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Phytoplankton and red tide

Learn about phytoplankton and red tide.

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Phytoplankton are single-cell plants. They're the 'vegetation' of the ocean. Their cells contain chlorophyll, and through photosynthesis, the sun's energy is harnessed to enable them to change inorganic carbon into organic carbon. This is how carbon, one of life's building blocks, gets into the ocean food web.

They live in the photic zone (surface layer) of the ocean, with a maximum concentration of cells at the thermocline-pycnocline. This is where the temperature, light and nutrient conditions best suit plant growth.

Red tide

Paralytic shellfish poisoning, also known as red tide, is the contamination of bivalve (clamshell) shellfish by micro-organisms called dinoflagellates. Red tides are caused by the explosive population growth of these minute, single-celled algae.

Some red tides are linked to shellfish poisoning. Eating shellfish contaminated with these micro-organisms could lead to death. The toxin produced is among the most potent natural poisons in the world and there is no known antidote.

Red tide doesn't always mean the water will appear red. The organism can have many different pigments, resulting in different colours in the water or no colour change at all.

Causes of contamination

The dinoflagellates are always in the water, but certain conditions can cause population explosions, known as blooms. These conditions include:

Red tides may be stimulated by excessive runoff of nutrients from land or by the discharge of untreated sewage into coastal waters. Both serve as fertilizers for the algae.

Some red tide organisms produce toxins that are concentrated by filter-feeding animals such as:

Little is known on how or why the organisms produce the toxin. As the hinged shellfish filter feed on the blooms, they store vast amounts of the toxin in their tissue without being affected.

Signs and symptoms

Symptoms of paralytic shellfish poisoning commonly begin with tingling or numbness beginning around the lips and spreading to the face and neck within 30 minutes of ingestion. This is followed by:

Similar sensations happen in the fingertips and tongue, followed by loss of voluntary movement. Rapid pulse and difficulty in breathing can occur.

Cardiac failure and paralysis of the chest and diaphragm can cause death within 12 hours.

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