Question: What causes the red tide?

Hayley Evers-King answered on 12 Mar 2013:

Hello 🙂 Thanks for asking a question about my favourite topic!!

The colour of the red tide is caused by tiny creatures which live in the ocean called phytoplankton and algae. They are kind of like plants – they photosynthesise. Sometimes they have red pigments in them and when huge numbers of them grow, they make the water red. Other times, the way they absorb and scatter light makes the water look red, even though they don’t have red pigments.

Red tides happen when lots and lots of phytoplankton or algae grow and reproduce. We call this a ‘bloom’. This happens when the best conditions for growth combine. This means that there has to be lots of food (nutrients) for the creatures, lots of light and they have to grow faster than they are eaten. Where I work in South Africa, we get lots of nutrients brought in to the coastal areas because of the way the ocean circulates here (it’s related to the wind, which stirs up cold water from the deep which is full of nutrients!). After the wind has mixed up the nutrient rich water, the wind needs to stop for a while for a bloom to happen. Other wise the phytoplankton/algae get mixed down too deep in the water, where its dark and they can’t get the light they need to photosynthesis and grow.

If all the conditions are right, a red tide happens and can cause a lot of problems for fish and us. But it can also cause beautiful blue waves to happen at night – this is called bioluminescence. There’s a brilliant video of it here: http://io9.com/5845782/watchers-surfers-ride-neon-blue-bioluminescent-waves-caused-by-red-tide