The green scum shown in these images is the worst algae bloom Lake Erie has experienced in decades. Such blooms were common in the lake's shallow western basin in the 1950s and 60s. Phosphorus from farms, sewage, and industry fertilised the waters so that huge algae blooms developed year after year. The blooms subsided during the 1970s, when regulations and improvements in agriculture and sewage treatment limited the amount of phosphorus that reached the lake. But in 2011, a giant bloom spread across the western basin once again.

Algae are one of the many single-celled organisms living in the Great Lakes. Certain types of algae can become abundant in shallow or undisturbed water due to excessive nutrient levels or sunlight in water. A dense population of algae is called a bloom and can discolour or result in surface scum layers in water. Blooms of the microscopic blue-green algae (cyanobacteria) Microcystis are common in parts of western Lake Erie, Saginaw Bay (Lake Huron) and some inland island lakes. The production of the hepatotoxin microcystin is of particular concern for some strains of Microcystis. Toxin producing blooms are referred to as Harmful Algal Blooms (HABs). Microcystins have been shown to be detrimental to the health of humans, animals and the ecosystem.

This picture taken on 01 September 2011, shows a large algae bloom in Lake Erie. The bloom comes from excess nitrogen that runs off into the lake from the many farm fields in the region of the corn belt. (Photo by @peteressick)

The following images were acquired by the Envisat satellite (Full Resolution, MERIS_FR) and show details of the area using three band combinations (7, 5, 2). This first image shows the largest algae bloom in 2011 extended from Toledo to beyond Cleveland and along the Ontario shore. The bloom extended over ten miles from the shores and in the central basin it was observed at a depth of 60 metres. The density of the bloom has also caused damage to the navigation because it causes disruption to boat motors.

Scientists know what causes the toxic blue-green algae that stain western Lake Erie every summer. It's phosphorus, a byproduct of sewage, fertilisers and manure. But which is to blame: treatment plants or farms? A new federal report points the finger at both and has prompted new questions from Ohio researchers about how to reduce pollution and curb algae growth. The U.S. Geological Survey report found that sewage-treatment plants in Detroit, Toledo and other lakeside communities put nearly the same amount of phosphorus into Lake Erie as the fertilisers and manure that rains wash off farm fields. That's a surprise for Ohio-based scientists who have estimated that farm fields can provide as much as 60 percent of the phosphorus in Lake Erie.

Lake Erie is the 11th largest freshwater lake in the world, with a surface area of 9,910 square miles and a shoreline of 871 miles. It is the fourth largest among the Great Lakes by surface area and the smallest of the Great Lakes by volume (116 cubic miles). Due to its comparatively small volume, it also has the shortest retention time - 2.7 years. Lake Erie is the shallowest, with a maximum depth of 210 feet, and because of its shallow depth, Lake Erie is the only Great Lake with its entire bottom above sea level. Morphologically, Lake Erie can be considered three lakes, with a shallow western basin, a deep eastern basin, and a central basin of moderate depth. These three Lake Erie basins exhibit differences beyond simply depth. They each have characteristic levels of biological productivity, fish populations and temperature profiles. The central basin is also the only one that exhibits "dead zones" where dissolved oxygen levels are zero (anoxic) or very low (hypoxic).
(From: Michigan State University).

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