What Is Red Tide and Where Does It Come From?

(Last Updated On: October 18, 2018)

What Is Red Tide? What Causes Ride Tide?
In this post, we’ll explore the following questions and more: What is red tide? What causes red tide? And is red tide dangerous?

What Is Red Tide?

Red tides are algal blooms large enough to cause discoloration of an entire area of coastline. They occur when a large amount of microscopic algae collects in a single area of the ocean, begin to reproduce, and eventually expand to a size great enough to contaminate the water and change the color. Most often it creates a rust color but, depending on the specifics, it can also appear closer to pink or orange or, in some cases, even brown or yellow. Scientists normally refer to these events as Harmful Algal Blooms, or HABs.

There are numerous varieties, and not all are harmful. Up to a certain point, excess algae can be useful as food for sea life. Once they reach HAB, or red tide, status, however, they typically decrease the oxygen level in the water and release harmful toxins. Both effects are harmful for fish, plants, and other sea life, and the toxins can be a problem for humans that come into contact with them.

Where Are Red Tides Found?

Red tides have occurred all over the world at different times, and since the 1980s they have become noticeably more frequent as the result of climate change. There are three common variations of red tide found in North America:

  • Alexandrium fundyense – along the coasts of New England and the Canadian Maritimes
  • Alexandrium catenella – along the entirety of the Pacific coast from California to Alaska
  • Karenia brevis – in the Gulf of Mexico

This last species is of particular concern when gulf hurricanes, such as Hurricane Michael, occur.

Are Red Tides Dangerous?

A small percentage of algae produces toxic material that can cause illness in fish, sea mammals, and humans. When enough of these algae congregate in one area and begin to multiply, it can affect the water enough to cause serious disease and even death.

One example is when a large algal bloom itself begins to die. When this happens, the process of decomposition can reduce the oxygen level enough to make it uninhabitable for marine life. Other symptoms occur as the result of natural toxins produced by the algae. Some of these can affect fish and krill directly. Others may not affect those lower on the food chain, but the toxins are consumed in large quantities and passed on to larger mammals and birds who feed on those creatures. In the past, HABs have been blamed for the death of large numbers of squid in California and manatees in Florida.

Red tides can be dangerous for humans who come into direct contact with them, either through physical contact or through consumption. Humans eating tainted shellfish can become severely ill with different variations of shellfish poisoning, some of which can be fatal.

What Causes a Red Tide?

Red tides occur when a complex combination of oceanic factors intersect in a specific manner. The following elements are among those that influence the creation of a red tide:

  • An increase in ocean temperatures
  • Calm seas
  • An unusually large presence of nutrients such as phosphorus or nitrogen (sometimes from pollution)
  • A change in sea currents
  • Lower than normal water salinity

As HABs have become more common and begun to last longer each year, scientists are working to determine specific formulas that will allow them to predict and, potentially, prevent them. General consensus in the scientific community is that as climate change continues to progress, both the frequency and severity of HABs will continue to grow.

What Does the Future Hold?

Improvements in technology are helping scientists to track and forecast algal blooms, allowing them to provide advance warning to affected areas (such as Florida in advance of Hurricane Michael) or, in some cases, attempt to lower the nutrient levels in the water to reduce the extent of the bloom. There are ongoing efforts to develop antidotes, although the variable nature of the HABs make it difficult to produce a comprehensive fix.

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