article_snippet: A red tide, or algal bloom, occurs when there is a sudden increase in the amount of algae in water.
article_snippet: When levels of nitrogen and phosphorus rise, which can be caused by underwater eruptions such as the one off the coast of Tonga, algae can multiply.
article_snippet: Typically, this causes water to turn green, but the effect also causes reds and yellows to be released.
article_snippet: In particular, a bloom of dinoflagellate algae causes water to turn blood red in colour.