Two people are dead and Tonga's government is advising the public to remain indoors after the eruption of the Hunga Tonga Hunga Ha'apai volcano, the World Health Organization (WHO) says.
Around 2 centimetres of volcanic ash and dust has fallen on Tongatapu —Tonga's main island— since Saturday's eruption, which triggered tsunami warnings across the Pacific Ocean.
Ash in the air and on the ground has raised concerns about air pollution and the potential contamination of food and water supplies, the WHO said.
Locals have been advised to drink bottled water and wear masks outdoors to avoid breathing in the ash.
"Thankfully, all health facilities on Tongatapu are fully functioning and clean-up efforts have been initiated," the WHO said.
It said initial reports were that around 100 houses had been damaged and 50 completely destroyed on Tongatapu.
"Many [Tongans] remain displaced, with 89 people taking shelter in evacuation centres on the island of 'Eua and many more seeking shelter with relatives," the WHO said.
"The Ha'apai and Vava'u island groups … remain out of contact with the capital [Nuku'alofa]."
There were particular concerns about the smaller and low-lying islands of Mango and Fonoi in the Ha'apai group, the WHO said.
Volcanic ash could 'could deliver long-lasting damage'
Scientists have warned that volcanic ash could deliver long-lasting damage to coral reefs, erode coastlines and disrupt fisheries.
The scientists have been studying satellite images and looking to the past to predict the future of the remote region.
Since the initial eruption, the volcano has been releasing sulfur dioxide and nitrogen oxide — two gases that create acid rain when they interact with water and oxygen in the atmosphere.
With Tonga's tropical climate, "there is likely to be acid rain around Tonga for a while to come", according to volcanologist Shane Cronin from the University of Auckland.
Acid rain causes widespread crop damage, and could ruin Tongan staples such as taro, corn, bananas and garden vegetables.
"Depending on how long the eruptions last, food security could be compromised," Professor Cronin said.
Satellite imagery shows the plume from the volcano spreading westward, which means Tonga could be spared some of this acid rain, although Fiji could then be in its path.
In a bulletin on Monday, the UN humanitarian affairs office said Fiji was monitoring its air quality, and had advised people to cover their household water tanks and stay indoors in the event of rain.
Tonga's exclusive economic zone of nearly 700,000 marine square kilometres is 1,000 times larger than its land area.
And most Tongans get their food — and livelihood — from the ocean.
While scientists have yet to investigate on the ground, "the few pictures that are available seem to show a blanket … of ash" on land, according to Marco Brenna, a geologist from the University of Otago in New Zealand.
Marine life could be 'poisonous or poisoned'
In the ocean, that ash can be harmful to marine life.
Weeks before Saturday's eruption, Tonga Geological Services had warned that nearby seawater was contaminated with toxic volcanic discharge, and that fishermen should "assume fish in these waters are poisoned or poisonous".
Inevitably, the weekend's huge eruption has made the situation worse.
Murky, ash-filled water near the volcano will deprive fish of food and wipe out spawning beds.
Some fish will perish, and survivors will be forced to migrate, scientists said.
Further, changes in the structure of the sea floor could create new obstacles for fishing vessels.
"It will be a while before the same or new fishing grounds will be restored," Dr Brenna said.
Coral 'buried and smothered' by ash
Falling ash can also smother coral reefs that, in Tonga, are the mainstay of a tourism industry that brought in up to $5 million per year before the COVID-19 pandemic.
Even before the eruption, Tonga's reefs were threatened by disease outbreaks and the effects of climate change, including coral bleaching and increasingly strong cyclones.
Now, "vast areas of the reefs in the immediate impact area at Hunga Tonga are probably buried and smothered by large deposits of volcanic ash", according to Tom Schils, a marine biologist from the University of Guam who has studied volcanic eruptions and corals in the Northern Mariana Islands.
Such eruptions also release more iron into the water, which can boost the growth of blue-green algae and sponges, which further degrade reefs.
Reefs may have to start over — a process that could take years, according to Brian Zgliczynski, a coral reef ecologist at the Scripps Institution of Oceanography.
"Species more tolerant of poor water quality will arrive first," Dr Zgliczynski said.
Meanwhile, hard corals and fish would take longer to return, he added.
A loss of coral reefs would also affect Tonga's ability to cope with rising waters and storm surges.
This is a concern for Tonga, where climate change is driving the sea level to rise by about 6 millimetres per year, double the global average.
In a 2015 report, Tonga valued its natural storm buffers — including coral reefs as well as coastal seagrasses and mangroves — at some $11 million annually.
With the latest eruption, a Tongan sea-level gauge recorded a tsunami wave of 1.19 metres before it stopped reporting.
Tsunamis are known to cause rapid coastal erosion.
And before communication systems went down, videos revealed damage to man-made seawalls.
"Coastal defences and reclaimed land could all be strongly impacted by the tsunami waves, leaving the islands more vulnerable," Professor Cronin said.
ABC/wires
