LEAMINGTON, ONT.—Early Wednesday morning, eight research vessels launched from the shores of Lake Erie. Five started from the American side, and three from the Canadian side. All had the same target: a slick of blue-green slime spreading across the lake's western basin.

Lake Erie is experiencing a harmful algal bloom that extends for roughly 50 kilometres, more than double the distance between Highway 427 and the Don Valley Parkway. Algae is a normal component of healthy freshwater ecosystems. But when certain species grow out of control, they can create dense, wide and sometimes poisonous mats of slime that threaten the whole food chain.

That includes the species at the top of the food chain: humans.

A bloom in Lake Erie in the summer of 2014 led to a multi-day drinking water ban in Toledo, Ohio, after officials detected algae-based toxins in the municipal water supply. Canadian researchers recently calculated that if Lake Erie's harmful algal blooms are left unchecked, the financial hit to tourism, fisheries and other markets in this country will amount to more than $5.3 billion over the coming decades.

The fifth anniversary of Toledo's drinking water crisis passed last week, but the problem is not getting better with time. The five worst harmful algal blooms on record in Lake Erie have all occurred since 2011.

This summer's bloom has scientists on high alert.

"This year's looking like it's going to be in the top couple. It's as bad as it has been," says Warren Currie, a Fisheries and Oceans Canada (DFO) research scientist with the Burlington-based Great Lakes Laboratory for Fisheries and Aquatic Sciences.

Wednesday's blitz was a co-ordinated, bi-national effort by more than a dozen academic and government research partners to characterize the physical, chemical and toxic properties of the bloom. Researchers refer to harmful algal blooms as HABs, and called the daylong effort the "HABs Grab."

Currie led one portion of the HABs Grab aboard the RV Cisco, a DFO boat specially equipped to survey the Great Lakes. Two other segments of Canadian waters were covered by boats from the University of Windsor's Great Lakes Institute for Environmental Research (GLIER).

Mike McKay, the executive director of GLIER, is Canadian, but spent 30 years working in the U.S. He notes that in Ohio, the harmful algal bloom has dominated the news for weeks. In Canada, the response has been muted.

"I think for a lot of people in Ontario it's sort of out of sight, out of mind," says McKay.

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Ontario benefits from some lucky features that concentrate algae blooms on the southern side of Lake Erie. In Ohio, "it's evident to anybody at a beach. You see the green water near shore."

But this province is not immune. On Monday, the bloom came within a kilometre of touching the shoreline in the town of Colchester, Ont. Forecasts predicted the bloom could reach Pelee Island over the weekend.

This was the first year Canada has participated in the HABs Grab. McKay says Canada has contributed to the problem of harmful algal blooms and it must be part of the solution, too.

"To solve this problem, we have to be proactive. And that's all in the watersheds."

The Great Lakes, as schoolchildren are taught, hold one-fifth of the world's total surface freshwater. More than 40 million people live within the Great Lakes basin, and one in four Canadians draws their drinking water directly from the Great Lakes.

Lake Erie is the smallest and shallowest of the five. At the same time, its major tributaries run through some of the most productive agricultural lands in both countries: hectares upon hectares of corn, soybean and other crops, as well as livestock operations.

When nutrients — fertilizers and manure, and in particular phosphorus — run off fields and into rivers, they are carried downstream and deposited in the lake. The phenomenon is known as nutrient loading. Instead of nourishing crops, these nutrients send the growth of naturally occurring algae species into overdrive.

The Maumee River in particular — which starts in Indiana and runs for more than 200 kilometres through land dominated by agriculture — deposits its waters in Lake Erie's southwestern corner. Those waters flow into the rest of the western basin of Lake Erie, which is extra shallow, and therefore extra warm. But Canada contributes too: the Thames River runs through highly productive lands and, via Lake St. Clair and the Detroit River, into the same basin.

"The situation in Lake Erie is a perfect storm," says McKay. "It's almost like an incubator." Algae blooms have been reported in other parts of the Great Lakes; toxin-producing blue-green algae was confirmed in the Hamilton Harbour in July. But because of Lake Erie's features, harmful algal blooms are a recurring summer phenomenon.

