For at least fifty years, phytoplankton and algae blooms have been a regular occurrence in summer on Lake Erie. The microscopic, floating plants generally start to flourish in June and July as the water warms and stratifies, and their numbers typically peak in August and September. But itís not every year that a bloom leads to the shutdown of water supplies in an American or Canadian city.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASAís Aqua satellite captured this true-color image of an algae bloom in the west end of Lake Erie on August 3, 2014. The algae has given the water a milky green tint.
The dominant organism in the current Lake Erie bloom is Microcystis spp., a type of freshwater blue-green algae that produces a toxin harmful to humans. If consumed, Microcystis can cause numbness, nausea, dizziness, and vomiting and lead to liver damage. (In rare cases, it can be deadly.) On August 2, 2014, environmental monitors for Toledo and surrounding towns in northwestern Ohio determined that public water supplies had levels of microcystin toxin that were higher than recommended by the World Health Organization (1.0 parts per billion). They warned residents not to drink or cook with tap water; boiling is not effective against the toxin. Though the bloom has continued, treatment facilities have since added extra filtering steps (including activated carbon), and public water sources were declared safe again on August 4.
Blooms of microcystis and other species of phytoplankton occur almost every summer on Lake Erie, where chemical nutrients (mainly phosphorous and nitrogen) are plentiful due to decades of runoff from farms and cities. Research suggests that the arrival of invasive zebra mussels in the lake has changed the food web in ways that may promote certain algal species by removing competitors. And as the southernmost and shallowest of the Great Lakes, Lake Erie warms quickly and the water column tends to stagnate in layers, without much mixing to distribute heat or nutrients. Microcystis thrives in this environment, and the blooms have reached record proportions several times in recent years.