An algal bloom is an accumulation of algae (typically microscopic) or a rapid increase in their amount in a body of water. Algal blooms may occur in fresh water as well as salt water. Algal bloom concentrations may reach millions of cells per milliliter of water. Algal blooms are often green, but they can also be other colors such as yellow-brown or red, depending on the species of algae.
Algal blooms are a big problem to ecosystems because, like any sudden unnatural growth, it harms the other animals in that ecosystem, usually decreasing their numbers quickly.