The distribution of phytoplankton biomass and primary production in the ocean can be studied from satellite platforms (MODIS and MERIS) using solar induced fluorescence (SIF, also called passive fluorescence) from chlorophyll a (ca. 675 nm band). Although the SIF algorithm is potentially most advantageous in Case 2 waters, where DOM and suspended sediments interfere with chlorophyll algorithms based on blue-green water leaving radiances, the application of SIF to understanding either phytoplankton physiology or carbon fixation on a global scale has not been achieved. Chlorophyll fluorescence from phytoplankton is strongly affected by physiological state and taxonomic composition. Basic photosynthesis research over the last decade tremendously improved our understanding of the physiological mechanisms that control the quantum yields of chlorophyll fluorescence, however, the contributions of these mechanisms to the observed variability of SIF in the ocean remain largely unknown. Although the variability in SIF yields is correlated with environmental forcing, the mechanisms responsible for this variability are not known due to very limited field studies of related processes. In this start-up project, we develop an instrumental package to directly measure fluorescence lifetimes and yields in the ocean and to study the physiological mechanisms of variability in the quantum yield of SIF with an overarching goal to develop improved models and algorithms for retrieving phytoplankton biomass, physiological status, and the rates of primary production. The quantum yield of chlorophyll fluorescence is directly derived from Time Correlated Single Photon Counting measurements of fluorescence lifetimes and is integrated with the variable fluorescence technique. We envision that this research project will provide the scientific background for the interpretation of solar induced fluorescence signals in the ocean, help to improve MODIS biomass algorithms, and provide crucial physiological information needed for better estimates of primary production from remote sensing of the ocean color.
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