Assessing Net Growth of Phytoplankton Biomass on Hourly to Annual Time Scales Using the Geostationary Ocean Color Instrument

Questions of whether diurnal changes in carbon fixation affect the global carbon budget cannot be answered using the present generation of polar orbiting ocean color sensors that can only retrieve one image daily. Here, we present novel satellite‐derived indices of chlorophyll‐based production based on the Geostationary Ocean Color Imager (GOCI), whose hourly imaging capability offer the potential for direct estimates of net phytoplankton growth over hourly to seasonal time scales. Our results reveal large variations in net chlorophyll growth in the GOCI study region, both over the day and between seasons. Hourly changes in chlorophyll concentration are highest during spring while growth rates show maxima during the winter. We show seasonal relationships between growth and photon flux. Our study suggests that Geostationary Ocean Color data can be used to constrain phytoplankton productivity on diurnal time scales and be an essential tool to better understand diurnal growth patterns over large spatial regions. Plain Language Summary The rate at which microscopic ocean plants, or phytoplankton, consume carbon dioxide represents a gap in scientific knowledge that needs to be filled in order to better model the earth system. To aid in this understanding we use a novel technique that allows us to track the growth behavior of phytoplankton in the Yellow Sea and the East Sea‐Japan Sea. This is enabled by using satellite data from the Geostationary Ocean Color Imager, which has the unprecedented ability to collect quality biological information from the ocean surface each daylight hour. We find that the results, while in agreement with local observations and other satellite studies, also contain information about how phytoplankton change over daily to annual cycles and how native communities adapt in response to the annual solar cycle. This information is useful to the ocean modeling community, that seeks to understand various ways in which phytoplankton communities affect the cycling of Earth’s carbon. Key Points The Geostationary Ocean Color Instrument tracks changes in net phytoplankton growth at hourly intervals Gains, losses, and net accumulations of chlorophyll show strong diurnal and seasonal variability Seasonality in net growth versus light provides evidence for seasonal evolution in phytoplankton community composition