information:extraction, structural:summarization
Eutrophication and cyanobacterial blooms

An ongoing concern is that nutrient overloading from fertilizers and human and animal waste, known as eutrophication, in which additional nitrogen and phosphorus enter the lake, will cause plant life to "run wild and multiply like crazy". Since fewer wetlands remain to filter nutrients, and greater channelization of waterways, nutrients in water can cause algal blooms to sprout, with "low-oxygen dead zones" in a complex interaction of natural forces. As of the 2010s, much of the phosphorus in the lake comes from fertilizer applied to no-till soybean and corn fields, but washed into streams by heavy rains. The algal blooms result from growth of Microcystis, a toxic blue-green alga that the zebra mussels, which infest the lake, do not eat.

Periodically, a dead zone, or region of low oxygen, occurs in the lake, the location of which varies. Scientists from the National Oceanic and Atmospheric Administration have been studying the lake's blue-green algal blooms and trying to find ways to predict when they are spreading or where they might make landfall; typically, the blooms arrive late each summer. This problem was extreme in the mid- and late 1960s, and the Lake Erie Wastewater Management Study conducted by the Buffalo District of the U.S. Army Corps of Engineers determined that the eutrophication was caused by point sources such as industrial outfalls and municipal sanitary and storm sewer outfalls, as well as diffuse sources, such as overland runoff from farm and forest land. All of these sources contribute nutrients, primarily phosphorus, to the lake. Growth of organisms in the lake is then spiked to the point that oxygen levels are depleted. Recommendations were made for reducing point-source outflows, and reducing farm contributions of phosphorus by changing fertilizer usage, employing no-till farming, and other conservative practices. Many industrial and municipal sources have since been greatly reduced. The improved farming practices, which were voluntary, were followed for a while, resulting in remarkable recovery of the lake in the 1970s.

The conservation practices are not monitored and have not been kept up. One recent account suggests that the seasonal algal blooms in Lake Erie were possibly caused by runoff from cities, fertilizers, zebra mussels, and livestock near water. A second report focuses on the zebra mussels as being the cause of dead zones, since they filter so much sediment that this produces an overgrowth of algae. One report suggests the oxygen-poor zone began about 1993 in the lake's central basin and becomes more pronounced during summer, but is somewhat of a mystery why this happens. Some scientists speculate that the dead zone is a naturally occurring phenomenon. Another report cites Ohio's Maumee River as the main source of polluted runoff of phosphorus from industries, municipalities, tributaries and agriculture, and in 2008, satellite images showed the algal bloom heading toward Pelee Island. Two two-year, $2 million studies are trying to understand the "growing zone", which was described as a 10-foot-thick layer of cold water at the bottom, 55 feet (17 m) in one area, which stretches 100 miles across the lake's center. It kills fish and microscopic creatures of the lake's food chain and fouls the water, and may cause further problems in later years for sport and commercial fishing.

Algal blooms continued in early 2013, but new farming techniques, climate change, and even a change in Lake Erie's ecosystem make phosphorus pollution more intractable. Blue-green algae (Cyanobacteria) bloom, were problematic in August 2019. According to a news report in August, "scientists fully expect to overwhelm much of western Lake Erie again this summer". By August 12, 2019, the bloom extended for roughly 50 kilometres (31 mi). A large bloom does not necessarily mean the cyanobacteria... will produce toxins", said Michael McKay, executive director of the Great Lakes Institute for Environmental Research at the University of Windsor. "Not enough is being done to stop fertilizer and phosphorus from getting into the lake and causing blooms," he added. Water testing was being conducted in August. The largest Lake Erie blooms to date occurred in 2015, exceeding the severity index at 10.5 and in 2011 at a 10, according to the NOAA. In early August, the 2019 bloom was expected to measure 7.5 on the severity index, but could range between 6 and 9. At that time, satellite images depicted a bloom stretching up to 1,300 square kilometres (500 sq mi) on Lake Erie, with the epicenter near Toledo, Ohio.