The fate of mangrove wetlands across northern Australia is deeply entwined with the wobble of the Moon, a new study suggests.
In 2015-16, a 1,000-kilometre stretch of mangroves in the Gulf of Carpentaria was wiped out.
The dieback was attributed to very low rainfall and steep drops in sea level that occurred during an intense El Niño.
But Neil Saintilan, an environmental scientist at Macquarie University, thought there could be more to the story.
"Why did you have this mortality in the Gulf of Carpentaria and yet elsewhere the mangroves survived?" he said.
To find out, Professor Saintilan teamed up with ecologists and oceanographers to analyse 30 years of satellite images of Australia's coastline.
Their data, reported today in the journal Science Advances, suggests increases and decreases of mangrove cover across Australia are driven by tidal changes as the Moon's orbit wobbles.
"The dominant factor is the lunar cycle, followed by an overarching trend of climate change, and then El Niño comes in and punctuates at different times," Professor Saintilan said.
But others believe that Moon-wobble-induced tidal changes, which also influence other marine habitats such as coral reefs, are just part of the picture, and shifts in climate play the central role.
Lunar cycles and tides
Tides are partially influenced by the changing tilt of the Moon's orbit, which produces a wobble that resembles the movement of a spinning coin, Professor Saintilan explained.
This wobble influences the pull of the Moon on the oceans over an 18.6-year cycle, dubbed the nodal cycle.
Tides are more exaggerated when the lunar plane is more closely aligned with Earth's equator.
This is different to king tides, which occur during the full and new moon phases and peak every 4.4 years.
But most of northern Australia's coast is dominated by the nodal cycle, Professor Saintilan said.
During the peak of the cycle, the coastline experiences very high tides, potentially causing flooding, and when it is in a trough, every 9.3 years, the tides are at their lowest.
Satellite images taken between 1987 and 2020 showed mangrove canopy over most of Northern Australia changed in sync with the nodal lunar cycle.
"If you look at the cover of the mangroves, it just starts moving into this 18-year cycle," Professor Saintilan said.
During the peak of the cycle, when tides were higher, mangroves were inundated more frequently and for longer, so they flourished.
During cycle troughs, they lost their leaves.
On top of this trend are changes to mangrove areas due to, for instance, rainfall and sea-level rise caused by global warming.
Moon wobble and El Niño a fatal combination
While mangroves in many parts of Northern Australia survived despite El Niño conditions in 2015-16, those in the Gulf of Carpentaria suffered.
This just happened to coincide with a nodal cycle trough for the Gulf.
Unlike other parts of the northern coast, which have two high tides a day, the Gulf only has one.
According to Professor Saintilan and his team, the Gulf goes through the minimum phase of the nodal cycle when areas that have two tides a day experience the peak phase of the cycle.
This means while mangroves in the Gulf are left high and dry, those elsewhere are sitting pretty.
During the low phase of the cycle, tidal levels can drop by nearly half a metre.
El Niño events also cause average sea levels to drop, but Professor Saintilan said the impact on mangroves was only short term.
While mangroves in Kakadu and north-west Western Australia were affected during the 2015 El Niño, they recovered.
"Mangroves survive and they bounce back really quickly," he said.
"But they didn't bounce back in the Gulf of Carpentaria in 2015, because you had the coincidence of the cycle trough and the El Niño together that really dropped inundation to the point where you had mass mortality."
Troughs in the nodal lunar cycle also coincided with earlier diebacks in the Gulf in 1998 and 1982.
"We also know that there are El Niños that don't coincide with the nodal cycle. You don't tend to find much mangrove dieback in those," Professor Saintilan said.
Another clue that El Niño was not the only culprit was the fact that the worst-affected mangroves in the Gulf were those most exposed to the tides.
"Even in the midst of the El Niño in the Gulf of Carpentaria, [mangroves in non-tidal areas] were still expanding and doing just fine," he said.
El Niño and climate change bigger factors
Ecologist Norman Duke from James Cook University, who led the previous paper implicating dieback with El Niño events, was not convinced that lunar cycles were the driving factor for changes in mangrove cover.
"Undoubtedly, tides are right there, but they're not the only thing," Dr Duke said.
"My hunch is that it is El Niño, with rainfall and tide influences after that [which are contributing factors]."
El Niño, marked by weakening tropical Pacific trade winds, can cause less rainfall in Australia's east and north, and extreme changes in sea level.
"That interruption of the trade winds is absolutely critical to sea level influences," Dr Duke said.
And unlike lunar cycles, the El Niño-Southern Oscillation (ENSO) pattern is affected by climate change, he added, noting that mangrove dieback in the Gulf in 1982 and 2015 also coincided with mass bleaching on the Great Barrier Reef.
Rainfall and sea level rise also causes mangrove areas to expand into salt marsh areas.
Oceanographer Charitha Pattiaratchi at the University of Western Australia's Ocean Institute also agreed that while lunar cycles played a role in mangroves' fate, the influence of ENSO was greater.
Professor Pattiaratchi, who was not involved in the new study, also said more data was needed to separate any effects on mangroves due to lunar cycles against underlying changes in sea-level rise due to climate change.
That's not to say that lunar cycles don't have an impact on other marine environments.
His team's research, released this week, found a similar impact of lunar cycles on reefs in areas such as north-western Australia, where lower tides during the middle of the day during the trough of the 18.6-year nodal cycle have contributed to coral bleaching during El Niño events.
Meanwhile, La Niña events cause higher sea levels.
While this might benefit mangroves, it can cause coastal flooding, especially during the peak of the nodal lunar cycle.
"Next year will be close [to the peak]. You will have higher water levels," Professor Pattiaratchi said.
