicks by. September passes, October passes, November is too risky. Finally there's a window of calm. On a cold Saturday morning in early December, Nalepa, a fit 50-something with thick hair and a broad, friendly face, meets the crew at Muskegon and they haul their gear on board. At 9:15 a.m., a crewman grabs a 3-foot pipe, pries loose the last thick, frozen knot tying the boat to the dock, and the 80-foot ship eases into Lake Michigan, heading due west to sample site C-5.
At 9.5 knots and in calm seas, the Laurentian will need about two and a half hours to reach C-5 in the middle of the lake. To kill time, Nalepa and aquatic biologist Dave Fanslow, who has worked with Nalepa since 1992, sit at a galley table and, above the diesel hum, talk about the sampling plan. On a 12-inch TV mounted in a corner above Nalepa, SpongeBob SquarePants, himself a resident of the benthos, argues with a clam. At another table, two diporeia researchers from Purdue University pop open laptops. Andy Yagiela, the ship's marine technician and defacto cook, has chili on the stove, coffee in the big urn, and the smell of each mingles in the cabin.
The deckhand, Jack Workman, asks the researchers to come into another room for a safety class. He lays an orange Mustang jumpsuit on the floor, explains how it will keep people afloat and how long they could survive if they were to slip off the deck while dipping for diporeia in raging, wintry seas. His spiel is not unlike an airline steward's safety demo, except Jack wears jeans and hasn't shaved.
To a landlubber, Lake Michigan is a giant basin of water. To a sailor, add to that layers of detail — points of land and bays and shoals and shelves. And to a benthos researcher, add one more layer, a set of 40 testing sites, each with a specific GPS coordinate. Scientists established the sites back in the 1960's to track changes in the benthos as Michigan's new pollution control laws were coming into force. Over the past 40-some years, these sites have been central to shaping the world's view on the health of Lake Michigan's bottomland.
These sites first revealed the decline that Nalepa discovered, and these are the sites where Nalepa and his crew are headed today. They hope to sample around the clock for three days straight, taking turns sleeping, or waking if needed on deck — that's if the quiet December weather holds, but already there's word that something might blow in on Monday afternoon.
Mention diporeia, and people might mistake it for an intestinal disorder. But among aquatic ecologists, the diporeia is a star in the Great Lakes ecosystem drama that has played out since the glaciers receded 11,000 years ago.
Diporeia live in what ecologists call the benthic zone. The term refers to the microenvironment at the bottom of a lake or other body of water. It includes the water just above the lake bottom, the surface of the lake bottom and the sediment itself. Diporeia burrow down in the sediment about one inch. They roam across the surface, seeking food. Sometimes they swim up into the water, for food and to mate. Clams also live in the benthic zone. Tiny worms. Crayfish. Snails. Myriad plants root in the benthic zone. The zebra mussel and the quagga mussel invaded this zone.
Before the crash in diporeia, when scientists calculated the total biomass of all the animals living in the benthos — all the clams, worms, crayfish, snails, diporeia, whatever — the diporeia made up over 2/3 of the total mass of animal life in most of Lake Michigan. In all the Great Lakes but Lake Superior, where the diporeia are doing okay, the story is the same — once plentiful, now scarce, in the course of
But diporeia aren't important just because there were once a lot of them. They provide a critical function in cycling energy through the Great Lakes food chain. When energy from the sun reaches the Great Lakes, microscopic plants called phytoplankton or algae convert the energy, through photosynthesis, to living tissue. Other microscopic animals then eat the phytoplankton, and energy from the sun then resides in their bodies, and so on, up the food chain.
But a tremendous amount of microscopic life drifts to the bottom of the lake uneaten. Most of that food energy would be locked up on the lake bottom if not for the diporeia. The diporeia eat the settling material, mostly algae called diatoms, transferring what was once the sun's energy to their own tissue, which is very high in fat and makes for fantastic, high-calorie fish food. As one researcher describes it, "The diporeia are like little packets of Crisco."
