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MAPPING LAKE VICTORIA
UMD's Large Lakes Observatory finds evidence
for the rapid evolution of 300 species of fish
Tom Johnson (center) and Mike Rosenmeier (right) are still studying the core samples in Duluth.
They are working with Ben Odhiambo (left), a student in the Ph.D. program at the
University of Nairobi who is in Duluth for one year.
The cockroaches on the boat are one and a half inches long. They come out during the night watch from about 11 p.m. until 4 a.m. scurrying about the floor, across the electronics on the table and up the walls. Tim and I are standing the night watch from 8-2 while Chris and Jim stand the 2-8. When we got done at 2 a.m., Tim and I tried sleeping on the flying bridge. I came down after about an hour of light dozing suffering from a headache from the diesel exhaust.
Tim came down shortly afterwards. The following 4 1/2 hours of sleep in the state room below was fitful. The room smells of stale air-almost like a sewer-and no air circulation reached the room. Shortly before it was time to wake up, someone accidentally stepped on me while crawling out of the bunk.
---From Tom Johnson's diary,
18 March, Lake Victoria, Africa
When Tom Johnson, director of the University of Minnesota Duluth Large Lakes Observatory in Duluth, led his team of 15 international researchers to Lake Victoria, his intent was to find the ancient shoreline of the Lake by studying layers of sediment. But, the discovery the team actually made reached far beyond finding the horizon of Lake Victoria.
The Large Lakes Observatory (LLO) is a center for international large lakes research and experimentation. It specializes in the geology, chemistry, and physics of Earth's largest lakes. When the LLO group went to Lake Victoria, they added a new dimension to the work already being done by biologists from around the world who study the cichlid fish, a species that are unique to this largest African lake.
The LLO team, funded by a grant from the National Science Foundation, was looking for the shoreline of the old lake, attempting to record sediment layers back through the last Ice Age.
Their observations would unveil something Lake Victoria has kept hidden for thousands of years: 12,000 years ago, at the end of the last Ice Age, the lake was an expanse of dry land. Since then, the basin has filled with water and over 300 species of the rare cichlid fish evolved.
In April 1995, on a 55-foot Uganda fisheries research boat named Ibis, the researchers sailed out attempting to map the lake bottom. They used a method called “seismic reflection profiling” which uses acoustic techniques to reveal the geology of the lake floor. This process sends out sound waves and records the way the waves bounce off the lake floor and the underlying sediment layers.
For three weeks, 24 hours a day, the operation proceeded.
“We traveled at a speed of 5 mph on a crowded, noisy, stinky fishing boat filled with tired researchers,” Johnson explained, “and we weren't finding any shoreline from the old lake anywhere.”
In fact, what they were discovering was particularly striking.
“We found that the lake floor was flat, incredibly flat. It was like sailing across a billiards table hour after hour. We were seeing something really hard underlying the softer, younger sediment layers,” said Johnson.
As week four of the expedition on Lake Victoria began, it was time to take sediment core samples to complete the study. Early on in the coring procedure the researchers found abnormalities. One of the first core samples they took actually bent the steel pipe of the core mechanism. Many questions arose among the scientists. What is the make-up of this compact substance that lies just beneath the soft muddy sediment? Could it be a layer of soil that, at some point, had been exposed to air?
The core samples brought them face to face with the incredible history of Lake Victoria and a discovery that would shatter a world record in the rate of species evolution.
“We broke the pipe apart and saw grass and other evidence to suggest we had actually penetrated through to a soil horizon. It implied that indeed this soil had at some point come into contact with air!” Johnson said. “Grass, however, is not a definite indicator of exposure to air because it can easily be carried by currents out onto the lake where it would sink into the sediment.”
As the team continued the coring technique, they kept finding hard sediment layers and evidence of grass.
“We moved to different sites on the lake and found the same evidence. We took core samples eight times and the same thing came up time and time again,” Johnson said.
By this time Johnson said they could not deny what they discovered on their geological expedition of Lake Victoria. The implications would make news in the world of evolutionary science.
The team didn't have sophisticated testing equipment on the ship to analyze the sediment, so they waited for verification from the samples shipped to the University of Minnesota's Limnological Research Center in Minneapolis. Back in Minnesota, Johnson and his colleagues from the Limnological Research Center, the University of Bergan, Norway, and Materera University, Uganda, found evidence that confirmed their suspicions. They found that Lake Victoria had completely dried out 12,000 years ago.
“We found grassland soil and clay with cracks in it showing that the lake had dried out,” Johnson said, “and there was pollen from cattails just overlying the soil, suggesting a marsh-like setting as the lake basin began to refill.”
Johnson said it took awhile for the reality of this discovery to sink in.
“The most exciting moment for many of us was when we realized the implications for our knowledge of evolution. It wasn't when we opened the core samples. It was after a period of thinking about the trip and the findings that we understood the significance of the evidence,” Johnson explained. “I had written in my notes when we decided to go on the expedition: 'Did this lake dry up completely? If true, then that had very important implications for rapid evolution of this fish.' After we got our results back and I re-read my notes, I must confess I didn't remember writing that question down.”
This year Johnson, after compiling a map from the seismic data, returned to Lake Victoria to take a core sample from the deepest point in the lake. Along with Johnson on the second trip to Africa was UMD graduate student, Mike Rosenmeier, who will receive his master's degree in geology in spring 1997.
“Our ultimate goal during the second trip to Lake Victoria was to penetrate even further into the soil layer to calculate how long the lake basin had actually been dry, as well as confirming that the lake was an expanse of land during the last Ice Age,” said Rosenmeier.
The team's initial research was supported by the evidence from the second trip and the discovery was recognized internationally in London's New Scientist, The New York Times and Science magazine.
Johnson plans on heading up another African expedition in May of 1998. “Our main interest is to get much longer records of change. We've been able to use our coring methods and get back in time typically on the order of 10 to 15 thousand years.
Now we're interested in increasing that capability by an order of magnitude,” Johnson explained. “We want to go back about 150 to 200 thousand years. If we do, it would take us back through two glacial cycles.”
Rosenmeier, who says his next task is to earn his Ph.D in geology, agrees with Johnson that there is a wide open market in the field of large lakes studies.
“The Large Lakes Observatory can make Duluth the hub of world research for large lakes,” said Johnson. As the planet becomes more crowded, more emphasis will be placed on keeping fresh water pure. “Lakes are incredibly important resources. They are Earth's beautiful jewels,” Johnson said. “Think about Lake Superior. We use it for drinking water, transportation, recreation and food. Maintaining that for decades to come is a momentous task. The Large Lakes Observatory can play a central role in insuring that this will happen.”
Preparing to take another core sample on Lake Victoria
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