Pink Rock Pool

January 2011

Why is this rock pool pink?

While beach combing near some rock pools; we came across this pool of water which is coloured pink. Is this pollution?

Answer

Pollution can affect the colour of fresh or marine water bodies; however there are some clues that suggest this is not chemical pollution. There is a visible line of salt crystals on the rock around the edge of the pool. This indicates that the water level as dropped from evaporation leaving the salt behind. Another clue is that no oily film can be seen on the water’s surface, so this is not petroleum or oil pollution.

So what is causing the water to be pink? With the isolation of this pool from the ocean the water has been slowly evaporated, leaving the salt within the pool. This has caused the salt concentration (or salinity) within the water to increase beyond the normal level for a marine environment.

In normal marine habitats animals can handle salinity levels of 30 – 40‰. Once the levels pass 45 – 52‰ most animal life is lost. Habitats showing over 50‰ salinity are called hypersaline.

There are some very small animal that can survive in hypersaline environments - bacteria. Once the habitat becomes hypersaline and loses most of the available oxygen; a bacteria known as anoxygenic phototrophic bacteria start to multiply. These bacteria love low oxygen and high salt environment and they are very unique. They are able to photosynthesis (like plants) without oxygen, instead they use hydrogen. Populations of these bacteria are often seen as purple or green pigmentation in the water.

This photo shows a hypersaline rock pool coloured pink by anoxygenic phototrophic bacteria.

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