The Lake Erie algae bloom this year was the biggest ever measured by the National Oceanic and Atmospheric Administration (NOAA). The scum from the algae bloom alone covered an area of 300 square miles - about the size of New York City - at its peak in August, CBS News reported.

Fed by excessive rainfall during summer and spring and rich fertilizer runoff from nearby Ohio and Indiana farms, the algae bloom grew so large that it left some areas of the lake depleted of oxygen and threatened the growth of fish. It peaked around mid-August and began to decrease by the end of September.

"The bloom in Sandusky Bay has also diminished, dropping below medium concentration for the first time this season. No other blooms are evident in the central and eastern basins," NOAA said in an October Bulletin. "We do not expect that the bloom will reappear." The agency reminded boaters to be careful.

Blooms occur when algae receives an excessive amount of phosphorus and nitrogen. Typically, the farmland runoff is driven into the lake in a short amount of time by a huge amount of rain. This year, Indiana and Ohio received more rain than the usual. The month of June made it among the top 20 wettest Junes on record in Toledo since 1880, the Washington Post reported.

Algae produces toxins called microcystins, and in large blooms, these toxins can reach high levels. Water contaminated with microcystins, when ingested, can cause vomiting, stomach pain, fever and extreme headache.

In 2014, another algal bloom contaminated the tap water in Toledo, affecting 400,000 residents. This year, however, the algal bloom was concentrated farther away from the shores, making the threat to tap water less severe.

In June, officials in Ohio, Michigan and Ontario agreed to take measures to ensure that the agriculture runoff spilling into Lake Erie will be reduced in 10 years, CBS News reported.