Volcanic ash may enhance phytoplankton growth in the ocean over 100 km away

A research group in Japan has suggested that ash released from volcanic eruptions on Nishinoshima Island—part of Japan's Ogasawara Islands—led to a temporary surge in phytoplankton levels in the seawater around Mukojima Island, which is located 130 km northeast of Nishinoshima and is also part of the Ogasawara Islands.

Mukojima lies within the subtropical gyre, a region known for low nutrient and low chlorophyll conditions. The study indicates that ash from the Nishinoshima eruptions was transported by wind and ocean currents to the waters around Mukojima, serving as a nutrient source for phytoplankton growth in that area.

The findings suggest that volcanic ash can enhance the productivity of marine waters even at considerable distances from the volcanic site. This study was published in the journal Progress in Earth and Planetary Science.

A large-scale volcanic eruption occurred at Nishinoshima between December 2019 and July 2020, releasing a substantial amount of ash into the ocean. Previous studies have shown higher levels of phytoplankton near the volcanic site. However, the effects of this ash on more distant areas were not well understood.

A group of researchers from Nagoya University, Tohoku University, Meiji University, and Waseda University, led by Professor Joji Ishizaka from the Institute for Space-Earth Environmental Research at Nagoya University, conducted a study to investigate the distant effects of ash from the Nishinoshima eruption.

The researchers analyzed satellite images of chlorophyll-a (Chl-a) around the Ogasawara Islands during the eruption of Nishinoshima. Generally, higher levels of Chl-a indicate a greater abundance of phytoplankton.

This study used two types of satellite image data. The first dataset was obtained from the Moderate Resolution Imaging Spectroradiometer sensor (MODIS) on NASA's Aqua satellite. Results showed that among the Ogasawara islands, Mukojima especially experienced a significant increase in Chl-a concentrations, rising from around 0.1 mg/m³ during the non-eruption period to a peak of nearly 0.2 mg/m³ during the eruption.

The second dataset was generated using data from the Japanese meteorological satellite Himawari-8, which was provided through the P-Tree System by JAXA. This dataset also confirmed an increase in Chl-a concentrations around Mukojima during the eruption period.

Based on these findings, the researchers hypothesized that the eruptions on Nishinoshima caused the increase in Chl-a around Mukojima. To verify this theory, they conducted numerical simulations using sea surface water velocity data from the Global Ocean Forecast System 3.1. They traced the movement of seawater with high Chl-a levels that reached the Mukojima area after transport of the ash by wind.

The results supported the hypothesis, demonstrating that the increase in Chl-a observed near Mukojima on July 4 was due to the volcanic ash released on Nishinoshima on June 28, six days prior.

Professor Ishizaka stated, "Our analyses of satellite data and numerical simulations suggest that the eruptions on Nishinoshima likely caused a phytoplankton bloom near Mukojima."

He explained, "Volcanic ash from the eruptions was released into the atmosphere and fell along the smoke to the northeast. The ash-containing seawater then traveled approximately 130 km to Mukojima. During this journey, the ash acted as a nutrient, promoting the growth of phytoplankton. Consequently, we observed a significant increase in phytoplankton around Mukojima six days later."

This study demonstrated that nutrients from volcanic eruptions can boost phytoplankton growth in areas with limited nutrients, even those far from the eruption site. These findings improve our understanding of ocean ecosystem dynamics and lay the groundwork for further research on the complex effects of volcanic activity on local ecosystems.

About the research team
This research was conducted over the course of more than two years by a team of students from Nagoya University, Tohoku University, Meiji University, and Waseda University, under the leadership of Professor Ishizaka. The students had formerly participated in the Advanced Satellite Training Course, which was organized by the Center for Orbital and Suborbital Observation at Nagoya University's Institute for Space-Earth Environmental Research.