Lago de Pátzcuaro during a cyanobacterial bloom

Eutrophication: Causes, Consequences, and Controls in Aquatic Ecosystems.
Nature Education Knowledge, 2013, 4(4):10

Eutrophication is a leading cause of impairment of many freshwater and coastal marine ecosystems in the world.
Why should we worry about eutrophication and how is this problem managed?

Some algal blooms pose an additional threat because they produce noxious toxins (e.g., microcystin and anatoxin-a.
Over the past century, harmful algal blooms (HABs) have been linked with
(1) degradation of water quality,
(2) destruction of economically important fisheries, and
(3) public health risks (Morris 1999).
Within freshwater ecosystems, cyanobacteria are the most important phytoplankton associated with HABs. Toxigenic cyanobacteria, including Anabaena, Cylindrospermopsis, Microcystis, and Oscillatoria (Planktothrix), tend to dominate nutrient-rich, freshwater systems due to their superior competitive abilities under high nutrient concentrations, low nitrogen-to-phosphorus ratios, low light levels, reduced mixing, and high temperatures.
Poisonings of domestic animals, wildlife, and even humans by blooms of toxic cyanobacteria have been documented throughout the world and date back to Francis’ (1878) first observation of dead livestock associated with a bloom of cyanobacteria.
Furthermore, cyanobacteria are responsible for several off-flavor compounds (e.g., methylisoborneal and geosmin) found in municipal drinking water systems