Single celled organisms such as algae and dinoflagellates thrive in oceans. They reproduce quickly into “blooms” if their environment has high nutrient. When some of these algae grow rapidly, become dense, and appear as patches near the water’s surface, a “red tide” occurs, owing to their reddish appearance.

“Red tide” is a colloquial term for harmful algal bloom (HAB) which is a dense aggregation of phytoplankton, algae, or cyanobacteria in marine or aquatic environment. Some species of cyanobacteria produce neurotoxins, hepatotoxins, cytotoxins, and endotoxins, making them dangerous to animals and humans.

Scientists clarify, however, that not all algal blooms are harmful, not all cause discoloration of water, and that these blooms are not associated with tides. What HABs are associated with, though, are large-scale marine mortality events and various types of shellfish poisonings.

When organisms produce neurotoxins that are harmful to shellfish and fish, these contaminated seafood become toxic food for humans. People who have eaten toxic fish and shellfish get contaminated with Paralytic Shellfish Poisoning (PSP).

Paralytic shellfish poisoning is caused by dinoflagellates that have a red-brown color, and can grow to such numbers that they cause red streaks to appear in the ocean called “red tides.” Shellfish that have caused this disease include mussels, cockles, clams, scallops, oysters, crabs, and lobsters.

Symptoms begin anywhere from 15 minutes to 10 hours after eating the contaminated shellfish, although usually within 2 hours. Symptoms are generally mild, and begin with numbness or tingling of the face, arms, and legs. This is followed by headache, dizziness, nausea, and muscular incoordination. Patients sometimes describe a floating sensation. In cases of severe poisoning, muscle paralysis and respiratory failure follow, and in these cases death may occur in 2 to 25 hours.

With a lack of comprehensive and conclusive studies, red tides are believed to be natural phenomena, not caused by humans. Red tides and harmful algae blooms sometimes occur where there is no apparent link to human activity. Some red tides and harmful algae blooms along the Pacific coast have been associated with cyclical El Nino weather patterns. Scientists have correlated the increase of Pacific red tides and other harmful algae blooms with a rise in ocean temperature of approximately one degree Celsius.

Other scientists, however, have correlated red tides with increased nutrients in coastal waters from sewage and fertilizers. These scientists generally believe that coastal pollution from human sewage, agricultural runoff, and other sources contribute to red tides, along with rising ocean temperatures.

With widespread human activity affecting the environment for the last hundreds of years, it is difficult to believe that red tides which eventually cause deaths among humans are not caused by people themselves.

Tags: Red Tide, Paralytic Shellfish Poisoning (PSP), Algae Blooms