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How did this algal bloom suddenly appear in the lake?

The effect of the torrential rain over the over the vast area of recently burnt alpine forest was to wash ash and soil rich in nitrogen and other nutrients into the Gippsland Lakes. Counter intuitively, the rain and floods also increased salinity in the Lakes as the higher water level facilitated greater mixing with seawater at Lakes Entrance.

[…]

…what you’re seeing here is a second generation of algae (Noctiluca Scintillans) which grew by eating the first generation of algae (Synechococcus), which itself bloomed by feeding on the runoff from the floods, which had been filled with nutrients from wildfires. Basically, this event was two years and two disasters in the making – and it was also completely harmless. The glowing algae are not toxic, and did not wind up choking off other life forms in the lakes.

via io9.com