Cyanobacteria (sometimes called blue-green algae) occur naturally in freshwater, but under certain conditions they can multiply quickly, creating a highly concentrated area known as an “algae bloom”. Algae blooms can be harmful to humans and animals. Some harmful cyanobacteria produce toxins (known as cyanotoxins) that are dangerous at very low levels in surface waterbodies. These harmful algae blooms (HABs) may occur at any time in Massachusetts, but are most common in late summer and early fall.