Toxic Ocean Blooms

Various species of dinoflagellates and other algae occasionally proliferate, forming what is called ocean blooms. Sometimes these blooms give a portion of the ocean a reddish color, resulting in a so-called "red tide." Ocean blooms can be toxic.

Distribution of Blooms

• Ocean blooms, both toxic and nontoxic, occur in coastal waters of the six inhabited continents. Blooms also occur, although rarely, in the middle of the oceans.

Frequency of Blooms

• Toxic ocean blooms have become more frequent in recent years, according to Woods Hole Oceanographic Institution. They have also spread to places where they never occurred before. For example, no recorded toxic blooms occurred near New England until 1972.

Toxic Bloom-Forming Species

• Most ocean blooms are nontoxic. But some algae form toxic blooms, such as the dinoflagellates Alexandrium tamarense and Gymnodinium catenatum and the diatom Pseudonitzschia. Shellfish, such as mussels, can imbibe the toxins. If people eat the affected mussels, the toxin sometimes causes paralysis and even death. The toxins can also kill such animals as fish and manatees. Some blue-green algae also form toxic blooms.

Factors Influencing Blooms

• Blooms tend to occur when sufficient nutrients, such as nitrates, are available. Higher temperatures also promote blooms. Life cycle is another factor. Alexandrium tamarense enters a resting stage after sexual reproduction and stops blooming, according to Institut de Ciències del Mar.