The water of Lake Superior along the rocky shoreline of northwestern Wisconsin's Apostle Islands is normally cold, clear and pristine. But late last week, kayakers and other visitors to the popular lakeshore found a scummy algae on Superior's surface.

While the bloom has largely dissipated, reports suggest it stretched in patches across a distance of about 50 miles along the shoreline from near Superior, Wis., to the Apostle Islands, said Robert Sterner, who directs the Large Lakes Observatory at the University of Minnesota Duluth. Sterner said the size of the algal bloom could be "unprecedented."

In some places, the algae covered the water in a thin layer of scum the color of pea-green soup. In other areas, the bloom broke up and looked more like flecks of pollen suspended in the water.

The blooms only started appearing in Lake Superior in 2012. Courtesy of Brenda Moraska Lafrancois

"They're very noticeable and very unexpected in a place like the deep and cold and generally low-nutrient Lake Superior," Sterner said.

Blue-green algae has become a lot more commonplace in recent years in lakes and rivers across the Midwest, including in parts of the Great Lakes. Warming lake temperatures are part of the reason. So is the increase in nutrients like phosphorus and nitrogen running off into water bodies from farmland and cities.

Sometimes, the algae can be toxic. In 2014, the city of Toledo, Ohio, had to stop pumping drinking water from Lake Erie for two days because algae blooms had poisoned the water.

Researchers don't know yet whether this most recent Lake Superior bloom was toxic. They've sent samples for testing and expect to hear back by the end of the week.

Meanwhile, they're trying to figure out why Lake Superior is suddenly experiencing these algae blooms.

Scientists don't have a definitive answer yet, but they believe the blooms are connected to several major storms that have socked the region in recent years.

The dominant algae in the blooms are examined under a microscope. Courtesy of Brenda Moraska Lafrancois

It started in 2012. That's the year a blue-green algae bloom was first detected in Lake Superior. It happened a few weeks after a storm in which 10 inches of rain drenched Duluth and the surrounding region, wrecking infrastructure and sending a plume of brown sediment into the lake.

It happened again in 2016 following storms that flooded Lake Superior tributaries in northern Wisconsin.

And it happened again this year, after historic storms in June dumped nearly a foot of rain across the region.

"In each of those cases, we've seen a significant algal bloom develop," said Brenda LaFrancois, a National Park Service aquatic ecologist.

Those huge rainstorms carry massive amounts of sediment into Lake Superior. And that sediment contains a lot of nutrients — from cities, lawns and farm runoff — which feed algae growth.

The increasing frequency in algal blooms is also tied to warmer lake temperatures. Scientists at the University of Minnesota Duluth have found that Lake Superior summer surface water temperatures have increased by 5 degrees over the past 30 years.

Warming waters: Superior nearly breaks water temperature record

"Lake Superior is a generally warming environment, and we see these blooms associated with relatively warm temperatures. We think that's an important factor," said Sterner.

Researchers expect to see more periodic algae blooms in Lake Superior, in part because of that increased water temperature.

On top of that, big rainstorms — the kind of events that researchers believe have triggered recent blooms — are expected to happen more frequently, especially in the Upper Midwest, an effect of climate change.

Researchers are working to understand what those changes in the lake and the climate mean for the formation of potentially harmful algae blooms in the future.

But in the meantime, they stress that the blooms that have occurred so far have had very little impact on the water quality of a lake that holds 10 percent of the world's fresh water.

"We're talking about a very tiny fraction of all of that water having this blue-green algae problem right now," Sterner said.