Spindly aquatic algae, once foul-smelling icons for Great Lakes pollution, are back. During the depths of the lakes' environmental troubles in the 1960s and '70s, the algae's population exploded. Vast clumps piled up on beaches, looking like untreated sewage and smelling like a pig farm.
Now, despite significantly cleaner lakes, the algae have staged a comeback in every lake but Lake Superior. In some areas, they appear to be more pervasive than before. Scientists are struggling to figure out why.
Some say Cladophora's resurgence is a nuisance algal bloom, not a harmful algal bloom, which can pose a direct danger to fisheries and public health. But the stakes are still high, specialists say.
The summertime appearance of dead clumps of odorous algae can close beaches and pinch property values. They also can trigger sudden shutdowns at nuclear power plants as algae clog pipes that suck cooling water from the lakes.
Even though it is often labeled a "nuisance," the algae's comeback raises public-health concerns, researchers add. As it decomposes, and as gulls stalk the mats looking for shellfish, levels of potentially harmful bacteria build.
"This is a huge issue for the entire region," says state Rep. Jonathan Richards (D) of Milwaukee, whose district includes a long stretch of shoreline. Public concern is not limited to lakefront property owners, he says, but reaches 10 to 20 miles inland.
The obvious suspect - nutrients from agricultural or urban runoff entering the lakes - may not tell the whole story.
"It's tempting to look at local sources - farms and industrial wastes," says Erica Young, an aquatic biologist at the University of Wisconsin at Milwaukee who is working on the problem. But although there are hot spots, "over the last couple of years, it's become clear that this is a lake-wide problem."
Cladophora's reemergence typifies a range of environmental challenges affecting lakes, rivers, and shorelines nationwide.
"I was a student on the first Earth Day, and I've been watching what's been going on around the globe ever since," says Marty Auer, an engineering professor at Michigan Technological University in Houghton, Mich. He's one of the sleuths credited with showing how Cladophora could be controlled in the 1980s. "It's amazing to me that as a society, we really do have the capability to engender massive changes in this ecosystem. This is another one of those cases."
Cladophora is an invasive species, notes Harvey Bootsma, a scientist at the University of Wisconsin at Milwaukee's Great Lakes Water Institute. It arrived in the 1800s. But it's not treated as invasive "because its genus is found all over the world," he explains.
Outbreaks during the last century were triggered when phosphates poured into the lakes, largely from household detergents and fertilizers. Dr. Auer and a colleague from the University of Michigan showed that when phosphate-laden water was properly treated, algae populations fell dramatically.
Phosphate levels fell after regulators put research results to work, and the US and Canada agreed to hold phosphate concentrations in the lakes to less-damaging levels.
Yet over the past decade, Cladophora has reemerged. Interestingly, despite deep-water phosphorus concentrations that still meet the conditions of the agreement, near-shore levels appear to be rising, says Robert Hecky, a researcher at the University of Waterloo in Ontario. Nonpoint sources of phosphates, such as regionwide fertilizer runoff, could be part of the reason, he and others say. So could storm runoff from urban areas. But the picture is complicated by what Dr. Hecky calls "an unholy alliance" between the algae and newer invasive species: zebra mussels and more recently introduced quagga mussels.
As phosphorus-enriched water flows into the lake, near-shore microalgae take up the nutrient. The mussels feed on the micro- algae. They deposit what they can't digest or the byproducts of what they can on the lake bottom. This tends to concentrate phosphorus where the mussels are. To Cladophora, this is heaven: plenty of food and hard shells on which to grow. And because mussels are filter feeders, they quickly turn murky water into clear water where they spread, allowing sunlight to penetrate to greater depths. This, in turn, expands the area over which Cladophora can grow. And where zebra mussels must attach themselves to rocks or other hard substrates, quagga mussels can grow on sand, laying the groundwork for further spreading.
Researchers say they are set to run experiments this summer to test this "phosphorus shunt" hypothesis.
For Lake Michigan, subtle changes in regional wind patterns may be another factor. Typically, prevailing winds in the summer pile up warm surface waters against the Michigan coast, drawing cold water up from the depths along the lakefronts of Illinois and Wisconsin. But during the past several years, the winds have shifted, sending warm water west. This may have provided Cladophora more favorable temperatures.
Ironically, the "unholy alliance" to humans may benefit fish. Dr. Bootsma says that the collection of shells and algae, which look like tiny shrubs, act as havens for small snails and crustaceans that larger fish feed on - much as a coral reef nurtures critical links in the ocean's food chain.
On the other hand, when mats of Cladophora die off and begin to decay, the process may create zones of water and lake sediment bereft of oxygen. In these small-scale "dead zones," harmful bacteria can thrive. Over the past five years, researchers have seen an increase in avian botulism in Lake Erie, which coincides with the resurgence of Cladophora, he adds.
One goal of US and Canadian research is to update a computer model Auer first developed to incorporate the new influences on Cladophora growth. The researchers say they hope the model will help forecast Cladophora outbreaks, as well as allow policymakers to weigh the costs and benefits of different approaches to managing the Cladophora problem.
Given the complex factors involved, it might seem that people in the Great Lakes region may have to learn to tolerate - if not love - their invasive, energetic algae. But noting that researchers and conservation managers have won other ecological battles, "I remain hopeful," Auer says. "We've got talented scientists, bright young people, and new technologies. I'm going to remain hopeful. That's what I do."