Algal blooms are a natural phenomenon, the occurrence of which may be increased by nutrient pollution. Algae can multiply quickly in waterways with an overabundance of nitrogen and phosphorus, particularly when the water is warm and the weather is calm. This proliferation causes "blooms" of algae that turn the water green, orange or red.
The ones that most folks take note of, of course, are the ones that turn the ocean to blood. Or, you know, "red tide", even though it has nothing to do with the tide. This is known as one of the 'harmful blooms', although not only the red and brown algea are harmful. These HABs can produce neurotoxins (which affect the nervous system) and hepatotoxins (which affect the liver). These toxins can potentially impact the health of people who come into contact with water where HABs are present in high numbers.
Even though these blooms have been around since before biblical times, there has been increased public awareness of the negative impacts of these blooms to marine resources -- such as strandings and deaths of marine mammals, birds, and sea turtles. In addition, scientists have determined that there are more toxic algal species, algal toxins, affected fisheries resources, food-web disruption, and economic losses from harmful algal blooms than ever before.