Algae form the base of our food web and thus can have widespread impact. Some organisms act as transporters (also known as "vectors") for the algal-derived toxins but are unaffected themselves. In many cases the toxins can be transported through the food web to humans. Depending on the toxin's potency, it may cause mild symptoms or death. In non-toxic HABs, the food web may still be impacted because of environmental changes; examples include "Brown Tides" and "Fish Kills."
TOXIC DINOFLAGELLATE BLOOMS
The dinoflagellate Gambierdiscus toxicus, which lives on coral and seaweed, is eaten by reef fish; these reef fish are eaten by carnivorous fish which are eaten by humans.
Certain dinoflagellates are concentrated by filter-feeding shellfish which are eaten by humans.
Karenia brevis is eaten by fish and concentrated by filter-feeding shellfish. Contaminated fish are eaten by birds, dolphins, and humans; humans also eat the shellfish. Manatees are affected by eating toxic tunicates (e.g., sea squirts).
Certain dinoflagellates are concentrated by shellfish and eaten by fish and invertebrates. Seals and fish are affected by consuming contaminated fish. Humans and birds are poisoned by the toxic shellfish.
 
TOXIC DIATOM BLOOMS
Certain diatoms are concentrated by filter-feeding shellfish and eaten by fish and invertebrates; these organisms are eaten by birds and humans. Sea lions are affected by consuming contaminated fish.
HARMFUL BLOOMS (non-toxic)
In mid-Atlantic waters, the presence of Aureococcus anophagefferens causes larval fish, shellfish and copepods to stop feeding and starve to death. In Texas, the eggs of fish such as the Red Drum (shown at right) will not hatch during massive Aureoumbra lagunensis blooms. Some scientists attribute these organisms' reactions to a "sugary coating" which is produced by the phytoplankton. During massive blooms, sunlight is blocked and seagrasses die in the murky, brown waters.
  • Chaetoceros-related (shown): The diatom Chaetoceros convolutus irritates gills, causing fish to produce enough mucous that they suffocate.
  • Heterosigma-related: Scientists are working to discover exactly how Heterosigma akashiwo kills fish.
  • Pfiesteria piscicida-related: Some studies indicate that this dinoflagellate is harmful only in the presence of fish and may actually attack them!