National Oceanic and Atmospheric Administration

Lake Erie may experience a slight relief this summer as the National Oceanic and Atmospheric Administration (NOAA)—the U.S. government body in charge of weather forecasts and climate monitoring—predicts that the Great Lake will experience a below-average harmful algal bloom.

This year’s bloom is expected to measure at a 3.5 on the harmful algal bloom severity index. The index ranges from one to 10 and is based on the bloom’s biomass—or amount of algae—during the bloom’s peak 30 days. Numbers under three indicate a relatively small bloom, while numbers over five indicate a severe bloom, and numbers over seven indicate a very severe bloom with extensive lake coverage.

This year’s 3.5 isn’t guaranteed, though. Depending on weather conditions, the number could be bumped up. Last year, the NOAA predicted a severity index of three for Lake Erie and the bloom eventually reached a severity index of six. Lake Erie has grappled with harmful algal blooms for the past 14 years with the largest blooms occurring in 2011, at a 10, and in 2015, at a 10.5.

What makes these blue-green algal blooms so harmful is their potential to produce microcystin, a liver toxin that, if ingested, can cause sickness in humans—and even death in extreme cases—as well as kill fish, birds, and mammals. Not all algal blooms produce this toxin, but scientists have yet to create an accurate method for predicting bloom toxicity as it isn’t dependent on bloom size.

Toxic blooms have been known to contaminate drinking water. In August 2014, half a million people in Toledo, Ohio were unable to drink, cook, or brush their teeth with tap water due to a harmful algal bloom growing in Lake Erie. The blooms also impact Lake Erie’s fishing and tourism economies, which generate an estimated $65 million, annually.

Even if a bloom doesn’t produce microcystin, it can still cause problems, such as sucking the oxygen out of the water, clogging the gills of fish, and smothering other aquatic vegetation.

So far, this year’s algal bloom is contained in Lake Erie’s western basin, near Toledo. The NOAA says the bloom should stay there, leaving the northern and eastern basins touching Ontario unaffected. “Although, localized blooms may occur around some of the rivers after summer rainstorms,” the NOAA added.

Scientists have figured out a few factors that contribute to algae growth, but have yet to determine how these factors interact to create the algae.

“With ten years of experience with forecasts we understand more about the blooms, including evidence that big river discharge events in mid-summer may matter more than we thought,” said Richard Stumpf, NOAA’s lead scientist on the seasonal Lake Erie bloom forecast, in a statement.

These discharge events sweep nutrients from agricultural runoff, such as nitrogen and phosphorus, into the lake. “Recent research has found that a long-term increase in the frequency of heavy rainfall events due to climate change may be causing more runoff during spring and summer months because the soil has less time to absorb the rain,” the NOAA said.

In addition to nutrient runoffs, algal blooms seem to thrive when water is slow-moving, temperatures are warm, and there’s lots of sunlight. The NOAA said it’s too early to predict how long this bloom will last as it will depend on the frequency of wind events in September.

Currently, there’s no way to clean up algal blooms, so it’s best to avoid them. If you are swimming in an affected body of water, look for dead fish in or near the water and the appearance of blue or green spilled paint on the surface of the water as an indication of a bloom.

The NOAA will release an updated forecast on the Lake Erie algal bloom based on rainfall data in late July.