Below-Average Harmful Algal Bloom Predicted for Western Lake Erie

The National Oceanic and Atmospheric Administration (NOAA) is forecasting that western Lake Erie will experience a smaller than average harmful algal bloom (HAB) this summer. Researchers said the HAB will be less severe than 2021 and more akin to what was seen in the lake in 2020.

This year's bloom is expected to measure 3.5, with a potential range of 2-4 on the severity index - whereas last year's bloom was measured at a 6. That 2021 projection had initially been for a severity of between 2 and 4.5. The index is based on the bloom's biomass -- meaning the amount of algae -- during the peak 30 days of the bloom. An index above 5 indicates more severe blooms. Blooms over 7 are particularly severe, with extensive scum formation and coverage affecting the lake. The largest Lake Erie blooms occurred in 2011, with a severity index of 10, and 2015, with a severity index of 10.5.

Lake Erie blooms consisting of cyanobacteria, a blue-green algae, are capable of producing a known liver toxin which poses a risk to humans and animals. These blooms can -- and have in the past -- forced cities and local governments to treat drinking water and prevent people from fishing, swimming, boating and visiting the shoreline. The blooms are heavily affected by wind and rainfall.

NOAA said it expects a start of the visible bloom in mid-to-late-July. The duration of the bloom depends on the frequency of wind events in September, which cannot be predicted this far in advance. The bloom will remain mostly in areas of the Western Basin. The Central and Eastern basins of the lake are usually unaffected, although localized blooms may occur around some of the rivers after summer rainstorms. July is expected to have average rainfall, but that is still uncertain. Larger rain events could result in increased river flow, and a higher severity index.

"With 10 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 National Centers for Coastal Ocean Science's (NCCOS's) lead scientist for the seasonal Lake Erie bloom forecast. "They create more uncertainty, but our models are improving as a result. We will also have to watch to see if these events become more common in the future."

Stumpf and several other researchers discussed the bloom Thursday during NOAA's annual forecast announcement held in partnership with Ohio Sea Grant/Ohio State University Stone Laboratory, among other research partners.

NOAAwill issue a seasonal forecast update in late July based on observed rainfall in the basin. 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. Combined with the increase in bioavailable phosphorus concentration in the early 2000s as a result of changes in agricultural practices, NOAA said, this rainfall trend may explain the higher-than-average phosphorus loads each spring over the last 14 years. The phosphorous loads are the driving force behind the blooms.

An updated HAB forecast and more information on the bloom can be found online here.