[Mike]

February 2008

(Basically unexplained, cause is just guessed at. I’m looking at global dimming for increased botulism strength, plus metals in the air weakening the birds’ immune systems, etc.) — MC

Science News –February 27, 2008
Great Lakes bird die-offs signal ecological changes

Widespread ecological change is occurring in the Great Lakes, driven by the synergistic impacts of introduced species.

Tens of thousands of birds have perished from botulism poisoning around the Great Lakes in less than a decade. Evidence points strongly to two culprits: the quagga mussel (Dreissena bugensis) and the round goby (Neogobius melanostomus), both aggressive invasive species that were presumably introduced to the region via ships’ ballast water. Researchers say that environmental changes brought about by the invaders are likely to worsen the situation.

Mark Breederland, Michigan Sea Grant
More than 1300 loons died during 2006–2007 from type-E botulism poisoning along the shores of northern Lake Michigan.Bird carcasses infected with the type-E strain of botulism first washed ashore around Lake Erie in 1999. Since then, mass bird mortalities have occurred annually, and the problem has spread to Lakes Ontario, Huron, and Michigan. Of the more than 50 avian species affected, diving birds such as loons, grebes, and many ducks—migrating southward from central Canada through the Great Lakes—have been hardest hit. But large numbers of shorebirds, songbirds, and raptors also have fallen prey. Rough estimates by the U.S. Geological Survey’s National Wildlife Health Center, based only on opportunistic collections of bird remains, peg the avian death toll at close to 70,000 from 1999 to 2006; no systematic surveys of bird mortalities on the Canadian side of the lakes have been conducted. Probably thousands more birds have expired on the water and have sunk, says Helen Domske of New York Sea Grant.

Quagga mussels and round gobies are commonly found in the stomachs of botulism-infected birds and fish. Because the quagga can flourish under a wide range of conditions, it is now displacing its invasive cousin, the zebra mussel (Dreissena polymorpha). The quagga wreaks havoc on lake food webs by consuming vast quantities of plankton, accumulating contaminants, and introducing the anaerobic bacterium Clostridium botulinum to the food chain. Birds may be infected directly by feeding on the infected mussels or indirectly by eating tainted gobies or other quagga-eating fish. “Gobies change color (PDF: 31 KB) after ingesting the botulism toxin,” (PDF: 31 KB) Domske says, “and this may act as a visual cue for birds, attracting them to the weakened fish.” When scavengers feed on contaminated fish and birds that land on coastlines, they can pick up the toxin.

The round goby has multiplied ferociously since its debut in the lakes in the early 1990s. “They’re like ants on the lake bottom,” says Mark Breederland of Michigan Sea Grant. “They breed four to five times a year and have voracious appetites for the eggs of our native fish.” Scientists recently discovered that newly hatched round gobies swim from lake-floor breeding sites to the surface; this enhances their chances of getting sucked into ballast tanks and transported by ships.

Despite the evidence implicating the goby and quagga in the massive bird die-offs, vexing questions remain. For one thing, type-E C. botulinum is ubiquitous in the lakes’ environment, and outbreaks did occur periodically before the current vectors arrived. However, it has proven difficult to show spores germinating in lake sediments. Alicia Perez-Fuentetaja of Buffalo State College recently measured significant levels of the genetic material coding for toxin production in lake sediments and in several invertebrates, including mussels, aquatic worms, and even fly larvae. Her work suggests that spore germination happens in localized, low-oxygen hot spots in the sediments that meet bacterial requirements for growth. More research is needed to better understand processes at the water–sediment she says, “but it appears there are potentially many routes of toxin transmission.”

Altered ecosystems have clearly augmented conditions favorable to botulism, says Breederland. The filtering action of zebra and quagga mussels has turned waters crystal clear in many areas of the lakes, allowing sunlight to penetrate deeper waters and significantly boost temperatures. In addition, an excess of phosphorus, which leads to more pervasive anaerobic conditions ideal for bacterial growth, may be responsible for a resurgence in the growth of the macroalga Cladophora. “We are seeing the long-term consequences of synergistic responses between multiple invasive species,” Breederland says.

Efforts are under way to tighten up restrictions on ballast-water discharge, curtail phosphorus runoff, and develop pheromones to disrupt goby reproduction, but researchers say no simple answer exists for realigning the food chain now dominated by the invaders. Still, Breederland says, “if dead birds lined up on beaches don’t get us passionate about addressing these problems, it’s hard to see what will.” —NOREEN PARKS

http://pubs.acs.org/subscribe/journals/esthag-w/2008/feb/science/np_birddieoff.html