Relation between eruption at Nishinoshima and chlorophyll-a concentration at Ogasawara Islands in 2020

Abstract Nishinoshima and Mukojima lie within the subtropical gyre, a region known for low-nutrient and low-chlorophyll (LNLC) conditions. These LNLC waters typically limit phytoplankton growth due to a lack of essential nutrients. The brightness temperature observed by Himawari-8/9 was correlated with volcanic activity at Nishinoshima, and we focus on the period from June to July 2020, when volcanic activity was most active. Data from the AQUA satellite’s Moderate Resolution Imaging Spectroradiometer (MODIS) and Himawari-8 daytime observations showed that the chlorophyll-a concentration (Chl-a) around Mukojima also peaked, with Seasonal and Trend decomposition using Loess (STL) analysis residuals increasing by 4.43% during the eruption period. Particle simulations of backward trajectories of seawater around Mukojima suggest that the increase in Chl-a observed near Mukojima on July 4 was attributable to volcanic ash released on June 28. These results indicate that volcanic ash spreading northeastward from Nishinoshima was carried by ocean currents to areas around Mukojima and within 100 km of Nishinoshima, resulting in increased Chl-a and phytoplankton growth over approximately six days.