The National Oceanic and Atmospheric Administration announced Tuesday the 2015 algal bloom that took place in Western Lake Erie over the summer was the most severe this century.

According to a NOAA press release, “The bloom severity index, which captures the amount of biomass in the bloom is 10.5 for 2015.” The index normally uses a 10.0 as the highest measure.

NOAA started tracking Lake Erie’s algal blooms in 2002, and since that point had considered a bloom in 2011 the worst recorded — the aforementioned former high of 10 in biomass amount.

Blooms can create a toxin called microcystin capable of poisoning wildlife and humans alike. Forming a paint-like sludge, the cyanobacteria that makes up the bloom can turn lake water green, burn skin, cause rashes, and create hypoxic zones—or “dead zones”—in the water, where lowered oxygen levels make conditions inhospitable to marine life. Excessive agricultural run-off (of nitrogen and phosphorus), wastewater, and stormwater that bring pollutants into Lake Erie are considered main causes behind the blooms.

However, the impact of the 2015 bloom may have been muted, as cold weather in September disrupted bloom growth, and due to its central location within the lake basin, there was less coastal damage than what occurred in 2011 (beaches were fouled and fish populations were damaged). The visible scum of the 2015 bloom reached its apex size of almost 300 square miles in August.

In comparison, the land area of the five boroughs of New York City equals 305 square miles.