When the diporeia roam the surface of the lake bottom or swim up into the water, fish eat them, and at that point they have served their final and critical role: Carrying energy from the bottom of the lake to the fish that live above them. From there, the food chain continues. The small fish are eaten by the big fish, and eventually there's a giant salmon or lake trout bending the end of a fishing pole into a perfect arc.
The diporeia perform their role with amazing efficiency and elegance, and what troubles aquatic ecologists most is there is no other animal in the system that can fill in with quite the same perfect performance once they are gone. In four of the five Great Lakes, they have all but vanished. Nalepa is determined to find out why.
And so 13 years after first pulling up a bottom sample with no diporeia, Nalepa is still motoring into the big lake, sample jars at the ready, obsessed with a tiny white creature of the deep. But unlike the mercurial Captain Ahab and his revenge, the mild-mannered scientist is motivated by a quest for knowledge and concern for the lakes he loves.
By the time the Laurentian reaches the first sample site, clouds smear the sky like gray frosting, and a black line of charcoal trims the western horizon. The water remains calm, "eerily calm" Nalepa says. His eyes scan the horizon, no boats or land anywhere, and he looks to the sky for clues as to what lies ahead.
To man the small crane that raises and lowers the bottom sampler, Workman sits in what looks like a discarded office chair bolted to the deck. He pulls the lever to drop the sampler, called a Ponar, but the grease has gummed up in the cold. Standing, he turns the cable wheel by hand to loosen it, and soon the Ponar is sinking to the bottom, the wheel whirring and clicking as the cable feeds down to 440 feet, the deepest sampling site on the route.
When the Ponar rises to break the surface five minutes later, ruddy mud streams back into the lake. Workman swings the crane over to Nalepa, working beside a waist-high wall at the edge of the boat. Nalepa sprays the Ponar with a hose and washes collected sediment through a fine screen. Soon all that's left is a cup of mud and a few pebbles, and in this is the diporeia.
Nalepa dumps the mix into a sample jar, and atop the murky water float a few diporeia, tiny white confetti of lake life. Nalepa pours from a jar labeled 10% formalin, screws on the black lid, writes C-5, A, 12/4/05 on a piece of masking tape and rubs it on the jar.
Workman drops the basket back in the water, and they do it all again — three times for each site. As soon as the third Ponar breaks the surface, the captain pokes his head out a door. "Is it a good one?" he yells. Nalepa nods.
"I'm out of here," the captain says. In a moment, the diesels surge, and the wake rolls out across the smooth, slate gray lake as the Laurentian heads to the next site, C-6.
On the way to C-6, word comes that the weather forecast has worsened. Winds will increase. Snow will fall. Nalepa takes in the news. He bounces the eraser of his pencil on a map spread on the table. "We'll abort these two sites," he says, and scratches a line through Q-13 and Q-45.
By 4 p.m., when the crew reaches the second site, a quarter inch of snow blankets the deck and everything on it — the crane, the chair, the tubs, the ropes, the hoses, the boots. The wind remains moderate, with waves at a foot or two. Jack again settles into the crane. Nalepa kneels by the low wall. And the sampling scene repeats. It's one these two men have done hundreds of times, and there's little talk as it all plays out. And again, as soon as the third sample breaks the surface, the captain checks and the boat moves on. The process reveals the steady drone of science, Ph.D factory work, but it's key to learning the nuances of our underwater world.
Nalepa first saw evidence of the diporeia decline back in the fall of 1992 at a site near the mouth of the St. Joseph River. "I remember that I asked the ship captain to make sure the coordinates are right and the depths are right because I'd look at the samples and I didn't see any diporeia. Before, they were just teeming in that region," he says. Over the course of that winter his lab assistants had picked through jars from the spring, summer and fall sampling trips. What they had found was the spring populations near St. Joseph were about normal, roughly 10,000 per square meter. By July the population had dropped to about 5,000 per square meter. "But by fall, the population had dropped to zero," Nalepa says. Other areas appeared stable.
