ANN ARBOR—This summer’s cyanobacteria bloom in western Lake Erie is expected to be a large one, but determining just how big it is likely to get was challenging for forecasters, including University of Michigan aquatic ecologist Don Scavia.

Scavia is a member of the federally funded research team that issues a Lake Erie cyanobacteria bloom forecast each year in early July. This year’s forecast calls for bloom that measures 7.5 on the severity index. The largest Lake Erie blooms on record occurred in 2011 (10) and 2015 (10.5).

The main driver of Lake Erie’s annual cyanobacteria bloom is elevated phosphorus from watersheds draining to Lake Erie’s western basin, particularly from the heavily agricultural Maumee River watershed. An estimated 85% of the phosphorus entering Lake Erie from the Maumee River comes from farm fertilizers and manure.

This spring, frequent heavy rainstorms across the region led researchers to consider two possible scenarios, said Scavia, a professor emeritus at the U-M School for Environment and Sustainability. The heavy rains could wash massive amounts of phosphorus into the Maumee River, leading to a record-size bloom.

“But many farmers were unable to plant and fertilize this spring because it was so wet, which created a second possibility,” Scavia said. “If less fertilizer was applied to croplands in the watershed this spring, maybe less phosphorus would end up in Lake Erie’s western basin, resulting in a smaller bloom.”

In the end, the forecast issued on July 11 called for a significant bloom, but not a record-breaker. When water discharges and phosphorus loads from March through early July were fed into computer models, the models pointed to a severity index reading of 7.5.

“There are two key lessons from all of this, assuming that our forecast is correct,” Scavia said. “First, extreme weather events can be smoothed out over the long March to July timeframe.

“Second, even without new fertilizer applications, there is sufficient phosphorus in the soils to produce a substantial load into western Lake Erie. And the big weather driver here may be the cold spring temperatures, which could truncate this year’s bloom.”

Because lake temperatures remained cool this spring due to higher-than-average rainfall, the bloom started later than it did last year, when exceptionally warm weather at the beginning of June triggered an early start.

More information:

• July 11, 2019, Michigan News story
• Don Scavia’s forecast page
• Don Scavia via The Conversation: How your diet contributes to nutrient pollution and dead zones in lakes and bay