Lake Erie’s toxic algae woes unlikely in Lake Superior
The lack of large-scale, intensive agriculture in the Lake Superior watershed, coupled with the big lake’s cold water and great depth, are enough to prevent the kind of toxic algae bloom seen in recent days on Lake Erie.
Nearly all residents who live around Lake Superior get their drinking water out of the big lake, including residents of Duluth, Superior and surrounding communities. But Lake Superior has avoided the kind of algae growth that has led to problems elsewhere, including over the weekend in Toledo, Ohio, which gets its water from Lake Erie.
“It’s all about the fact Lake Superior is so low in nutrients, especially phosphorus, and because it’s so cold,’’ said Robert Sterner, director of the Large Lakes Observatory at the University of Minnesota Duluth.
Sterner said it’s likely that agricultural phosphorus running through tributaries, such as the Maumee River, is over-fertilizing Lake Erie. Because Lake Erie is relatively warm, that gives algae a chance to grow, including into its sometimes toxic blue-green stage. And because Erie is relatively shallow, older phosphorus on the lake bottom can be more easily stirred up for algae to feed on.
But there is very little agricultural phosphorus running into Lake Superior — some 95 percent of its shoreline is forested. And because the lake is so cold, algae simply don’t get the energy to grow.
Lake Erie also has a large watershed that stretches for miles, allowing more rivers to flow more miles and carry more nutrients into the lake. Lake Superior’s watershed, by comparison, is much smaller in relative size to the huge lake. Much of the water in Lake Superior falls onto the lake itself as snow or rain, Sterner noted, and doesn’t have the chance to pick up nutrients on shore.
“It’s not to say there is no nutrient load from the watershed, but relative to the size of Lake Superior, it’s quite small,’’ Sterner said.
Algae blooms do happen in the Northland, however. The St. Louis River estuary saw a large algae bloom last summer after tons of nutrients washed into the lower river in the June 2012 flood. That provided that catalyst for a major algae outbreak in the harbor in 2013, said Euan Reavie, an aquatic ecologist and algae expert at UMD’s Natural Resources Research Institute.
Unlike the blue green algae of Lake Erie, however, the diatoms in the St. Louis River aren’t a health issue.
“It’s not that we can’t have algae blooms here, it’s just that we don’t have the same conditions as Lake Erie ... and we have different species of algae that are usually not toxic,’’ Reavie said.
Howard Jacobson, Duluth’s utility operations manager, said the raw Lake Superior water coming into the city water treatment plant is cleaner than many cities’ treated water. Still, the city has operated a filter system since the 1970s to remove large particles and add a small amount of chlorine to kill any bacteria that make it through.
“We sample the intake water coming into the plant regularly, and spot-check samples from throughout the system to make sure we don’t miss anything,’’ Jacobson said.
On Saturday, officials told a half-million residents served by the Toledo, Ohio, water supply not to drink, brush or cook with water from the tap, and warned them not to give it to their pets. They urged older and sick people not to bathe in the water.
Tests at the Toledo water treatment plant had found unsafe levels of microcystin, a toxin that can cause diarrhea, vomiting and even abnormal liver function. Officials blamed an algae bloom on Lake Erie. The blooms are believed to be caused by excess phosphorus washed into the lake from farm fields and lawns, malfunctioning septic systems or livestock pens. The ban was lifted on Monday after tests showed that toxins were no longer at dangerous levels.