Evaluation of Temporal Variability in Surface Chlorophyll-a Concentration Estimated by GCOM-C/SGLI in Onagawa Bay

概要

SGLI (Second-generation Global Imager) is a multispectral sensor mounted on the GCOM-C satellite which has 250 m spatial resolution for monitoring chlorophyll a (Chl-a) concentration in the ocean. We evaluated the usefulness of the GCOM-C/SGLI data products for estimating Chl-a variability in Onagawa Bay located along the Sanriku Coast on the Pacific Ocean of northern Japan. Spatio-temporal patterns in the distribution of Chl-a concentration is useful information for sustainable aquaculture operations because the surface chlorophyll data indicates the amount and the location of naturally occurring phytoplankton on which cultured species such as shellfish and ascidians feed. However, estimating Chl-a variability using satellite data in shallow coastal waters, particularly nearshore areas, can be challenging due to possible interferences originating from benthic and terrestrial substrates.

　In this study, we compared monthly transitions of Chl-a concentrations estimated by the G-COMC/SGLI with in situ observation data obtained at 12 sites in Onagawa Bay between January 2018 to February 2021. At sampling stations where the distance to the nearest land was ≧ 500 m, the SGLI estimates correlated significantly with in situ Chl-a measurements. This suggests the GCOM-C/ SGLI product can be used reliably for estimating the spatio-temporal Chl-a variability so as long the sampling stations are located sufficiently far from the coastline. Moreover, we analysed the Chl-a variability in association with changes in the monthly average of sea surface temperature (SST) based on “JAXA Himawari Monitor” to assess the potential influent of the Oyashio Current. The Oyashio Current comes from the Arctic Ocean and flows southward passing through the Bering Strait transporting cold and nutrient- rich water into the Pacific Ocean.

　Our study indicated the Chl-a values estimated by the SGLI were consistently higher when the waters of the Oyashio Current flew in or passed around the coast of Onagawa Bay, suggesting higher primary productivity observed in Spring months may be driven primarily by the intrusion events of the Oyashio Current.