First and foremost, the many species of phytoplankton are not all toxic. Some diatoms and dinoflagellates contain a biological toxin that affects the nervous system of the organisms that consume them. This neurotoxin poisoning can lead to paralysis, convulsions or even death. These microscopic algae occur naturally in the environment. However, favourable weather conditions when they bloom (heavy rainfall, decreased salinity, increased water temperature) cause a proliferation called a “red tide”, in reference to the very high density of these algae at the water surface. These tides are visible and can be tracked from the air.
Although cetaceans, pinnipeds and seabirds do not feed directly on these toxic algae, their prey have fed on these algae or have consumed organisms that have ingested them. This is the principle of bioaccumulation. As whales must engulf huge quantities of prey to meet their needs, the concentration of the neurotoxin increases exponentially.
This red tide phenomenon is extraordinarily rare, but as a result of human activities, eutrophication, algae present in the ballast water of ships and climate change, it has been tending to occur more frequently worldwide.
One of the most remarkable events in the history of red tides occurred in 1998, when more than 400 California sea lions died due to poisoning from the alga Pseudo-nitzschia australis. Toxins were identified in anchovies, sea lions’ main prey, as well as in the body fluids of the latter. In 2008, in the St. Lawrence, a red tide spanning 600 km2 caused the death of nearly a dozen belugas, around a hundred seals, and several thousand seabirds and fish. During the summer of 2015, in the Gulf of Alaska, some thirty great whale carcasses were discovered. Scientists suspect toxic algae poisoning, though no evidence has been reported to date.
In humans, these algae are only toxic if consumed. Fish purchased in fish shops do not contain this toxin, as they were caught outside the areas affected by the red tide. Some shellfish harvesting areas are closed during the critical period and it is strongly recommended not to consume shellfish, fish viscera or the offal of marine birds in May and June, when the risk of blooms is highest.