Red Tides Revealed – What is a red tide and why does it occur?

At the moment we’re experiencing a “red tide”  in Gordon’s Bay.  You often might hear about “red tide” in the media and see browny orange patches of water.  But what is a red tide and how does it develop?

What is a red tide?

The term red tide is a bit misleading because the water is not always red – sometimes it can be brown, orange, purple or yellow. The colouration varies with the type of phytoplankton, its pigments, size and concentration – even the time of day and angle of the sun that you see it!

Most of the red tides we experience are caused by a high concentration of phytoplankton called dinoflagellates.

What causes a red tide?

Southerly winds during summer or early autumn cause cold, nutrient-rich water from the deeper parts of the ocean to rise up to the surface.  This process is called upwelling.  Dinoflagellates cysts (the resting state of the organism) which have been lying dormant in the sediments on the sea floor are carried towards the surface during this upwelling.  High quantities of nutrients in the upwelled water, ideal temperature, salinity and light, causes the cysts to germinate, grow and divide.  This rapid increase in dinoflagellate numbers is known as a “bloom” of phytoplankton.  A concentration of the bloom in one area due to wind and currents, as well as the dinoflagellates’ ability to swim to the surface, causes the formation of a red tide.

If the nutrients are depleted, or the bloom is dispersed by wind and currents, the dinoflagellates will form dormant cysts again which sink to the sea floor – no more red tide… until conditions are favourable once again.


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