An algal bloom in 2004 turned Ford Lake green (Credit: John Lehman)
Ford Lake in Southeast Michigan has suffered from nuisance algal blooms since Henry Ford built the dam on the Huron River that created the impoundment in the 1930s. The effort to uncover the cause of the blooms has all the thrilling elements of a well-spun science yarn, from the pursuit of reproducible results to a natural intervention that supplied elusive control data.
Until recently, there had been two competing hypotheses about what caused the algal blooms. The first was that phosphorus flowing into the lake from the Huron River provided the algae with the nutrients they needed to run amok. The second was that the phosphorus came from within the lake, released from the sediment and stirred up when the lake mixed after periods of anoxia.
John Lehman, a professor in the University of Michigan’s Department of Ecology and Evolutionary Biology, had found the second hypothesis more viable since one August day when he and a student were out on the lake during a thunderstorm.
“A cold front came through and all of a sudden we started smelling hydrogen sulfide,” Lehman said, referring to the rotten egg smell produced by another microbial consequence of hypoxia. “And I said, ‘Oh man, this lake is turning over right now.’ And then subsequently we started seeing the algae growing.”
To test that hypothesis, Lehman and other researchers got permission from Ypsilanti Township in 2006 to manipulate the lake by controlling how water flowed through the dam. Rather than drawing water from the lake’s surface, their plan was to selectively pass water through floodgates at the bottom of the dam. That would keep the temperatures similar throughout the column, preventing the stratification that promotes anoxia and unlocks phosphorus bound up in the sediment.
The lake responded well and went without a nuisance bloom that summer. They treated 2007 as a control year and reverted to drawing water mainly from the lake’s upper layer. The blooms came back.
In 2008 the researchers installed a mooring to monitor water temperature and dissolved oxygen at multiple depths throughout the water column. Measurements taken every 30 minutes would give them a much clearer picture of what was going on in the lake. They rebooted the selective withdrawal plan, and the lake went without a bloom from that year until 2011.
While all that was going on, local governments were tackling the other hypothesized cause of the algae blooms phosphorus loading from the Huron River. Lawn fertilizer restrictions were passed in 2007, which Lehman said they detected a drop in phosphorus concentrations in the Huron;
“Lo and behold, we were actually able to document it, And that turned out to be a big deal nationally,” he said. “Around the country, it’s had a big effect in terms of environmental legislation.”
That was good news for environmental policy advocates, but it left Lehman with a scientific conundrum. Both the restrictions and selective withdrawal went into effect in 2008. Sure, the lake was in better shape, but why? Was it the fertilizer ordinance, the dam operations, or both?
“That’s not a satisfying place to be in science,” Lehman said.
To clear things up, Lehman would need another control year without selective withdrawal. If the blooms returned despite lower fertilizer inputs, he could be sure that it was the new dam operations that cleared up the lake. So in 2009, he went back to the township and asked them to forego selective withdrawal.
“And they said ‘Not on your life,’” Lehman said. They had a bloom-free lake and weren’t willing to risk losing it in the name of scientific inquiry.
But then came the dry summer of 2012. Drought conditions prevented dam operators from discharging much of anything from the lake, making selective withdrawal impractical. That effectively gave Lehman a control year. The water quality mooring and other sampling efforts showed that the lake stratified for 35 days in June and July. Dissolved oxygen dropped to near zero and the sediment released phosphorus. A cold front in late July and August mixed the lake, churning the nutrient up to the surface where it triggered an algae bloom.
“In that case we could say it wasn’t the reduction in phosphorus loading from the outside, it was the fact that the phosphorus built up in the anoxic water and then the subsequent mixing events brought it up into the surface,” Lehman said. “I think it was a pretty clean demonstration of what the real causes were.”
Top image: An algal bloom in 2004 turned Ford Lake green (Credit: John Lehman)