Some of the algae that makes up an algal bloom can be toxic.
This would probably be the main difference between an algal bloom
and humans. Hope this helps!
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Algal Blooms are caused by over fertilization, runoff.
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There is an article on algal bloom in Wikipedia which is very
comprehensive. Bloom that is not of a floral nature usually refers
to algal bloom which occurs in acquatic environments where the
oxygen levels are depleted.