At first, Nalepa suspected disease or toxin because the decline was so focused in one region. "I thought, Was there a new river input of some kind, has anything changed in that watershed?" But nothing popped out, particularly something that could have decimated the population within six months.
The zebra mussel was an obvious suspect because it had been first documented in the Great Lakes basin in 1988 — an invasive species introduced by an oceangoing ship — and had been making a rapid advance throughout the system. But Nalepa dismissed the mussel initially because, in Lake Michigan in 1992, the zebra mussel was still clustered near Chicago, 50 lake miles away from St. Joseph.
NOAA then deployed more resources to figure out why the diporeia was disappearing. They sampled the benthos more frequently, studied the shrimp's physiology and looked for infection — bacteria, virus. "It's fairly common for disease to sweep through saltwater shrimp populations," Fanslow says. "But crustacean disease is not well understood, and even if you knew there was a disease, it is very difficult to determine what it is." The researchers found nothing.
The diporeia themselves didn't make the answer easier to come by. For one, there were no corpses to study. Diporeia are so delicate and food-rich that microorganisms in the benthos consume the bodies within 24 to 48 hours. Also, diporeia are notoriously difficult to breed in the laboratory. They're very sensitive to changes in light, temperature and pressure. "Basically they love living in dark, 39-degree water and mud on the bottom of the lake," Fanslow says.
What live experiments the scientists did perform only complicated the answer. In one test, researchers kept diporeia in sediment from a site near St. Joseph for several months, and the animals did fine. But meanwhile, animals at the sample site in the lake declined by 70 percent. In another test, researchers put diporeia in a tank with zebra mussels — thinking perhaps the mussels put off a toxin or carried a disease — again, no meaningful decline.
Some suspected that the zebra mussel was filtering so much food from the water that the diporeia were starving. Nalepa recalls a study he did in the Saginaw Bay in 1991. As part of its natural cycle, the bay turns opaque with green algae. "That spring, as usual, you couldn't see your hand if you reached in to your wrist," Nalepa says. "But by that fall, after the zebra mussel had reproduced, you could see bottom in 15 feet of water, they had filtered that much food from the water. It was unbelievable."
Nalepa opens a notebook and turns to a full-page photo of a diporeia. He runs a finger over an orange band. "That's lipids and some other stuff," he says. "If the diporeia were starving, you'd think their lipid content would be lower, but that's not the case." Apparently the diporeia, at least the surviving ones, were getting what they needed to eat. As he runs through all the research dead ends for what's probably the 100 thousandth time, a cloud of frustration darkens his mien. "There's buts everywhere you look at it," Nalepa says.
In '94 and '95, Nalepa added 120 sampling sites in central and northern Lake Michigan to the 40 sites in southern Lake Michigan, and conducted a whole lake survey. That year showed populations stable except for some spreading of the collapse around St. Joseph. But the apparent stability didn't last long. Testing in subsequent years showed the diporeia decline radiating outward from St. Joseph, and no part of the lake seemed safe.
Despite the murky evidence researchers have come to agree that the chief culprits are the zebra mussel and quagga mussel, because the diporeia decline began as the mussels flourished. "But what the specific cause and effect mechanism is — that's the elusive thing," Nalepa says.
By nightfall, the Laurentian is working in a cluster of sample sites just off the coast of Wisconsin. The team moves quickly, but the weather changes even more quickly. Up in the pilothouse, a computer radar screen displays the boat's location, each of the sample sites and Wisconsin's Wind Point protruding from the west. Snow shows as an ephemeral green Milky Way flickering across the glowing map. And outside, the wind increases.
