Water And Oceanography

What Type of Water Conditions cause a Red Tide

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Red tide is a phenomenon that occurs all across the world along the coasts. It is formed when phytoplankton or algae accumulate in such large numbers in a given area that they discolor the water. The preferred name by scientists is harmful algal blooms (HABs). They are designated as such because the term ‘red tide’ includes both harmful and non-harmful, but most people associate red tide with only the harmful blooms, so it is important to have a better designation that will specify whether the bloom is harmful or not.

In the United States alone, nearly every coastal state has reported algal blooms and HABs. The harmful blooms have the potential to harm or kill shellfish, fish, birds, marine mammals, and human beings. These are the blooms that people are most concerned about because they have the potential to harm people and damage the economy in certain areas. HABs are formed in the same manner as all algal blooms are formed and stem from the occurrence of specific conditions in the coastal water.

Basic information on HABs

Since around the 1980s, HAB events have increased globally in almost thirty countries. While HABs occur all over the world, most algal blooms are not harmful or produce any toxins from their growth. Of the sixty types of phytoplankton that are associated with red tide, only four or five lead to the release of toxins into the water. Small nontoxic algal blooms are even beneficial as fish eat the plant material. The term ‘red tide’ came from the color that the classic HABs have been associated with, which is a reddish brown. The blooms can, however, be many different colors and typically aren’t red as they can be anywhere from green, red or orange, and even brown in color.

The blooms can deplete the amount of dissolved oxygen in the water and even release harmful toxins that are dangerous to marine life as well as humans if we consume seafood tainted with HAB toxins. The species in the United States that produce the harmful toxins include: karenia brevis which is found in the gulf coast, alexandrium catenella which is found along the Pacific coast, and alexandrium fundyense which is found along the northern Atlantic coast.

Conditions under which blooms occur

HABs occur naturally in the environment and as a result of human disturbance to a coastal environment. They respond to conditions like a warm surface temperature of the ocean in combination with low salinity, and calm seas. A recent storm followed by several sunny days in the summer months can also lead to the creation of HABs.

Probably the biggest increase in the appearance of algal blooms along the coasts of the world has to do with the presence of high nutrient contents in the waters. Nutrient content is really just the presence of phosphorous containing compounds and nitrogen containing compounds. The presence of both serves as a food source for all algal blooms, so the more nutrients present and the more favorable the weather and water conditions are, the more growth potential the algae has in the water.

While the causes of such high levels of nutrients can occur naturally, they are mostly a result of the interaction of people and the environment. Overpopulation of areas creates nutrient rich runoff from storm events, as does the presence of industrial effluent and agricultural sources. High levels of nitrogen and phosphorus are found in all of these sources and they are all the result of human interaction with the environment.


The wind, ocean currents, storms, and even the passage of ships all have the potential to carry HABs over very long distances to new areas where the conditions may be even more favorable for their growth. Wind and ocean currents also have the potential to concentrate HABs in a given area and increase the health risk to humans and aquatic organisms. HABs can even remain in a dormant state and even travel in the sediments on the ocean floor and survive the much colder temperatures

What is being done?

It is likely that HABs will never completely go away completely because the conditions under which they form occur naturally as well as a result of people all over the globe. While there may not be a method to get rid of them, scientists are looking into a treatment for the toxins produced by HABs, better detection methods and even systems that will warn and alert people to the presence of HABs in coastal waters so appropriate action can be taken to limit the health risk to people.

More about this author: Blaise Pascal

From Around the Web

  • InfoBoxCallToAction ActionArrowhttp://oceanservice.noaa.gov/facts/redtide.html
  • InfoBoxCallToAction ActionArrowhttp://serc.carleton.edu/microbelife/topics/redtide/
  • InfoBoxCallToAction ActionArrowhttp://www.mass.gov/?pageID=eohhs2modulechunk&L=4&L0=Home&L1=Provider&L2=Guidance+for+Businesses&L3=Food+Safety&sid=Eeohhs2&b=terminalcontent&f=dph_environmental_foodsafety_p_red_tide&csid=Eeohhs2
  • InfoBoxCallToAction ActionArrowhttp://oceanservice.noaa.gov/facts/redtide.html