Largest freshwater blue-green algal bloom

Algal blooms are notoriously difficult to measure, with many different metrics having been employed to define their size, including area, density and weight. In terms of surface coverage. Lake Erie – one of the five great lakes in North America – has regularly played host to large-scale algal blooms. In 2011, Erie experienced the largest harmful algal bloom in its recorded history, with a peak intensity over three times greater than any previously observed bloom. At its peak in early October, the extent of the bloom was estimated to be more than 5,000 km2 (1,930 sq mi). Taxonomic analysis of the phytoplankton community confirmed that composition was almost entirely Microcystis, a toxic cyanobacteria.

Another record-worthy blue-green algal bloom occurred during 1991–92 along the Barwon-Darling River in Australia. Dominated by Anabaena circinalis, which secretes dangerous neurotoxins, the bloom spread for over 1,000 km (620 mi) - surface area unknown - along the river, killing domesticated livestock that drank the contaminated water, and leading to a State of Emergency being declared.