O'Malley Robert T.

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Robert T.

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  • Article
    Satellite-detected fluorescence reveals global physiology of ocean phytoplankton
    (Copernicus Publications on behalf of the European Geosciences Union, 2009-05-08) Behrenfeld, Michael J. ; Westberry, Toby K. ; Boss, Emmanuel S. ; O'Malley, Robert T. ; Siegel, David A. ; Wiggert, Jerry D. ; Franz, Bryan A. ; McClain, Charles R. ; Feldman, G. C. ; Doney, Scott C. ; Moore, J. Keith ; Dall'Olmo, Giorgio ; Milligan, A. J. ; Lima, Ivan D. ; Mahowald, Natalie M.
    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.
  • Preprint
    Assessing the uncertainties of model estimates of primary productivity in the tropical Pacific Ocean
    ( 2008-03) Friedrichs, Marjorie A. M. ; Carr, Mary-Elena ; Barber, Richard T. ; Scardi, Michele ; Antoine, David ; Armstrong, Robert A. ; Asanuma, Ichio ; Behrenfeld, Michael J. ; Buitenhuis, Erik T. ; Chai, Fei ; Christian, James R. ; Ciotti, Aurea M. ; Doney, Scott C. ; Dowell, Mark ; Dunne, John P. ; Gentili, Bernard ; Gregg, Watson ; Hoepffner, Nicolas ; Ishizaka, Joji ; Kameda, Takahiko ; Lima, Ivan D. ; Marra, John F. ; Melin, Frederic ; Moore, J. Keith ; Morel, Andre ; O'Malley, Robert T. ; O'Reilly, Jay ; Saba, Vincent S. ; Schmeltz, Marjorie ; Smyth, Tim J. ; Tjiputra, Jerry ; Waters, Kirk ; Westberry, Toby K. ; Winguth, Arne
    Depth-integrated primary productivity (PP) estimates obtained from satellite ocean color based models (SatPPMs) and those generated from biogeochemical ocean general circulation models (BOGCMs) represent a key resource for biogeochemical and ecological studies at global as well as regional scales. Calibration and validation of these PP models are not straightforward, however, and comparative studies show large differences between model estimates. The goal of this paper is to compare PP estimates obtained from 30 different models (21 SatPPMs and 9 BOGCMs) to a tropical Pacific PP database consisting of ~1000 14C measurements spanning more than a decade (1983- 1996). Primary findings include: skill varied significantly between models, but performance was not a function of model complexity or type (i.e. SatPPM vs. BOGCM); nearly all models underestimated the observed variance of PP, specifically yielding too few low PP (< 0.2 gC m-2d-2) values; more than half of the total root-mean-squared model-data differences associated with the satellite-based PP models might be accounted for by uncertainties in the input variables and/or the PP data; and the tropical Pacific database captures a broad scale shift from low biomass-normalized productivity in the 1980s to higher biomass-normalized productivity in the 1990s, which was not successfully captured by any of the models. This latter result suggests that interdecadal and global changes will be a significant challenge for both SatPPMs and BOGCMs. Finally, average root-mean-squared differences between in situ PP data on the equator at 140°W and PP estimates from the satellite-based productivity models were 58% lower than analogous values computed in a previous PP model comparison six years ago. The success of these types of comparison exercises is illustrated by the continual modification and improvement of the participating models and the resulting increase in model skill.