The shores of Lake Erie. (Photo by Ty Wright for The Washington Post)

On Monday morning, two days after residents of Toledo, Ohio, were ordered to stop drinking tap water, the ban was lifted and the water declared safe to drink. Adding a dose of theatricality to the announcement that the water was drinkable once again, Toledo Mayor D. Michael Collins stood in front of reporters, called the water safe and downed a glass himself.

The notice to stop using tap water, first issued by Toledo on Saturday, affected about half a million people. Toledo’s system produces 26 billion gallons of drinking water per year, water that is taken from Lake Erie to the Collins Park Water Treatment Plant for purification, according to last year’s annual report from the Toledo Department of Public Utilities.

It was at this treatment plant that chemists found samples showing higher levels of the toxin microcystin, which causes abdominal pain, vomiting and kidney damage. Toledo’s water notice on Saturday said that the problem may have been due to algae at Lake Erie, the source of the city’s drinking water. (Microcystin-LR, considered one of the most toxic types of microcystin, is also also one of the most common strains seen in the Great Lakes, according to the National Oceanic and Atmospheric Administration.) Once the water ban went into effect, city and state officials began sending tens of thousands of gallons of water to distribution centers around Toledo and the surrounding area.

Testing at Collins Park and across the system showed that by Monday morning, the microcystin levels in the water were acceptable once again, the Ohio Environmental Protection Agency announced. (More on precisely what “acceptable levels” are at the bottom of the post.)

The algae bloom at Lake Erie strengthened since last Thursday, NOAA’s Great Lakes Environmental Research Laboratory and the National Centers for Coastal Ocean Sciences said in a bulletin on Monday. These blooms have been centered in the western portion of the lake, with the heaviest concentration seen in the part of the lake near Toledo. (This surge was predicted by NOAA last month, though it noted that the bloom was expected to be smaller than was seen last year.)

In the summer, these algal blooms are a regular occurrence along the western portion of Lake Erie, according to EPA reports. While the water ban in this case only lasted for two days, it speaks to another issue: More than 250 million people in the United States depend on fresh water from lakes and rivers for drinking water, the EPA said, resources that are susceptible to problems stemming from algae and pollution-related chemicals.

The water issue in the U.S. pales in comparison to the clean water shortages in other parts of the globe. There are 783 million people without access to clean water around the world, according to the United Nations. But the Toledo ban still speaks to the sensitivity of water systems in the U.S., which are relied upon by hundreds of millions of people and can be severely affected by natural occurrences or outside contaminants.

Earlier this year, residents in West Virginia had endured a 10-day ban on using tap water after a chemical involved in coal processing leaked into the water supply. In Portland, Ore., officials announced in April that they would dump about 38 millions of drinking water after a teenager urinated into the reservoir. A month later, Portland issued a boil-water alert for hundreds of thousands people and drained two reservoirs after water samples were found to be contaminated (its third boil-water alert in the last six years). And less than a year ago, Carroll Township in Ohio — not far from Toledo — also issued a tap water ban after higher levels of microcystin were spotted in the water.

And, as the EPA notes, there is also the issue of climate change; this could significantly affect the water levels in the Great Lakes, which would in turn affect the people who rely on this water. The National Wildlife Federation said in a report last year that the changing global climate and fluctuations in the weather will lead to more algal blooms at Lake Erie.

“Over the past two days we’ve been reminded of the importance of our crown jewe l— Lake Erie — to our everyday lives,” Ohio Gov. John Kasich (R) said in a statement Monday. “We must remain vigilant in our ongoing efforts to protect it.”

It is not clear what, exactly, caused the bloom and contamination and what will happen next. Craig Butler, head of the Ohio Environmental Protection Agency, said Monday that the agency would work with the city of Toledo to figure out what happened. The surge in algae is believed to be due to fertilizer used around the lake that is pushed into the lake by rain. The International Joint Commission, a U.S.-Canadian agency, said in a report issued earlier this year that fertilizer limits would be needed.

Meanwhile, the city of Toledo said on Monday that “steps are being taken statewide” to create a regulated microcystin testing system. The acceptable level of microcystin in Toledo’s water is 1 microgram per liter, a number that comes from the World Health Organization. Most of the guidelines governing acceptable microcystin levels in the U.S. come from WHO because there are no federal regulations governing microsystin.