Only a few dozen of the many thousands of species of microscopic and macroscopic algae are repeatedly associated with toxic or harmful blooms. Some species, such as the dinoflagellate Alexandrium tamarense (left) and the diatom Pseudo-nitzschia australis (below) produce potent toxins which are liberated when the algae are eaten.
Other species kill without toxins, like this Chaetoceros species (left) which has spines with serrated edges which can lodge in fish gill tissues, causing irritation, over production of mucous, and eventual death. Each of these species, and many others, need careful study at the organismal level if we are to understand the population dynamics and trophic impacts of HAB's throughout the U.S. Photos by L. Fritz, J. Rines, and R. Horner.