Algal blooms can be harmful without being toxic. Some of these non-toxic HABs can kill fish and other marine organisms by creating unhealthy ecological conditions. For example, during "Brown Tides," grazers that normally eat certain species of phytoplankton stop doing so and starve to death. This results in less food being available for those organisms higher up the food web. Another example of a non-toxic HAB is the Chaetoceros-related bloom which mechanically injures or kills fish. The exact causes of other non-toxic HABs -- such as Heterosigma-related and Pfiesteria-related "Fish Kills" -- are still unclear.

HARMFUL (non-toxic) BLOOMS
The presence of certain types of phytoplankton blooms -- including Aureococcus anophagefferens in mid-Atlantic waters and Aureoumbra lagunensis off of Texas -- causes larval fish, shellfish and copepods to stop feeding and starve to death. In Texas "Brown Tides", the eggs of fish such as the Red Drum (shown at right) will not hatch. Some scientists attribute these organisms' reactions to a "sugary coating" which is produced by the phytoplankton. During massive "Brown Tides," sunlight is blocked and seagrasses die in the murky, brown waters.
  • During Chaetoceros-related Fish Kills (shown at right), barbed diatom spines become lodged in fish gills. When the spines break off, they cause an inflammatory response in fish and large amounts of mucous are produced. This eventually suffocates the fish or makes it susceptible to infection
  • Scientists are working to discover exactly how Heterosigma akashiwo kills fish.
  • Although researchers are working to isolate a toxin associated with Pfiesteria piscicida, some studies indicate that this dinoflagellate is harmful only in the presence of fish and may actually attack them!