A new study suggests that the fate of mangrove wetlands in northern Australia is deeply tied to the wobble of the moon.
Key points:
- New study has analyzed 30 years of satellite data to see how Australia’s mangrove wetlands have changed
- The study found that mangrove wetlands expanded and contracted in an 18.6-year cycle that coincides with the wobble of the Moon.
- Researchers say this could help explain why mangroves died out in the Gulf of Carpentaria
In 2015-16, a stretch of 1,000 kilometers of mangroves was removed in the Gulf of Carpentaria.
The death was attributed to very low rainfall and sharp drops in sea level that occurred during an intense El Niño.
But Neil Saintilan, an environmental scientist at Macquarie University, thought there might be more to it.
“Why did you have this mortality in the Gulf of Carpentaria and yet mangroves survived elsewhere?” he said
To find out, Professor Saintilan teamed up with ecologists and oceanographers to analyze 30 years of satellite images of the Australian coast.
Their data, published today in the journal Science Advances, suggests that increases and decreases in mangrove cover in Australia are driven by tidal changes as the moon’s orbit moves.
“The dominant factor is the lunar cycle, followed by a general trend of climate change, and then El Niño comes in and kicks in at different times,” Professor Saintilan said.
But others believe that tidal changes induced by the moon’s wobble, which also influence other marine habitats such as coral reefs, are only part of the picture and climate changes play a central role.
Lunar cycles and tides
The tides are partially influenced by the variation in the inclination of the Moon’s orbit, which produces an oscillation that resembles the movement of a spinning coin, Professor Saintilan explained.
The lunar nodes are the points where the Moon’s path crosses the ecliptic, the plane of Earth’s orbit that is shown as the view of the Sun from Earth over the course of a year. (Wikimedia Commons)
This oscillation influences the Moon’s pull on the oceans during an 18.6-year cycle, called the nodal cycle.
The tides are most exaggerated when the lunar plane is more aligned with the Earth’s equator.
This is different from king tides, which occur during full and new moon phases and peak every 4.4 years.
But most of Australia’s northern coast is dominated by the nodal cycle, Professor Saintilan said.
During the peak of the cycle, most of the coastline experiences very high tides, which can cause flooding, and when it is in a trough, every 9.3 years, the tides are at their lowest level.
Satellite images taken between 1987 and 2020 showed that the mangrove canopy over most of northern Australia changed in sync with the nodal lunar cycle.
“If you look at mangrove cover, it’s just starting to move into this 18-year cycle,” Professor Saintilan said.
Mangrove cover changed over an 18-year cycle. (Provided by: Neil Saintilan)
During the peak of the cycle, when the tides were higher, the mangroves were flooded more often and for longer, so they flourished.
During the troughs of the cycle, they lose their leaves.
Adding to this trend are changes in mangrove areas due to, for example, rainfall and rising sea levels caused by global warming.
The wobbly moon 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.
Hundreds of kilometers of mangroves died in the Gulf of Carpentaria in 2015/16. (Provided by: James Cook University)
This just happened to coincide with a gulf nodal cycle.
Unlike other parts of the north coast, which have two high tides a day, the Gulf only has one.
According to Professor Saintilan and his team, this means that the gulf goes through the minimum phase of the nodal cycle when areas that have two tides a day experience the maximum phase of the cycle.
This means that while the 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 almost 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 the mangroves of Kakadu and northwestern Western Australia were affected during the 2015 El Niño, they recovered.
“Mangroves survive and recover very quickly,” he said.
“But they didn’t recover in the Gulf of Carpentaria in 2015, because you had the coincidence of the cycle and El Niño together that really brought the flood down to the point where you had mass mortality.”
Troughs of the nodal lunar cycle also coincided with earlier disappearances 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 usually find a lot of mangrove deaths there,” Professor Saintilan said.
7,500 hectares of mangroves have died on the southern and western sides of the Gulf of Carpentaria in Karumba. (ABC North West Queensland: Lucy Murray)
Another indication that El Niño was not the only culprit was the fact that the most affected mangroves in the gulf were the most exposed to the tides.
“Even in the midst of El Niño in the Gulf of Carpentaria, [mangroves in non-tidal areas] they were still expanding and doing well,” he said.
El Niño and climate change most important factors
Ecologist Norman Duke of James Cook University, who led the earlier paper implicating the die-off with El Niño events, was not convinced that lunar cycles were the driving factor behind changes in mangrove cover.
“Tides are definitely there, but they’re not the only thing,” Professor Duke said.
“My hunch is that it’s El Niño, with rain and tidal influences following that [which are contributing factors].”
El Niño, marked by the weakening of tropical Pacific trade winds, can lead to less rainfall in eastern and northern Australia and extreme changes in sea level.
“This disruption of the trade winds is absolutely critical for sea level influences,” Professor Duke said.
And unlike lunar cycles, the El Niño-Southern Oscillation (ENSO) pattern is affected by climate change, he added, noting that the 1982 and 2015 gulf mangrove die-offs also coincided with the mass bleaching of the Great Barrier Reef.
The death of the mangroves corresponded to mass bleaching events in the Great Barrier Reef. (OBJECTIVE: Roger Steene)
Rainfall and rising sea levels are also causing mangrove areas to expand into salt marsh areas.
Oceanographer Charitha Pattiaratchi of the University of Western Australia’s Ocean Institute also agreed that while lunar cycles played a role in the fate of mangroves, ENSO’s influence 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 from underlying changes in sea level rise due to climate change.
This is not to say that lunar cycles do not have an impact on other marine environments.
His team’s research, published this week, found a similar impact of lunar cycles on reefs in areas such as northwestern Australia, where the lowest tides during the middle of the day during the trough of the 18 nodal cycle ,6 years have contributed to the bleaching of corals. during El Niño events.
Meanwhile, La Niña events cause sea levels to rise.
While this could benefit the mangroves, it can lead to 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.
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