Imagine, for a moment, a perfect summer day in southern Manitoba: The sky is free of clouds, there's a gentle breeze and you're on your way to the lake.

As you roll up to the water, you're excited to lie on the beach, laze in the shallows or maybe launch a boat — until you see the thick green sludge on Lake Winnipeg's surface.

The algae bloom is as bright as a lime Slurpee, as odoriferous as an old hockey bag and as thick as the shag carpet at your uncle Sherman's house. If only you had known about the bloom before you hit the road.

This scenario is what Paul Cooley wants to help avoid. The remote-sensing scientist is the brains behind Lake Winnipeg Bloomfinder, a website that uses satellite images to show boaters and beachgoers where algae blooms are visible at the surface.

"We can get an image up in less than a day and people can see what the lake looks like right now," said Cooley, the president of Winnipeg company NextGen Environmental Research.

Algae lined the shore of Balsam Bay, on the east side of Lake Winnipeg's southern basin, on June 25, 2019. (Submitted by Roger Rempel)

Cooley said his website, soon to be up and running for a second season, is intended to help Manitobans make decisions about when they'll visit any given location along Lake Winnipeg.

But he also said he wants to promote a better understanding about the environmental health of the world's 11th-largest lake, which is threatened by eutrophication, an ecological imbalance caused by too many nutrients within its waters.

"I was convinced people are interested," he said, referring to both the health of the lake and the science that explains what's happening within it. "I think it's really important that the community engages in conversations about this and learns more about the lake and becomes more adaptive and resilient."

Blue-greens are the worst

Algae blooms are nothing new for Lake Winnipeg, which drains a vast watershed that stretches from the Rocky Mountains in the west to Dakotas-Minnesota border in the south and the boreal forest of Ontario to the east.

Nutrients that flow into the lake, primarily phosphorus and nitrogen, promote the growth of thousands of species of algae. When the conditions are right, algae can grow into blooms capable of covering vast expanses of the lake.

The brown stains below the ice at the end of the winter are usually caused by diatoms, tiny organisms with shells that resemble glass, said Hedy Kling, a Manitoba lake ecologist and one of world's leading experts on identifying algae.

Stringy blooms that you can pick up in your hands are probably blooms of green algae, which makes great garden fertilizer, she said.

But paint-like blooms of green or indigo algae, too watery to be held in your hands, are likely blue-green algae, she said.

These are the species of greatest concern to people who use the lake. Blue-greens can produce dangerous toxins, under certain environmental conditions.

When blue-green algae start running out of food, they may emit poisons that kill off part of the bloom in order to free up nutrients for the rest of the algae.

"Some of them die, so the others can live," Kling explained.

Phosphorus is the culprit

Blue-green algae are also notable for their ability to get some of their food directly from the air. When there isn't enough nitrogen in the lake, blue-greens can grab it straight from the atmosphere, which is 80 per cent nitrogen.

That's why environmental scientists have focused mostly on trying to limit the flow of phosphorus — the other main nutrient that promotes the growth of algae — into Lake Winnipeg.

Manitoba Sustainable Development would like to see Lake Winnipeg limit phosphorus to 50 parts per million. But over the past five years, phosphorus concentrations in the lake's southern basin have averaged around 100 parts per million.

"We're at about double of what we'd like to see," said Nicole Armstrong, director of water science and water management for Manitoba Sustainable Development. "This isn't surprising. Nutrients are coming from a huge watershed."

This satellite image from July 30, 2018, shows relatively clear water entering Lake Winnipeg from the mouths of the Red River — and an algae bloom off Grand Beach, to the upper right. (European Space Agency image modified by NextGen Environmental)

About half the phosphorus that winds up in Lake Winnipeg flows into the lake from other Canadian provinces and the U.S., where the health of Manitoba's largest lake isn't anywhere near as much of a top-of-mind concern as it is in communities along the lake.

Major floods also increase the load of phosphorus into the lake, said University of Manitoba geographer Greg McCullough, who's spent decades studying the way nutrients flow into Lake Winnipeg and what happens afterward.

This is partly a simple matter of more water bringing more nutrients into the lake. But about a third of the additional phosphorus that winds up in the lake in soggy years can be attributed to floodwaters soaking up phosphorus from agricultural fertilizers and sewage, McCullough said.

"If you didn't have a flood, it wouldn't get off the land and you wouldn't have high concentrations," he said.

Climate change is also a problem, McCullough said. Warmer temperatures make Lake Winnipeg more hospitable to blue-green algae, which prefer balmier conditions, he said.

Climate change is also making the Minnesota portion of the Lake Winnipeg drainage basin wetter, creating more of the overland flooding that brings nutrients into Manitoba, he added.

Most of the phosphorus that enters the lake stays in the the lake, where it's either suspended in the water or deposited in lake-bottom sediments that get churned up.

Paul Cooley says he was inspired to take action about Lake Winnipeg after growing up alongside the lake. (Submitted by Paul Cooley)

All of these factors have contributed to something cottagers, boaters, commercial fishers and beachgoers have noticed: bigger algae blooms, more often.

"What we're seeing, generally, is more frequent and intense algal blooms over the last several years," Armstrong said.

While the province monitors the lake for algae blooms, Armstrong said she welcomes Paul Cooley's efforts to place satellite images of blooms online.

"We simply can't be everywhere at once," Armstrong said.

The remote-sensing scientist said he was inspired to act after growing up along the lake at the family cabin in the Victoria Beach area.

"It took several generations to get the lake into the state that it's in now and it will likely be multigenerational during this recovery," he said.

"Everyone is trying hard, but I think at this point in time that the challenge is large enough where it's sort of an all in situation."