NASA Photo: Lake Erie Algal Bloom Is so Severe It Can Be Seen From Space and Covers an Area Bigger Than Houston

The algal bloom that appeared in Lake Erie in July is so severe it can be seen from space, with a NASA image showing how a vast area of water has turned green. Experts with the National Oceanic Atmospheric Administration (NOAA) had predicted 2019 could see a particularly severe bloom, and it turned out to be one of the biggest in recent years.

Algal blooms happen when there is a rapid increase in the population of algae in a body of water. They tend to happen when certain wind and water conditions are favorable. The water changes color because of pigments in the algae.

These blooms can be harmful. Some species drain the water of oxygen, meaning other marine animals living there suffocate to death. Other species, like in Lake Erie, produce toxins that can kill fish, birds and mammals. In extreme cases, people can also die.

lake erie
Lake Erie algal bloom as seen from space. NASA Earth Observatory

Microcystis cyanobacteria, the organism responsible for the Lake Erie bloom, produces a toxin that can cause vomiting, dizziness, numbness and liver damage. This year's bloom began last month. The NASA image was taken on July 29 at the point when it covered about 300 square miles. By August 13, it had spread out over 620 square miles.

"Green patches show where the bloom was most dense and where toxicity levels were unsafe for recreational activities," NASA said in a statement.

On Thursday, the NOAA put out a notice warning people of the danger posed. It said analysis of the bloom was ongoing and that toxin concentrations were decreasing, "but may continue to exceed the recreational threshold where the bloom is most dense."

It continued: "Keep pets and yourself out of the water in areas where scum is forming. The persistent cyanobacteria bloom in Sandusky Bay continues. No other blooms are present in Lake Erie."

NASA said the bloom in Lake Erie had been the result of calm winds in July that allowed algal toxins to build up—normally, they would have been dispersed. This was combined with heavy rains that may have transported nutrients to the lake from farms.

The NOAA bulletin said winds forecast over the weekend could result in the bloom mixing and moving eastwards towards the Bass Islands.

lake erie blooms
Algae blooms at Lake Erie. NOAA

Over the last 10 years, only algal blooms at Lake Erie have been considered mild. The rest were either significant or severe. "The severity index is based on a bloom's biomass—the amount of its harmful algae—over a sustained period," the NOAA said. "The largest blooms, 2011 and 2015, were 10 and 10.5, respectively. Last year's bloom had a severity of 3.6 considered a mild bloom."