Tuesday, January 31st, 2023
Tuesday, January 31st, 2023

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Mussels destroying link in Lake Michigan food web

Traverse City, Mich. (AP) — A tiny, shrimplike creature that
forms a crucial link in the Great Lakes food web has all but
disappeared from Lake Michigan because of competition from invasive
foreign mussels, scientists reported Wednesday.

Observations over a decade have documented a 96-percent drop-off
of the amphipod species known as diporeia, according to scientists
with NOAA’s Great Lakes Environmental Research Laboratory in Ann
Arbor.

During that period, the lake’s population of quagga mussels rose
dramatically, said the researchers, who described their findings in
the journal Freshwater Biology.

“It’s pretty astounding what changes occurred the lake in just
10 years,” said Tom Nalepa, a biologist who led the study for the
National Oceanic and Atmospheric Administration lab.

The quagga mussel, a thumbnail-sized invader from Eastern
Europe, is believed to have hitched a ride to the Great Lakes in
ballast water of a trans-Atlantic cargo ship in 1989 – three years
after the arrival of its better-known cousin, the zebra mussel.

Both have spread rapidly across most of the lakes, with the
quaggas inhabiting cold, dark waters that zebra mussels avoid. They
eat the same types of algae on which diporeia relies. Diporeia is a
dietary staple for bottom feeders such as whitefish – standard fare
in many of the region’s restaurants.

Previously, diporeia made up 70-80 percent of the whitefish
diet. Now they are getting skinnier and less abundant as they
subsist on the mussels, which are the aquatic equivalent of junk
food – lower in calories than diporeia, their shells devoid of
nutritional value.

“The quagga mussels are basically sucking all the energy out of
the lake,” Napela said. “Not only is diporeia declining. The lake
cannot support all this biomass of quagga mussels without other
components of the food web losing populations.”

Also slumping amid the mussel onslaught are prey fish such as
the alewife, bloater and sculpin, which are crucial food for
salmon, trout and other popular sport species.

Similar trends have been noted in Lakes Huron and Ontario and
parts of Lake Erie. The mussels have a far lesser presence in Lake
Superior, probably because its calcium concentrations are too low
to meet their needs, Nalepa said.

Biologists first noted a diporeia drop-off in southern Lake
Michigan in the early 1990s, as the zebra mussel rapidly colonized
the area to a depth of about 55 yards.

The latest study documents the virtual lakewide replacement of a
native species by an invader and suggests the change will
fundamentally alter the ecosystem, the scientists said.

Napela and colleagues began taking sediment samples from dozens
of sites around the lake in 1994-95 and repeated the operation in
2000 and 2005. They did extra sampling annually near the southern
end.

Diporeia were particularly abundant in lower depths. Its biggest
declines happened after the quagga mussel’s rapid expansion in the
late 1990s, Napela said.

The scientists have continued their annual sampling in selected
locations, turning up no reason evidence that the trends are
slowing. Only when the mussel populations finally stabilize will
their long-term effects on fish and the rest of the ecosystem be
known, Napela said.

Other studies are exploring whether food competition is the only
reason for the mussels’ devastating effect on diporeia.

Environmentalists have long campaigned for stronger laws to
prevent exotic species invasions in the lakes – particularly from
ship ballast. The study illustrates the continuing fallout from
governments’ failure to act sooner, said Joel Brammeier, vice
president for policy of the Chicago-based Alliance for the Great
Lakes.

“The lesson here is that if we don’t focus on prevention, there
will always be an invader around the corner that will be 10 or 100
times more dangerous than the one we saw before,” Brammeier
said.

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