Bloom, baby, bloom: Dramatic growth of phytoplankton follows Tonga eruption

A new study by a team of scientists from the University of Hawai‘i at Mānoa and Oregon State University has revealed that the largest underwater volcanic eruption of the century led to a dramatic bloom of microscopic marine life that covered an area nearly 40 times the size of Oʻahu.

The explosion of phytoplankton growth north of the island of Tongatapu in the Kingdom of Tonga happened within 48 hours of the eruption of Hunga Tonga-Hunga Haʻapai. Phytoplankton are tiny photosynthetic organisms that produce oxygen and serve as the base of the marine food web. The growth of these microbes can increase rapidly when nutrients become available.

“Even though the Hunga Tonga-Hunga Haʻapai eruption was submarine, a large plume of ash reached a height of tens of kilometers into the atmosphere,” Benedetto Barone, lead author of the study and research oceanographer at the Center for Microbial Oceanography: Research and Education in UH-Mānoa’s School of Ocean and Earth Science and Technology, said in a press release. “The ash fallout supplied nutrients that stimulated the growth of phytoplankton, which reached concentrations well beyond the typical values observed in the region.”

Three of the study authors previously assessed and sampled a smaller phytoplankton bloom linked with the 2018 eruption of Kīlauea on the Big Island.

“It was fairly straightforward to modify the computer code that I had written to analyze the satellite measurements around Hawaiʻi to determine the impact of the Tonga eruption on the nearby ocean ecosystem,” Barone said in the press release. “From the first moment of seeing the results of the analysis, it was clear that there had been a fast phytoplankton response in a large region.”

The research team was led by the School of Ocean and Earth Science and Technology. It analyzed satellite images, emission of radiation and light reflection at the sea surface and determined that the deposition of volcanic ash following the Hunga Tonga-Hunga Haʻapai eruption was likely the most important source of nutrients responsible for massive phytoplankton growth.

“We were impressed to observe the large region with high chlorophyll concentrations within such a short time after the eruption,” Dave Karl, study co-author and director of the Center for Microbial Oceanography: Research and Education. “This shows how quickly the ecosystem can respond to nutrient fertilization.”

The eruption was a natural fertilization event that revealed the capacity of phytoplankton to respond fast — when the right conditions arise.

“A casual observer might see seemingly very different parts of the environment — in this case, a volcano producing a large eruption and a major shift in the ecology of the oceans nearby,” Ken Rubin, study co-author and volcanologist in the School of Ocean and Earth Science and Technology Department of Earth Sciences. said in the press release. “However, our observations illustrate the broad interconnectedness and interdependence of different aspects of the environment, perhaps even indicating an underappreciated link between volcanism and shallow marine ecosystems globally.”

Phytoplankton pull carbon dioxide, the culprit behind the warming of the planet, from the atmosphere. Barone said the dynamics of the enormous phytoplankton bloom following the Tonga eruption can help predict the behavior of pelagic environments, when nutrients are added to nutrient-impoverished regions of the ocean.

“This knowledge can prove useful in the discussion about the impacts of carbon dioxide removal technologies based on ocean fertilization,” Barone said.