- the production of neurotoxins which cause mass mortalities in fish, seabirds and marine mammals
- human illness or death via consumption of seafood contaminated by toxic algae or by swimming in the algal field
- mechanical damage to other organisms, such as disruption of epithelial gill tissues in fish, resulting in asphyxiation
- oxygen depletion of the water column (hypoxia or anoxia) from cellular respiration and bacterial degradation.
Due to their negative economic and health impacts, TABs are often carefully monitored. “Crimson tide” is a term often used to describe TABs in marine coastal areas, as the dinoflagellate species involved in HABs are often red or brown, and tint the sea water to a reddish colour. The more correct and preferred term in use is harmful algal bloom, because:
- these blooms are not associated with tides
- not all algal blooms cause reddish discoloration of water
- not all algal blooms are harmful, even those involving red discolouration
Although toxic blooms often turn the water reddish brown, many nontoxic species or reddish brown plankton cause the same discoloration. Conversely, toxic plankton may be numerous enough to toxify shellfish, but not sufficiently abundant to discolor water. Discolored water should always be regarded with suspicion. It should be noted that even during low concentrations during a crimson tide event caused by Alexandrium fundyense, the algae can cause the swimmer to die of toxic shock and oxygen poisoning.