May 30, 2008 -- The freshwater Great Lakes on the Canadian-U.S. border used to be packed with a tiny, yet ecologically important, crustacean called Diporeia, but this krill-like animal has been disappearing in droves from all but one of the lakes, according to at least two teams of researchers.
The Diporeia disappearance has likely led to the physical shrinkage of alewives (a species of herring), from 10 inches in length to 8 inches or lower, since this fish relies upon the rice grain-sized crustacean as a food source. Diporeia also provides Chinook salmon and other commercially important species, such as lake whitefish, trout and walleye, with up to 80 percent of their food.
The chain of events foreshadows further population declines, scientists say. Pollution and disease could be contributing to the problem, but scientists suspect an insidious invader is primarily responsible for the deaths.
"The loss of Diporeia in lakes Michigan, Erie, Huron and Ontario coincided with the introduction and expansion of zebra and quagga mussels," collaborating researcher Thomas Nalepa told Discovery News.
Nalepa, a research biologist at the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory, explained that the mussels probably came to the lakes as stowaways on European and Asian vessels.
"A ship bound for the Great Lakes will take on freshwater ballast for stability at an overseas port before its transoceanic journey," he said. "Once in the Great Lakes, it will discharge this ballast water before it takes on cargo. When this happens, freshwater organisms that survive the journey will then be introduced into the new system."
The mussels, once in the lakes, muscle in on Diporeia's turf. They settle above the small crustacean's sediment homes and filter out algal plankton, which Diporeia must feed upon. They then leave behind copious amounts of waste -- literally defecating on the hapless crustaceans -- and transmit disease.
Freshwater mussels aren't even palatable to humans, according to Nalepa, who described them as having a "musky, ammonia taste."
He added, "I tried cooking some zebra mussels after they were first introduced, and they stunk up the whole house."
While the scientists cannot yet directly prove the invasive mussels are pushing out both Diporeia and native Great Lakes mussels, which are also on the decline, one key clue is that the tiny, local crustaceans still have a stable population in Lake Superior, which contains low calcium levels. Mussels need calcium for shell development, so the foreign mussels haven't spread as much there.
Climate change probably is not directly affecting Diporeia, according to Tomas Hook, a Purdue University scientist who is also investigating the problem.
"These animals live on the bottom of lakes in very deep areas where temperatures are rather stable and cold," Hook told Discovery News, before adding that climate change might, however, adversely change the lakes' algal communities, and these perturbations could ripple down the food chain.
Next month, a paper authored by a team of researchers, led by Purdue's Marisol Sepulveda, will be published in the journal Aquatic Toxicology. It outlines how Sepulveda's group has been identifying substances involved in Diporeia metabolism. These are chemicals that allow the organisms to respond to stress.
Although the study could not point a finger at the zebra and quagga mussels, it did find that commonly used pesticides, such as atrazine, decrease the stress response in Diporeia, leaving it more susceptible to the mussel invasion and the diseases they spread.
Sepulveda, an assistant professor of ecotoxicology and aquatic animal health, told Discovery News that there is reason for hope, because she is confidant "that our multidisciplinary team of scientists will help us find some answers that will pinpoint the definitive cause(s) of Diporeia declines."
Related Links:
Jennifer Viegas' blog: Born Animal
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