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Red Tide

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What is Red Tide?

Harmful algal blooms, or HABs, basically occur when colonies of algae—simple plants that live in both saltwater as well as in freshwater—grow out of control, causing toxic or harmful effects on humans, fish, shellfish, marine mammals, as well as birds. Human illnesses caused by HABs are rare, but they can be debilitating or even fatal.


While many people refer to these blooms as "red tides," scientists prefer to refer to them as "harmful algal blooms." Every summer, one of the most well-known HABs in the country occurs along Florida's Gulf Coast. This bloom, like many others, is caused by microscopic algae that produce toxins that kill fish and make shellfish unsafe to consume. Toxins may also make it difficult to breathe the surrounding air.


HABs have been reported in every U.S. coastal state, as well as their occurrence may be on the rise. HABs are known to be a national concern because they affect not only the health of people and marine ecosystems but also the 'health' of local and regional economies.


What Causes Red Tide?

Let's look for the source of the red tide. This bloom, like many others, is caused by microscopic algae that produce toxins that kill fish and make shellfish unsafe to consume. Toxins may also make it difficult to breathe the surrounding air. Algae bloom frequently turn the water red, as the name implies.

Red Tides in California

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In its most basic form, a red tide is a phytoplankton bloom. Phytoplankton is microscopic and single-celled plants that live in our coastal waters. A “bloom” generally occurs when a specific species of phytoplankton begin rapidly reproducing, resulting in millions of cells in each gallon of water. Phytoplankton species do not all produce visible blooms. Red tides are caused by a type of phytoplankton known as dinoflagellates, which appear to prefer warmer and calmer waters.

What Causes the Colour in Red Tides?

The phytoplankton cells that cause a red tide contain pigments that allow them to capture sunlight, which is required for cell nourishment, growth, and reproduction. When millions of cells are concentrated in each gallon of seawater along our coast, these pigments can emit a reddish colour. If you looked at just one of these cells under a microscope, you might notice that it has a golden-brown colour.

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Are Red Tides Always Red?

In some parts of the world, red tides can appear in a variety of colours, ranging from brown to burgundy, to bright red, and even yellow. The colour of a bloom is determined by the phytoplankton species that are blooming, as well as other factors such as light intensity and the angle at which the sun shines on the water.

Where Do Red Tides Occur?

Red tides occur all over the world, from the tropics to Alaska. Red tides are most frequently observed in California between Santa Barbara and San Diego.

When is Red Tide?

These algae typically bloom in the spring and summer, with the more toxic blooms occurring more frequently in the spring. Alexandrium species can also be found in California, where they were discovered in 1927 following an outbreak of paralytic shellfish poisoning.

Dinoflagellates Red Tide

Let's look at how dinoflagellates contribute to red tides. Red tides are a common occurrence in warm, polluted coastal oceans. They form when populations of dinoflagellate algae explode to massive proportions. The waters turn red because the dinoflagellates have red plastids. Dinoflagellates exploit harsh environmental conditions that kill off other organisms.


Dinoflagellate, (division Dinoflagellata), any of a variety of one-celled aquatic organisms with two distinct flagella and characteristics of both plants and animals. The majority are marine, but some live in freshwater habitats. In all but the colder seas, the group is an important component of phytoplankton and a key link in the food chain. Dinoflagellates are also responsible for some of the bioluminescence seen in the sea. Several species can reproduce rapidly under certain conditions, resulting in water blooms or red tides that discolour the water and may poison fish and other animals.


Dinoflagellates in Brief

Dinoflagellates have sizes ranging from 5 to 2,000 micrometres (0.0002 to 0.08 inch). The vast majority are microscopic, but some form visible colonies. Dinoflagellate nutrition is autotrophic, heterotrophic, or mixed; some species are parasitic or commensal. About one-half of the species are photosynthetic; however, many of these are also predatory. Although sexual processes have been observed in a few genera, reproduction is primarily accomplished through binary or multiple fission. Dinoflagellate populations can reach 60 million organisms per litre of water under ideal conditions.


The annulus, which contains a flagellum, is a median or coiled groove that bands the dinoflagellate cell. The sulcus is a longitudinal groove that extends posteriorly from the annulus to the point where a second flagellum is attached. Armoured dinoflagellates have cellulose plates that can have long spiny extensions; some species without armour have a thin pellicle (protective layer). Dinoflagellates with photosynthetic plastids (that is pigment-containing bodies) can store food in the form of starches, starchlike compounds, or also in the form of oils.

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FAQs on Red Tide

Question 1: Define Red Tide. Red Tides are Caused by Which Algae?

Answer: Let's look at the definition of red tide. Red tide is a phenomenon caused by algal blooms, in which algae proliferate to the point of discolouring coastal waters (hence the name "red tide"). The algal bloom may also deplete oxygen levels in the water and/or release toxins that can make humans and other animals sick.

At least three dinoflagellate species and one diatom species are to blame for the toxic mess that is red tides in the United States. Toxins produced by these microscopic forms of algae can sicken humans and be fatal to marine animals.

Question 2: Why Does Red Tide Glow at Night?

Answer: “On sunny days, the organisms swim to the surface, where they congregate, resulting in an intensified colouration of the water – and the origin of the term 'red tide.' When agitated by waves or other movements in the water at night, phytoplankton emits a dazzling neon blue glow.”

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