Abstract
Abstract. Phytoplankton photosynthesis links global ocean biology and climate-driven fluctuations in the physical environment. These interactions are largely expressed through changes in phytoplankton physiology, but physiological status has proven extremely challenging to characterize globally. Phytoplankton fluorescence does provide a rich source of physiological information long exploited in laboratory and field studies, and is now observed from space. Here we evaluate the physiological underpinnings of global variations in satellite-based phytoplankton chlorophyll fluorescence. The three dominant factors influencing fluorescence distributions are chlorophyll concentration, pigment packaging effects on light absorption, and light-dependent energy-quenching processes. After accounting for these three factors, resultant global distributions of quenching-corrected fluorescence quantum yields reveal a striking consistency with anticipated patterns of iron availability. High fluorescence quantum yields are typically found in low iron waters, while low quantum yields dominate regions where other environmental factors are most limiting to phytoplankton growth. Specific properties of photosynthetic membranes are discussed that provide a mechanistic view linking iron stress to satellite-detected fluorescence. Our results present satellite-based fluorescence as a valuable tool for evaluating nutrient stress predictions in ocean ecosystem models and give the first synoptic observational evidence that iron plays an important role in seasonal phytoplankton dynamics of the Indian Ocean. Satellite fluorescence may also provide a path for monitoring climate-phytoplankton physiology interactions and improving descriptions of phytoplankton light use efficiencies in ocean productivity models.
Highlights
Phytoplankton are taxonomically-diverse, single-celled organisms that populate the upper sunlit layer of most water bodies on Earth, and they are photosynthetic
Iron stress is found to be a key factor influencing satellite-based quantum yields, consistent with physiological investigations of iron stress effects (Sakshaug and Holm-Hansen, 1977; Rueter and Ades, 1987; Behrenfeld et al, 2006b, 2008). These results indicate that satellite fluorescence quantum yields may prove an important new tool for characterizing iron stress in the global oceans, improving ocean photosynthesis estimates, and resolving climatephytoplankton interactions
Spatial patterns in Fsat are correlated to first order with chlorophyll estimates (Chlsat) (Neville and Gower, 1977; Gower and Borstad, 1990), as illustrated in Fig. 1 using July 2004 images
Summary
Phytoplankton are taxonomically-diverse, single-celled organisms that populate the upper sunlit layer of most water bodies on Earth, and they are photosynthetic. Incident irradiance and pigment absorption strongly influence this “natural fluorescence” signal (e.g., Babin et al, 1996; Cullen et al, 1997; Ostrowska et al, 1997; Maritorena et al, 2000; Morrison, 2003; Westberry et al, 2003; Laney et al, 2005; Huot et al, 2007; Schallenberg et al, 2008), which was proposed originally as a tool for deriving phytoplankton photosynthetic rates (Topliss and Platt, 1986; Kiefer et al, 1989) and can register physiological variability (Letelier et al, 1997; Morrison, 2003) Such field observations were instrumental to the development of satellite fluorescence detection capabilities. These results indicate that satellite fluorescence quantum yields may prove an important new tool for characterizing iron stress in the global oceans, improving ocean photosynthesis estimates, and resolving climatephytoplankton interactions
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