Salinity in shallow estuaries rose as freshwater inflows dropped with the drought, and then the bushfires brought additional nitrogen and phosphorous – including from fire retardants – that spurred cyanobacteria growth. The big storms provided the final blow for much of the aquatic life, Mr Fallon said.
Deeper than about eight metres or in areas where waters more easily mix, such as much of Sydney Harbour, sea life continues largely unaffected, he said.
The Abyss Project’s report is scheduled to be made public on Monday. The loss of so-called foundational species would likely cause “feedback loops occurring throughout the entire ecosystem”, said Nathalie Simmonds, head of marine science at the project.
Some species are recovering faster than others, with potentially long-lasting impacts.
At places such as Monterey in Botany Bay, highly varied species previously found attached to nets and other submerged objects had begun to be replaced by an unidentified algal species.
“They are all completely gone except for this one algae that has literally covered everything,” Mr Fallon said.
David Booth, a marine ecologist at University of Technology Sydney, said poor water quality and then coronavirus pandemic restrictions had limited the ability of scientists to examine the impact of “this chemical event”.
To lose kelp from many areas and also urchins – normally “two opposing forces” – was very unusual, Dr Booth said. Kelp is typically resilient while urchins “are pretty good at hunkering down” but both were hammered.
Effects on fish may take a while to show up. For instance, some grazing species may fare better than others depending on the mix of plant species that return.
The Abyss Project’s report noted that “climate change will see an increase in the intensity and frequency of extreme weather events”, but also worse droughts and bushfires.
“[O]ur observational and scientific data will provide a baseline for future researchers to enable these coastal environments to be protected for future generations,” it said.
Professor Booth said those increasing climate stresses in the future could “just decouple everything” in the marine ecosystems around Sydney and beyond.
In wake of the recent destruction of aquatic life, the Berejiklian government should revisit plans for a Sydney Marine Park, he said.
The government ditched the plan in September 2018 – six months out from last year’s state elections – even before the public consultation period had closed after complaints from anglers and others.
“It shouldn’t have dropped off the agenda,” Professor Booth said, adding the originally proposed marine park comprised 17 “sanctuary zones” covering just 2.4 per cent of the waters around Sydney.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.