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What Is a Red Tide?

A red tide occurs when certain types of algae—plant-like organisms that live in the water—grow out of control. The name “red tide” comes from the fact that overgrowth of algae can cause the color of the water to turn red, as well as green or brown.

A red tide blooms off the coast of Texas.

A red tide blooms off the coast of Texas. Credit: NOAA News Archive 120910

What Causes a Red Tide?

Red tides are caused by algae, which are tiny, microscopic organisms that grow in the water. Almost all bodies of water have some algae, but in a red tide, there is a lot more algae in the water than usual. In fact, the water changes color in a red tide because the population of algae living in the water becomes so dense.

Red tides have been around since long before humans. However, certain human activities are making them more frequent.

Chemicals from farming, factories, sewage treatment plants and other sources can become dissolved in water on the land. This water, called runoff, eventually flows into the ocean and can cause algae to grow faster, leading to red tides.

Image of a stream running through a grassy field with a building in the background.

Nutrient-filled water, called runoff, can flow into lakes and oceans, contributing to algal blooms such as red tides. However, certain farming practices can reduce the amount of runoff that flows into streams and rivers, thus helping to prevent red tides. Credit: Chesapeake Bay Program CC BY-NC 2.0

Why Are Red Tides Dangerous?

Red tides are sometimes also called harmful algal blooms. Some of the algae that causes a red tide produce powerful toxins, which are harmful chemicals that can kill fish, shellfish, mammals and birds.

If people eat fish or shellfish that have been in the water with toxic algae, they will also ingest the toxins, which can make them sick. Many regions restrict fishing during a red tide for this reason. Nearby restaurants take local fish and shellfish off the menu, too.

Other types of harmful algal blooms are caused by nontoxic algae species that still make trouble. For example, when giant masses of algae bloom, they eventually die and start to decompose. As they decay, the oxygen levels in the water begin to decrease. The water can become so low in oxygen that animals in the water either swim away to healthier waters or die off.

During a red tide, beaches are sometimes covered in dead fish and other animals that either ingested toxins or couldn’t get enough oxygen.

Dead fish on beach near Sarasota, Florida.

Dead fish on beach near Sarasota, Florida, killed by toxins from a 2018 red tide. Credit: NOAA National Centers for Coastal Ocean Science

How Are We Monitoring Red Tides?

Satellites orbiting high above Earth can help us keep an eye on red tides. Although each individual algae organism is quite small, an entire algal bloom is big enough that it can be seen from space.

Image of an algal bloom in Lake Erie.

The NOAA-20 satellite captured this image of an algal bloom in Lake Erie in September 2017. Credit: NOAA

NOAA’s GOES-East satellite captures color images of the eastern United States every five minutes. The satellite’s Advanced Baseline Imager (ABI) instrument can help track the movement of red tides in real time. NOAA-20, a satellite that observes the whole surface of the planet twice each day, has an instrument called VIIRS—short for Visible Infrared Imaging Radiometer Suite—that can observe and detect changes in the color of oceans and lakes.

These satellites also track weather patterns that can make red tides more likely. For instance, heavy rain, flooding runoff from hurricanes and excessive heat can all contribute to red tides.

The goal is to use these observations from satellites to detect and forecast red tides before they reach the coast. That way, communities are better prepared to plan for the health and environmental impacts of red tides.

If you want to check out the red tide forecast for an upcoming trip to the beach, see NOAA’s Harmful Algal Bloom forecast online:

Red tide just off Florida’s southwestern coast.

NOAA’s GOES-East satellite spotted this red tide (just off Florida’s southwestern coast), which caused a state of emergency in August 2018. Credit: NOAA