How the Moon Devastated a Mangrove Forest

How the Moon Devastated a Mangrove Forest

In 2015 the moon’s wobble and an El Nino teamed up to kill off tens of millions of Australian mangroves

Gray mangroves are sensitive to water level. Credit: Gary Bell/Oceanwide/Minden Pictures

The mystery emerged in 2015, when nearly 10 percent of the seemingly healthy mangrove forest along northern Australia’s Gulf of Carpentaria suddenly died. Scientists initially blamed this crucial ecosystem‘s die-off solely on an unusually strong El Nino, a weather pattern that periodically siphons water away from the western Pacific and lowers local tides. But a new study published in Science Advances reveals that El Nino had a stealthy accomplice: the moon.

Researchers analyzed more than 30 years of national satellite data to narrow down the suspect list. The study’s lead author Neil Saintilan is a biogeographer at Macquarie University, Australia. He says that it was “just the most phenomenal data set.” A pattern quickly emerged–about every 18 to 19 years, mangrove tree cover along the Gulf of Carpentaria thinned out significantly before bouncing back to normal within a couple of years. The mangrove canopies grew unusually dense nine years after each such dieoff.

This regularity provided the researchers with an important clue. Saintilan states that nature is often chaotic. “If something is superregular it’s likely to be an orbital cycle .”

“The 18.6-year cycle is essentially driven by what we call a ‘wobble'” in the moon’s orbit, says climate dynamics researcher Sophie Wilmes, who studies tides at Bangor University in Wales and was not involved in the mangrove investigation. Daily ocean tides are affected by lunar gravity. As the moon’s orbit oscillates, or wobbles, over 18.6 years, it creates regular, sustained periods of unusually high or low tides in certain places. This effect is especially strong in the Gulf of Carpentaria; its low tides can drop by an average of 40 centimeters because of its location relative to the equator and the shape of Australia’s coastline.

Indeed, the researchers found that Australia’s 2015 mangrove die-off fell 18 and a half years after the previous one. And because of the 2015 El Nino, the trees were hit with a low-tide double whammy: El Nino decreased tides an additional 40 centimeters, a fatal blow to water-loving mangroves.

Although the orbital mechanics and wobble of the moon have been extensively studied, there has not been much work on its impact on ecology. Saintilan and his collaborators hope to investigate whether this phenomenon affects mangroves in other parts. They also plan to study the effects of climate change on sea-level rise and natural ecological patterns. A moderate rise might reduce some of the tidal drops, which could help to preserve mangrove forests. However, an extreme rise could flood the trees at the cycle’s highest tidal point. Saintilan states that it is possible to predict when or if mangroves will be facing major problems.

This article was originally published with the title “Mangrove Moon” in Scientific American 327, 6, 22 (December 2022)

doi: 10. 1038/scientificamerican1222-22b


    Joanna Thompson is an insect enthusiast and former Scientific American intern. She is based New York City. Follow Thompson on Twitter @jojofoshosho0

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