Diporeia Is Declining And We Should Be Concerned

September 2021 Multimedia ShoreMaster

A long, metal arm reaches down, nearly 200 meters into the depths of Lake Michigan. Upon reaching the lake’s floor a contraption on the end opens up and slams itself into the dirt, trapping small creatures in its clutches. The arm then retracts, carrying its prey back towards the light, and to the scientists waiting in a boat above.

In 1980, those scientists would’ve found a plethora of diporeia, a small amphipod common to the Great Lakes. In 2021, scientists are lucky to find any.

This is called a survey, a yearly tradition for Ashley Elgin, a benthic ecologist at the NOAA Great Lakes Environmental Research Laboratory since 2014. Every five years they conduct a whole lake survey, but according to Elgin some species can explode and decline within a five-year span, so yearly surveys can be better when possible. Elgin conducts these in the southern-third portion of Lake Michigan every year.

That contraption obtaining the samples is called a Ponar grab. It’s 50 pounds and lowered from a boat on a winch. It captures a twentieth of a meter, or about one-half square foot, of sediment, which scientists carefully study to pick out living organisms, like a kid poking through mud in search of worms. Elgin’s predecessor began documenting the decline of diporeia in these samples and Elgin has taken up the cause.

What’s The Big Deal?

Diporeia are small invertebrates that contain high amounts of lipid molecules, also known as fat, making them a high-energy food source for small, “prey” fish which, in turn, feed the larger salmon and trout that are the backbone of a billion-dollar fishery. When a critical member of the food web slowly declines in numbers, the effects start to appear in other fish.

David “Bo” Bunnell is a 16-year research fishery biologist at the US Geological Survey Great Lakes Science Center. Bunnell focuses his work on prey fish and how they support recreational fish, like lake trout.

Bunnell indicated that bloater, a native prey fish, used to have a diet of 50 percent diporeia. Alewife, a non-native, could have a diet of up to 25 percent diporeia, depending on the season. A large portion of their diets is declining, which becomes a problem for even bigger fish, like whitefish.

“Diporeia was a really important component in sustaining these prey fish populations,” Bunnell explained, “not only because of its abundance but because it provides a lot of lipids. So it’s a really high-quality prey item.”

Losing this high-energy prey causes prey fish to become skinnier, meaning salmon and trout have to eat even more prey fish to keep from losing weight. For an angler or commercial fisher, the heavier the fish the better.

“As diporeia became less abundant in the late 90s and early 2000s, we began to see clear evidence of reduced growth and reduced condition,” Bunnell continued. “The fatter a fish is, the more calories can be transferred up the food chain into the salmon and trout. There’s very clear evidence that as diporeia diminish in the Great Lakes, a lot of these prey fish became much skinnier than what they used to be.”

According to Elgin, even charter boat captains have noted a decline in fish

Why Are Diporeia Declining?

Elgin and Bunnell agree with an unfortunate truth: no one really knows why there’s a decline in diporeia. A perhaps irritating pill to swallow for Elgin as a scientist because it’s her job to know, and right now there’s just no way to know for sure.

“A lot of people want to know,” Elgin said. “It’s frustrating for me that as scientists we don’t have a strong answer for it.”

Elgin suggests that the speed and coordination of the diporeia decline could offer disease causation. A more popular theory points to quagga and zebra mussels. The arrival of these mussels, invasive species to the Great Lakes, corresponded with the decline of diporeia, though that’s not enough to put all the blame on them.

“The ecosystem is so complex, there are many moving parts and other stressors in addition to mussels. And other things have changed in the last few decades besides mussels arriving,” Elgin explained. “There’s been some controversy and no consensus on what’s happened and why the decline has happened.”

It’s believed that mussels came to the Great Lakes in ballast water while in their free-swimming larval phase. International freighters carry water in their ballast to weigh them down in rough seas. According to Bunnell, in the 80’s and 90’s, without full understanding of the potential dangers, large freighters would pick up water containing these larvae in places like the Caspian Sea, where they are native. The freighters would then carry them all the way to the Great Lakes and dump the water where it was shallow, allowing the mussels to invade and ultimately spread.

Ships now undertake strategies to prevent ballast water from contaminating the Great Lakes with new invasive species. They will stop in the Atlantic Ocean to replace their ballast water, or treat their ballast water to kill any living organisms. Despite these efforts, the damage was already done, with the exception of Lake Superior.

Diporeia levels in Lake Superior continue to remain strong, and guess what? There are close to no mussels. Though Bunnell calls this circumstantial evidence, it could point to a correlation. But then again, maybe not.

“Every lake has its own personality and its own unique challenges,” Elgin expressed, and she should know. She has been around the lakes since she was a kid, growing up on Lake Superior.

As both the coldest and deepest of the lakes, Lake Superior is different and brings too many variables to know for sure where to send the blame.

What Can Be Done?

Assuming quagga and zebra mussels are indeed guilty of instigating the decline of diporeia, there could be hope in the long run. The Environmental Protection Agency approved Zequanox for use, a product designed to help control zebra and quagga mussels that is less toxic to other organisms. The only problem is then applying it to an entire lakebed.

Efforts of organizations such as the Invasive Mussel Collaborative and the work of scientists like Elgin and Bunnell continue in an attempt to help save diporeia and other native species in the Great Lakes.

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