A foremost objective for NASA�s ocean color missions has been the quantification of global productivity. One of the primary challenges in this quest is the accurate characterization of phytoplankton physiological variability. Recognition of this limitation was one of the key factors supporting the development of solar-stimulated fluorescence measurement capabilities on the MODIS sensors. While the utility of MODIS fluorescence data as an alternative measure of phytoplankton chlorophyll biomass has been demonstrated, we have failed to achieve the initial goal of linking fluorescence to physiology. This failure reflects shortcomings in the analysis of MODIS data and misunderstandings within the oceanographic community regarding fluorescence-physiology links. Here we propose a research project aimed at retrieving physiological information from MODIS fluorescence data. Our approach and underlying objectives, however, are distinctly different than earlier attempts. Instead of pursuing a general relationship between satellite fluorescence data an algal growth rates or photosynthetic efficiencies, we intend to use MODIS data to specifically characterize the distribution of iron-limited phytoplankton populations. We build upon a solid understanding of iron-stress effects on phytoplankton fluorescence and center this study on a new, unparalleled field data set for model development and validation.
For the past 12 years, we have conducted extensive field measurements of phytoplankton physiological characteristics in the tropical Pacific ocean. These studies, which will continue through 2007, have revealed unique fluorescence characteristics that can be used to identify specific growth constraints over vast areas. Physiological processes underlying these dramatic fluorescence diagnostics have been resolved and are linked to iron stress and the relative availability of reduced nitrogen sources. We have identified a fluorescence diagnostic that is caused by synthesis of special iron-stress-induced pigment-protein complexes with greatly elevated fluorescence yields. In areas where these complexes are abundant, their enhanced fluorescence properties should be clearly seen in MODIS fluorescence data. Detecting the signal, however, will require significant improvements in remote sensing retrievals of phytoplankton fluorescence efficiencies. Our proposed project accordingly begins with this prerequisite algorithm development and progresses toward a remote sensing capability to assess iron limitation in the ocean. Results from this work will contribute to a better understanding of changing growth constraints in natural marine phytoplankton populations, improved satellite-based estimates of ocean productivity, more complete descriptions of land-atmosphere-ocean feedbacks, and hopefully a resurgence of interest in remote sensing fluorescence measurements as a powerful tool for studying our global ocean ecosystems.