Nalepa talks to Purdue researcher Kim Ralston Hooper. She's an eco-toxicologist studying the possibility that the pesticide atrazene is playing a role in the diporeia decline. Even though the zebra and quagga are prime suspects, with nothing definitive, researchers must still pursue other avenues. Nalepa says the boat will turn home as soon as Hooper has her samples. But she needs 2,000 diporeia, so the crew heads to a site south of Racine, Wisconsin, where diporeia populations are still present.
It's nearly 11 p.m. when the crew reaches Hooper's sample site. The waves are nearing four feet, and snowflakes the size of nickels careen into Nalepa's face as he kneels again by the side of the boat. Workman is asleep, so Yagiela runs the crane. Nearly everybody moves in the orange protection of a Mustang suit. A half dozen spotlights aim into the snowy night and cast the deck in a mix of harsh whiteness and dark shadow. It's another snow globe scene, but this time the researchers are in the rocking diorama and the boundary is defined not by glass, but by blackness.
In the old days, fetching 2,000 diporeia might have taken four drops of the Ponar. Not so anymore. The sampler goes down again and again and again, 50 times, in a process that will take four hours. People make conversation to pass the time. "I've actually built two saunas," Fanslow says, snow pelting him in the face as he hangs onto a pole for stability. "One at my home in Ann Arbor and another at my place on Batchawana Bay."
About midnight, Fanslow takes over for Nalepa, manning the Ponar station until 3 a.m., when the snow measures 4 inches deep. The work is exhausting, and when it's done, the researchers find their bunks and slump in for the rocky four-hour trip home. The bad weather forced Nalepa to abandon all but a quarter of his sites. He'll finish up in fall of 2006. In the meantime, he'll study the spring and summer results and keep spreading the word about the decline of the diporeia.
In the Great Lakes Environmental Research Laboratory there's a closet, a typical closet — about 6 feet by 4 feet, gray metal door, one ceiling tile broken in half and stained with water. And in the closet is every diporeia sample that Nalepa has ever collected. Vials bundled in rubber bands. Pint-sized jars stacked in plastic tubs. Samples on chairs, on shelves, on the floor. Each has a label telling place and time, like Musk 15A 10-29-99. The early ones are filled with formalin and white flecks of diporeia. About the early to mid-90's zebra mussel shells appear, filling some jars to a third or halfway. Later years, some vials are crammed to the lid with shells, and the diporeia are gone.
The closet is a museum of sorts, a chemically preserved tribute to a species in decline, but what will happen in the years to come? Nalepa suspects that the future might be revealing itself in Lake Huron, where populations of alewives have crashed. Big game fish like salmon and lake trout don't eat diporeia directly, but diporeia are very important to alewives — which do not eat zebra mussels or quagga mussels. With fewer forage fish, Michigan is stocking 50 percent fewer game fish in Lake Huron this year. "Why stock them if there's nothing for them to eat?" Nalepa asks.
Whitefish are also showing the strain, although less dramatically. Diporeia made up 50 to 70 percent of the whitefish diet, but now the fish eat primarily zebra mussels and quagga mussels. The mussels take more energy to eat — crunching through shells — and provide less energy per gram. "It's like eating a rice cake instead of a cheeseburger," said one researcher. Whitefish are now skinnier and have less value in the market.
Is there hope for the diporeia? Nalepa considers the question for a moment and describes what's happening in the Finger Lakes of New York. The lakes have zebra mussels, but the diporeia have somehow hung on. Researchers don't know why, but their stability might reveal a resiliency that could somehow prove important in the future. The hope is tenuous, but that's all there is.
Without diporeia, the Great Lakes will, of course, still exist, and animals will live in the waters. But which species will survive the change? "What worries me," Nalepa says, "is instead of a diverse fishery, what we could have is a biological community consisting mostly of mussels and very few fish."
Jeff Smith is editor of Traverse, Northern Michigan's Magazine
Note: This article was first published in August 2006 and was updated for the web February 2008.