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A “magical” swarm of moon jellyfish colliding with algae dazzled onlookers this week, but it comes with a warning.

The bloom of jellyfish – temporary increases in populations – has occurred over the past few weeks in Storm Bay, east of Hobart, and as far as halfway up Tasmania’s east coast. The biologist and jellyfish expert Lisa-ann Gershwin said the population growth was “unprecedented” and had “stepped up dramatically” last month.

She said filming the Aurelia aurita – moon jellyfish – swimming through bioluminescent organisms on Thursday was “the most magical thing I’ve seen in my life”, but also a sign of something wrong.

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This week’s events form part of a “fascinating sequence”, Gershwin said, that began in early December when there was a bloom of salps – small jellyfish-like creatures – across Tasmania’s Storm Bay and halfway up the east coast.

“It was just like nothing I’d ever seen before,” she said.

“Then those died off, and created a pulse of nutrients that led to an epic, epic bloom of Noctiluca scintillans which is the bioluminescent algae,” she said.

Jellyfish blooming is a natural part of their lifecycle, but the large numbers are drivers of harm in the ocean, Lisa-ann Gershwin says. Photograph: Lisa-ann Gershwin

“It has been truly, truly epic. Those are now kind of diminishing a bit, and we’re getting the jellyfish building.”

Gershwin, who runs “glow tours” of nature, took a group to the Hobart waterfront on Thursday night.

Despite the spectacular visual display, she said the jellyfish bloom was part of a sequence that pointed to “something is wrong in the ocean”.

“Each of them is a visible indicator of this and also a driver of more damage. But combined, one after the other, after the other, it’s insane,” she said.

How long the jellyfish will linger depends on access to food, and the weather, Gershwin says. Photograph: Lisa-ann Gershwin

Gershwin said while jellyfish blooming was a natural part of their lifecycle, the large numbers were drivers of harm in the ocean.

“What they do is they eat the eggs and larvae of other species, so that those species don’t have the next generation … the jellyfish just eat them all,” she said.

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Gershwin said salps consumed “massive” amounts of phytoplankton so “nothing else has anything to eat”.

“They’re able to just take all of these ecosystems in these explosions of populations,” she said.

Bioluminescence in Montagu Bay Reserve, Tasmania in December. Photograph: Jenny Kathy, Bioluminescence Tasmania

Bioluminescent algae also gorges on phytoplankton and sucks oxygen out of the water, Gershwin said.

Both the jellyfish and algae were “terrible pests” in their own right, but were also a very conspicuous sign that “something’s out of balance”, she said.

How long the jellyfish could linger depends on access to food, and the weather, Gershwin said.

“Nobody knows, we are literally in unprecedented circumstances.”

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