Thinking like military historians, limnologists at Cornell University have documented the invasion by an exotic species Daphnia exilis in one of North America's dirtiest lakes -- an invasion that ultimately failed when a polluting industry cleaned up its act.
Their report in the May 1999 issue of the journal Limnology and Oceanography describes how diapausing (or dormant) eggs of the crustacean Daphnia exilis were:
- Hauled from the toxic bottom of New York state's Onondaga Lake by Cornell students,
- hatched in the laboratory to yield healthy crustaceans, and
- genetically traced to one pioneering organism that hitchhiked east in the 1920s.
"These little guys are real survivors, considering that they successfully colonized a lake far from their natural habitat and banked their eggs for the future in a place that became a Superfund toxic cleanup site," says Nelson G. Hairston Jr., the Frank H.T. Rhodes Professor of Environmental Science at Cornell.
Hairston (pronounced "HAHRS-ton") was leading a team of Cornell biology students and professional limnologists in a U.S. Environmental Protection Agency-funded survey of organisms in lake-bottom sediments when Meghan Q. Fellows found the eggs of one creature that "didn't belong" in Onondaga Lake -- or anywhere east of the Mississippi.
The Daphnia discovered by Fellows, a Cornell undergraduate at the time and now a graduate student at San Diego State University, were known to inhabit shallow, fishless salt ponds in the western United States. Like other diapausing organisms, adult Daphnia produce eggs that can "sleep" for years until they sense favorable conditions for hatching, a useful trait for repopulating ponds that periodically go dry. Adult Daphnia are less than 1/8th inch long, and their diapausing eggs, called ephippial cases, are microscopic.
Hairston's laboratory in the Cornell Section of Ecology and Systematics has considerable experience with diapausing aquatic life. Its researchers once revived 300-year-old copepod eggs from a Rhode Island pond lake. So awakening decades-old eggs from Onondaga Lake was not a problem, once the scientists found the right combination of temperature, water chemistry and light.
But the crustacean Rip van Winkles provided no clues about how their salt-loving ancestors got to a freshwater lake in upstate New York until a few historic facts came to light, and were further illuminated by a genetics-tracing technique similar to DNA fingerprinting:
- For decades in the 1900s, Onondaga Lake was about 1/10th as salty as sea water because a lakeshore factory used geologic deposits of sodium chloride salt and limestone to manufacture soda ash. Massive waste beds of calcium carbonate and sodium choloride were left at the water's edge, raising the salinity of Onondaga Lake to a level that Daphnia would find comfortable. Indeed, even before this salt pollution of the industrial era, sodium chloride deposits from ancient inland seas were exploited by Native Americans and European settlers, giving Syracuse the nickname Salt City.
- The soda ash factory, known locally as the Solvay Process plant, was owned by Rowland Hazard, who in the 1920s also owned a lead mine in Missouri, at the eastern edge of the natural range of the exotic Daphnia. Mine operators sometimes transferred machinery from one site to another, and with the equipment could come debris from the previous location.
- Genetic analysis of animals hatched from the dormant eggs showed that they -- and probably all the billions of Daphnia that temporarily thrived in Onondaga Lake -- were related to a single, pioneering individual, a kind of Adam (or Eve) of a salty water garden.
"Our hypothesis is that a few Daphnia eggs hitched a ride in the dry mud on Mr. Hazard's lead-mining machinery, all the way from Missouri, and one lucky egg found a suitable home here and started it all," Hairston says. "Since they can reproduce without sex, all it takes is one." He notes that a year-by-year analysis of sediments from the lake bottom indicates the presence of millions of Daphnia the first year, an unexplained absence of Daphnia eggs from 1927 to 1940, and a major resurgence that peaked around 1970, when there could have been billions of Daphnia in the lake.
Then the Daphnia declined and disappeared from the lake water by the early 1980s, leaving only diapausing eggs as a record of their failed invasion. The chemical industry had stopped depositing waste salts along the shore of the lake. Today the salinity level of Onondaga Lake is close to normal.
However, other toxic pollutants remain, and one -- mercury -- could account for some of Daphnia's reproductive difficulties. Beginning in 1946, mercury waste was created from a process that produced chlorine gas, and hundreds of pounds of the toxic heavy metal were dumped in the lake each year until 1970. Of all the Daphnia eggs recovered from lake-bottom sediments by the Cornell scientists, only those deposited less than 25 years ago are still viable and can be hatched. The rest probably are dead.
Daphnia eggs will rise again, Hairston predicts, every time a boat anchor stirs up sediment on the lake bottom. But the invaders won't get far. Unless the Salt City's lake turns saline again, the best place to see swimming D. exilis east of the Mississippi is in a Cornell laboratory.
Authors of the Limnology and Oceanography journal report, "Population of a failed invasion," are Hairston; Fellows; Linda J. Perry, a Cornell undergraduate at the time of the research and now a graduate student at Emory University; Andrew J. Bohonak, who obtained his Ph.D. at Cornell and is now at the University of Hawaii's Center for Conservation Research and Training; Colleen M. Kearns, technician in the Cornell Section of Ecology and Systematics; and Daniel R. Engstrom at the Science Museum of Minnesota's St. Croix Watershed Research Station. In addition to the EPA, the study was supported, in part, by the National Science Foundation, the U.S. Department of Agriculture and the Upstate Freshwater Institute in Syracuse.