Phytoplankton natural fluorescence variability in the Sargasso Sea |
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| Institution: | 1. College of Marine Science, University of South Florida, St. Petersburg, FL, United States of America;2. National Satellite Ocean Application Service, Ministry of Natural Resources, Beijing, China;3. State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China;1. Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bussestr. 24, 27570 Bremerhaven, Germany;2. Institute of Environmental Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany;1. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling, Shaanxi 712100, China;2. School of Instrumentation Science and Opto-Electronics Engineering, Beihang University, Beijing 100083, China;3. Key Laboratory of Digital Earth Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China;4. Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China;5. Key Laboratory of Agricultural Soil and Water Engineering in Arid Area of Ministry of Education, Northwest A&F University, Yangling, China;6. Institute of Water Saving Agriculture in Arid Regions of China (IWSA), Northwest A&F University, Yangling, China;1. Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR, USA;2. Horn Point Laboratory, Center for Environmental Science, University of Maryland, Cambridge, MD, USA |
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| Abstract: | Phytoplankton fluorescence has been used historically as a means of assessing phytoplankton biomass, rates of primary production (PP) and physiological status in laboratory, in situ, and satellite based investigations. Assumptions about the quantum yield of phytoplankton fluorescence, φf, are often overlooked and can become problematic when fluorescence based methods are applied. A time series of φf observations from the northwestern Sargasso Sea is presented with the goal of understanding the controls on fluorescence and its applicability for assessing upper ocean biological processes. Accurate estimates of φf require accounting for Raman scattering and the conversion of planar to scalar irradiance. Variability in φf occurs on both seasonal and episodic time scales. Seasonal variations show maxima in the surface layer during summer months while lower, more uniform values are found throughout the winter when deep mixing occurs. Large episodic variations in φf are observed throughout the record which dwarf seasonal changes. Predictions of depth-dependent and depth-integrated PP rates using φf and natural fluorescence fluxes are only marginally successful (r2～50%), although comparable with results from global bio-optical models for the Sargasso Sea. Improvements in PP predictions are hindered by weak statistical relationships with other parameters. φf is largely decoupled from the quantum yield of carbon assimilation, φc, indicating that an inverse relationship between fluorescence and photosynthesis does not exist. Consequently, variability in the quantum yield of thermal de-excitation, φh, is found to be of similar magnitude as φf on the timescales observed. These observations show that assumptions about photochemical energy flow through the phytoplankton community must be made carefully and that the fluorescence–photosynthesis relationship is not straightforward. |
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