Harmful algal blooms, which often produce a toxin and occur in both salt and fresh water, are known to kill fish, marine mammals, and birds; they can contaminate shellfish with toxins and harm human health, sometimes resulting in fatalities. They shut down fisheries, sideline fishermen and drive tourists away from resorts.
Blooms can come from algae that range from microscopic single-celled organisms to macroscopic seaweed, and cause harm through the production of toxins or by accumulation of the plants. Even when a toxin is not produced harmful algal blooms can be deadly when overgrowth alters marine habitats by blocking light, clogging fish gills or smothering corals or other life. In addition, when large blooms die and decompose, the oxygen is stripped from the water resulting in uninhabitable “dead zones” (hypoxia). Nitrogen pollution and excess nutrients are known to cause many harmful algal blooms and “dead zones”.
For up-to-date detailed information on harmful algal bloom outbreaks in the United States and around the world: https://www.oceanchampions.org/?p=2516