Millions of tiny single-cell dynamos, most likely to be dinoflagellates, have been visible as waves break along Manly shores.

"It's a good signal of spring in coastal waters," said Iain Suthers, an expert in fish and plankton-like sea creatures at the University of NSW.

Ocean upwellings of nutrients combined with favourable coastal winds and the warming conditions of spring are likely to have prompted the algal-like bloom, Professor Suthers said.

The action of the waves – and even the movement of fish or crustaceans through the water – provides the agitation needed to trigger the luminescent glow.

"The cells might have enough energy to produce two or three flashes and then they need to recharge," he said.

The species may well be noctiluca scintillans – commonly known as sea sparkle - which can appear like fish eggs with a diameter of 1-2 millimetres and which leave a brownish red smear on the surface during the day.

At night, though, the chemicals in the tiny creatures can be triggered, letting off a bluish glow. Nights with little moonlight for competition can make the best viewing times.

A 2010 paper by US-based expert Edith Widder identified reasons that creatures release light, noting that some animals use it to coat predators with luminescent slime, making them easier targets for other predators.

For the dinoflagellates off Manly, though, the role of the luminescence is likely to be ward off predators by suggesting they won't make a tasty meal.

Large blooms of noctiluca may indeed not be palatable since the ammonia ions they use to stay afloat can be lethal if they drift into salmon farms and are consumed in quantity, Professor Suthers said.

Noctiluca are now being found as far south as Tasmanian waters, and are one indication that climate change is warming the world's oceans, he said.

Spring time

Waters off Sydney are running at close to 18 degrees, or about 1-2 degrees warmer than usual for this time of year, said Brett Dutschke, senior meteorologist with Weatherzone.

Other signs of spring's arrival can be seen in the fewer cold days, although the past week with rain on most days has masked some of the warming.

More such conditions are likely in coming days with another 40-80 millimetres of rain possible over the Sydney region, Mr Dutschke said.

That rain should lift the monthly rainfall tally above 200 millimetres – well over double the average for the month – making it the wettest August since 1998, Mr Dutschke said.

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