They are the seas' silent killers, toxin-laden patches of algae that amass along coastal shores and wreck havoc on marine ecosystems. They appear with no warning and outbreaks have become more frequent. Virtually every coastal country in the world has suffered from their effects.
These are harmful algal blooms (HABs), more commonly known as "red tides" because their ominous presence is sometimes characterized by a massive red patch of water encroaching the shorelines. Often outbreaks are invisible and thus pose an even greater threat. HABs occur when colonies of algae - simple ocean plants that live in the sea - grow out of control and produce toxins that can poison fish, shellfish and other marine life, and pose a major threat to people's health and fishermen's livelihoods.
via Detecting a Killer Toxin - IAEA.org.