More than three years after the cataclysmic eruption of Tonga’s Hunga volcano — the most powerful anywhere in more than a century — scientists say life is beginning to creep back across an ash-smothered seafloor that still shifts like liquid.
An international team led by Associate Professor Rebecca Carey of the University of Tasmania has been surveying the submarine volcano, about 65 kilometres north of Tongatapu, from CSIRO research vessel Investigator. Working alongside Tonga Geological Services, the researchers are mapping the reshaped volcanic structure and cataloguing early stages of ecological recovery on the seabed.
The eruption buried the surrounding seafloor beneath ultra-fine ash that, when disturbed, behaves less like sediment and more like a slurry. Currents have repeatedly reworked the flour-like material into soft, unconsolidated muds and silts, pooling in depressions to form thick ponds. Scattered among these plains are house-sized boulders blasted from the volcano and transported across the bottom by sediment flows, now perched like rocky islands above a dark grey expanse.
Despite the desolate scene, the team reports clear, if patchy, signs of renewal. Using a suite of tools — including a deep-towed camera capable of filming at 4,000 metres, a benthic sled, a multicorer for one‑metre sediment samples and a giant piston corer that can retrieve cores up to 24 metres long — scientists have identified two distinct communities: species clinging to hard surfaces and opportunists exploiting the mud.
On the boulders, colonies of bryozoans and other bottom-dwellers have begun to establish miniature reefs, turning the rocks into stepping stones for recolonisation. Elsewhere, sea cucumbers and other scavengers pick over the soft substrate. Stalked hydroids punctuate the otherwise sparse landscape.
Below the surface, the recovery story is more advanced at a microscopic scale. In the upper centimetres of mud, researchers found thriving populations of porcelaneous miliolid foraminifera — single‑celled organisms well suited to soft sediments — alongside tube‑forming agglutinated foraminifera, which construct tiny armoured casings from fine grains and sit vertically with their feeding structures extended above the mud. Shell remains of floating and free‑swimming microfossils, including planktonic foraminifera and pteropods, are more abundant in the top layers than deeper down, suggesting the “sediment soup” is beginning to compact and stabilise.
Even so, the team cautions that progress is uneven. The mud remains so soft that many species cannot gain a foothold, and the seabed appears prone to resurfacing — potentially being liquefied and remobilised by earthquakes — which could bury nascent communities and reset the clock.
For Tonga, the work is as much about hazard and livelihoods as it is about natural history. Tonga Geological Services senior geologist Mele Manu said the voyage is delivering vital new information on post‑eruption seabed changes and risks, and bolstering the country’s ability to monitor and prepare for future events. “The research also enhances our understanding of ecosystem recovery and the return of marine life, supporting Tonga’s biodiversity and livelihoods that depend on them,” Ms Manu said.
The observations will help answer a pressing regional question: how long marine ecosystems take to rebound after large‑scale disturbances. That knowledge matters across the Pacific, where communities rely heavily on coastal and offshore waters for food security and income, and where resilience to natural disasters is a constant priority.
Sea time for the expedition was provided through CSIRO’s Marine National Facility, supported by the National Collaborative Research Infrastructure Strategy. CSIRO scientists are also working in the Kingdom of Tonga on an ACIAR‑funded project aimed at building farm resilience through improved soil management.
