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What causes red tides and other harmful algae blooms?

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Question: What causes red tides and other harmful algae blooms?
Answer: There is no one single cause of red tides and other harmful algae blooms. Scientists generally believe that coastal pollution from human sewage, agricultural runoff and other sources contribute to red tides, along with rising ocean temperatures.

On the Pacific Coast of the United States, for example, red tide occurrences have been increasing since about 1991. 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 as well as increased nutrients in coastal waters from sewage and fertilizers.

Some red tides and harmful algae blooms along the Pacific coast also have been associated with cyclical El Nino weather patterns. On the other hand, red tides and harmful algae blooms sometimes occur where there is no apparent link to human activity.

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