Nearly 500,000 people in northwestern Ohio have been warned not to drink or boil their tap water since Saturday. (Update: On Monday, the mayor of Toledo lifted the drinking-water ban, saying tests had shown the water was safe to drink again.)

A large algae bloom is planted on the western edge of Lake Erie

The reason for the ban? Water officials in Toledo found evidence of microcystin — a toxin created by blue-green algae that can cause nausea and liver damage.

Officials are still conducting tests to figure out what happened, but the most likely source of the toxin would be the large algae bloom that's parked itself on the western edge of Lake Erie — where Toledo gets its drinking water.

Here's what the lake looked like as of July 31, 2014, according to a satellite image from the National Oceanic and Atmospheric Administration (NOAA). The bloom is on the western side, right by Toledo:

Experts say the bloom, which was nurtured by excess phosphorus runoff, appears to be pinned to the western edge of the lake by a combination of winds, waves, and currents.

So far, the city of Toledo, which provides water to about 500,000 people in the region, has been the most severely affected. The Toledo Blade reports that city officials are still running tests and trying to figure out how to make the water drinkable again. They're still not sure how the toxin could have gotten through the plant's treatment systems — or how widespread the contamination actually is.

Where do toxic algae blooms come from?

Algae blooms in Lake Erie typically arrive in the summer and are fed by excess nutrients in the lake — especially phosphorus.

The blooms are fed by Fertilizer runoff from farms and lawns

That phosphorus can come from fertilizer runoff from farms and lawns as well as from livestock pens or malfunctioning septic systems. A bout of strong sunshine this summer also appears to have helped the algae bloom grow.

These algae blooms were quite common in Lake Erie back in the 1960s and 1980s, and then faded away for 20 years or so. (In the 1970s, things got so bad that the United States and Canada made a concerted effort to limit the total amount of phosphorus that entered the lake.)

Recently, however, the blooms have been making a comeback — possible reasons include heavy phosphorus runoff from farms in northern Ohio, an invasion of foreign zebra mussels that have removed competitors to the blue-green algae, and heavier rainfall in the spring. This year's bloom is forecast by NOAA to be the fourth-worst since 2000:

Severity of algae blooms in Lake Erie

These cyanobacterial blooms can inflict plenty of environmental damage. For one, when the blooms decay, they consume a lot of oxygen in the lake, killing off other plants and animals.

What's more, under certain conditions, some species of cyanobacteria — the blue-green algae — can create a variety of cyanotoxins that can cause damage to the human liver and nervous system. Here's a list:

According to the Environmental Protection Agency, these cyanotoxins can be fairly tricky to detect and remove from drinking water supplies. Certain screening techniques and treatments work for some toxins but not others.

Cyanotoxins can be tricky to remove from the water supply

"Therefore, drinking water operators must know the growth patterns and species of cyanobacteria that dominates the bloom, the properties of the cyanotoxins… and the most effective treatment process," the EPA notes. "Applying the wrong treatment process at a specific state in treatment could damage cells and result in the release rather than removal of cyanotoxins."

In the case of Toledo, there are still lots of unanswered questions: Is there actually microcystin in the water supply? And if so, how did it get there? Why didn't existing treatment methods work? After all, microcystin has appeared at the plant before, but it usually been filtered out.

Ohio officials have warned that microcystin-tainted water is unlikely to be fatal, but the toxin can cause nausea and liver damage, so they're warning residents not to drink — or even boil — the tap water. It's not yet clear when the water will be safe to drink again, they say.

In the meantime, residents around Toledo are currently rushing far and wide to stock up on bottled water (and prices, predictably, are surging). The state is currently trying to get more bottled water to the region.

Can we prevent these toxic algae blooms?

Experts have suggested a couple of different steps over the years for dealing with the resurgence of toxic blooms.

Some experts have called for stricter limits on phosphorus into Lake Erie

One straightforward way to prevent future algae blooms would be to put less phosphorus into the lake. That's what the region did back in the 1970s after Lake Erie was utterly clogged by algae.

At the time, the United States and Canada spent some $8 billion upgrading sewage systems, banning certain types of phosphorus-heavy laundry detergents, and pushing to modernize farming practices so as to prevent so much soil and stormwater runoff. The ultimate goal was to limit the phosphorus load in Lake Erie to 11,000 metric tons per year — half what it was at the peak between 1967 and 1972. That was thought to solve the problem.

But now that Ohio's farms are expanding and the blooms are returning, some groups have been calling for stricter controls. The Lake Erie Ecosystem Priority recently issued a report arguing for even stricter limits on the phosphorus load. That could be achieved, the report noted, via things like a ban on farmers spreading manure on frozen ground in the winter, or improved infrastructure to reduce urban storm-water runoff.

Other experts have called for improved methods to control the spread of cyanobacterial blooms or better techniques to filter out harmful microcystins. A 2012 paper in the Polish Journal of Environmental Studies, for instance, described a technique for a sand filter that contained bacteria that could degrade microcystin.

That paper noted that toxic algae blooms have become "one of the most serious health risks of the 21st century," cropping up in places like China, Japan, Brazil, and Australia.

Further reading: