TRAVERSE CITY, Mich. – Call it the return of the green slime.
Back in the 1960s, foul gobs of algae along Great Lakes shorelines made swimmers and sunbathers miserable before a crackdown on phosphorus pollution repelled the invasion.
Now, the algae are mounting a comeback and controlling it may be tougher this time, according to the Michigan Environmental Council, an umbrella organization for a host of environmental and public interest organizations in the state.
"The nightmare may be poised to repeat itself," the council said in a statement accompanying a report released Wednesday.
Algae blooms have been on the rise since the mid-1990s in parts of all of the Great Lakes except Lake Superior, whose icy waters are not as hospitable to the slimy aquatic plants.
The problem has worsened recently and is particularly severe on shallow, warm Lake Erie, experts said.
"It's very much the same story on our coast" on Lake Michigan, said John Berges, a biologist with the University of Wisconsin at Milwaukee's Great Lakes Water Institute.
Out-of-control algae look bad and smell worse but there are more serious dangers, the environmental council's report said.
Swimmers and pets who accidentally swallow algae-choked water can get sick. Algae blooms can reduce oxygen levels in the waters, causing fish kills. Some clumps are thick enough to block water intake pipes.
The surge four decades ago was blamed on excessive phosphorus, an essential plant nutrient. A single pound of phosphorus can stimulate growth of up to 500 pounds of algae, the report said.
Legislatures in Michigan and some neighboring states imposed limits on phosphate laundry detergents in the 1970s that were credited with significantly reducing the algae buildup.
But phosphorus continues flowing into the lakes from fertilizer runoff from farms and residential lawns, pet and livestock waste and leaky residential sewage systems, the report said.
Also, although Michigan was among the first states to virtually ban phosphorus in laundry soaps, it exempted dishwashing machine detergents — a loophole the environmental council wants the state Legislature to close.
But Berges said he was skeptical that dishwashers are a primary culprit in the green slime's return.
A more likely cause — and the reason the problem may be harder to solve this time — is the arrival in the 1980s of two exotic species of mussels, the zebra mussel and its cousin, the quagga mussel, he said.
The mussels filter water, making it clearer, allowing sunlight to penetrate deep into the lakes, possibly enabling algae to thrive at greater depths than before, Berges said. The mussels also eat microscopic algae and excrete nutrient-rich wastes, he said.
Phosphorus levels are lower in the lakes than they were in the 1960s, increasing the likelihood the mussels are the leading cause of the algae's resurgence, he said.
The report recommends banning phosphorus in lawn fertilizer and dishwasher detergents, helping farmers and landowners create strips of natural vegetation along waterways to absorb excess phosphorus and patching up septic and municipal sewage systems.