Scientists are still trying to understand why some algae species respond so spectacularly to nutrient loading and not others. Lake Erie's blooms are dominated by blue-green algae, or cyanobacteria. Some cyanobacteria produces a toxin called microcystin, which is harmful to humans, pets and livestock, and can even be lethal.

In 2014, officials in Toledo detected microcystin in a city water treatment plant. The mayor warned the half-million residents served by the municipal water supply not to drink from the tap, or even brush their teeth with city water.

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"When you lose access to a resource you take for granted, such as clean safe water, hysteria follows," says McKay. "We saw shelves being cleaned out of bottled water supplies around Toledo for probably a hundred-mile radius. So that really increased awareness, at least in U.S. states that border the Great Lakes."

Ontario is less at risk of an algal bloom drinking water crisis for a few reasons. The Maumee River, the biggest nutrient contributor, discharges near Toledo, and the circulation patterns of the lake push those waters south. The Ontario shoreline has just a handful of municipal water intakes, and municipal treatment plants do a good job of removing microcystin from the water supply, McKay says. Property owners who rely on wells may be at a higher risk, however. The Windsor-Essex County Health Unit, the Essex-Region Conservation Authority, the Ministry of Environment and Climate Change and municipalities are all monitoring water quality in different ways.

But other lake-related industries are at risk: commercial fisheries, recreational fisheries and tourism are together worth billions, as Canadian researchers recently calculated.

As DFO's Currie puts it, "People don't want to bring their boats and put them in the water knowing that they're going to get slimed." As if to illustrate his point, as researchers unloaded water samples from the DFO research vessel, two young men were unloading beach towels and half-eaten bags of chips from a pleasure craft.

One factor in the dramatic uptick of harmful algal blooms in Lake Erie may be climate change. Strong spring rains drive more nutrients from fields into Lake Erie's tributaries, and climate change is associated with more severe rainstorms. But changes in farming techniques may also play a role.

McKay refers to harmful algal blooms as a "field-to-faucet" problem and says the solutions need to be proactive, not reactive: once the nutrient loading hits the lake, it's hard to reverse.

On Wednesday, federal Environment Minister Catherine McKenna was in Burlington to pledge $1.06 million in funding over three years for 10 clean water projects, part of $44.8 million for the Great Lakes Protection Initiative announced in the 2017 budget. Some of the projects will address phosphorous loading and harmful algal blooms in Lake Erie.

"It's nowhere near enough money to accomplish the goals, but they're spending it in the right place," says McKay.

Ultimately, researchers say, both countries need to work together. In 2016, the U.S. and Canada committed to reducing phosphorus runoff into Lake Erie's western and central basins by 40 per cent; last year, the federal government and the province released their plan for achieving that target. Environmental groups criticized the plan for relying too much on weak, voluntary measures, however.

In 2002, the U.S. National Oceanic and Atmospheric Administration (NOAA) began issuing a forecast for the severity of algae blooms in western Lake Erie on a scale of 1 to 10. In 2011, the bloom measured as a 10; the worst year on record, 2015, broke the scale at 10.5.

This year's prediction, issued in July, was roughly an 8.

NOAA provides a daily Lake Erie Harmful Algal Bloom bulletin, showing satellite imagery of the extent of the bloom. But that data is a proxy measurement: it detects blue-green colour in the water; it cannot measure the presence of algae directly.

Last week's HABs Grab was an effort to sample nearly 200 sites across the western basin of Lake Erie directly. Part of the goal of the day was to validate the satellite's algorithm, and part of it was to provide a more complete portrait of the characteristics of the bloom. While the samples were collected by eight different vessels and processed by their respective teams, they will all be analyzed at the University of Toledo to ensure accuracy.

The bloom won't peak for a few more weeks and it could underperform predictions. But when DFO's Warren Currie set out on the lake Wednesday morning, he was surprised to see slime well beyond where satellites had predicted it would be.

"It's really expanded out. We saw green water in a lot more of the basin," he says.