The algae in Lake Erie was more severe and more highly concentrated this summer than in any summer since NOAA began measuring the blooms in 2002. This year’s harmful green bloom was due to excessive Midwest rainfall in spring and summer, and the fertilizer that rain picks up and carries to rivers which empty into the lake.

The algae isn’t just an eyesore — it’s incredibly harmful to humans. It produces a toxin called microcystin that, in August of 2014, reached such high levels that officials declared a water ban in Toledo — you couldn’t even drink it if you boiled it. When swallowed, mycrocystin can cause stomach pain, nausea, vomiting, severe headaches and fever.

The toxic algae feed off of nutrients that run into Lake Erie primarily from the Maumee River, which snakes through the fertile farmlands of Ohio and Indiana. The runoff from these farms contains nitrogen from fertilizer and phosphorus from livestock manure and sewage, which sends the algae in Lake Erie into overdrive.

This year the rain over Ohio and Indiana was truly excessive. The Maumee River watershed received as much as eight inches more than normal in the month of June. It was also the fourth wettest June in Toledo, Ohio, and one of the top 20 wettest months since records began in 1880.

It was also the Maumee River’s highest discharge for the month of June, a full 30 percent higher than the previous discharged record in 1980, and third highest discharge month out of any month on record, according to Rick Stumpf, an oceanographer and toxic algae expert at NOAA.

The timing was also critical in this year’s peak — the rains came at just the right time for the algae in Lake Erie to take the most advantage of the nutrients in the runoff.

The algae began to fester in July, and reached peak biomass in mid-August, says NOAA. “Over a 40-day period from late July to the end of August, the biomass detected from satellite exceeded that of any other time period we have monitored, except for the first week of October, 2011,” Stumpf and his co-author, Tim Wynne, wrote in NOAA’s algal bloom bulletin. “On August 5th, dense scum covered up to 300 square miles of the western basin; this occurred again on August 15th.”

During that time, the biomass index exceeded the previous record of 10 set in 2011 and peaked at 10.5. The scale itself had been a zero to 10 scale until this year, and the 10.5 index came as a surprise after initial forecasts suggested it would max out at around nine. NOAA says that a severity above five indicates blooms of particular concern.