How to recognise blue-green algae
Blue-green algae is most common in late summer. Small amounts of blue-green algae look like tiny yellowish or greenish blobs floating in the water. In calm conditions, blue-green algae can form a thin greenish film on the surface of the water. Different kinds of blooms
Blooms of blue-green algae may vary greatly in appearance, depending on their intensity and age, as well as local conditions. More intense blooms can colour the water greenish, making it look like green paint; or they may form a greenish-yellow sludge resembling pea soup on the surface or along the shoreline. Only when it dries out does blue-green algae actually turn bluish or even purplish in colour, when colourful phycocyanins are released as its cells decompose. Blue-green algae may also give off a characteristic musty or mouldy smell.
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Mass blooms of blue-green algae may make the water look like green paint.
© Reija Jokipii
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Blue-green algae can turn bluish, grey or even bright turquoise as it gets older.
© Reija Jokipii
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Two simple tests
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Put some algae in a jar, and let it stand undisturbed for about an hour. If greenish particles rise to the surface, the algae is blue-green algae. Photo: Johanna Issakainen
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Test using a stick whether the algae is a solid mass that can be lifted out of the water. If strands of algae remain on the stick, the algae is harmless filamentous algae; but if the algae disperses into particles when you touch it with the stick, it is probably blue-green algae. Photo: Johanna Issakainen
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